E-Type Asteroids: Spectroscopic Investigation on the 0.5μm Absorption Band

Icarus 152, 127–133 (2001) doi:10.1006/icar.2001.6623, available online at http://www.idealibrary.com on

E-Type Asteroids: Spectroscopic Investigation on the 0.5 µm Absorption Band1 Sonia Fornasier2 and Monica Lazzarin Dipartimento di Astronomia, Vic. dell’Osservatorio 5, 35122 Padova, Italy E-mail: [email protected] Received May 17, 2000

Visible spectra of five E-type asteroids have been obtained at the European Southern Observatory with the 1.5-m telescope. Three of these objects reveal a peculiar absorption band centered around 0.5 µm whose origin is not easily attributable. A possible source might be troilite, which is also a known constituent of aubrite meteorites, as it has a spectral behavior consistent with the visible and near infrared spectrum of these objects and may be responsible not only for the 0.5-µm band but probably also for the 3-µm band observed on the surface of these objects. We report in this paper the results of our investigation and a discussion regarding the possible composition of E-type asteroids. °c 2001 Academic Press Key Words: asteroids; spectroscopy.

1. INTRODUCTION

Following Tholen (1989) classification, E-type asteroids have a spectral behavior quite flat or a little reddish, with at most the presence of weak absorption features. Their peculiar characteristic is the highest albedo than any other asteroidal class (around 0.4–0.6), which distinguishes them from the spectrally similar, but characterized by a lower albedo, M- and P-type asteroids. Tholen classified 14 asteroids in the E class (Tholen 1989). Most of the members of the class lie around 1.9 AU, in the Hungaria region, just outside the inner edge of the main belt, even if the two largest E-type objects, 44 Nysa and 64 Angelina, are located farther away, inside the main belt, with semimajor axes of 2.48 and 2.68 AU, respectively. The surface composition of E-type objects, on the basis of their spectral behavior and high albedo, seems to be dominated by iron-free or iron-poor silicates such as enstatite, forsterite, or feldspar, and resembles the aubrite meteorites spectra (Gaffey et al. 1989, 1992, Zellner et al. 1977). For this reason E-type asteroids are believed to be the source of enstatite achondrites: both 44 Nysa and 434 Hungaria, from dynamical considerations, seem to be favorably located for me1 Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 64.S-0205 and 62.S-0173. 2 To whom correspondence should be addressed.

teorites production, and their polarimetric properties are consistent with laboratory polarimetry of the Norton County aubrite (Zellner et al. 1977). 3103 Eger, the only known E-type asteroid among the near Earth asteroids population, has a spectral behavior similar to the aubrites. Moreover the orbital intersection between 3103 Eger and the Earth seems to be a favorable source for the aubrite meteorites (Gaffey et al. 1992, Binzel 1995). Following the Bell classification (Bell et al. 1989), E-class asteroids are considered to be igneous bodies owing to their small heliocentric distance, and probably formed by crystallization of melted materials. Moreover their mineralogy dominated by iron-poor silicates implies that they formed and differentiated under relatively reducing conditions, with parent bodies heated to at least 1580◦ C (the melting point of enstatite) to produce enstatite (Keil 1989). However, the identification of an absorption feature around 3 µm on the spectra of some E asteroids, that is usually found also on hydrated asteroids, has opened a debate regarding the nature of E-type asteroids: if the 3-µm band is due to hydrated minerals, then high albedo asteroids might not be all anhydrous and igneous as previously believed (Rivkin et al. 1995, Rivkin 1997). Burbine et al. (1998) also made a comparison with numerous meteorites and mineral spectra databases finding that troilite seems to represent the best match to E-type asteroids both in the visible and in the near infrared regions (Burbine et al. 1998, Cloutis et al. 1999). Here we present the visible spectra of five E-type asteroids. Three of them reveal a peculiar and wide absorption band centered around 0.5 µm whose origin is not yet well understood. We report in this paper the results of our investigation and a discussion regarding the possible composition of E-type asteroids. 2. OBSERVATIONS AND DATA REDUCTION

The data presented here have been obtained during two observing runs performed in 1999 at the European Southern Observatory of La Silla (Chile), with the 1.5-m (ESO) telescope equipped with a Boller and Chivens spectrograph and a Loral Lesser CCD as detector (2048 × 2048 pixels). The grating used ˚ was 225 gr/mm, with a dispersion of 331 A/mm in the first order.

127 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 Observational and Physical Characteristics of the Observed Asteroids Asteroid

Date

mv

S. An. (airm.)

64 Angelina 317 Roxane 1251 Hedera 2035 Stearns 3103 Eger

11/04/99 11/05/99 11/04/99 11/04/99 03/16/99

12.3 14.1 14.9 15.3 18.2

HD20630 (1.19) Hyades64 (1.47) Hyades64 (1.45) Hyades64 (1.43) HD6060 (1.16)

A D Airm. (AU) (Km) Alb. 1.24 1.54 1.42 1.22 1.39

2.68 2.28 2.71 2.13 1.40

53.8 22.6 14.6 5.5 3.2

0.43 0.49 ? ? 0.63

Note. Objects, date of observations, visual magnitude of the asteroids, solar analog stars used for reduction with their airmasses, airmasses of the asteroids, semimajor axis (AU), diameter (km), and albedo of the objects derived from IRAS data and Mc Fadden et al. (1989). Question marks indicate unavailable data.

The CCD has 15 µm square pixels, giving a dispersion of about ˚ 5 A/pixel in the wavelength direction. The spectral range is about ˚ 0.4 < λ < 0.9 µm with a FWHM of about 10 A. Each spectrum was recorded through a slit oriented in the East–West direction. The slit was opened to about 8 arcsec in order to reduce effects due to differential refraction and the possibility of losing signal due to guiding errors of the tele˚ in scope. We also used a filter cutting the signal below 4000 A order to prevent overlapping of the second spectral order on the spectrum. In Table I we report the circumstances of the observations, the visual magnitude of the asteroids, the solar analog stars used to remove the solar contribution with relative airmasses, the airmasses of the asteroids, and some physical characteristics of the observed objects: semimajor axis (AU), diameter (km) and albedo (derived from IRAS data and Mc Fadden et al. 1989). During each night we also recorded bias, flat-field, calibration lamp and several (six to seven) solar analog stars spectra at different intervals throughout the night. In particular each asteroid observation was followed by solar analog star observations taken through similar airmasses and under similar sky conditions as reported in Table I. The spectra were reduced using ordinary procedures of data reduction (see Luu and Jewitt 1990) with the software packages Midas and IDL. These procedures include: subtraction of the bias from the raw data, flat field correction, cosmic rays removal, background subtraction, collapsing the two-dimensional spectra to one dimension, wavelength calibration and atmospheric extinction correction (using La Silla atmospheric extinction coefficients). Wavelength calibration was made using a lamp with He, Ar, Fe, and Ne emission lines. The calibration was made in the following way: some lines were previously recognized by the user and these values were compared with those present in a table with all the real lines locations. The dispersion relation, with a tolerance of 3 pixels, was then fitted by a third-order bivariate polynomial. The residuals of the wavelength calibra˚ After these procedures we have normalized tion were ≤3 A.

all the spectra, both of asteroids and of solar analog stars, at 1 ˚ The reflectivity of the asteroids was then obaround 5500 A. tained by dividing the spectra of the objects by a solar analog spectrum. Solar analog stars are fundamental in the final step of the reduction procedure to remove the solar contribution from the spectra of the asteroids and to obtain the asteroidal reflectivities. Eight solar analog stars, chosen on the basis of their spectral similarity to the Sun (Hardorp 1978), have been observed: Hyades 64, HD 1835, HR 2208, HD 44594, HD 20630, HD 76151, HD 89010, and HR 6060. Their choice is connected with the observational period and with their closeness in time and in airmass to the asteroids. Anyway we compared also the reduced solar analog spectra each other finding negligible differ˚ The comences in reflectivity gradient, on order of 1%/103 A. patibility of the solar analogs observed places an upper limit of ˚ on reflectivity gradient errors that could be due about 1%/103 A to division by different solar analogs. Spectra used in these studies have been smoothed with a median filter technique, using a box of 40 (spectral direction) × 3 (spatial direction) pixels for each point of the spectrum. Threshold was set to 0.1, meaning that the original value was replaced by the median value if the median value differs by more than 0.1 from the original value. Even if we took as much care as possible in data reduction, some spurious features due to an incomplete removal of sky lines ˚ and of the water (in particular of the O2 A band around 7600 A ˚ ˚ telluric bands around 7200 A and 8300 A) are present on the asteroidal spectra. Anyway these features are easily recognizable and were disregarded in the spectral analysis. Moreover the telluric H2 O absorption at 0.82 µm plus the intense water absorptions beginning near 0.90 µm, coupled with the drop in responsivity of the CCD detector at those wavelengths, have affected a clear identification of the 0.80–0.90 µm features that seem to be present on some spectra. Future observations concentrating on removal of telluric water effects at these wavelengths should improve the clear identification of these absorption bands. We computed the slope of the continuum for each asteroid spectrum using a linear fitting to the data from the beginning of ˚ excluding the data points of the the spectrum to about 8000 A, 0.5-µm absorption band if it was present on the spectrum. Values of absorption bands FWHM, position, and depth were obtained by a Gaussian fitting to the data points in each absorption feature, after straight line division. We also compared the spectra of each asteroid exhibiting the 0.5-µm absorption band with all the solar analog spectra obtained each night, finding negligible differences. The repeatability of the band using different solar analog stars, the absence of relevant sky features around the 0.5-µm region, and the fact that solar absorption lines in this spectral region are much narrower than the peculiar absorption feature identified on some E-asteroidal spectra confirm the reality of this absorption feature. Table II presents E-class asteroidal observations with relative bands identified on their spectra.

E-TYPE ASTEROIDS SPECTROSCOPY

TABLE II E-class Asteroids up to Now Observed with the Relative Bands Identified on Their Spectra Asteroid

Visible range bands

3-µm band

44 Nysa

no

yes

64 Angelina

0.5 µm 0.92 µm

yes

214 Aschera

no data available

yes

317 Roxane

yes

620 Drakonia

0.5 µm 0.92 µm no data available

no data available no

1251 Hedera

no

2035 Stearns

0.5 µm 0.43 µm 0.5 µm 0.92 µm

no data available no

424 Hungaria

3103 Eger

no data available

Reference Rivkin 1997, Zellner et al. 1985, Gaffey et al. 1989 This paper; Burbine et al. 1998, Rivkin 1997 Rivkin 1997 This paper, Rivkin 1997, Burbine et al. 1998 Rivkin, 1997 This paper This paper, Rivkin 1997 This paper, Burbine et al. 1998, Gaffey et al. 1992

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confirms the observations of Burbine et al. (1998). The spectrum ˚ a flat red has a sharp turnup in reflectance from 5000 and 5500 A, ˚ (slope 5.39 ± 0.03%/103 A), ˚ continuum from 5500 to 7800 A ˚ This object shows and a sharp turnoff in reflectance past 7800 A. ˚ but, also a shallow absorption feature centered around 8200 A, due to its faintness, coupled with the drop in the sensitivity of the detector and with the presence of water absorption bands beyond 0.80 µm, we think that other observations are needed to confirm its presence. On 64 Angelina Burbine et al. (1998) found also a shallow absorption feature around 0.92 µm and Rivkin (1997) found an absorption band around 3 µm. 3.2. 317 Roxane This object does not show the 0.5-µm absorption band; it ˚ (slope 2.83 ± has a flat red slope continuum until 8000 A 3 ˚ ˚ 0.03%/10 A) and a shallow absorption band around 8600 A (Fig. 2). As for 64 Angelina, we think that other data are needed to confirm the presence of this band. Rivkin (1997) found a sharp feature at 3 µm with a depth of 29% ± 19%. This detection reveals that Roxane, with a semimajor axis of 2.29 AU, is the asteroid closest to the Sun up to now investigated that presents the 3-µm band (Rivkin 1997, Rivkin, personal communication).

3. RESULTS

3.3. 1251 Hedera 3.1. 64 Angelina Our spectrum reveals a deep absorption band centered at ˚ with a FWHM of 497.78 ± 11.28 A ˚ and 4918.10 ± 3.80 A a depth of about 8% with respect to the continuum (Fig. 1) that

˚ (slope 2.58 ± A flat red spectrum is present until 8000 A 3 ˚ 0.02%/10 A) (Fig. 3); it does not present any absorption fea˚ are due respecture (the features around 7600, 7300, and 8300 A tively to an incomplete removal of O2 A and H2 O telluric bands).

FIG. 1. Reflectance spectrum of 64 Angelina.

FIG. 2. Reflectance spectrum of 317 Roxane.

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FIG. 3. Reflectance spectrum of 1251 Hedera.

FIG. 4. Reflectance spectrum of 2035 Stear.

No other spectroscopic results on Hedera are present in the literature.

˚ are due to an incomplete removal of telluric 7600, and 8300 A absorption bands. 3103 Eger has a low signal-to-noise ratio as respect to that of the other E-type asteroids here reported, owing to its faintness at the time of observation (m v = 18.2). The near-infrared spectrum of 3103 Eger (0.8–2.5 µm) obtained by Gaffey et al. (1992)

3.4. 2035 Stearn The spectrum has an absorption band centered at 4904.84 ± ˚ with a FWHM of 599.89 ± 28.21 A ˚ and a depth of about 6.04 A 4% with respect to the continuum (Fig. 4); it seems to present ˚ with a FWHM also a shallow band around 4323.24 ± 3.03 A ˚ The spectrum has a flat red continuum from of 86.67 ± 8.37 A. ˚ (the feature centered ˚ (slope 4.72 ± 0.04%/103 A) 5500 to 8500 A ˚ is due to an incomplete removal of telluric water around 8300 A band). ˚ is not easily attributable: The band identified around 4300 A it has been associated to pyroxenic minerals such us pigeonite or augite (Hazen et al. 1978) by Busarev (1998) on M-asteroids, to jarosite by Vilas et al. (1993) on low-albedo asteroids, and to chlorites and Mg-rich serpentines by King and Clark (1989) on enstatite chondrites. Anyway most of these minerals present a 3-µm band that was not detected until now on 2035 Stearn (Rivkin 1997, Rivkin, personal communication), so we think that other observations are needed to clearly identify the minerals ˚ band. responsible of the 4300-A 3.5. 3103 Eger It is evident on the spectrum (Fig. 5) the presence of a deep ˚ with a FWHM of absorption band centered at 4934.51 ± 5.35 A ˚ and a depth of 9% as respect to the continuum. 493.48 ± 22.84 A This band was previously identified also by Burbine et al. (1998). ˚ The spectrum shows a flat red continuum from 5400 to 8400 A ˚ and spurious features around 7200, (slope 7.28 ± 0.19%/103 A)

FIG. 5. Reflectance spectrum of 3103 Eger.

E-TYPE ASTEROIDS SPECTROSCOPY

does not reveal any specific mineral absorption feature, while Burbine et al. (1998) found also a shallow feature that extends past 0.92 µm. 4. DISCUSSION AND CONCLUSION

Three of the five E-type asteroids observed (64 Angelina, 2035 Stearn, and 3103 Eger) revealed a peculiar absorption band centered around 0.5 µm. The origin of this band is not yet easily attributable. Burbine et al. (1998) suggested that a possible source of this band may be troilite (FeS), as it has a spectral behavior consistent with the visible and near infrared spectrum of these objects. In fact, as reported by Cloutis and Burbine (1999), synthetic troilite reveals an absorption band near 0.47 µm quite similar to that observed in the visible spectra of some E-type asteroids, a sharp absorption edge in the 0.5–0.6-µm region, and a weak Fe2+ transition absorption feature centered around 0.9 µm, all due to ferrous iron transitions. Moreover troilite presents also an absorption band near 3.1 µm (Burbine et al. 1998, Cloutis and Burbine 1999, Vaughan and Craig 1978, Cloutis and Gaffey 1993, 1994), a feature found also on some E-type asteroids (Rivkin et al. 1995, Rivkin 1997). Another reason for which troilite may be a reasonable candidate for the 0.5-µm absorption feature is that troilite is a known constituent of aubrites (Olsen et al. 1977, Watters and Prinz 1979, Keil et al. 1989, Cloutis and Gaffey 1993, 1994), the meteorites that seem to have the E-type asteroids as principal candidates for parent bodies. Moreover Salisbury et al. (1991) observed also the infrared band around 3 µm in different aubrite spectra. A problem associated with this kind of interpretation is the fact that until now only synthetic troilite and not meteoritic troilite samples revealed the 0.5-µm band. However, troilite is a complex mineral, exhibiting structural and compositional variations, which result in a spectral behavior quite variable (Vaughan and Craig 1978, Ribbe 1974). For instance, the bands centered around 0.5 and 3 µm do not appear in all troilite spectra (Britt and Pieters 1990, Cloutis and Gaffey 1993, Cloutis, personal communication), and their wavelength position changes from sample to sample, probably due to variation in the crystal structure of individual samples. Another problem is the fact that troilite is a darkening agent, not easily compatible with the high albedo of the E asteroids. Anyhow, following Cloutis et al. (1990), enstatite is a highly transparent mineral, so a small amount of a spectrally featured material mixed in with, or adjacent to, enstatite would be detectable. For example, different aubrites such as Shallowater, Cumberland Falls, and Allan Hills 78113 contain xenoliths (composed predominantly of low iron olivine, orthopyroxene, and opaque phases (Graham et al. 1977, Neal and Lipschutz 1981, Verkouteren and Lipschutz 1983, Lipschutz et al. 1988, Keil et al. 1989)) darker than the host materials, suggesting that also high albedo E-type asteroids may contain dark inclusions on their surfaces. Considering the data regarding our objects, we made a comparison with different minerals spectra available in the litera-

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ture, finding that troilite, embedded with iron-free or iron-poor silicates, represents at the moment a plausible source of the 0.5- and 0.92-µm band observed, and possibly also of the 3µm band. In particular, for the 3-µm band, it has been found that troilite samples exposed to air for some days, and heated, still show evidence of the 3-µm band, suggesting that this absorption feature may be due, at least in part, to Fe2+ electronic transitions (Vaughan and Craig 1978, Cloutis and Burbine 1999). This interpretation is consistent with the definition of these asteroids as igneous bodies that have experienced high temperatures during their history. However, there is another interpretation of the infrared band as some authors attribute it to the presence of aqueous altered materials (Rivkin et al. 1995, Rivkin 1997, Britt et al. 1994). Indeed hydrated materials that are not stable at temperatures higher than 500 K are not expected to be present on igneous bodies (Jones et al. 1990) such as E-type asteroids. Anyway water is an extremely intense absorber in the 3-µm region and very small amounts of altered minerals (less than a few tenths of a percent by weight) as well as the presence of fluid inclusions in otherwise anhydrous minerals can result in a strong 3-µm band, as reported by Roush et al. (1987) who found strong 3-µm absorption bands on the spectra of apparently unaltered minerals. If the interpretation of the 3-µm band as due to hydrated materials were correct, this might imply that some E-class asteroids are not igneous and/or that igneous bodies, after melting and formation, may have successively experienced a not yet well known type of aqueous alteration process. The following explanations regarding the possible presence of oxidized and hydrated materials on high albedo E-type asteroids have been given: • Rivkin et al. (1995) and Busarev and Krugly (1995) have suggested that high albedo hydrated asteroids could be a mixture of different materials such as high-albedo anhydrous minerals and low-albedo hydrated minerals with different origins, probably as a result of low velocity collisional impacts. Such kind of events might have taken place during the period of Jupiter growth, when large ice bodies might have been scattered and have collided with asteroids of the inner belt, allowing the hydration of some igneous bodies and/or leaving on them hydrated materials such as phyllosilicates. Such type of co-existence was also found for example on the meteoritic breccia Kaidun, which presents different type of meteoritic materials (El3, EH5, C1, CR2) mixed together with hydrated materials (Ivanov 1989, 1992, Zolensky et al. 1994). • Rivkin et al. (1995) have also suggested that the observed 3-µm band on the spectra of E-type asteroids could be explained with the presence of veins of high-albedo hydrated materials on their surface. These kinds of mineral veins, which were also found on some CI chondrites (Zolensky et al. 1988), might have reached the asteroid surfaces during the bodies formation and allow us to explain both the typical high albedo observed on

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E-type asteroids and the possible presence of hydration bands on their surfaces. • Vilas (1994) proposed that high-albedo asteroids whose spectra present the 3-µm band may represent objects that are in a later stage of aqueous alteration process, when iron is leached out from iron oxides and phyllosilicates and is stored in magnetite grains in greater proportions. This depletion of iron oxide molecules should decrease their efficiency as darkening agents, raise the albedo of the materials, and make the disappearance of the 0.7-µm oxidized iron feature. So she proposed that the surface of these high-albedo hydrated objects may consist of iron-depleted phyllosilicates, opaques such as magnetite, or iron sulfides, and elementary iron. For instance Cyr et al. (1998) show that ice crystal and water might have been drifted by diffusive processes into a region of the solar nebula closer to the Sun than previous believed, allowing water ice crystals to be included also inside E-type asteroids and the aqueous alteration process to occur on them. From the data obtained for E-type objects up to now, we think that a clear composition responsible for the 0.5-, 0.9-, and 3-µm bands has not yet been identified. At the moment troilite would explain well both the visible and near infrared bands identified on E-type asteroids. However, because of the low reflectance of troilite and the fact that only synthetic samples revealed until now the absorption features, we do not exclude the possibility of a different origin for all the three bands or for some of them, in particular for the infrared band that could be due to hydrated materials. At the moment, other materials are under examination also by other authors (Burbine, personal communication, Kelley, personal communication) as possible sources of the 0.5-µm band. So we think that more data and more comparisons with meteorite spectra and laboratory mixtures of silicates (such as enstatite, forsterite, and feldspar), sulfides, and chondritic inclusions are needed to clearly identify the composition and evolution of the E-type asteroids and to obtain a complete picture of the whole group of E-type objects up to now identified. ACKNOWLEDGMENTS The authors thank F. Vilas, M. S. Kelley, T. H. Burbine, G. Consolmagno, and A. S. Rivkin for their useful suggestions.

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