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Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups
Icarus 149, 173–189 (2001) doi:10.1006/icar.2000.6512, available online at http://www.idealibrary.com on
Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups1 Jorge M´arcio Carvano, Daniela Lazzaro, Thais Moth´e-Diniz, and Cl´audia A. Angeli Departamento de Astrof´ısica, Observat´orio Nacional, 20921 Rio de Janeiro, Brazil E-mail: [email protected]
and Marcos Florczak Departamento de F´ısica, CEFET, 80000 Curitiba, Brazil Received March 31, 2000; revised August 8, 2000
We observed 29 Hungaria and 31 Phocaea asteroids at the European Southern Observatory (Chile) in the wavelength range 4900– ˚ The Phocaea and the Hungaria are both high-inclination 9200A. groups, located at the inner edge of the main belt near the ν5 , ν6 , and ν16 secular resonances. We confirm that the Hungaria group is composed mainly of E-type asteroids but with a relatively large number of S-types. The Phocaea group, on the other hand, is composed mainly of S-type asteroids. We discuss the possible implications of the observed composition distribution on the origin and evolution of these high-inclination groups. °c 2001 Academic Press Key Words: asteroids; asteroids, composition; spectroscopy; surfaces, asteroids.
1. INTRODUCTION
The Hungaria and Phocaea dynamical groups are located at the inner edge of the main belt of asteroids. They are composed of high-inclination (and high-eccentricity for the Phocaea group) asteroids located in a region with very complex dynamics (Scholl and Froeschl´e 1986). The Hungaria group lies near the edges of the ν5 and ν16 secular resonances while the Phocaea group is virtually surrounded by the ν5 , ν6 , and ν16 secular resonances, as can be seen in Fig. 1. In addition, a number of low-order mean motion resonances also occur in the proximity of these groups. By their location, they might be, as recently suggested (Migliorini et al. 1998, Michel et al. 2000), possible sources of the asteroids that must be replenishing the short-living Marscrosser population. The expression “dynamical group” is used to mean asteroids that are currently clustered in the orbital element space, in opposition to “family,” which designates a cluster of asteroids in the 1 Based on observations made with the 1.52-m telescope at the European Southern Observatory (La Silla, Chile) under the agreement with the CNPq/Observat´orio Nacional (Brazil).
proper element space. The origin of these two kinds of clusters is assumed to be much different: while the former is due to some long-term dynamical process, the latter is probably the outcome of the collisional break-up of a larger asteroid. Several authors, using different sets of proper elements as well as distinct clustering techniques and criteria, have regarded several asteroids in the Phocaea region as members of a possible but doubtful family (Carusi and Valsecchi 1982, and references therein). The analysis of the taxonomies of a small sample revealed (Bell 1989) the presence of cosmochemical incompatible compositions ruling out a common genetic origin. Zappal`a et al. (1990), using a refined proper elements set (Milani and Knezevic 1990) and a hierarchical clustering algorithm, also failed to identify a family in the Phocaea region, but the authors acknowledge that this result may be due to the poor accuracy of the proper elements. More recent works on family identification (Zappal`a et al. 1994, Bendjoya 1993) rely on the same improved set of proper elements by Milani and Knezevic (1992) which, however, is still not suitable for high-inclination and/or -eccentricity orbits. For such objects the semi-analytic theory by Lemaitre and Morbidelli (1994) should be preferred (Knezevic et al. 1995) but it has not been yet thoroughly applied to these groups, specifically searching for families. For the Hungaria group Lemaitre (1994) seems to identify some dynamical clustering, possibly indicating the presence of a family, and we will discuss these results farther on. On the other hand, the compositional analysis of a statistically significant number of members of these groups may provide interesting data to aid in the assessment of the reality of the dynamical groupings and of collisional families. The composition sampling of these regions of complex dynamics in the inner edge of the main belt may also allow a better understanding of the transport mechanisms from the main belt to planet-crossing orbits. With these purposes in mind, we have performed a spectral survey of asteroid members of the Phocaea and Hungaria groups. Here we present the analysis of visible CCD spectra of
173 0019-1035/01 $35.00 c 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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TABLE I Hungaria Group—Circumstances of Observation Asteroid 434 Hungaria
FIG. 1. The inner part of the main belt of asteroids indicating the locations of the Hugaria and Phocaea dynamical groups along with the approximate position of the main resonances.
29 and 31 members of the Hungaria and the Phocaea group, respectively. In Section 2 we describe the observations and the reduction of the data. The results for each group are discussed separately in Sections 3 and 4. In Section 5 we summarize the obtained results and discuss some of their implications. 2. OBSERVATIONS
The observations were carried out at the European Southern Observatory at La Silla (ESO-Chile). We used a 1.5-m telescope equiped with a Boller and Chivens spectrograph and a CCD 2048 × 2048 pixels with a readout noise of ±7 electrons. ˚ A grating of 225 g/mm with a dispersion of 330 A/mm in the first order was used. The CCD has a square 15-µm pixel, giv˚ ing a dispersion of about 5 A/pixel in the wavelength direction. ˚ with an The useful spectral range is about 4900 < λ < 9200 A, ˚ The spectra were taken through a 5-arcsec slit FWHM of 10 A. oriented in the east–west direction. The slit width was chosen in order to minimize the effects of atmospheric differential refraction and reduce the loss of light at both ends of the spectrum. All of the observations were made as near as possible to the meridian of the asteroid. The spectra of the Hungarias and Phocaeas were taken in eight observing runs, from November 1996 to June 1999, with atmospheric conditions good to excellent during most of the observations. The spectral data reduction was performed using the Image Reduction and Analysis Facility package following standard procedures. A full description of the data reduction process is given in Florczak et al. (1998).
UT date
1997/01/06 1997/01/06 1025 Riema 1997/01/06 1996/11/17 1355 Magoeba 1998/03/08 1998/03/08 1509 Esclangona 1998/03/07 1998/03/08 1998/03/08 1600 Vyssotsky 1999/06/18 1919 Clemence 1997/03/17 1997/03/17 2001 Einstein 1998/09/02 1998/09/03 1998/08/30 1998/08/30 2150 Nyctimene 1997/01/07 2272 Montezuma 1997/01/08 2491 Tvashtri 1997/01/09 3043 San Diego 1997/03/20 3101 Goldberger 1997/01/06 3169 Ostro 1997/12/27 3309 Brorfelde 1997/03/17 1997/03/17 3400 Aotearoa 1997/01/08 3447 Burckhalter 1998/09/03 1998/08/29 1998/08/29 3880 Kaiserman 1997/07/07 1997/07/07 1997/07/13 3940 Larion 1998/01/01 1998/01/04 4116 Elachi 1998/12/21 1998/12/22 4125 Lew Allen 1997/03/19 4483 Petofi 1997/12/21 1997/12/21 1997/12/24 1997/12/24 1997/12/24 4713 Steel 1997/07/07 1997/07/15 4764 Joneberhart 1998/12/16 5639 1989 PE 1998/01/03 1997/12/27 1997/12/28 6310 Jankonke 1997/01/07 1997/01/07 6394 1990 QM2 1997/01/11 6447 Terrycole 1997/01/09 6461 1993 VB5 1997/01/07 6493 Cathybennett 1997/01/08
R (AU) 1 (AU) Mag 2.06 2.06 1.92 1.94 1.77 1.77 1.80 1.80 1.80 1.87 1.89 1.89 2.05 2.05 2.05 2.05 1.85 1.71 1.82 2.07 1.99 1.88 1.75 1.75 1.75 2.04 2.04 2.04 1.88 1.88 1.87 1.87 1.87 1.73 1.73 2.08 1.85 1.85 1.85 1.85 1.85 1.82 1.83 2.01 1.88 1.88 1.88 1.87 1.87 2.04 1.97 1.78 1.83
1.29 1.29 1.10 1.31 0.99 0.99 1.02 1.02 1.02 1.26 0.89 0.89 1.07 1.07 1.09 1.09 1.27 1.33 1.37 1.14 1.13 1.58 0.87 0.87 0.92 1.06 1.06 1.06 0.98 0.98 1.01 1.14 1.12 1.21 1.20 1.22 1.09 1.09 1.10 1.10 1.10 0.90 0.88 1.23 1.06 1.06 1.06 1.41 1.41 1.17 1.20 0.94 1.27
14.39 14.39 15.23 15.81 15.51 15.51 15.16 15.14 15.14 15.10 14.86 14.86 15.22 15.19 15.31 15.31 16.56 17.19 17.03 16.25 16.93 16.45 15.86 15.85 16.28 14.87 14.87 14.87 16.14 16.14 16.29 15.55 15.49 16.22 16.20 16.47 14.59 14.59 14.64 14.64 14.64 14.87 14.71 16.65 16.68 16.66 16.66 16.84 16.84 16.20 16.56 15.54 16.26
α
Solar analog
21.58 21.58 21.19 27.32 27.03 27.02 25.95 25.79 25.79 30.58 2.27 2.26 9.83 9.22 11.60 11.62 29.80 34.78 32.07 13.15 18.34 31.42 20.89 20.89 23.98 8.41 8.53 8.52 20.17 20.18 22.80 25.80 25.18 33.58 33.48 17.88 25.41 25.41 25.80 25.80 25.80 19.36 16.22 22.09 22.00 21.66 21.68 31.04 31.04 17.06 22.58 23.27 30.70
HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD1835 HD9562 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD9562 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD144585 HD1835 HD160417 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594
Note. For each spectrum are given, in column 1, the observation date, in columns 2 to 5, respectively, the geocentric and heliocentric distances, the visual magnitude, and the solar phase angle, and in column 6, the solar analog used to obtain the reflactance spectra. The ephemeris were taken from JPL’s HORIZONS System (http://ssd.jpl.nasa.gov/horizons.html).
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TABLE II Phocaea Group—Circumstances of Observation
TABLE II—Continued 4533 Orth
Asteroid 25 Phocaea
UT date
1998/03/06 1998/03/07 1998/03/08 1998/03/09 1997/01/11 105 Artemis 1997/01/02 1997/01/02 1996/11/17 265 Anna 1997/07/08 1997/07/08 1997/07/15 273 Atropos 1997/12/24 1997/12/31 1997/12/31 1997/12/31 323 Brucia 1997/07/08 1997/07/08 1997/07/14 1997/07/14 502 Sigune 1997/12/28 1997/12/28 914 Palisana 1998/01/04 1997/12/23 1997/12/23 1997/12/23 950 Ahrensa 1996/11/19 1108 Demeter 1997/07/06 1997/07/06 1997/07/13 1318 Nerina 1997/03/17 1997/03/17 1997/03/19 1322 Coppernicus 1997/01/11 1367 Nongoma 1998/03/08 1998/03/09 1568 Aisleen 1997/12/23 1573 Vaisala 1998/03/07 1998/03/08 1575 Winifred 1996/11/18 1591 Baize 1996/11/17 1883 Rimito 1998/08/29 1998/08/30 2014 Vasilevskis 1997/01/12 2050 Francis 1999/06/19 2105 Gudy 1998/03/09 1998/03/09 1997/12/22 2965 Surikov 1997/01/12 3888 Hoyt 1999/06/20 3913 Chemin 1996/11/17 4121 Carlin 1997/07/08 4132 Bartok 1998/12/12 1998/12/14 1998/12/14 1998/12/15 4340 Dence 1997/07/07 1997/07/13 4511 Rembrandt 1998/08/29 1998/08/29 1998/08/30
R (AU) 1 (AU) Mag 2.39 2.38 2.38 2.38 3.01 2.68 2.68 2.62 2.26 2.26 2.28 2.64 2.64 2.65 2.64 2.31 2.31 2.29 2.29 2.58 2.58 2.88 2.86 2.86 2.86 2.40 1.95 1.95 1.94 1.85 1.85 1.85 2.96 2.34 2.34 2.14 2.60 2.60 2.41 2.54 2.00 2.00 2.69 1.82 2.35 2.35 2.23 2.27 1.79 2.81 2.05 1.87 1.88 1.88 1.88 1.83 1.83 1.81 1.81 1.81
1.69 1.68 1.67 1.65 2.14 2.59 2.59 1.92 1.54 1.54 1.49 2.01 2.09 2.09 2.09 1.29 1.29 1.28 1.28 2.48 2.49 1.93 1.89 1.89 1.89 1.53 1.17 1.17 1.22 0.86 0.86 0.85 2.03 1.47 1.47 1.27 1.80 1.81 1.72 1.98 1.18 1.18 2.33 0.85 1.55 1.55 1.77 1.30 0.93 1.97 1.35 1.37 1.35 1.35 1.35 1.08 1.13 0.89 0.89 0.90
11.88 11.86 11.83 11.80 12.56 13.90 13.90 13.08 14.99 14.99 14.87 14.86 14.98 14.98 14.98 12.42 12.42 12.49 12.49 15.88 15.88 12.99 12.79 12.79 12.79 15.21 14.86 14.86 14.99 13.06 13.06 13.08 17.14 16.52 16.52 15.14 16.53 16.54 16.39 16.25 16.05 16.04 16.72 14.46 15.04 15.04 15.45 16.40 15.16 16.68 15.80 15.07 15.05 15.05 15.03 15.83 15.97 14.23 14.23 14.25
α
Solar analog
20.30 20.09 19.88 19.65 10.58 21.37 21.37 17.92 22.23 22.23 20.00 18.91 19.86 19.86 19.86 2.94 2.94 4.93 4.93 22.26 22.26 6.45 3.76 3.76 3.76 13.54 24.56 24.57 26.85 0.85 0.86 1.16 7.35 14.80 14.97 15.88 15.82 16.00 19.90 20.94 22.00 21.98 21.02 13.86 17.58 17.58 25.30 5.44 24.17 12.62 25.50 30.56 30.23 30.23 30.06 27.89 29.38 19.21 19.21 19.37
HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD144585 HD144585 HD44594 HD144585 HD144585 HD44594 HD44594 HD144585 HD144585 HD44594 HD144585 HD144585 HD144585 HD44594 HD44594 HD144585 HD44594 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585
4826 Wilhelms 6084 Bascom 6560 Pravdo
1997/03/17 1997/03/17 1997/03/20 1997/01/11 1996/11/18 1998/12/10 1998/12/16 1997/01/11
1.93 1.93 1.93 2.29 2.18 2.02 2.04 2.60
0.99 0.99 1.01 1.99 1.40 1.44 1.39 1.62
15.01 15.01 15.10 16.67 15.65 16.34 16.24 15.49
13.71 13.71 14.99 25.26 20.28 26.92 25.47 3.77
HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594
Note. See Table I note.
Asteroids and analogs were observed usually at airmasses close to 1.0 and not greater than 1.3, with a few exceptions. Atmospheric extinctions were corrected using the mean extinction curve of La Silla (Tug 1977). Different solar analogs were observed in each observing run in order to compute reflectivities. On each night, at least two solar analogs were observed in order to estimate the quality of the night. The ratios between the spectra of the solar analogs for each night do not show any substantial variation. Tables I and II show, for the Hungaria and Phocaea groups, respectively, the date of observation, the geocentric and heliocentric distances, and the visual magnitude, and solar phase angle of each asteroid discussed in this work, as well as the solar analog used to obtain the divided spectrum. Whenever more than one spectrum was available, the least noisy was chosen to be representative of the superficial mineralogy of the asteroid and it is used in all plots as representative of the asteroid. All asteroid spectra were normal˚ by convention. The resulting spectra are shown in ized at 5500 A Appendixes A and B for the Hungaria and Phocaea groups, respectively. No correction was made for phase-reddening effects, since all asteroids were observed at phase angles smaller than 35◦ and therefore any phase-induced effect should be smaller than 1% (Luu and Jewitt 1990). 3. THE HUNGARIA GROUP
The spectra were visually classified according to template spectra defined in Tholen’s taxonomy. Classification was made according to general similarities to the template spectra, determined by means of visual inspection and with the aid of albedo data, whenever available. Our sample of the Hungaria group is composed of a majority of X-type (18) asteroids, a relatively large proportion of S-types (8), two distinct C-types, and an A-type. In Fig. 2 we present running box averages of the spectra of the Hungaria sample grouped according to their classification in the present work. We recall that the X-type classification designates a spectrum characteristic of the classes E, M, or P. These classes, as defined in Tholen’s taxonomy, are said to be spectrally degenerated since the information available on the visible and NIR spectra is not sufficient to distinguish between them. These three classes have nearly identical spectra and, according to Tholen, can only be distinguished by their albedos, which
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FIG. 2. Spectra of the observed asteroids in the Hungaria group, sorted by spectral type. Along with the S-type asteroids is shown the spectrum of the A-type, 1600 Vyssotsky. All spectra are normalized at 5500 A˚ and averaged by a running box.
are high for E-, intermediate for M-, and low for P-type asteroids. The superficial mineralogies implied by each of these classes are considerably different: carbon/organic-rich silicates for the P-class, metal with possibly traces of silicates or metal + enstatite for the M-class, and enstatite or other iron-free silicates for the E-types (Gaffey et al. 1989). The inferred presence of enstatite and/or metal in the E-type implies that their par-
ent bodies should have been heated to temperatures higher than 1500◦ C. This interpretation is, however, in contradiction with the discovery of hydration absorption bands around 3 µm in the spectra of 44 Nysa and (possibly) of 64 Angelina, both E-types, and of the M-types 92 Undina and 201 Penelope, among others (Rivkin et al. 1995). These results imply formation temperatures no greater than 500◦ C for the parent bodies (Jones et al. 1990). Since no albedo is available for any of the X-types in our sample, it is not possible to determine their classification according to Tholen template spectra. The location of the Hungaria group in the inner edge of the main belt seems to favor a mineralogy associated with the E- or M-classes, especially considering that 6 of the 13 E-types classified by Tholen are in this region. Recent works (Burbine et al. 1998, Bus 1999) have revealed subtle spectral characteristics that apparently are distinctive of this class. These consist of an absorption feature from 0.44 to 0.51 µm, a sharp turn-up in reflectance from 0.51 to 0.55 µm, a flat red-sloped spectrum from 0.55 to 0.70 µm, and a shallow feature that extends past 0.92 µm. All of these features have been observed by Burbine and co-workers in the spectra of 64 Angelina, 434 Hungaria, and 3103 Eger, previously classified as E-type. On his feature-based taxonomy, Bus (1999) defines an Xe-class described as having and overall slope that is slightly to moderately red with a decrease in spectral slope longward of 0.75 µm and several subtle absorption features, including an absorption band shortward of 0.55 µm and a very shallow absorption feature around 0.6 µm. This class encompasses most of the asteroids classified as E-type in Tholen. Visual inspection reveals that some, or all, of these features are present on the majority of the X-type spectra in our sample as shown in Table III. In particular, the turn-up in reflectance between 0.51 and 0.55 µm present on most of the spectra can be seen even on the somewhat noisy spectrum of 3400 Aotearoa. Based on the presence of such features we were able to assign the Xe classification to 16 of the 18 X-types. Among the 2 remaining, 4764 Joneberhart may also be an Xe-type, but its spectrum shortward of 0.51 µm cannot be trusted due to problems with the solar analog. In Table IV, the physical parameters such as the diameter, the albedo (whenever available), the mean orbital elements, and the taxonomic classification given by Tholen (1989), Bus (1999), and the in present work are given for each observed asteroid. Considering the inherent diversity of the S-type asteroids spectra, we made use of four spectral parameters, namely slope A, slope B, position of the maximum, and apparent depth, in order to better characterize our spectra. Slope A is computed as ˚ and is an a linear fit of the spectra between 5400 and 7200 A indicator of the redness of the spectra as introduced by Luu and Jewitt (1990). Slope B is calculated as a linear fit between 8000 ˚ and is indicative of the steepness of the decrease and 9000 A in reflectance after the maximum. Finally, the apparent depth is computed as the division of the normalized reflectivity at 7400 ˚ and gives an indication of the real depth. It is noteand at 9200 A worthy that these three parameters (position of the maximum, slope B, and apparent depth) are related to the position of the center and to the depth of the 1-µm olivine–pyroxene absorption
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TABLE III Hungaria Group—Physical and Dynamical Parameters Asteroid
D (km) Alb. a (AU)
e
TABLE IV Hungaria Group—Spectral Features among the X-Types
I (deg) Classa Classb Classc Asteroid
434 Hungaria 1025 Riema 1355 Magoeba 1509 Esclangona 1600 Vyssotsky 1919 Clemence 2001 Einstein 2150 Nyctimene 2272 Montezuma 2491 Tvashtri 3043 San Diego 3101 Goldberger 3169 Ostro 3309 Brorfelde 3400 Aotearoa 3447 Burckhalter 3880 Kaiserman 3940 Larion 4116 Elachi 4125 Lew Allen 4483 Petofi 4713 Steel 4764 Joneberhart 5639 1989 PE 6310 Jankonke 6394 1990 QM2 6447 Terrycole 6461 1993 VB5 6493 Cathybennett
10.5 — 1.94 0.074 22.50 5.7 — 1.97 0.038 26.85 9.4 — 1.85 0.044 22.82 8.2 0.233 1.86 0.032 22.31 15.8 — 1.84 00.037 21.17 7.8 — 1.93 0.094 19.33 10.3 — 1.93 0.098 22.68 7.9 — 1.91 0.056 25.32 4.9 — 1.86 0.089 24.32 7.0 — 1.87 0.054 22.86 7.3 — 1.92 0.106 21.79 5.5 — 1.97 0.046 28.54 16.7 — 1.89 0.066 24.90 6.4 — 1.81 0.053 21.14 5.8 — 1.93 0.098 20.23 11.5 — 1.99 0.028 20.71 6.6 — 1.94 0.082 17.56 11.0 — 1.98 0.056 22.83 8.8 — 1.87 0.080 24.11 7.3 — 1.92 0.118 20.44 15.8 — 1.92 0.083 26.72 10.5 — 1.92 0.073 22.66 7.3 — 1.93 0.047 24.83 5.8 — 1.85 0.022 26.60 7.9 — 1.91 0.031 23.59 8.0 — 1.93 0.093 22.81 7.3 — 1.95 0.085 19.77 8.4 — 1.95 0.102 23.41 9.2 — 1.94 0.083 24.71
E E X S — X X — S X — — TS — — — — — — — — — — — — — — — —
Xe Xe — — A — Xe — — — — — — — — — — — Sl — — A — — — — — — —
Xe Xe Xe S A Xe Xe S S Xe S Xe C S Xe Xe Xe Xe S S X S X C Xe Xe Xe Xe Xe
Note. For each asteroid, columns 2 and 3 show the diameters and albedo, columns 4 to 6 show the osculating semi-major axis, eccentricity, and inclination, and columns 7 to 9 show the classification according to Tholen (1989), Bus (1999), and the present work. a Tholen (1989). b Bus (1999). c Present work.
band which are associated with the mineralogical composition. However, due to insufficient spectral coverage and the onset of ˚ we are unable to put any useful contelluric bands at 9200 A, straint on the position of the band I center. The computed values of all of these parameters are presented in Table V. Due to the ˚ and the noise genpresence of an atmospheric O2 band at 7619 A ˚ the erated by the onset of the H2 O atmospheric bands at 9200 A, uncertainty on the maximum position can be fairly large and we ˚ for all S-type asteroids assigned the conservative value of 250 A in our sample. The spectra of S-types in the Hungaria group have maxima ˚ and show an apparent bimodality conbetween 7818 and 7995 A cerning the spectral slope A: four asteroids present values dis˚ while the remaining cluster around tributed around 18% 103 A 3 ˚ 13% 10 A. Both groups, however, span the same range of maximum position and slope B. There appears to be a slight correlation between the values of slope B and the maximum as well as an anti-correlation between slope A and the diameter. The latter
Turnup in reflectance Band around Decrease in spectral around 0.5 µm 0.6 µm slope after 0.75 µm
434 Hungaria 1025 Riema 1355 Magoeba 1919 Clemence 2001 Einstein 2491 Tvashtri 3101 Goldberger 3400 Aotearoa 3447 Burckhalter 3880 Kaiserman 3940 Larion 4483 Petofi 4764 Joneberhart 6310 Jankonke 6394 1990 QM2 6447 Terrycole 6461 1993 VB5 6493 Cathybennett
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No ? Yes Yes Yes Yes Yes
? ? ? ? Yes ? ? ? ? Yes Yes No ? ? ? ? ? ?
Yes Yes Yes Yes Yes Yes Yes ? ? Yes Yes No ? ? Yes No ? Yes
Note. The presence or absence of the spectral features indentified by Burbine et al. (1998) and Bus (1999) are summarized. The question mark (?) identifies cases where positive identification was not possible due to noisy spectra.
might be easily understood as the presence of more regoliths on larger asteroids but we see no obvious explanation for the first correlation. On the other hand, since we are dealing with a very small sample, any observed trend should be taken with much caution. Asteroid 4713 Steel has one of the smallest spectral slopes A in our sample but it has been classified as an A-type by Bus
TABLE V Hungaria Group—S Class Spectral Parameters
Asteroid
Slope A ˚ (% 103 A) ±5
Slope B ˚ (% 103 A) ±1
˚ Maximum (A) ±250
Apparent depth ±0.05
1509 Esclangona 2150 Nyctimene 2272 Montezuma 3043 San Diego 3309 Brorfelde 4116 Elachi 4125 Lew Allen 4713 Steel
14.22 19.30 13.47 20.54 18.20 13.75 16.93 12.72
−1.64 −11.18 −9.74 −4.81 −7.24 −5.23 −3.99 −4.82
7995 7778 7818 7834 7822 7854 7887 7897
0.97 1.07 1.04 1.07 1.03 0.97 0.99 1.03
Note. Slope A and Slope B are calculated as linear fits in the ranges 5400– ˚ , respectively. The errors quoted are typical values of 7200 and 8000–9000 A the standard deviation. The position of the maximum is given as the wavelength where a running box average with bin size of 100 is maximum. The uncertanty is estimated as the larger of the differences among values obtained with different ˚ bin sizes. The apparent depth is the ratio of the reflectances at 7400 and 9200 A of running box averages with a bin size of 100. The uncertanty is the typical value of the difference obtained with different bins.
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(1999). There is another asteroid, 1600 Vyssotsky, also classified as an A-type by Bus and confirmed as such by our observations. On the other hand, both of our spectra of 4713 Steel (observed on different nights) are typical of S-types with moderate spectral slope. The spectra are very similar to that of 4116 Elachi, which is classified as a Sl in Bus taxonomy. We ignore the cause of this discrepancy but the coherence between our two spectra is reassuring regarding the quality of our observations. Clearly, further observations are needed in order to determine the true nature of this asteroid. The two asteroids classified as C-types in our Hungaria sample, 3169 Ostro and 5639 1986PE, exhibit very similar spectra: essentially flat, with no drop off in reflectance toward the UV and with a shallow absorption feature between 0.50 and 0.70 µm. 3169 Ostro’s previous classification as a TS in Tholen’s taxonomy was made based on ECAS photometry covering only the 0.3- to 0.7-µm range (as seen in the archives of the Planetary Data System: http://pdssbn.astro.umd.edu). This incomplete coverage was probably due to problems during the observation of this particular asteroid, which may also explain the greater redness of the ECAS spectrum of 3169 Ostro. In order to analyze the spatial distribution of compositions in the Hungaria group we searched for all members with a taxonomical and/or spectral classification. First, using the 1999 version of Bowell’s catalog (Bowell et al. 1994), we identified all members of the Hungaria group defined as those asteroids with semi-major axis in the range 1.98–2.0 AU, inclinations between 16◦ and 40◦ , and aphelion distances less than 1.666 AU. Among the 130 numbered asteroids satisfying these criteria, 37 do have a compositional classification, representing around 30% of the total sample. Of these, around 60% are classified as E, X, or Xe, nearly 30% as S and the remainder as C or A. The distribution in osculating orbital elements of these asteroids is shown in Fig. 3. No clear trend is apparent except that the X-type (here including the E and Xe) seem to be located preferentially in the outer rim of the group, at distances greater than 1.92 AU. An analysis of compositional distribution in the proper elements space is more difficult due to the problems already mentioned for these high-inclination and/or -eccentricity orbits. In the upper part of Fig. 4 we show the distribution of compositions in the proper element phase space using three different data sets based on the theories of Lemaitre and Morbidelli (1994), Williams (1989), and Milani and Knezevic (1994). The red symbols represent asteroids with proper elements computed through the first theory (Lemaitre, personal communication) while the blue and green are those of Williams and Milani and Knezevic, respectively. We note again a predominance of X-types on the outer part of the group as well as some small clusters. In particular, one of these clusters corresponds to four of the eight members of the dynamical family suggested by Lemaitre (1994). These asteroids are 434 Hungaria, 1919 Clemence, 1920 Sarmiento, and 3880 Kaiserman. We note that other asteroids, such as 2048 Dwornik, 2001 Einstein, 3400 Aotearoa, and 4764 Joneberhart, also lie very near to this cluster. If this cluster indeed represents
FIG. 3. Distribution in the mean orbital elements space of all of the asteroids of the Hungaria group having a taxonomic and/or spectral classification.
a family, more precise analysis taking into account the background density, will be needed. We can only confirm that, from a mineralogical point of view, there is a coherence between these asteroids that supports the hypothesis of the collisional breakup of a larger parent body. 4. THE PHOCAEA GROUP
Among the 31 asteroids observed in the Phocaea region, 24 turned out to be S-types, with 3 C-types, 3 X-types, and a D-type, as shown in Table VI. Running box averages of the spectra of the Phocaea sample, grouped according to their classification, are shown in Fig. 5. One of the X-type asteroids, 1318 Nerina, was found to be an Xe-type, following the criteria outlined in the previous section, but all have albedos compatible with M-type asteroids. The larger asteroid in the group, 105 Artemis, is among the three C-type asteroids in our sample. Its spectrum is very similar to that of 914 Palisana, both showing no substantial drop of reflectivity toward the UV and a shallow band between 0.55 and 0.8 µm, which is more prominent on the spectrum of 105 Artemis. This band may be an indication of a process of aqueous alteration (Vilas et al. 1994, Barucci et al. 1998). The remaining C-type, 1108 Demeter, has a flat, featureless spectrum. The diversity within the S-types in the Phocaea region is comparable to that observed in the nearby Flora family (Florczak et al. 1998). The spectra present a slope A between 6 and 16%
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FIG. 4. Distribution in the proper orbital elements space of all asteroid members of the Hungaria (a) and Phocaea (b) groups having a taxonomical and/or spectral classification. The red symbols represent objects whose elements were computed using the theory of Lemaitre of Morbidelli (1994). The green and blue symbols represent asteroids that do not have proper elements computed through the above theory and for which the theories of Williams (1989) or Milani and Knezevic (1994) were used, respectively.
˚ a slope B between −20 and −6% 103 A, ˚ a maximum 103 A, ˚ ranging from 7450 to 7900 A , and an apparent depth between 1.03 and 1.24. All of the computed values are presented in Table VII where we can see no apparent correlation between any of these parameters and the diameter. This result is somewhat to be expected, given the poor range of diameters. On the other hand, there is a slight anti-correlation between the values of slope A and of the apparent depth: as the former increases the latter decreases. This can be an indication of a space weathering process which darkens the surface of the objects, increasing its redness (slope A) and decreasing the depth of the absorption band (Wetherill and Chapman 1988, Pieters and McFadden 1994, Doressoundiram et al. 1998). We caution that the apparent depth as computed in the present work is just indicative of the depth of the pyroxene–olivine band at ∼1 µm and any conclusion driven from this parameter should be taken with some caution. Within our sample of S-type asteroids we can see an almost continuous trend with the greatest asteroid, 25 Phocaea, in the upper part of the distribution (Fig. 5). In the lower part of
the distribution there are two asteroids, 950 Ahrensa and 1575 Winifred, which have distinctive spectra, with an increase of reflectance before the maximum and a decrease immediately after it. These spectra cannot be properly fitted by a straight line since the steepness decreases just before and after the maximum, giving the spectra an aspect like a “saw tooth”. Comparison with ordinary chondrite (OC) meteorite spectra measured by Gaffey (1976) shows that there are striking similarities between the spectra of these asteroids and some OC meteorites of the L4, L5, LL4, and LL5 types. Figure 6 gives running box averages of the spectra of all of the S-type asteroids of our sample along with those of some OC meteorites (Gaffey 1976) which appear in the lower part of the distribution. We recall that Gaffey et al. (1993), using a composite spectra of 25 Phocaea covering 0.4 to 2.5 µm, concluded that the silicate content of this asteroid (derived from the position of the 1-µm band and the ratio of the depths of the 1- and 2-µm bands) is indeed consistent with a OC-like composition. Moreover, analysis of the rotational variations on the spectrum of 25 Phocaea (Moth´e-Diniz et al. 2000) indicates that
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TABLE VI Phocaea Group—Physical and Dynamical Parameters Asteroid 25 Phocaea 105 Artemis 265 Anna 273 Atropos 323 Brucia 502 Sigune 914 Palisana 950 Ahrensa 1108 Demeter 1318 Nerina 1322 Coppernicus 1367 Nongoma 1568 Aisleen 1573 Vaisala 1575 Winifred 1591 Baize 1883 Rimito 2050 Francis 2014 Vasilevskis 2105 Gudy 2965 Surikov 3888 Hoyt 3913 Chemin 4121 Carlin 4132 Bartok 4340 Dence 4511 Rembrandt 4533 Orth 4826 Wilhelms 6084 Bascom 6560 Pravdo
D (km)
Alb.
a (AU)
e
I (deg)
Class a
Class b
Class c
75.1 119.1 23.7 29.3 35.8 16.0 76.6 15.0 25.6 13.0 11.0 9.6 14.5 9.8 9.5 18.7 9.2 8.9 17.5 22.5 8.6 10.1 13.8 6.7 10.5 9.2 9.1 10.5 13.8 10.5 15.1
0.231 0.047 0.105 0.162 0.176 0.340 0.094 0.179 0.046 0.181 — — — 0.223 0.245 0.106 — — — 0.183 0.086 — — 0.354 0.305 — 0.310 — — — —
2.40 2.37 2.41 2.39 2.38 2.38 2.45 2.37 2.42 2.30 2.42 2.34 2.35 2.37 2.37 2.39 2.41 2.32 2.40 2.38 2.39 2.39 2.35 2.37 2.40 2.39 2.39 2.36 2.37 2.31 2.35
0.255 0.176 0.268 0.160 0.301 0.179 0.211 0.159 0.257 0.202 0.233 0.129 0.254 0.232 0.177 0.175 0.262 0.237 0.282 0.150 0.217 0.250 0.223 0.254 0.287 0.233 0.253 0.245 0.191 0.235 0.114
21.57 21.48 25.62 20.41 24.22 25.00 25.24 23.48 24.93 24.64 23.32 22.45 24.89 24.56 24.78 24.77 25.49 26.57 21.40 29.30 24.23 22.19 23.91 23.09 23.31 25.19 22.74 22.65 25.28 22.99 23.23
S C — SCTU S S CU — CX — — — — — — — — S — — — — — — — — — — — — —
S Ch — — — — — Sa — — — — — — — — — — — — — — — — — S — — — — —
S C X X S S C S C Xe S S S S S S S S S D S S S S S S S S S S S
Note. See Table III note. Tholen (1989). b Bus (1999). c Present work.
a
this asteroid has an homogeneous surface, as should be expected if it is composed of undifferentiated chondritic material. Therefore, judging from the observed spectral slopes, aparent depths, and overall shapes of the spectra, we can conclude that the S-type asteroids of the Phocaea group span a continuous distribution bounded by OC-like objects, i.e., 25 Phocaea in the upper part and OC meteorites in the lower part. This distribution is exactly what would be expected if an ongoing space weathering process was acting on fragments of a disrupted homogeneous body. It should be stressed, however, that this assessment is only tentative, since we do not have any information on the position of the band I center. This parameter is closely related to the superficial mineralogy of an S-type asteroid and usually is regarded as being unffected by space weathering processes. Although this last statement has been put into question by laboratory experiments envolving laser bombardment of meteoritic samples (Moroz et al. 1996), our case would greatly benefit if one could show that there is no significant scatter in the band center of the S-types in the Phocaea group.
The distribution of compositions among the Phocaea group has also been analyzed in the same way as in the case of the Hungaria, discussed above. In the Phocaea case we found a taxonomic\spectroscopic classification for 43 asteroids, among a total of 154 members of the group. Among these, nearly 75% have a S-type classification while around 15% are C-types and the remainder are of other classes. The distribution of these asteroids in mean orbital elements is shown in Fig. 7. The observed predominance of S-type asteroids in the Phocaea group is expected, being the most common type in this region of the main belt. Yet, it would also be compatible with a common origin through collisional breakup. To see whether the distribution in proper elements is also compatible with this hypothesis we then examined this point using the same sets of proper elements as in the case of the Hungaria group. The majority of the S-type asteroids seem very tightly clustered while the remaining spectral types are distributed around the periphery, as shown in the lower part of Fig. 4. A notable exception is again the large C-type 105 Artemis, which falls near the center
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TABLE VII Phocaea Group—S Class Spectral Parameters
Asteroid
Slope A ˚ (% 103 A) ±5
Slope B ˚ (% 103 A) ±1
˚ Maximum (A) ±250
Apparent depth ±0.05
25 Phocaea 323 Brucia 502 Sigune 950 Ahrensa 1322 Coppernicus 1367 Nongoma 1568 Aisleen 1573 Vaisala 1575 Winifred 1591 Baize 1883 Rimito 2050 Francis 2014 Vasilevskis 2965 Surikov 3888 Hoyt 3913 Chemin 4121 Carlin 4132 Bartok 4340 Dence 4511 Rembrandt 4533 Orth 4826 Wilhelms 6084 Bascom 6560 Pravdo
16.89 9.74 10.54 12.48 6.39 11.48 10.43 10.77 9.16 8.81 9.85 9.31 8.18 10.60 11.86 7.72 11.32 11.02 11.92 12.27 9.26 8.33 12.64 14.05
−9.90 −6.66 −10.04 −15.65 −20.25 −8.13 −9.96 −7.20 −18.79 −14.90 −10.84 −6.45 −15.05 −15.55 −6.93 −12.08 −11.60 −9.63 −13.47 −8.87 −15.49 −18.92 −14.42 −9.10
7697 7707 7702 7586 7578 7691 7687 7794 7590 7467 7700 7664 7558 7568 7672 7517 7892 7684 7682 7720 7502 7773 7533 7763
1.06 1.06 1.08 1.16 1.24 1.03 1.08 1.03 1.20 1.14 1.08 1.06 1.14 1.16 1.05 1.14 1.08 1.08 1.12 1.06 1.13 1.14 1.14 1.10
Note. See Table V note for details.
FIG. 5. Spectra of the observed asteroids in the Phocaea group, sorted by spectral type. Along with the spectra of the X-types is shown the spectrum of ˚ and averaged by a the D-type 2105 Gudy. All spectra are normalized at 5500 A running box.
of the S-type cluster. The presence of such a large asteroid with implied mineralogy incompatible with the predominant S-types was the chief evidence pointed out by Bell (1989) against the reality of a Phocaea family. However, we recall that a similar argument was used to question an Eunomia family, but a recent spectroscopic survey (Lazzaro et al. 1999) indicates that large C-type asteroids are very common in this part of the inner
FIG. 6. Spectra of the S-type asteroids in the Phocaea group obtained in this work are compared with four ordinary chondrites’ spectra. The Mezo-Mandaras, Cynthiana, and Knyahinya are L3, L4, and, L5, respectively, and Soko-Banja is ˚ and LL5-type (Gaffey 1976). All of the spectra are normalized around 5500 A are averaged by a running box.
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FIG. 7. Distribution in the mean orbital elements space of all the asteroid members of the Phocaea group having a taxonomical and/or spectral classification.
belt. This might be just the result of a selection effect, since the C-type asteroids do have darker albedo, but it might also be due to some other effect such as, for example, a less friable material. Therefore, the large C-type-like asteroids in a Phocaea family may be just background objects, if a family is indeed confirmed by accurate clustering techniques. 5. SUMMARY AND DISCUSSION
In the present work we have analyzed the surface composition of a significant number of asteroid members of the Hungaria and Phocaea groups. As a general result we can say that this analysis pointed out the possible presence of some families, both in the Hungaria and in the Phocaea groups, which need now to be confirmed through refined dynamical and clustering techniques. In particular, we have found that for both groups there exist clusters of asteroids which have spatial distribution and composition compatible with the fragmentation of an larger parent body. The concentration of a great number of E/Xe-types in a relatively small region, as is the case in the Hungarias, raises several possibilities regarding its origin: (a) it could be the result of the collisional breakup of a larger E-type parent body; (b) the conditions that led to the formation of this class of asteroids occurred
only on a narrow portion of the primordial nebula, around the present Hungaria region; (c) the conditions that led to the formation of this class of asteroids occurred on a larger portion than the present Hungaria region, but asteroids in other locations were later removed due to some dynamical process; (d) the E-types in the Hungaria region were formed elsewhere in the main belt and later brought to their present position by some dynamical process. The first hypothesis seems unlikely since all of the E/Xe asteroids in the Hungaria region are small. This kind of size distribution is not the usual outcome of fragmentation theories, which expect one or two larger bodies, maybe resulting from a reaccumulation after fragmentation, and a great number of small ones (Tanga et al. 1999, Marzari et al. 1999, Benz and Asphaug 1999, and references therein ). It should be noted, however, that the physics involved in the collisional fragmentation of asteroids is not thoroughly known and it is possible that poorly constrained parameters such as the strengh of the E-type parent-body material may account for such unusual size distribution. On the other hand, the distribution in the proper elements space seems also incompatible with a unique breakup. There might exist some smaller families, as proposed by Lemaitre (1994) and discussed above, but we do not believe that all of the E-type like asteroids in this region resulted from the fragmentation of a unique body, unless there exists some other dynamical process not yet considered in the computation of the proper elements we used. In this context it is important to stress the importance of a mineralogical analysis of the remaining members of the family proposed by Lemaitre. Hypothesis (b) and, to a lesser extent (c), suffers from the fact that currently we can find E-type asteroids in other regions of the main belt. In particular, the two largest E-type, 44 Nysa and 64 Angelina, are at semi-major axes of 2.42 and 2.68 AU, respectively. On the other hand, the discovery of hydration bands on 44 Nysa and, possibly, on 64 Angelina (Rivkin et al. 1995) opens the possibility that there are actually two mineralogies associated with E-type asteroids. It would be conceivable then that the E-type asteroids in the Hungaria region have a mineralogy consistent with enstatite achondrite meteorites while E-types farther away from the Sun present a mineralogy consistent with the presence of hydration bands. Unfortunately, we cannot search for this band in our spectra, due to insufficient spectral coverage. In any case, it would remain to be shown for hypothesis (c) that the Hungaria region is indeed more dynamically stable than its surroundings. The same problem applies to hypothesis (d). This last hypothesis is conjured by the presence of large E-type asteroids at greater heliocentric distances and the fact that the distribution of taxonomic classes does have a heliocentric trend. Therefore, hypothesis (d) would gain some weight if hydratation bands were found on the E-type asteroids of the Hungaria region. On the other hand, the dynamical problems involved in transfering these asteroids from farther away from the Sun into their present orbits are huge.
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As it is, all hypotheses considered here have problems. It seems to us that a simpler solution would be a mix of hypothesis (a) with (b) or (c), as already suggested by Michel et al. (2000). Definite answers to this question, however, would depend on detailed dynamical studies regarding the reality of the proposed families and the overall stability of the Hungaria region, as well as observation campaigns designed to search for aqueous alteration on the E-types in the Hungaria group. For the Phocaea group we have shown that most of the observed objects have spectra similar to that of S-type asteroids, with just a minor percentage of different compositions. The spectra show common characteristics: a maximum around λ = ˚ and a reflectivity gradient spanning a continuous but lim7500 A ited range. We found a trend between the apparent depth of the 1-µm band and the redness of the spectra which seems indicative of an ongoing space weathering effect acting on the surface of asteroids having a similar composition. Moreover, the distribution of spectra is bounded in the upper part by the spectra of the largest asteroid of the sample, 25 Phocaea, which was already identified (Gaffey et al. 1993) as having a surface composition compatible with OC material. In the lower part of the distribution the asteroid spectra are similar to those of some OC meteorites. This distribution is, therefore, in agreement with that expected from the fragmentation of a homogenous OC-like parent body and where the largest fragment, 25 Phocaea, would retain more efficiently regoliths and, thus, have a redder spectrum. We recall that there are several lines of evidence that suggest that regolith accumulation can effectively produce such a reddening effect (Chapman 1996, and references therein). On the other hand, the clear anti-correlation between spectral slope and diameter found by Gaffey (1993) for a subsample of S-type asteroids with OClike mineralogy and diameters greater than 100 km seems to contradict this interpretation. Since the definitions of spectral slope on the two works, although different, do correlate, this inconsis-
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tency may be related to the difference of diameter range of the samples. It is noteworthy that the above scenario does not require a unique fragmentation, but just a common origin of the objects. In this sense we recall that several authors (Williams 1992, Carusi and Valsecchi 1982, and references therein) already recognized a cluster in the Phocaea region indicating a family of collisional origin. Here we have shown that the spectra of these asteroids also reflect a coherent mineralogical composition, probably affected by a variable degree of space weathering. In this scenario we suppose that the large C-type asteroid, 105 Artemis, is an interloper. If a dynamical analysis does show that the S-type asteroids in the Phocaea region are indeed the result of a collisional breakup, then the fact that some of them present a OC-like composition definitively links these asteroids to the OC meteorites. The specific mechanism able to transport fragments from the Phocaea region to Earth is to be investigated but is probably associated with one or more resonances as already suggested (Migliorini et al. 1998, Michel et al. 2000) . It is important also to note that most asteroids in the Phocaea group have a composition similar to those of the nearby Flora family (Florczak et al. 1998). Can that indicate a common origin? It is difficult to say, but since we are in a region of very complex dynamics a migration of objects from low- to high-inclination orbits cannot be ruled out, despite the presence of the µ5 secular resonance. Note that the asteroids in the Phocaea group have also higheccentricity orbits which are in agreement with a resonant transport mechanism through a mean motion resonance (Migliorini et al. 1998, Michel et al. 2000). Although we believe to have considerably increased the knowledge on the composition of the high-inclination groups of Hungaria and Phocaea, the main result of the present work is to point out the necessity and urgency of a detailed dynamical and cluster analysis of these regions which will be fundamental to addressing the important question of their origin.
APPENDIX A. Relative reflectivity of 29 asteroid members of the Hungaria group. The spectra are presented by increasing asteroid number. They are ˚ by convention. normalized around 5500 A
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APPENDIX A—Continued
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APPENDIX A—Continued
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APPENDIX A—Continued
APPENDIX B. Relative reflectivity of 31 asteroid members of the Phocaea group. The spectra are presented by increasing asteroid number. They are ˚ by convention. normalized around 5500 A
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APPENDIX B—Continued
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APPENDIX B—Continued
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ACKNOWLEDGMENTS The authors acknowledge the technical staff of ESO for their prompt help whenever needed and A. Betzler for his cooperation with some of the reductions. We are grateful to Dr. Anne Lemaitre for providing us with the proper elements set used in this paper. The present work has been supported by CNPq, CAPES, and FAPERJ through diverse fellowships and grants.
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History (Y. Kozai, R. P. Binzel, and T. Hirayama, Eds.), pp. 140–145. Astronomical Society of the Pacific Conference Series Vol. 63, San Francisco. Lemaitre, A., and A. Morbidelli 1994. Proper elements for highly inclined asteroidal orbits. Celest. Mech. 60, 29–56. Luu, J., and D. Jewitt 1990. Charge couple device of asteroids. 1. Near-Earth and 3:1 resonance asteroids. Astron. J. 99, 1985–2011. Marzari, F., P. Farinella, and D.R. Davis 1999. Origin, aging, and death of asteroid families. Icarus 142, 63–77. Michel, P., F. Migliorini, A. Morbidelli, and V. Zappal`a 2000. The population of Mars-Crossers: Classification and dynamical evolution. Icarus, in press. Migliorini, F., P. Michel, A. Morbidelli, D. Nesvorny, and V. Zappal`a 1998, Origin of Earth-crossing asteroids: A quantitative simulation. Science 281, 2022–2024. Milani, A., and Z. Knezevic 1990. Secular perturbation theory and computation of asteroidal proper elements. Celest. Mech. 49, 347–412. Milani, A., and Z. Knezevic 1992. Asteroid proper elements and secular resonances. Icarus 98, 211–232. Milani, A., and Z. Knezevic 1994. Asteroid proper elements and the dynamical structure of the asteroid main belt. Icarus 107, 219–254. Moroz, L. V., A. V. Fisenko, L. F. Semjonova, C. M. Pieters, and N. N. Korotaeva 1997. Optical effects of regolith processes on S-asteroids as simulated by laser shots on ordinary chondrite and other mafic materials. Icarus 122, 366– 382. Moth´e-Diniz, T., D. Lazzaro, J. M. Carvano, and M. Florczak 2000. Rotationally resolved spectra of some S-type asteroids. Icarus 148, in press. Pieters, C. M., and L. A. McFadden 1994. Meteorite and asteroid reflectance spectroscopy: Clues to early Solar System process. Ann. Rev. Earth Planet. Sci. 22, 457–497. Rivikin, A. S., E. S. Howell, D. T. Britt, L. A. Lebofsky, M. C. Nolan, and D. D. Branston 1995. 3 µm spectrophotometric survey of M- and E-class asteroids. Icarus 317, 90–100. Scholl, H., and Ch. Froeschl´e 1986. The effects of the secular resonances ν16 and ν5 on asteroidal orbits. Astron. Astrophys. 170, 138–145. Tanga, P., A. Cellino, P. Michel, and V. Zappal´a, 1999. On the size distribution of asteroid families: The role of geometry. Icarus 141, 65–78. Tholen, D. J. 1989. Asteroid taxonomic classifications. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 1139–1150. Univ. of Arizona Press, Tucson. Tug, H. 1977. Vertical extinction on La Silla Messenger 11, 7–8. Vilas, F., K. S. Jarvis, and M. S. Gaffey 1994. Iron alteration minerals in the visible and near-infrared spectra of low-albedo asteroids. Icarus 109, 274– 283. Wetherill, G. W., and C. R. Chapman 1988. Asteroids and meteorites. In Meteorites and the Early Solar System (J. Kerridge and M. S. Matthews, Eds.), pp. 35–67. Univ. of Arizona Press, Tucson. Williams, J. G. 1989. Proper elements and family membership of the asteroids. In Asteroids II (R. P. Binzel, T. Grehels, and M. S. Matthews, Eds.), pp. 1034– 1072. Univ. of Arizona Press, Tucson. Williams, J. G. 1992. Asteroid families—An initial search. Icarus 96, 251–280. Zappal`a, V., A. Cellino, P. Farinella, and Z. Knezevic 1990. Asteroid families: I. Identification by hierarchical clustering and reliability assessment. Astron. J. 100, 2030–2046. Zappal`a, V., A. Cellino, P. Farinella, and A. Milani 1994. Asteroid families: Extension to unnumbered multi-opposition asteroids. Astron. J. 107, 772– 801.
Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups1 Jorge M´arcio Carvano, Daniela Lazzaro, Thais Moth´e-Diniz, and Cl´audia A. Angeli Departamento de Astrof´ısica, Observat´orio Nacional, 20921 Rio de Janeiro, Brazil E-mail: [email protected]
and Marcos Florczak Departamento de F´ısica, CEFET, 80000 Curitiba, Brazil Received March 31, 2000; revised August 8, 2000
We observed 29 Hungaria and 31 Phocaea asteroids at the European Southern Observatory (Chile) in the wavelength range 4900– ˚ The Phocaea and the Hungaria are both high-inclination 9200A. groups, located at the inner edge of the main belt near the ν5 , ν6 , and ν16 secular resonances. We confirm that the Hungaria group is composed mainly of E-type asteroids but with a relatively large number of S-types. The Phocaea group, on the other hand, is composed mainly of S-type asteroids. We discuss the possible implications of the observed composition distribution on the origin and evolution of these high-inclination groups. °c 2001 Academic Press Key Words: asteroids; asteroids, composition; spectroscopy; surfaces, asteroids.
1. INTRODUCTION
The Hungaria and Phocaea dynamical groups are located at the inner edge of the main belt of asteroids. They are composed of high-inclination (and high-eccentricity for the Phocaea group) asteroids located in a region with very complex dynamics (Scholl and Froeschl´e 1986). The Hungaria group lies near the edges of the ν5 and ν16 secular resonances while the Phocaea group is virtually surrounded by the ν5 , ν6 , and ν16 secular resonances, as can be seen in Fig. 1. In addition, a number of low-order mean motion resonances also occur in the proximity of these groups. By their location, they might be, as recently suggested (Migliorini et al. 1998, Michel et al. 2000), possible sources of the asteroids that must be replenishing the short-living Marscrosser population. The expression “dynamical group” is used to mean asteroids that are currently clustered in the orbital element space, in opposition to “family,” which designates a cluster of asteroids in the 1 Based on observations made with the 1.52-m telescope at the European Southern Observatory (La Silla, Chile) under the agreement with the CNPq/Observat´orio Nacional (Brazil).
proper element space. The origin of these two kinds of clusters is assumed to be much different: while the former is due to some long-term dynamical process, the latter is probably the outcome of the collisional break-up of a larger asteroid. Several authors, using different sets of proper elements as well as distinct clustering techniques and criteria, have regarded several asteroids in the Phocaea region as members of a possible but doubtful family (Carusi and Valsecchi 1982, and references therein). The analysis of the taxonomies of a small sample revealed (Bell 1989) the presence of cosmochemical incompatible compositions ruling out a common genetic origin. Zappal`a et al. (1990), using a refined proper elements set (Milani and Knezevic 1990) and a hierarchical clustering algorithm, also failed to identify a family in the Phocaea region, but the authors acknowledge that this result may be due to the poor accuracy of the proper elements. More recent works on family identification (Zappal`a et al. 1994, Bendjoya 1993) rely on the same improved set of proper elements by Milani and Knezevic (1992) which, however, is still not suitable for high-inclination and/or -eccentricity orbits. For such objects the semi-analytic theory by Lemaitre and Morbidelli (1994) should be preferred (Knezevic et al. 1995) but it has not been yet thoroughly applied to these groups, specifically searching for families. For the Hungaria group Lemaitre (1994) seems to identify some dynamical clustering, possibly indicating the presence of a family, and we will discuss these results farther on. On the other hand, the compositional analysis of a statistically significant number of members of these groups may provide interesting data to aid in the assessment of the reality of the dynamical groupings and of collisional families. The composition sampling of these regions of complex dynamics in the inner edge of the main belt may also allow a better understanding of the transport mechanisms from the main belt to planet-crossing orbits. With these purposes in mind, we have performed a spectral survey of asteroid members of the Phocaea and Hungaria groups. Here we present the analysis of visible CCD spectra of
173 0019-1035/01 $35.00 c 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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TABLE I Hungaria Group—Circumstances of Observation Asteroid 434 Hungaria
FIG. 1. The inner part of the main belt of asteroids indicating the locations of the Hugaria and Phocaea dynamical groups along with the approximate position of the main resonances.
29 and 31 members of the Hungaria and the Phocaea group, respectively. In Section 2 we describe the observations and the reduction of the data. The results for each group are discussed separately in Sections 3 and 4. In Section 5 we summarize the obtained results and discuss some of their implications. 2. OBSERVATIONS
The observations were carried out at the European Southern Observatory at La Silla (ESO-Chile). We used a 1.5-m telescope equiped with a Boller and Chivens spectrograph and a CCD 2048 × 2048 pixels with a readout noise of ±7 electrons. ˚ A grating of 225 g/mm with a dispersion of 330 A/mm in the first order was used. The CCD has a square 15-µm pixel, giv˚ ing a dispersion of about 5 A/pixel in the wavelength direction. ˚ with an The useful spectral range is about 4900 < λ < 9200 A, ˚ The spectra were taken through a 5-arcsec slit FWHM of 10 A. oriented in the east–west direction. The slit width was chosen in order to minimize the effects of atmospheric differential refraction and reduce the loss of light at both ends of the spectrum. All of the observations were made as near as possible to the meridian of the asteroid. The spectra of the Hungarias and Phocaeas were taken in eight observing runs, from November 1996 to June 1999, with atmospheric conditions good to excellent during most of the observations. The spectral data reduction was performed using the Image Reduction and Analysis Facility package following standard procedures. A full description of the data reduction process is given in Florczak et al. (1998).
UT date
1997/01/06 1997/01/06 1025 Riema 1997/01/06 1996/11/17 1355 Magoeba 1998/03/08 1998/03/08 1509 Esclangona 1998/03/07 1998/03/08 1998/03/08 1600 Vyssotsky 1999/06/18 1919 Clemence 1997/03/17 1997/03/17 2001 Einstein 1998/09/02 1998/09/03 1998/08/30 1998/08/30 2150 Nyctimene 1997/01/07 2272 Montezuma 1997/01/08 2491 Tvashtri 1997/01/09 3043 San Diego 1997/03/20 3101 Goldberger 1997/01/06 3169 Ostro 1997/12/27 3309 Brorfelde 1997/03/17 1997/03/17 3400 Aotearoa 1997/01/08 3447 Burckhalter 1998/09/03 1998/08/29 1998/08/29 3880 Kaiserman 1997/07/07 1997/07/07 1997/07/13 3940 Larion 1998/01/01 1998/01/04 4116 Elachi 1998/12/21 1998/12/22 4125 Lew Allen 1997/03/19 4483 Petofi 1997/12/21 1997/12/21 1997/12/24 1997/12/24 1997/12/24 4713 Steel 1997/07/07 1997/07/15 4764 Joneberhart 1998/12/16 5639 1989 PE 1998/01/03 1997/12/27 1997/12/28 6310 Jankonke 1997/01/07 1997/01/07 6394 1990 QM2 1997/01/11 6447 Terrycole 1997/01/09 6461 1993 VB5 1997/01/07 6493 Cathybennett 1997/01/08
R (AU) 1 (AU) Mag 2.06 2.06 1.92 1.94 1.77 1.77 1.80 1.80 1.80 1.87 1.89 1.89 2.05 2.05 2.05 2.05 1.85 1.71 1.82 2.07 1.99 1.88 1.75 1.75 1.75 2.04 2.04 2.04 1.88 1.88 1.87 1.87 1.87 1.73 1.73 2.08 1.85 1.85 1.85 1.85 1.85 1.82 1.83 2.01 1.88 1.88 1.88 1.87 1.87 2.04 1.97 1.78 1.83
1.29 1.29 1.10 1.31 0.99 0.99 1.02 1.02 1.02 1.26 0.89 0.89 1.07 1.07 1.09 1.09 1.27 1.33 1.37 1.14 1.13 1.58 0.87 0.87 0.92 1.06 1.06 1.06 0.98 0.98 1.01 1.14 1.12 1.21 1.20 1.22 1.09 1.09 1.10 1.10 1.10 0.90 0.88 1.23 1.06 1.06 1.06 1.41 1.41 1.17 1.20 0.94 1.27
14.39 14.39 15.23 15.81 15.51 15.51 15.16 15.14 15.14 15.10 14.86 14.86 15.22 15.19 15.31 15.31 16.56 17.19 17.03 16.25 16.93 16.45 15.86 15.85 16.28 14.87 14.87 14.87 16.14 16.14 16.29 15.55 15.49 16.22 16.20 16.47 14.59 14.59 14.64 14.64 14.64 14.87 14.71 16.65 16.68 16.66 16.66 16.84 16.84 16.20 16.56 15.54 16.26
α
Solar analog
21.58 21.58 21.19 27.32 27.03 27.02 25.95 25.79 25.79 30.58 2.27 2.26 9.83 9.22 11.60 11.62 29.80 34.78 32.07 13.15 18.34 31.42 20.89 20.89 23.98 8.41 8.53 8.52 20.17 20.18 22.80 25.80 25.18 33.58 33.48 17.88 25.41 25.41 25.80 25.80 25.80 19.36 16.22 22.09 22.00 21.66 21.68 31.04 31.04 17.06 22.58 23.27 30.70
HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD1835 HD9562 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD9562 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD144585 HD1835 HD160417 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594
Note. For each spectrum are given, in column 1, the observation date, in columns 2 to 5, respectively, the geocentric and heliocentric distances, the visual magnitude, and the solar phase angle, and in column 6, the solar analog used to obtain the reflactance spectra. The ephemeris were taken from JPL’s HORIZONS System (http://ssd.jpl.nasa.gov/horizons.html).
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TABLE II Phocaea Group—Circumstances of Observation
TABLE II—Continued 4533 Orth
Asteroid 25 Phocaea
UT date
1998/03/06 1998/03/07 1998/03/08 1998/03/09 1997/01/11 105 Artemis 1997/01/02 1997/01/02 1996/11/17 265 Anna 1997/07/08 1997/07/08 1997/07/15 273 Atropos 1997/12/24 1997/12/31 1997/12/31 1997/12/31 323 Brucia 1997/07/08 1997/07/08 1997/07/14 1997/07/14 502 Sigune 1997/12/28 1997/12/28 914 Palisana 1998/01/04 1997/12/23 1997/12/23 1997/12/23 950 Ahrensa 1996/11/19 1108 Demeter 1997/07/06 1997/07/06 1997/07/13 1318 Nerina 1997/03/17 1997/03/17 1997/03/19 1322 Coppernicus 1997/01/11 1367 Nongoma 1998/03/08 1998/03/09 1568 Aisleen 1997/12/23 1573 Vaisala 1998/03/07 1998/03/08 1575 Winifred 1996/11/18 1591 Baize 1996/11/17 1883 Rimito 1998/08/29 1998/08/30 2014 Vasilevskis 1997/01/12 2050 Francis 1999/06/19 2105 Gudy 1998/03/09 1998/03/09 1997/12/22 2965 Surikov 1997/01/12 3888 Hoyt 1999/06/20 3913 Chemin 1996/11/17 4121 Carlin 1997/07/08 4132 Bartok 1998/12/12 1998/12/14 1998/12/14 1998/12/15 4340 Dence 1997/07/07 1997/07/13 4511 Rembrandt 1998/08/29 1998/08/29 1998/08/30
R (AU) 1 (AU) Mag 2.39 2.38 2.38 2.38 3.01 2.68 2.68 2.62 2.26 2.26 2.28 2.64 2.64 2.65 2.64 2.31 2.31 2.29 2.29 2.58 2.58 2.88 2.86 2.86 2.86 2.40 1.95 1.95 1.94 1.85 1.85 1.85 2.96 2.34 2.34 2.14 2.60 2.60 2.41 2.54 2.00 2.00 2.69 1.82 2.35 2.35 2.23 2.27 1.79 2.81 2.05 1.87 1.88 1.88 1.88 1.83 1.83 1.81 1.81 1.81
1.69 1.68 1.67 1.65 2.14 2.59 2.59 1.92 1.54 1.54 1.49 2.01 2.09 2.09 2.09 1.29 1.29 1.28 1.28 2.48 2.49 1.93 1.89 1.89 1.89 1.53 1.17 1.17 1.22 0.86 0.86 0.85 2.03 1.47 1.47 1.27 1.80 1.81 1.72 1.98 1.18 1.18 2.33 0.85 1.55 1.55 1.77 1.30 0.93 1.97 1.35 1.37 1.35 1.35 1.35 1.08 1.13 0.89 0.89 0.90
11.88 11.86 11.83 11.80 12.56 13.90 13.90 13.08 14.99 14.99 14.87 14.86 14.98 14.98 14.98 12.42 12.42 12.49 12.49 15.88 15.88 12.99 12.79 12.79 12.79 15.21 14.86 14.86 14.99 13.06 13.06 13.08 17.14 16.52 16.52 15.14 16.53 16.54 16.39 16.25 16.05 16.04 16.72 14.46 15.04 15.04 15.45 16.40 15.16 16.68 15.80 15.07 15.05 15.05 15.03 15.83 15.97 14.23 14.23 14.25
α
Solar analog
20.30 20.09 19.88 19.65 10.58 21.37 21.37 17.92 22.23 22.23 20.00 18.91 19.86 19.86 19.86 2.94 2.94 4.93 4.93 22.26 22.26 6.45 3.76 3.76 3.76 13.54 24.56 24.57 26.85 0.85 0.86 1.16 7.35 14.80 14.97 15.88 15.82 16.00 19.90 20.94 22.00 21.98 21.02 13.86 17.58 17.58 25.30 5.44 24.17 12.62 25.50 30.56 30.23 30.23 30.06 27.89 29.38 19.21 19.21 19.37
HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585 HD144585 HD44594 HD144585 HD144585 HD44594 HD144585 HD144585 HD44594 HD44594 HD144585 HD144585 HD44594 HD144585 HD144585 HD144585 HD44594 HD44594 HD144585 HD44594 HD144585 HD44594 HD44594 HD44594 HD44594 HD144585 HD144585 HD144585 HD144585 HD144585
4826 Wilhelms 6084 Bascom 6560 Pravdo
1997/03/17 1997/03/17 1997/03/20 1997/01/11 1996/11/18 1998/12/10 1998/12/16 1997/01/11
1.93 1.93 1.93 2.29 2.18 2.02 2.04 2.60
0.99 0.99 1.01 1.99 1.40 1.44 1.39 1.62
15.01 15.01 15.10 16.67 15.65 16.34 16.24 15.49
13.71 13.71 14.99 25.26 20.28 26.92 25.47 3.77
HD144585 HD144585 HD144585 HD44594 HD44594 HD44594 HD44594 HD44594
Note. See Table I note.
Asteroids and analogs were observed usually at airmasses close to 1.0 and not greater than 1.3, with a few exceptions. Atmospheric extinctions were corrected using the mean extinction curve of La Silla (Tug 1977). Different solar analogs were observed in each observing run in order to compute reflectivities. On each night, at least two solar analogs were observed in order to estimate the quality of the night. The ratios between the spectra of the solar analogs for each night do not show any substantial variation. Tables I and II show, for the Hungaria and Phocaea groups, respectively, the date of observation, the geocentric and heliocentric distances, and the visual magnitude, and solar phase angle of each asteroid discussed in this work, as well as the solar analog used to obtain the divided spectrum. Whenever more than one spectrum was available, the least noisy was chosen to be representative of the superficial mineralogy of the asteroid and it is used in all plots as representative of the asteroid. All asteroid spectra were normal˚ by convention. The resulting spectra are shown in ized at 5500 A Appendixes A and B for the Hungaria and Phocaea groups, respectively. No correction was made for phase-reddening effects, since all asteroids were observed at phase angles smaller than 35◦ and therefore any phase-induced effect should be smaller than 1% (Luu and Jewitt 1990). 3. THE HUNGARIA GROUP
The spectra were visually classified according to template spectra defined in Tholen’s taxonomy. Classification was made according to general similarities to the template spectra, determined by means of visual inspection and with the aid of albedo data, whenever available. Our sample of the Hungaria group is composed of a majority of X-type (18) asteroids, a relatively large proportion of S-types (8), two distinct C-types, and an A-type. In Fig. 2 we present running box averages of the spectra of the Hungaria sample grouped according to their classification in the present work. We recall that the X-type classification designates a spectrum characteristic of the classes E, M, or P. These classes, as defined in Tholen’s taxonomy, are said to be spectrally degenerated since the information available on the visible and NIR spectra is not sufficient to distinguish between them. These three classes have nearly identical spectra and, according to Tholen, can only be distinguished by their albedos, which
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FIG. 2. Spectra of the observed asteroids in the Hungaria group, sorted by spectral type. Along with the S-type asteroids is shown the spectrum of the A-type, 1600 Vyssotsky. All spectra are normalized at 5500 A˚ and averaged by a running box.
are high for E-, intermediate for M-, and low for P-type asteroids. The superficial mineralogies implied by each of these classes are considerably different: carbon/organic-rich silicates for the P-class, metal with possibly traces of silicates or metal + enstatite for the M-class, and enstatite or other iron-free silicates for the E-types (Gaffey et al. 1989). The inferred presence of enstatite and/or metal in the E-type implies that their par-
ent bodies should have been heated to temperatures higher than 1500◦ C. This interpretation is, however, in contradiction with the discovery of hydration absorption bands around 3 µm in the spectra of 44 Nysa and (possibly) of 64 Angelina, both E-types, and of the M-types 92 Undina and 201 Penelope, among others (Rivkin et al. 1995). These results imply formation temperatures no greater than 500◦ C for the parent bodies (Jones et al. 1990). Since no albedo is available for any of the X-types in our sample, it is not possible to determine their classification according to Tholen template spectra. The location of the Hungaria group in the inner edge of the main belt seems to favor a mineralogy associated with the E- or M-classes, especially considering that 6 of the 13 E-types classified by Tholen are in this region. Recent works (Burbine et al. 1998, Bus 1999) have revealed subtle spectral characteristics that apparently are distinctive of this class. These consist of an absorption feature from 0.44 to 0.51 µm, a sharp turn-up in reflectance from 0.51 to 0.55 µm, a flat red-sloped spectrum from 0.55 to 0.70 µm, and a shallow feature that extends past 0.92 µm. All of these features have been observed by Burbine and co-workers in the spectra of 64 Angelina, 434 Hungaria, and 3103 Eger, previously classified as E-type. On his feature-based taxonomy, Bus (1999) defines an Xe-class described as having and overall slope that is slightly to moderately red with a decrease in spectral slope longward of 0.75 µm and several subtle absorption features, including an absorption band shortward of 0.55 µm and a very shallow absorption feature around 0.6 µm. This class encompasses most of the asteroids classified as E-type in Tholen. Visual inspection reveals that some, or all, of these features are present on the majority of the X-type spectra in our sample as shown in Table III. In particular, the turn-up in reflectance between 0.51 and 0.55 µm present on most of the spectra can be seen even on the somewhat noisy spectrum of 3400 Aotearoa. Based on the presence of such features we were able to assign the Xe classification to 16 of the 18 X-types. Among the 2 remaining, 4764 Joneberhart may also be an Xe-type, but its spectrum shortward of 0.51 µm cannot be trusted due to problems with the solar analog. In Table IV, the physical parameters such as the diameter, the albedo (whenever available), the mean orbital elements, and the taxonomic classification given by Tholen (1989), Bus (1999), and the in present work are given for each observed asteroid. Considering the inherent diversity of the S-type asteroids spectra, we made use of four spectral parameters, namely slope A, slope B, position of the maximum, and apparent depth, in order to better characterize our spectra. Slope A is computed as ˚ and is an a linear fit of the spectra between 5400 and 7200 A indicator of the redness of the spectra as introduced by Luu and Jewitt (1990). Slope B is calculated as a linear fit between 8000 ˚ and is indicative of the steepness of the decrease and 9000 A in reflectance after the maximum. Finally, the apparent depth is computed as the division of the normalized reflectivity at 7400 ˚ and gives an indication of the real depth. It is noteand at 9200 A worthy that these three parameters (position of the maximum, slope B, and apparent depth) are related to the position of the center and to the depth of the 1-µm olivine–pyroxene absorption
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TABLE III Hungaria Group—Physical and Dynamical Parameters Asteroid
D (km) Alb. a (AU)
e
TABLE IV Hungaria Group—Spectral Features among the X-Types
I (deg) Classa Classb Classc Asteroid
434 Hungaria 1025 Riema 1355 Magoeba 1509 Esclangona 1600 Vyssotsky 1919 Clemence 2001 Einstein 2150 Nyctimene 2272 Montezuma 2491 Tvashtri 3043 San Diego 3101 Goldberger 3169 Ostro 3309 Brorfelde 3400 Aotearoa 3447 Burckhalter 3880 Kaiserman 3940 Larion 4116 Elachi 4125 Lew Allen 4483 Petofi 4713 Steel 4764 Joneberhart 5639 1989 PE 6310 Jankonke 6394 1990 QM2 6447 Terrycole 6461 1993 VB5 6493 Cathybennett
10.5 — 1.94 0.074 22.50 5.7 — 1.97 0.038 26.85 9.4 — 1.85 0.044 22.82 8.2 0.233 1.86 0.032 22.31 15.8 — 1.84 00.037 21.17 7.8 — 1.93 0.094 19.33 10.3 — 1.93 0.098 22.68 7.9 — 1.91 0.056 25.32 4.9 — 1.86 0.089 24.32 7.0 — 1.87 0.054 22.86 7.3 — 1.92 0.106 21.79 5.5 — 1.97 0.046 28.54 16.7 — 1.89 0.066 24.90 6.4 — 1.81 0.053 21.14 5.8 — 1.93 0.098 20.23 11.5 — 1.99 0.028 20.71 6.6 — 1.94 0.082 17.56 11.0 — 1.98 0.056 22.83 8.8 — 1.87 0.080 24.11 7.3 — 1.92 0.118 20.44 15.8 — 1.92 0.083 26.72 10.5 — 1.92 0.073 22.66 7.3 — 1.93 0.047 24.83 5.8 — 1.85 0.022 26.60 7.9 — 1.91 0.031 23.59 8.0 — 1.93 0.093 22.81 7.3 — 1.95 0.085 19.77 8.4 — 1.95 0.102 23.41 9.2 — 1.94 0.083 24.71
E E X S — X X — S X — — TS — — — — — — — — — — — — — — — —
Xe Xe — — A — Xe — — — — — — — — — — — Sl — — A — — — — — — —
Xe Xe Xe S A Xe Xe S S Xe S Xe C S Xe Xe Xe Xe S S X S X C Xe Xe Xe Xe Xe
Note. For each asteroid, columns 2 and 3 show the diameters and albedo, columns 4 to 6 show the osculating semi-major axis, eccentricity, and inclination, and columns 7 to 9 show the classification according to Tholen (1989), Bus (1999), and the present work. a Tholen (1989). b Bus (1999). c Present work.
band which are associated with the mineralogical composition. However, due to insufficient spectral coverage and the onset of ˚ we are unable to put any useful contelluric bands at 9200 A, straint on the position of the band I center. The computed values of all of these parameters are presented in Table V. Due to the ˚ and the noise genpresence of an atmospheric O2 band at 7619 A ˚ the erated by the onset of the H2 O atmospheric bands at 9200 A, uncertainty on the maximum position can be fairly large and we ˚ for all S-type asteroids assigned the conservative value of 250 A in our sample. The spectra of S-types in the Hungaria group have maxima ˚ and show an apparent bimodality conbetween 7818 and 7995 A cerning the spectral slope A: four asteroids present values dis˚ while the remaining cluster around tributed around 18% 103 A 3 ˚ 13% 10 A. Both groups, however, span the same range of maximum position and slope B. There appears to be a slight correlation between the values of slope B and the maximum as well as an anti-correlation between slope A and the diameter. The latter
Turnup in reflectance Band around Decrease in spectral around 0.5 µm 0.6 µm slope after 0.75 µm
434 Hungaria 1025 Riema 1355 Magoeba 1919 Clemence 2001 Einstein 2491 Tvashtri 3101 Goldberger 3400 Aotearoa 3447 Burckhalter 3880 Kaiserman 3940 Larion 4483 Petofi 4764 Joneberhart 6310 Jankonke 6394 1990 QM2 6447 Terrycole 6461 1993 VB5 6493 Cathybennett
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No ? Yes Yes Yes Yes Yes
? ? ? ? Yes ? ? ? ? Yes Yes No ? ? ? ? ? ?
Yes Yes Yes Yes Yes Yes Yes ? ? Yes Yes No ? ? Yes No ? Yes
Note. The presence or absence of the spectral features indentified by Burbine et al. (1998) and Bus (1999) are summarized. The question mark (?) identifies cases where positive identification was not possible due to noisy spectra.
might be easily understood as the presence of more regoliths on larger asteroids but we see no obvious explanation for the first correlation. On the other hand, since we are dealing with a very small sample, any observed trend should be taken with much caution. Asteroid 4713 Steel has one of the smallest spectral slopes A in our sample but it has been classified as an A-type by Bus
TABLE V Hungaria Group—S Class Spectral Parameters
Asteroid
Slope A ˚ (% 103 A) ±5
Slope B ˚ (% 103 A) ±1
˚ Maximum (A) ±250
Apparent depth ±0.05
1509 Esclangona 2150 Nyctimene 2272 Montezuma 3043 San Diego 3309 Brorfelde 4116 Elachi 4125 Lew Allen 4713 Steel
14.22 19.30 13.47 20.54 18.20 13.75 16.93 12.72
−1.64 −11.18 −9.74 −4.81 −7.24 −5.23 −3.99 −4.82
7995 7778 7818 7834 7822 7854 7887 7897
0.97 1.07 1.04 1.07 1.03 0.97 0.99 1.03
Note. Slope A and Slope B are calculated as linear fits in the ranges 5400– ˚ , respectively. The errors quoted are typical values of 7200 and 8000–9000 A the standard deviation. The position of the maximum is given as the wavelength where a running box average with bin size of 100 is maximum. The uncertanty is estimated as the larger of the differences among values obtained with different ˚ bin sizes. The apparent depth is the ratio of the reflectances at 7400 and 9200 A of running box averages with a bin size of 100. The uncertanty is the typical value of the difference obtained with different bins.
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(1999). There is another asteroid, 1600 Vyssotsky, also classified as an A-type by Bus and confirmed as such by our observations. On the other hand, both of our spectra of 4713 Steel (observed on different nights) are typical of S-types with moderate spectral slope. The spectra are very similar to that of 4116 Elachi, which is classified as a Sl in Bus taxonomy. We ignore the cause of this discrepancy but the coherence between our two spectra is reassuring regarding the quality of our observations. Clearly, further observations are needed in order to determine the true nature of this asteroid. The two asteroids classified as C-types in our Hungaria sample, 3169 Ostro and 5639 1986PE, exhibit very similar spectra: essentially flat, with no drop off in reflectance toward the UV and with a shallow absorption feature between 0.50 and 0.70 µm. 3169 Ostro’s previous classification as a TS in Tholen’s taxonomy was made based on ECAS photometry covering only the 0.3- to 0.7-µm range (as seen in the archives of the Planetary Data System: http://pdssbn.astro.umd.edu). This incomplete coverage was probably due to problems during the observation of this particular asteroid, which may also explain the greater redness of the ECAS spectrum of 3169 Ostro. In order to analyze the spatial distribution of compositions in the Hungaria group we searched for all members with a taxonomical and/or spectral classification. First, using the 1999 version of Bowell’s catalog (Bowell et al. 1994), we identified all members of the Hungaria group defined as those asteroids with semi-major axis in the range 1.98–2.0 AU, inclinations between 16◦ and 40◦ , and aphelion distances less than 1.666 AU. Among the 130 numbered asteroids satisfying these criteria, 37 do have a compositional classification, representing around 30% of the total sample. Of these, around 60% are classified as E, X, or Xe, nearly 30% as S and the remainder as C or A. The distribution in osculating orbital elements of these asteroids is shown in Fig. 3. No clear trend is apparent except that the X-type (here including the E and Xe) seem to be located preferentially in the outer rim of the group, at distances greater than 1.92 AU. An analysis of compositional distribution in the proper elements space is more difficult due to the problems already mentioned for these high-inclination and/or -eccentricity orbits. In the upper part of Fig. 4 we show the distribution of compositions in the proper element phase space using three different data sets based on the theories of Lemaitre and Morbidelli (1994), Williams (1989), and Milani and Knezevic (1994). The red symbols represent asteroids with proper elements computed through the first theory (Lemaitre, personal communication) while the blue and green are those of Williams and Milani and Knezevic, respectively. We note again a predominance of X-types on the outer part of the group as well as some small clusters. In particular, one of these clusters corresponds to four of the eight members of the dynamical family suggested by Lemaitre (1994). These asteroids are 434 Hungaria, 1919 Clemence, 1920 Sarmiento, and 3880 Kaiserman. We note that other asteroids, such as 2048 Dwornik, 2001 Einstein, 3400 Aotearoa, and 4764 Joneberhart, also lie very near to this cluster. If this cluster indeed represents
FIG. 3. Distribution in the mean orbital elements space of all of the asteroids of the Hungaria group having a taxonomic and/or spectral classification.
a family, more precise analysis taking into account the background density, will be needed. We can only confirm that, from a mineralogical point of view, there is a coherence between these asteroids that supports the hypothesis of the collisional breakup of a larger parent body. 4. THE PHOCAEA GROUP
Among the 31 asteroids observed in the Phocaea region, 24 turned out to be S-types, with 3 C-types, 3 X-types, and a D-type, as shown in Table VI. Running box averages of the spectra of the Phocaea sample, grouped according to their classification, are shown in Fig. 5. One of the X-type asteroids, 1318 Nerina, was found to be an Xe-type, following the criteria outlined in the previous section, but all have albedos compatible with M-type asteroids. The larger asteroid in the group, 105 Artemis, is among the three C-type asteroids in our sample. Its spectrum is very similar to that of 914 Palisana, both showing no substantial drop of reflectivity toward the UV and a shallow band between 0.55 and 0.8 µm, which is more prominent on the spectrum of 105 Artemis. This band may be an indication of a process of aqueous alteration (Vilas et al. 1994, Barucci et al. 1998). The remaining C-type, 1108 Demeter, has a flat, featureless spectrum. The diversity within the S-types in the Phocaea region is comparable to that observed in the nearby Flora family (Florczak et al. 1998). The spectra present a slope A between 6 and 16%
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FIG. 4. Distribution in the proper orbital elements space of all asteroid members of the Hungaria (a) and Phocaea (b) groups having a taxonomical and/or spectral classification. The red symbols represent objects whose elements were computed using the theory of Lemaitre of Morbidelli (1994). The green and blue symbols represent asteroids that do not have proper elements computed through the above theory and for which the theories of Williams (1989) or Milani and Knezevic (1994) were used, respectively.
˚ a slope B between −20 and −6% 103 A, ˚ a maximum 103 A, ˚ ranging from 7450 to 7900 A , and an apparent depth between 1.03 and 1.24. All of the computed values are presented in Table VII where we can see no apparent correlation between any of these parameters and the diameter. This result is somewhat to be expected, given the poor range of diameters. On the other hand, there is a slight anti-correlation between the values of slope A and of the apparent depth: as the former increases the latter decreases. This can be an indication of a space weathering process which darkens the surface of the objects, increasing its redness (slope A) and decreasing the depth of the absorption band (Wetherill and Chapman 1988, Pieters and McFadden 1994, Doressoundiram et al. 1998). We caution that the apparent depth as computed in the present work is just indicative of the depth of the pyroxene–olivine band at ∼1 µm and any conclusion driven from this parameter should be taken with some caution. Within our sample of S-type asteroids we can see an almost continuous trend with the greatest asteroid, 25 Phocaea, in the upper part of the distribution (Fig. 5). In the lower part of
the distribution there are two asteroids, 950 Ahrensa and 1575 Winifred, which have distinctive spectra, with an increase of reflectance before the maximum and a decrease immediately after it. These spectra cannot be properly fitted by a straight line since the steepness decreases just before and after the maximum, giving the spectra an aspect like a “saw tooth”. Comparison with ordinary chondrite (OC) meteorite spectra measured by Gaffey (1976) shows that there are striking similarities between the spectra of these asteroids and some OC meteorites of the L4, L5, LL4, and LL5 types. Figure 6 gives running box averages of the spectra of all of the S-type asteroids of our sample along with those of some OC meteorites (Gaffey 1976) which appear in the lower part of the distribution. We recall that Gaffey et al. (1993), using a composite spectra of 25 Phocaea covering 0.4 to 2.5 µm, concluded that the silicate content of this asteroid (derived from the position of the 1-µm band and the ratio of the depths of the 1- and 2-µm bands) is indeed consistent with a OC-like composition. Moreover, analysis of the rotational variations on the spectrum of 25 Phocaea (Moth´e-Diniz et al. 2000) indicates that
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TABLE VI Phocaea Group—Physical and Dynamical Parameters Asteroid 25 Phocaea 105 Artemis 265 Anna 273 Atropos 323 Brucia 502 Sigune 914 Palisana 950 Ahrensa 1108 Demeter 1318 Nerina 1322 Coppernicus 1367 Nongoma 1568 Aisleen 1573 Vaisala 1575 Winifred 1591 Baize 1883 Rimito 2050 Francis 2014 Vasilevskis 2105 Gudy 2965 Surikov 3888 Hoyt 3913 Chemin 4121 Carlin 4132 Bartok 4340 Dence 4511 Rembrandt 4533 Orth 4826 Wilhelms 6084 Bascom 6560 Pravdo
D (km)
Alb.
a (AU)
e
I (deg)
Class a
Class b
Class c
75.1 119.1 23.7 29.3 35.8 16.0 76.6 15.0 25.6 13.0 11.0 9.6 14.5 9.8 9.5 18.7 9.2 8.9 17.5 22.5 8.6 10.1 13.8 6.7 10.5 9.2 9.1 10.5 13.8 10.5 15.1
0.231 0.047 0.105 0.162 0.176 0.340 0.094 0.179 0.046 0.181 — — — 0.223 0.245 0.106 — — — 0.183 0.086 — — 0.354 0.305 — 0.310 — — — —
2.40 2.37 2.41 2.39 2.38 2.38 2.45 2.37 2.42 2.30 2.42 2.34 2.35 2.37 2.37 2.39 2.41 2.32 2.40 2.38 2.39 2.39 2.35 2.37 2.40 2.39 2.39 2.36 2.37 2.31 2.35
0.255 0.176 0.268 0.160 0.301 0.179 0.211 0.159 0.257 0.202 0.233 0.129 0.254 0.232 0.177 0.175 0.262 0.237 0.282 0.150 0.217 0.250 0.223 0.254 0.287 0.233 0.253 0.245 0.191 0.235 0.114
21.57 21.48 25.62 20.41 24.22 25.00 25.24 23.48 24.93 24.64 23.32 22.45 24.89 24.56 24.78 24.77 25.49 26.57 21.40 29.30 24.23 22.19 23.91 23.09 23.31 25.19 22.74 22.65 25.28 22.99 23.23
S C — SCTU S S CU — CX — — — — — — — — S — — — — — — — — — — — — —
S Ch — — — — — Sa — — — — — — — — — — — — — — — — — S — — — — —
S C X X S S C S C Xe S S S S S S S S S D S S S S S S S S S S S
Note. See Table III note. Tholen (1989). b Bus (1999). c Present work.
a
this asteroid has an homogeneous surface, as should be expected if it is composed of undifferentiated chondritic material. Therefore, judging from the observed spectral slopes, aparent depths, and overall shapes of the spectra, we can conclude that the S-type asteroids of the Phocaea group span a continuous distribution bounded by OC-like objects, i.e., 25 Phocaea in the upper part and OC meteorites in the lower part. This distribution is exactly what would be expected if an ongoing space weathering process was acting on fragments of a disrupted homogeneous body. It should be stressed, however, that this assessment is only tentative, since we do not have any information on the position of the band I center. This parameter is closely related to the superficial mineralogy of an S-type asteroid and usually is regarded as being unffected by space weathering processes. Although this last statement has been put into question by laboratory experiments envolving laser bombardment of meteoritic samples (Moroz et al. 1996), our case would greatly benefit if one could show that there is no significant scatter in the band center of the S-types in the Phocaea group.
The distribution of compositions among the Phocaea group has also been analyzed in the same way as in the case of the Hungaria, discussed above. In the Phocaea case we found a taxonomic\spectroscopic classification for 43 asteroids, among a total of 154 members of the group. Among these, nearly 75% have a S-type classification while around 15% are C-types and the remainder are of other classes. The distribution of these asteroids in mean orbital elements is shown in Fig. 7. The observed predominance of S-type asteroids in the Phocaea group is expected, being the most common type in this region of the main belt. Yet, it would also be compatible with a common origin through collisional breakup. To see whether the distribution in proper elements is also compatible with this hypothesis we then examined this point using the same sets of proper elements as in the case of the Hungaria group. The majority of the S-type asteroids seem very tightly clustered while the remaining spectral types are distributed around the periphery, as shown in the lower part of Fig. 4. A notable exception is again the large C-type 105 Artemis, which falls near the center
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TABLE VII Phocaea Group—S Class Spectral Parameters
Asteroid
Slope A ˚ (% 103 A) ±5
Slope B ˚ (% 103 A) ±1
˚ Maximum (A) ±250
Apparent depth ±0.05
25 Phocaea 323 Brucia 502 Sigune 950 Ahrensa 1322 Coppernicus 1367 Nongoma 1568 Aisleen 1573 Vaisala 1575 Winifred 1591 Baize 1883 Rimito 2050 Francis 2014 Vasilevskis 2965 Surikov 3888 Hoyt 3913 Chemin 4121 Carlin 4132 Bartok 4340 Dence 4511 Rembrandt 4533 Orth 4826 Wilhelms 6084 Bascom 6560 Pravdo
16.89 9.74 10.54 12.48 6.39 11.48 10.43 10.77 9.16 8.81 9.85 9.31 8.18 10.60 11.86 7.72 11.32 11.02 11.92 12.27 9.26 8.33 12.64 14.05
−9.90 −6.66 −10.04 −15.65 −20.25 −8.13 −9.96 −7.20 −18.79 −14.90 −10.84 −6.45 −15.05 −15.55 −6.93 −12.08 −11.60 −9.63 −13.47 −8.87 −15.49 −18.92 −14.42 −9.10
7697 7707 7702 7586 7578 7691 7687 7794 7590 7467 7700 7664 7558 7568 7672 7517 7892 7684 7682 7720 7502 7773 7533 7763
1.06 1.06 1.08 1.16 1.24 1.03 1.08 1.03 1.20 1.14 1.08 1.06 1.14 1.16 1.05 1.14 1.08 1.08 1.12 1.06 1.13 1.14 1.14 1.10
Note. See Table V note for details.
FIG. 5. Spectra of the observed asteroids in the Phocaea group, sorted by spectral type. Along with the spectra of the X-types is shown the spectrum of ˚ and averaged by a the D-type 2105 Gudy. All spectra are normalized at 5500 A running box.
of the S-type cluster. The presence of such a large asteroid with implied mineralogy incompatible with the predominant S-types was the chief evidence pointed out by Bell (1989) against the reality of a Phocaea family. However, we recall that a similar argument was used to question an Eunomia family, but a recent spectroscopic survey (Lazzaro et al. 1999) indicates that large C-type asteroids are very common in this part of the inner
FIG. 6. Spectra of the S-type asteroids in the Phocaea group obtained in this work are compared with four ordinary chondrites’ spectra. The Mezo-Mandaras, Cynthiana, and Knyahinya are L3, L4, and, L5, respectively, and Soko-Banja is ˚ and LL5-type (Gaffey 1976). All of the spectra are normalized around 5500 A are averaged by a running box.
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FIG. 7. Distribution in the mean orbital elements space of all the asteroid members of the Phocaea group having a taxonomical and/or spectral classification.
belt. This might be just the result of a selection effect, since the C-type asteroids do have darker albedo, but it might also be due to some other effect such as, for example, a less friable material. Therefore, the large C-type-like asteroids in a Phocaea family may be just background objects, if a family is indeed confirmed by accurate clustering techniques. 5. SUMMARY AND DISCUSSION
In the present work we have analyzed the surface composition of a significant number of asteroid members of the Hungaria and Phocaea groups. As a general result we can say that this analysis pointed out the possible presence of some families, both in the Hungaria and in the Phocaea groups, which need now to be confirmed through refined dynamical and clustering techniques. In particular, we have found that for both groups there exist clusters of asteroids which have spatial distribution and composition compatible with the fragmentation of an larger parent body. The concentration of a great number of E/Xe-types in a relatively small region, as is the case in the Hungarias, raises several possibilities regarding its origin: (a) it could be the result of the collisional breakup of a larger E-type parent body; (b) the conditions that led to the formation of this class of asteroids occurred
only on a narrow portion of the primordial nebula, around the present Hungaria region; (c) the conditions that led to the formation of this class of asteroids occurred on a larger portion than the present Hungaria region, but asteroids in other locations were later removed due to some dynamical process; (d) the E-types in the Hungaria region were formed elsewhere in the main belt and later brought to their present position by some dynamical process. The first hypothesis seems unlikely since all of the E/Xe asteroids in the Hungaria region are small. This kind of size distribution is not the usual outcome of fragmentation theories, which expect one or two larger bodies, maybe resulting from a reaccumulation after fragmentation, and a great number of small ones (Tanga et al. 1999, Marzari et al. 1999, Benz and Asphaug 1999, and references therein ). It should be noted, however, that the physics involved in the collisional fragmentation of asteroids is not thoroughly known and it is possible that poorly constrained parameters such as the strengh of the E-type parent-body material may account for such unusual size distribution. On the other hand, the distribution in the proper elements space seems also incompatible with a unique breakup. There might exist some smaller families, as proposed by Lemaitre (1994) and discussed above, but we do not believe that all of the E-type like asteroids in this region resulted from the fragmentation of a unique body, unless there exists some other dynamical process not yet considered in the computation of the proper elements we used. In this context it is important to stress the importance of a mineralogical analysis of the remaining members of the family proposed by Lemaitre. Hypothesis (b) and, to a lesser extent (c), suffers from the fact that currently we can find E-type asteroids in other regions of the main belt. In particular, the two largest E-type, 44 Nysa and 64 Angelina, are at semi-major axes of 2.42 and 2.68 AU, respectively. On the other hand, the discovery of hydration bands on 44 Nysa and, possibly, on 64 Angelina (Rivkin et al. 1995) opens the possibility that there are actually two mineralogies associated with E-type asteroids. It would be conceivable then that the E-type asteroids in the Hungaria region have a mineralogy consistent with enstatite achondrite meteorites while E-types farther away from the Sun present a mineralogy consistent with the presence of hydration bands. Unfortunately, we cannot search for this band in our spectra, due to insufficient spectral coverage. In any case, it would remain to be shown for hypothesis (c) that the Hungaria region is indeed more dynamically stable than its surroundings. The same problem applies to hypothesis (d). This last hypothesis is conjured by the presence of large E-type asteroids at greater heliocentric distances and the fact that the distribution of taxonomic classes does have a heliocentric trend. Therefore, hypothesis (d) would gain some weight if hydratation bands were found on the E-type asteroids of the Hungaria region. On the other hand, the dynamical problems involved in transfering these asteroids from farther away from the Sun into their present orbits are huge.
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As it is, all hypotheses considered here have problems. It seems to us that a simpler solution would be a mix of hypothesis (a) with (b) or (c), as already suggested by Michel et al. (2000). Definite answers to this question, however, would depend on detailed dynamical studies regarding the reality of the proposed families and the overall stability of the Hungaria region, as well as observation campaigns designed to search for aqueous alteration on the E-types in the Hungaria group. For the Phocaea group we have shown that most of the observed objects have spectra similar to that of S-type asteroids, with just a minor percentage of different compositions. The spectra show common characteristics: a maximum around λ = ˚ and a reflectivity gradient spanning a continuous but lim7500 A ited range. We found a trend between the apparent depth of the 1-µm band and the redness of the spectra which seems indicative of an ongoing space weathering effect acting on the surface of asteroids having a similar composition. Moreover, the distribution of spectra is bounded in the upper part by the spectra of the largest asteroid of the sample, 25 Phocaea, which was already identified (Gaffey et al. 1993) as having a surface composition compatible with OC material. In the lower part of the distribution the asteroid spectra are similar to those of some OC meteorites. This distribution is, therefore, in agreement with that expected from the fragmentation of a homogenous OC-like parent body and where the largest fragment, 25 Phocaea, would retain more efficiently regoliths and, thus, have a redder spectrum. We recall that there are several lines of evidence that suggest that regolith accumulation can effectively produce such a reddening effect (Chapman 1996, and references therein). On the other hand, the clear anti-correlation between spectral slope and diameter found by Gaffey (1993) for a subsample of S-type asteroids with OClike mineralogy and diameters greater than 100 km seems to contradict this interpretation. Since the definitions of spectral slope on the two works, although different, do correlate, this inconsis-
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tency may be related to the difference of diameter range of the samples. It is noteworthy that the above scenario does not require a unique fragmentation, but just a common origin of the objects. In this sense we recall that several authors (Williams 1992, Carusi and Valsecchi 1982, and references therein) already recognized a cluster in the Phocaea region indicating a family of collisional origin. Here we have shown that the spectra of these asteroids also reflect a coherent mineralogical composition, probably affected by a variable degree of space weathering. In this scenario we suppose that the large C-type asteroid, 105 Artemis, is an interloper. If a dynamical analysis does show that the S-type asteroids in the Phocaea region are indeed the result of a collisional breakup, then the fact that some of them present a OC-like composition definitively links these asteroids to the OC meteorites. The specific mechanism able to transport fragments from the Phocaea region to Earth is to be investigated but is probably associated with one or more resonances as already suggested (Migliorini et al. 1998, Michel et al. 2000) . It is important also to note that most asteroids in the Phocaea group have a composition similar to those of the nearby Flora family (Florczak et al. 1998). Can that indicate a common origin? It is difficult to say, but since we are in a region of very complex dynamics a migration of objects from low- to high-inclination orbits cannot be ruled out, despite the presence of the µ5 secular resonance. Note that the asteroids in the Phocaea group have also higheccentricity orbits which are in agreement with a resonant transport mechanism through a mean motion resonance (Migliorini et al. 1998, Michel et al. 2000). Although we believe to have considerably increased the knowledge on the composition of the high-inclination groups of Hungaria and Phocaea, the main result of the present work is to point out the necessity and urgency of a detailed dynamical and cluster analysis of these regions which will be fundamental to addressing the important question of their origin.
APPENDIX A. Relative reflectivity of 29 asteroid members of the Hungaria group. The spectra are presented by increasing asteroid number. They are ˚ by convention. normalized around 5500 A
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APPENDIX A—Continued
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APPENDIX A—Continued
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APPENDIX A—Continued
APPENDIX B. Relative reflectivity of 31 asteroid members of the Phocaea group. The spectra are presented by increasing asteroid number. They are ˚ by convention. normalized around 5500 A
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APPENDIX B—Continued
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APPENDIX B—Continued
SURVEY OF HUNGARIA AND PHOCAEA DYNAMICAL GROUPS
ACKNOWLEDGMENTS The authors acknowledge the technical staff of ESO for their prompt help whenever needed and A. Betzler for his cooperation with some of the reductions. We are grateful to Dr. Anne Lemaitre for providing us with the proper elements set used in this paper. The present work has been supported by CNPq, CAPES, and FAPERJ through diverse fellowships and grants.
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