- Email: [email protected]
Spectrophotometry of J8, J9, and four Trojan asteroids from 0.32 to 1.05 μm
|CARUS 46, 108-113 (1981)
Spectrophotometry of J8, J9, and Four Trojan Asteroids from 0.32 to 1.05/~m DALE W. SMITH 1"2 Department of Astronomy, University of Washington, Seattle, Washington 98195
PAUL E. JOHNSON l'a'4 Jet Propulsion Laboratory, Pasadena, California 91103 AND
RICHARD W. SHORTHILL 1 Geospace Sciences Laboratory, University of Utah Research Institute, 420 Chipeta Way, Suite 160, Salt Lake City, Utah 84108 Received August 12, 1980; revised March 2, 1981 New 30-channel narrowband photometry from 0.32 to 1.05 ~zm of the retrograde Jovian satellites J9 (to 0.7/xm) and J8 and the trailing Trojan asteroids 617,884, 1172, and 1173 is presented. The data confirm previous measurements of J8,617,884, 884, and 1172 at h < 0.8/xm, but the extension into the infrared shows that the normalized spectral reflectance of these objects rises steadily from - 0 . 8 at 0.4/zm to - 1 . 4 at 1.05/~m, suggesting they are too bright in the near infrared to be C-type asteroids. The C classification of 1173 is confirmed. J9 is markedly redder than J8 at visible wavelengths. The results indicate a greater taxonomic contrast between these distant objects and main-belt asteroids than previously thought. INTRODUCTION
The outer satellites of Jupiter and the Trojan asteroids are the most distant known concentrations of asteroidal objects. As such, they are the principal probes of the history of the outer asteroid belt. Although the relationship between the satellites and Trojans and between these groups and the main asteroid belt has long been a subject of interest, its investigation has been delayed until recently by the faint magnitudes of these objects. The focus of current photometry has been to make taxonomic classification of Guest Investigator, Hale Observatories. .2 Present address: Department of Physics and Astronomy, Western Washington University, Bellingham, Washington 98225. 3 NASA-NRC Resident Research Associate. 4 Present address: Royal Observatory, Blackford Hill, Edinburgh EH9 3H J, Scotland.
the satellites and Trojans. Most main-belt asteroids can be classified in one of five taxonomic classes defined empirically using photometric properties (Bowell et al., 1978). About 75% of main-belt asteroids belong to class C, characterized by a neutral reflectance at h < 0.6/~m; the remainder belong to classes S, M, E, or R, the last having reddish spectra and high albedo, or none of the classes (class U). Taxonomic classification is useful in comparing the photometric properties of these distant objects to those of the main belt and in testing the photometric homogeneity of each of the physical groups, including the leading and trailing Trojan clouds and the prograde and retrograde satellite groups. Recent UBVRI photometry by Degewij and van Houten (1979) (DvH) shows that the colors of the prograde satellites J6 and J7 and the retrograde satellite J8 are those 108
0019-1035/'81/040108-06502.00/0 Copyright© 1981by AcademicPress, Inc. All rightsof reproductionin any formreserved.
SPECTROPHOTOMETRY OF J8, J9, AND TROJANS of C-type asteroids. Eight of twelve sampled Trojans are also found to possess Ctype colors, while four h a v e reddish colors but a low albedo and hence populate a new class R D (red, dark) which does not o c c u r in the main belt. Several Trojans have b e e n o b s e r v e d b y C h a p m a n and Gaffey (1979, hereafter CG) in the course of a p r o g r a m of spectrophotometric r e c o n n a i s s a n c e of the asteroids. Their data e n c o m p a s s the spectral range 0.32 to - 0 . 8 ~ m and reveal a mixture o f Clike and unclassified spectra. We h a v e made s p e c t r o p h o t o m e t r i c measurements of the retrograde satellites J8 Pasiphae and J9 Sinope and the trailing Trojan asteroids 617 Patroclus, 884 Priamus, 1172 Aneas, and 1173 Anchises in order to refine their t a x o n o m i c classification and thus enable i m p r o v e d taxonomic c o m p a r i s o n within and a m o n g the physical groups. T h e s e data are the first photoelectric m e a s u r e m e n t s of J9 and extend the wavelength c o v e r a g e of the remaining objects redward f r o m 0.8 to 1.05 ~m. OBSERVATIONS The observations w e r e m a d e on the nights of Dec. 20 and 21, 1978, using the 200-in. telescope and Oke multichannel s p e c t r o m e t e r (MCSP) (Oke, 1969). The M C S P is a dual-aperture instrument which simultaneously m e a s u r e s the object and a sky spot 40 arcsec a w a y in 30 channels in the wavelength range 0.32 to 1.05 tzm. S p e c t r o m e t e r apertures of 14 arcsec diameter w e r e e m p l o y e d . The p r o g r a m objects are listed in Table I with the total integration time and air m a s s for each night. The objects w e r e identified at the telescope b y c o m p a r i s o n of the television monitor and same-scale enlargements o f the P a l o m a r O b s e r v a t o r y Sky Survey prints on which the calculated path o f the object was drawn. The satellite e p h e m e r i d e s were taken f r o m the tables of H e r g e t (1968) and the Trojan ephemerides were t a k e n f r o m the E p h e m e r i s for Minor Planets (1978).
109
TABLE I OBSERVING PARAMETERS
Object
617 617 884 1172 1172 1173 J8 J9 P/Tempel-2 ~
UT date
1978 Dec. 21.4 1978 Dec. 22.4 1978 Dec. 21.2 1978 Dec. 21.4 1978 Dec. 22.4 1978 Dec. 22.5 1978 Dec. 21.3 1978 Dec. 22.3 1978Dec. 22.2
Reported in Johnson
et al.
Integration time (rain)
Air mass
20 20 20 28 22 38 20 20 30
1.6 1.6 1.0 1.5 1.5 1.4 1.7 1.4 1.1
(1981).
The identifications were confirmed after the integration period b y observing the object had m o v e d with the e x p e c t e d direction and rate. The object and sky were alternately observed in both s p e c t r o m e t e r apertures in order to determine the relative sensitivity of the two apertures. The nominal sequence was to p e r f o r m five 2-min integrations with the object in one aperture and then to place the object in the other aperture and repeat. The sky brightness was r e m o v e d b y differencing the count rates in the object and sky apertures, with channel-by-channel allowance for the relative sensitivity in the two apertures. The spectral reflectance was obtained b y taking the ratio, in each channel, of the count rate of the p r o g r a m object to the H y a d e s star HD28992. This star, whose spectral type is G1V, has the median color in the group c o m p o s e d of itself and the H y a d e s stars HD26736 (G3V) and HD27836 (G1V); its B - V equals that used by D v H . The normalized fluxes of HD28992 and HD26736 were within 2% of each other in all channels; the normalized fluxes of HD28992 and HD27836 showed m o r e variation in s o m e channels, but in none did the variation exceed 10%. H y a d e s G stars were chosen b e c a u s e b o t h their spectral type and metal a b u n d a n c e , and thus their colors,
110
SMITH, JOHNSON, AND SHORTHILL
would closely match those of the Sun. Correction for extinction was neglected because the difference in extinction between the program objects and the Hyades stars was less than 1%. The spectral reflectances were normalized to unity at 0.56/~m. Use of solar-type stars in the H y a d e s is simpler and more direct than the method of CG, in which A-, B-, and solar-type standards previously calibrated to a L y r a e are observed. In turn, c~ L y r a e is tied to the Sun using a theoretical model whose results are checked b y a comparison of lunar observations and lunar samples. The consistency of the two methods was tested by measuring the spectral reflectance of the bright asteroid 14 Irene on both nights. The two nights' spectra were internally consistent to 1% and the average is compared in Fig. 1 with a spectrum measured by CG. At wavelengths between 0.45 and 1.05 /zm, the two methods exhibit no systematic difference and no 2o- difference in any channel. Some disagreement is present near 0 . 4 / z m , perhaps due to the Balmer discontinuity. RESULTS AND DISCUSSION The normalized reflectances of the four trailing Trojan asteroids 617, 884, 1172, and 1173 are listed in Table II and shown in Fig. 2. The error bars are a best estimate of the lcr internal uncertainty in the reflectances; 1.6
1.6
t.4
1-4 1.2
t.2
1"x'z ,r.xg° Illxxll t4
t,0
I •
•
t.0
O°B
0.B 0.6
}I-
0.4 0.3
O°B
t ,
,
0.4
0.5
, 0.6
, 0.7
' 0.B
' 0°9
~ l°0
0.4 l°t
I~VEI.~TI4 ( n l ~ )
FIG. 1. N o r m a l i z e d s p e c t r a l r e f l e c t a n c e o f 14 I r e n e . T h e e r r o r b a r s a r e l~r c o n f i d e n c e limits f o r o u r d a t a . The squares show the data of Chapman and Gaffey (1979); t h e u n c e r t a i n t y in t h e i r d a t a p o i n t s is - ---0.03 in t h e v i s i b l e , a s s h o w n b y t h e s a m p l e e r r o r b a r , a n d is greater at the extreme wavelengths.
TABLE NORMALIZED
Band center (p.m) 0.3240 0.3400 0.3560 0.3720 0.3880 0.4040 0.4200 0.4360 0.4520 0.4680 0.4840 0.5000 0.5160 0.5320 0.5480 0.5640 0.5820 0.6180 0.6540 0.6900 0.7260 0.7620 0.7980 0.8340 0.8700 0.9060 0.9420 0.9780 1.0140 1.0500
617
II
SPECTRAL
884
0.62 0.82 0.66 1.02 0.78 1.11 0.78 1.06 --0.89 1.01 0.89 0.95 0.90 1.00 0.91 0.92 0.92 0.94 0.96 0.97 0.98 0.98 0.99 1.01 0.99 1.00 0.99 1.01 -1.01 1.02 1.02 1.07 1.06 1.08 1.11 1.07 1.12 1.16 1.21 . . . 1.01 1.03 1.19 1.26 1.24 1.28 1.39 1.55 1.10 1.11 1.26 1.43 1.31 1.55 1.37 1.47
REFLECTANCES
1172
1173
J8
0.58 0.68 0.81 0.80 -0.84 0.84 0.87 0.87 0.88 0.92 0.95 0.97 0.98 0.97 -1.02 1.08 1.11 1.13 1.22 . 1.09 1.37 1.40 1.51 1.22 1.46 -1.54
0.71 -0.69 -0.71 -0.74 0.78 0.75 -0.86 0.98 0.85 0.88 0.87 0.94 0.90 0.90 0.93 0.98 0.96 1.00 0.96 1.03 0.95 1.02 0.96 1.02 0.97 0.99 --1.02 1.02 1.02 1.03 1.03 1.07 0.99 1.12 1.07 1.17 . . 0.94 1.34 1.10 1.24 1.19 1.48 1.21 -1.13 -1.11 1.45 1.23 1.44 1.15 --
J9
----0.58 0.61 0.74 0.59 0.71 0.57 0.69 0.67 0.71 0.82 0.94 1.08 1.00 1.02 1.03 1.16 1.24 ---------
the average uncertainty is obtained from the point-to-point variance and the wavelength dependence is determined by the counting statistics. Some additional systematic error may also be present, but the good agreement of our spectrum of 14 Irene with that measured by CG indicates that any systematic error is probably small. The reflectances of 617 and 1172 are the average of the separate results obtained on two nights. The spectrophotometry of CG and the UBVRI measurements of D v H are also shown. The spectrophotometry is based on one night's observation of each object. The broadband colors were converted to normalized reflectances using the solar colors
SPECTROPHOTOMETRY OF J8, J9, AND TROJANS •
1-4
1.2 l.O
•
.t m~r.i , . x lr/V ~t7a --&--
I£
• ~
t.4
. iIiiIi
1°0
0.8
II III[.,
0.6 6-4
,Tz f
I
1-0
1.2
I I I ,., l°B
da~ 0.B
• •
o .6 I
06v,T IF ,-°l."
°"o22 o:,
i
i z
1.2
I
I
i.I. z':, " i" o'911 'o 6,,
, .z
o.,
o:, I~qVEL.ENOTHCMI(~3H8 )
FIG. 2. Normalized spectral reflectance of asteroids 617, 884, 1172, and 1173. Legends and error bars as in Fig. 1, with the addition that data of Degewij and van Houten (1979) are given by × 's. The uncertainty in the data of Chapman and Gaffey (1979) is --+0.03 in the visible. The sample error bar with × shows the typical uncertainty in the data of Degewij and van Houten.
listed b y DvH: U - B = 0.10, B - V = 0.63, V - R = 0.31, a n d V - I = 0.73. The U B V data are based on observations on three to five nights and the VRI data on observations on one night. The three sets of data on 617 Patroclus are in close agreement from 0.4 to 0.7/zm, but possible differences arise at greater wavelengths. The broadband points suggest a smaller reflectance at R and I than the sets of s p e c t r o p h o t o m e t r y do. In the range 0.7 to 0.95 ttm the data of CG show a neutral reflectance which, combined with the slope in the visible, indicates that 617 is a C-type object. While our data below 0 . 9 5 / z m can be interpreted similarly, the high reflectance near and above 1.0/~m suggests that the reflectance m a y rise through the
111
range 0.7 to 1.05/xm in a way not characteristic of C asteroids. This spectral behavior almost surely arises from surface mineralogy rather than from maturing o f the soil by impact, for Matson et al. (1977) have demonstrated an absence of the latter effect on other asteroids. The two sets o f narrowband reflectances o f 884 Priamus show a similar slope throughout the c o m m o n wavelength range, 0.4 to 0.8/zm. The reflectance rises steadily redward, and CG assign an M or E type to 884. Our data indicate that the rise in reflectance continues at least to 1.05 ~m, confirming that 884 is not a C-type object. The U B V data also show 884 to be red and D v H assign it type RD. All three data sets of 1172 Aneas show excellent agreement in the c o m m o n wavelength range, 0.4 to 0.8/zm, in exhibiting a rapid increase in reflectance with increasing wavelength. Our data indicate this trend continues to at least 1.05 /zm, and thus suggest that the assignment of type C to 1172 made by CG and D v H must be revised. The red color and low albedo of 0.04 (DvH) suggest possible membership in the RD class. The spectrum of 1173 Anchises is similar in all three data sets. Our data confirm the steep slope in the reflectance curve at wavelengths less than 0.5/zm as well as the more gradual slope at greater wavelengths and extend the latter from 0.8 on to 1.05 /xm, and hence support the assignment o f class C to 1173. The complete (0.32 to 1.05/xm) spectra of the trailing Trojans 617, 884, and 1172 presented here resembled the complete spectra o f the leading Trojans 588, 624, 911 measured b y CG and classified as R-like or U. The confirmation of 884 as a non- C asteroid and the indication o f the complete spectra that 617 and 1172 should be classified as RD or U (rather than C as indicated b y the spectra reaching only to - 0 . 8 / z m ) further accentuate the taxonomic difference between the Trojan clouds and the main belt already noted by CG and D v H . The
112
SMITH, JOHNSON, AND SHORTHILL 1.4
1-4
:III:
1-2
I
1.0 .9. 0.B 0.6
IIIiI:II:
w ~ t.2
:I.:
0.6 0.3
1-g 1-0 1.B
, I I
1.2
ii=i,/J/I II ~8
1.0 0.8
I
I"
1.0 0-B
t i
0.4
i
i
i
i
i
0.5 0.6 0-7 0.8 0.9 Wi:IVELENOTH CIIICRO~8]
' 1.0
[1.6 I,I
FIG. 3. Normalized spectral reflectance of J8 and J9. Legends and sample error bar are as in Fig. 2. The J9 reflectances have been smoothed by a 3-point sliding average. confirmation of 1173 as type C, h o w e v e r , indicates that the Trojans are not a homogeneous t a x o n o m i c group. The normalized reflectances of J8 and J9 are s h o w n in Fig. 3 along with the U B V R I m e a s u r e m e n t s of J8 b y D v H . The reflectances are b a s e d on data taken on one night and the b r o a d b a n d colors are b a s e d on data taken on two nights. The n a r r o w b a n d and b r o a d b a n d measurements of J8 Pasiphae show good agreement at k < 0.8 ~ m , but diverge at greater wavelengths. Although the b r o a d b a n d point in I indicates a fiat reflectance consistent with a C-type s p e c t r u m , the narrowband data indicate a rise in reflectance at h > 0 . 6 / z m similar to that seen in 617,884, and 1172 and suggest that J8 is too red to be a C-type object. It is not possible at present to determine which data are correct, and further observations are needed. The s p e c t r u m of J9 Sinope is m a r k e d l y different f r o m that of J8 in being m u c h redder at visible wavelengths. D a t a a b o v e 0.75/.tm are v e r y noisy and are not shown. The steep slope of the reflectance curve in the visible is similar to those of s o m e R-like spectra m e a s u r e d b y CG; the closest m a t c h is provided b y 246 A s p o r i n a but is p r o b a b l y fortuitous. Speculation on possible mineralogy is unwarranted at this time b e c a u s e the near-infrared data are missing and the visi-
ble data are b a s e d on only one night of observation. The s p e c t r o p h o t o m e t r y reported here indicates that only one o f the six objects o b s e r v e d has a C-type s p e c t r u m , and thus that the Trojan asteroids and outer Jovian satellites m a y differ f r o m the main-belt asteroids m o r e than suggested by previous work. The leading and trailing Trojan clouds a p p e a r to share a similar t a x o n o m y with a p r e p o n d e r a n c e of objects whose reflectance increases steadily with wavelength and which are bright in near infrared, but one Trojan (1173) is confirmed as a Ctype object. At least s o m e similarity between the outer satellites and the Trojans is d e m o n s t r a t e d b y the r e s e m b l a n c e of the s p e c t r u m of the retrograde satellite J8 to those of the Trojans. But J9, also a retrograde satellite, appears to be m u c h redder than J8 in the visible and does not r e s e m b l e the Trojans. Thus the retrograde group shows t a x o n o m i c heterogeneity. Further, the prograde and retrograde satellite groups m a y differ f r o m each other if the C classification of the prograde satellites J6 and J7 b y D v H is sustained b y future work. While the a p p a r e n t t a x o n o m i c inhomogeneity of both the trailing Trojan and the retrograde satellite groups precludes a unique interpretation and indicates the need for further observations, especially of the satellites, the apparent t a x o n o m i c differences of these distant objects f r o m the main-belt asteroids does suggest that they m a y not h a v e originated in the main belt. ACKNOWLEDGMENTS We thank Hale Observatories for providing the observing time and for assistance with the observations. We thank Drs. Clark Chapman and Johan Degewij for helpful discussions. This work was supported in part by NSF Grant AST-7824847 and NASA Grants NAGW-ll and NGR 48-002-142. REFERENCES BOWELL, E.,
C H A P M A N , C . R . , G R A D I E , J. C., MORRISON, D . , AND Z E L L N E R , B . (1978). T a x o n -
omy
of asteroids.
CHAPMAN,
C.
R.,
Icarus AND
35, 313-335. GAFFEY,
M.
J.
(1979).
Reflectance spectra for 277 asteroids. In Asteroids (T. Gehrels, Ed.), pp. 655-687. Univ. of Arizona Press, Tucson.
SPECTROPHOTOMETRY DEGEW1J, J., AND VAN HOUTEN, C. J. (1979). Distant asteroids and outer Jovian satellites. In Asteroids (T. Gehrels, Ed.), pp. 417-435. Univ. of Arizona Press, Tuscon. Ephemerides o f Minor Planets (1978). Soviet Academy of the Sciences, USSR. HERGET, P. (1968). Ephemerides o f Comet Schwassmann-Wachmann I and the Outer Satellites o f Jupiter. Publications of the Cincinnati Observatory No. 23.
O F JS, J9, A N D T R O J A N S
113
JOHNSON, P. E., SMITH, D. W., AND SHORTHILL, R. W. (1981). An outburst of comet Tempel-2 observed spectrophotometrically. Nature 289, 155-156. MATSON, D., JOHNSON, T. V., AND VEEDER, G. (1977). Soft.maturity and planetary regoliths: Moon, Mercury, and the asteroids. Proc. 8th Lunar Sci. Conf. 1001-1011. OKE, J. B. (1969). A multichannel photoelectric spectrometer. Proc. Astron. Soc. Pacific 81, 11-22.
Spectrophotometry of J8, J9, and Four Trojan Asteroids from 0.32 to 1.05/~m DALE W. SMITH 1"2 Department of Astronomy, University of Washington, Seattle, Washington 98195
PAUL E. JOHNSON l'a'4 Jet Propulsion Laboratory, Pasadena, California 91103 AND
RICHARD W. SHORTHILL 1 Geospace Sciences Laboratory, University of Utah Research Institute, 420 Chipeta Way, Suite 160, Salt Lake City, Utah 84108 Received August 12, 1980; revised March 2, 1981 New 30-channel narrowband photometry from 0.32 to 1.05 ~zm of the retrograde Jovian satellites J9 (to 0.7/xm) and J8 and the trailing Trojan asteroids 617,884, 1172, and 1173 is presented. The data confirm previous measurements of J8,617,884, 884, and 1172 at h < 0.8/xm, but the extension into the infrared shows that the normalized spectral reflectance of these objects rises steadily from - 0 . 8 at 0.4/zm to - 1 . 4 at 1.05/~m, suggesting they are too bright in the near infrared to be C-type asteroids. The C classification of 1173 is confirmed. J9 is markedly redder than J8 at visible wavelengths. The results indicate a greater taxonomic contrast between these distant objects and main-belt asteroids than previously thought. INTRODUCTION
The outer satellites of Jupiter and the Trojan asteroids are the most distant known concentrations of asteroidal objects. As such, they are the principal probes of the history of the outer asteroid belt. Although the relationship between the satellites and Trojans and between these groups and the main asteroid belt has long been a subject of interest, its investigation has been delayed until recently by the faint magnitudes of these objects. The focus of current photometry has been to make taxonomic classification of Guest Investigator, Hale Observatories. .2 Present address: Department of Physics and Astronomy, Western Washington University, Bellingham, Washington 98225. 3 NASA-NRC Resident Research Associate. 4 Present address: Royal Observatory, Blackford Hill, Edinburgh EH9 3H J, Scotland.
the satellites and Trojans. Most main-belt asteroids can be classified in one of five taxonomic classes defined empirically using photometric properties (Bowell et al., 1978). About 75% of main-belt asteroids belong to class C, characterized by a neutral reflectance at h < 0.6/~m; the remainder belong to classes S, M, E, or R, the last having reddish spectra and high albedo, or none of the classes (class U). Taxonomic classification is useful in comparing the photometric properties of these distant objects to those of the main belt and in testing the photometric homogeneity of each of the physical groups, including the leading and trailing Trojan clouds and the prograde and retrograde satellite groups. Recent UBVRI photometry by Degewij and van Houten (1979) (DvH) shows that the colors of the prograde satellites J6 and J7 and the retrograde satellite J8 are those 108
0019-1035/'81/040108-06502.00/0 Copyright© 1981by AcademicPress, Inc. All rightsof reproductionin any formreserved.
SPECTROPHOTOMETRY OF J8, J9, AND TROJANS of C-type asteroids. Eight of twelve sampled Trojans are also found to possess Ctype colors, while four h a v e reddish colors but a low albedo and hence populate a new class R D (red, dark) which does not o c c u r in the main belt. Several Trojans have b e e n o b s e r v e d b y C h a p m a n and Gaffey (1979, hereafter CG) in the course of a p r o g r a m of spectrophotometric r e c o n n a i s s a n c e of the asteroids. Their data e n c o m p a s s the spectral range 0.32 to - 0 . 8 ~ m and reveal a mixture o f Clike and unclassified spectra. We h a v e made s p e c t r o p h o t o m e t r i c measurements of the retrograde satellites J8 Pasiphae and J9 Sinope and the trailing Trojan asteroids 617 Patroclus, 884 Priamus, 1172 Aneas, and 1173 Anchises in order to refine their t a x o n o m i c classification and thus enable i m p r o v e d taxonomic c o m p a r i s o n within and a m o n g the physical groups. T h e s e data are the first photoelectric m e a s u r e m e n t s of J9 and extend the wavelength c o v e r a g e of the remaining objects redward f r o m 0.8 to 1.05 ~m. OBSERVATIONS The observations w e r e m a d e on the nights of Dec. 20 and 21, 1978, using the 200-in. telescope and Oke multichannel s p e c t r o m e t e r (MCSP) (Oke, 1969). The M C S P is a dual-aperture instrument which simultaneously m e a s u r e s the object and a sky spot 40 arcsec a w a y in 30 channels in the wavelength range 0.32 to 1.05 tzm. S p e c t r o m e t e r apertures of 14 arcsec diameter w e r e e m p l o y e d . The p r o g r a m objects are listed in Table I with the total integration time and air m a s s for each night. The objects w e r e identified at the telescope b y c o m p a r i s o n of the television monitor and same-scale enlargements o f the P a l o m a r O b s e r v a t o r y Sky Survey prints on which the calculated path o f the object was drawn. The satellite e p h e m e r i d e s were taken f r o m the tables of H e r g e t (1968) and the Trojan ephemerides were t a k e n f r o m the E p h e m e r i s for Minor Planets (1978).
109
TABLE I OBSERVING PARAMETERS
Object
617 617 884 1172 1172 1173 J8 J9 P/Tempel-2 ~
UT date
1978 Dec. 21.4 1978 Dec. 22.4 1978 Dec. 21.2 1978 Dec. 21.4 1978 Dec. 22.4 1978 Dec. 22.5 1978 Dec. 21.3 1978 Dec. 22.3 1978Dec. 22.2
Reported in Johnson
et al.
Integration time (rain)
Air mass
20 20 20 28 22 38 20 20 30
1.6 1.6 1.0 1.5 1.5 1.4 1.7 1.4 1.1
(1981).
The identifications were confirmed after the integration period b y observing the object had m o v e d with the e x p e c t e d direction and rate. The object and sky were alternately observed in both s p e c t r o m e t e r apertures in order to determine the relative sensitivity of the two apertures. The nominal sequence was to p e r f o r m five 2-min integrations with the object in one aperture and then to place the object in the other aperture and repeat. The sky brightness was r e m o v e d b y differencing the count rates in the object and sky apertures, with channel-by-channel allowance for the relative sensitivity in the two apertures. The spectral reflectance was obtained b y taking the ratio, in each channel, of the count rate of the p r o g r a m object to the H y a d e s star HD28992. This star, whose spectral type is G1V, has the median color in the group c o m p o s e d of itself and the H y a d e s stars HD26736 (G3V) and HD27836 (G1V); its B - V equals that used by D v H . The normalized fluxes of HD28992 and HD26736 were within 2% of each other in all channels; the normalized fluxes of HD28992 and HD27836 showed m o r e variation in s o m e channels, but in none did the variation exceed 10%. H y a d e s G stars were chosen b e c a u s e b o t h their spectral type and metal a b u n d a n c e , and thus their colors,
110
SMITH, JOHNSON, AND SHORTHILL
would closely match those of the Sun. Correction for extinction was neglected because the difference in extinction between the program objects and the Hyades stars was less than 1%. The spectral reflectances were normalized to unity at 0.56/~m. Use of solar-type stars in the H y a d e s is simpler and more direct than the method of CG, in which A-, B-, and solar-type standards previously calibrated to a L y r a e are observed. In turn, c~ L y r a e is tied to the Sun using a theoretical model whose results are checked b y a comparison of lunar observations and lunar samples. The consistency of the two methods was tested by measuring the spectral reflectance of the bright asteroid 14 Irene on both nights. The two nights' spectra were internally consistent to 1% and the average is compared in Fig. 1 with a spectrum measured by CG. At wavelengths between 0.45 and 1.05 /zm, the two methods exhibit no systematic difference and no 2o- difference in any channel. Some disagreement is present near 0 . 4 / z m , perhaps due to the Balmer discontinuity. RESULTS AND DISCUSSION The normalized reflectances of the four trailing Trojan asteroids 617, 884, 1172, and 1173 are listed in Table II and shown in Fig. 2. The error bars are a best estimate of the lcr internal uncertainty in the reflectances; 1.6
1.6
t.4
1-4 1.2
t.2
1"x'z ,r.xg° Illxxll t4
t,0
I •
•
t.0
O°B
0.B 0.6
}I-
0.4 0.3
O°B
t ,
,
0.4
0.5
, 0.6
, 0.7
' 0.B
' 0°9
~ l°0
0.4 l°t
I~VEI.~TI4 ( n l ~ )
FIG. 1. N o r m a l i z e d s p e c t r a l r e f l e c t a n c e o f 14 I r e n e . T h e e r r o r b a r s a r e l~r c o n f i d e n c e limits f o r o u r d a t a . The squares show the data of Chapman and Gaffey (1979); t h e u n c e r t a i n t y in t h e i r d a t a p o i n t s is - ---0.03 in t h e v i s i b l e , a s s h o w n b y t h e s a m p l e e r r o r b a r , a n d is greater at the extreme wavelengths.
TABLE NORMALIZED
Band center (p.m) 0.3240 0.3400 0.3560 0.3720 0.3880 0.4040 0.4200 0.4360 0.4520 0.4680 0.4840 0.5000 0.5160 0.5320 0.5480 0.5640 0.5820 0.6180 0.6540 0.6900 0.7260 0.7620 0.7980 0.8340 0.8700 0.9060 0.9420 0.9780 1.0140 1.0500
617
II
SPECTRAL
884
0.62 0.82 0.66 1.02 0.78 1.11 0.78 1.06 --0.89 1.01 0.89 0.95 0.90 1.00 0.91 0.92 0.92 0.94 0.96 0.97 0.98 0.98 0.99 1.01 0.99 1.00 0.99 1.01 -1.01 1.02 1.02 1.07 1.06 1.08 1.11 1.07 1.12 1.16 1.21 . . . 1.01 1.03 1.19 1.26 1.24 1.28 1.39 1.55 1.10 1.11 1.26 1.43 1.31 1.55 1.37 1.47
REFLECTANCES
1172
1173
J8
0.58 0.68 0.81 0.80 -0.84 0.84 0.87 0.87 0.88 0.92 0.95 0.97 0.98 0.97 -1.02 1.08 1.11 1.13 1.22 . 1.09 1.37 1.40 1.51 1.22 1.46 -1.54
0.71 -0.69 -0.71 -0.74 0.78 0.75 -0.86 0.98 0.85 0.88 0.87 0.94 0.90 0.90 0.93 0.98 0.96 1.00 0.96 1.03 0.95 1.02 0.96 1.02 0.97 0.99 --1.02 1.02 1.02 1.03 1.03 1.07 0.99 1.12 1.07 1.17 . . 0.94 1.34 1.10 1.24 1.19 1.48 1.21 -1.13 -1.11 1.45 1.23 1.44 1.15 --
J9
----0.58 0.61 0.74 0.59 0.71 0.57 0.69 0.67 0.71 0.82 0.94 1.08 1.00 1.02 1.03 1.16 1.24 ---------
the average uncertainty is obtained from the point-to-point variance and the wavelength dependence is determined by the counting statistics. Some additional systematic error may also be present, but the good agreement of our spectrum of 14 Irene with that measured by CG indicates that any systematic error is probably small. The reflectances of 617 and 1172 are the average of the separate results obtained on two nights. The spectrophotometry of CG and the UBVRI measurements of D v H are also shown. The spectrophotometry is based on one night's observation of each object. The broadband colors were converted to normalized reflectances using the solar colors
SPECTROPHOTOMETRY OF J8, J9, AND TROJANS •
1-4
1.2 l.O
•
.t m~r.i , . x lr/V ~t7a --&--
I£
• ~
t.4
. iIiiIi
1°0
0.8
II III[.,
0.6 6-4
,Tz f
I
1-0
1.2
I I I ,., l°B
da~ 0.B
• •
o .6 I
06v,T IF ,-°l."
°"o22 o:,
i
i z
1.2
I
I
i.I. z':, " i" o'911 'o 6,,
, .z
o.,
o:, I~qVEL.ENOTHCMI(~3H8 )
FIG. 2. Normalized spectral reflectance of asteroids 617, 884, 1172, and 1173. Legends and error bars as in Fig. 1, with the addition that data of Degewij and van Houten (1979) are given by × 's. The uncertainty in the data of Chapman and Gaffey (1979) is --+0.03 in the visible. The sample error bar with × shows the typical uncertainty in the data of Degewij and van Houten.
listed b y DvH: U - B = 0.10, B - V = 0.63, V - R = 0.31, a n d V - I = 0.73. The U B V data are based on observations on three to five nights and the VRI data on observations on one night. The three sets of data on 617 Patroclus are in close agreement from 0.4 to 0.7/zm, but possible differences arise at greater wavelengths. The broadband points suggest a smaller reflectance at R and I than the sets of s p e c t r o p h o t o m e t r y do. In the range 0.7 to 0.95 ttm the data of CG show a neutral reflectance which, combined with the slope in the visible, indicates that 617 is a C-type object. While our data below 0 . 9 5 / z m can be interpreted similarly, the high reflectance near and above 1.0/~m suggests that the reflectance m a y rise through the
111
range 0.7 to 1.05/xm in a way not characteristic of C asteroids. This spectral behavior almost surely arises from surface mineralogy rather than from maturing o f the soil by impact, for Matson et al. (1977) have demonstrated an absence of the latter effect on other asteroids. The two sets o f narrowband reflectances o f 884 Priamus show a similar slope throughout the c o m m o n wavelength range, 0.4 to 0.8/zm. The reflectance rises steadily redward, and CG assign an M or E type to 884. Our data indicate that the rise in reflectance continues at least to 1.05 ~m, confirming that 884 is not a C-type object. The U B V data also show 884 to be red and D v H assign it type RD. All three data sets of 1172 Aneas show excellent agreement in the c o m m o n wavelength range, 0.4 to 0.8/zm, in exhibiting a rapid increase in reflectance with increasing wavelength. Our data indicate this trend continues to at least 1.05 /zm, and thus suggest that the assignment of type C to 1172 made by CG and D v H must be revised. The red color and low albedo of 0.04 (DvH) suggest possible membership in the RD class. The spectrum of 1173 Anchises is similar in all three data sets. Our data confirm the steep slope in the reflectance curve at wavelengths less than 0.5/zm as well as the more gradual slope at greater wavelengths and extend the latter from 0.8 on to 1.05 /xm, and hence support the assignment o f class C to 1173. The complete (0.32 to 1.05/xm) spectra of the trailing Trojans 617, 884, and 1172 presented here resembled the complete spectra o f the leading Trojans 588, 624, 911 measured b y CG and classified as R-like or U. The confirmation of 884 as a non- C asteroid and the indication o f the complete spectra that 617 and 1172 should be classified as RD or U (rather than C as indicated b y the spectra reaching only to - 0 . 8 / z m ) further accentuate the taxonomic difference between the Trojan clouds and the main belt already noted by CG and D v H . The
112
SMITH, JOHNSON, AND SHORTHILL 1.4
1-4
:III:
1-2
I
1.0 .9. 0.B 0.6
IIIiI:II:
w ~ t.2
:I.:
0.6 0.3
1-g 1-0 1.B
, I I
1.2
ii=i,/J/I II ~8
1.0 0.8
I
I"
1.0 0-B
t i
0.4
i
i
i
i
i
0.5 0.6 0-7 0.8 0.9 Wi:IVELENOTH CIIICRO~8]
' 1.0
[1.6 I,I
FIG. 3. Normalized spectral reflectance of J8 and J9. Legends and sample error bar are as in Fig. 2. The J9 reflectances have been smoothed by a 3-point sliding average. confirmation of 1173 as type C, h o w e v e r , indicates that the Trojans are not a homogeneous t a x o n o m i c group. The normalized reflectances of J8 and J9 are s h o w n in Fig. 3 along with the U B V R I m e a s u r e m e n t s of J8 b y D v H . The reflectances are b a s e d on data taken on one night and the b r o a d b a n d colors are b a s e d on data taken on two nights. The n a r r o w b a n d and b r o a d b a n d measurements of J8 Pasiphae show good agreement at k < 0.8 ~ m , but diverge at greater wavelengths. Although the b r o a d b a n d point in I indicates a fiat reflectance consistent with a C-type s p e c t r u m , the narrowband data indicate a rise in reflectance at h > 0 . 6 / z m similar to that seen in 617,884, and 1172 and suggest that J8 is too red to be a C-type object. It is not possible at present to determine which data are correct, and further observations are needed. The s p e c t r u m of J9 Sinope is m a r k e d l y different f r o m that of J8 in being m u c h redder at visible wavelengths. D a t a a b o v e 0.75/.tm are v e r y noisy and are not shown. The steep slope of the reflectance curve in the visible is similar to those of s o m e R-like spectra m e a s u r e d b y CG; the closest m a t c h is provided b y 246 A s p o r i n a but is p r o b a b l y fortuitous. Speculation on possible mineralogy is unwarranted at this time b e c a u s e the near-infrared data are missing and the visi-
ble data are b a s e d on only one night of observation. The s p e c t r o p h o t o m e t r y reported here indicates that only one o f the six objects o b s e r v e d has a C-type s p e c t r u m , and thus that the Trojan asteroids and outer Jovian satellites m a y differ f r o m the main-belt asteroids m o r e than suggested by previous work. The leading and trailing Trojan clouds a p p e a r to share a similar t a x o n o m y with a p r e p o n d e r a n c e of objects whose reflectance increases steadily with wavelength and which are bright in near infrared, but one Trojan (1173) is confirmed as a Ctype object. At least s o m e similarity between the outer satellites and the Trojans is d e m o n s t r a t e d b y the r e s e m b l a n c e of the s p e c t r u m of the retrograde satellite J8 to those of the Trojans. But J9, also a retrograde satellite, appears to be m u c h redder than J8 in the visible and does not r e s e m b l e the Trojans. Thus the retrograde group shows t a x o n o m i c heterogeneity. Further, the prograde and retrograde satellite groups m a y differ f r o m each other if the C classification of the prograde satellites J6 and J7 b y D v H is sustained b y future work. While the a p p a r e n t t a x o n o m i c inhomogeneity of both the trailing Trojan and the retrograde satellite groups precludes a unique interpretation and indicates the need for further observations, especially of the satellites, the apparent t a x o n o m i c differences of these distant objects f r o m the main-belt asteroids does suggest that they m a y not h a v e originated in the main belt. ACKNOWLEDGMENTS We thank Hale Observatories for providing the observing time and for assistance with the observations. We thank Drs. Clark Chapman and Johan Degewij for helpful discussions. This work was supported in part by NSF Grant AST-7824847 and NASA Grants NAGW-ll and NGR 48-002-142. REFERENCES BOWELL, E.,
C H A P M A N , C . R . , G R A D I E , J. C., MORRISON, D . , AND Z E L L N E R , B . (1978). T a x o n -
omy
of asteroids.
CHAPMAN,
C.
R.,
Icarus AND
35, 313-335. GAFFEY,
M.
J.
(1979).
Reflectance spectra for 277 asteroids. In Asteroids (T. Gehrels, Ed.), pp. 655-687. Univ. of Arizona Press, Tucson.
SPECTROPHOTOMETRY DEGEW1J, J., AND VAN HOUTEN, C. J. (1979). Distant asteroids and outer Jovian satellites. In Asteroids (T. Gehrels, Ed.), pp. 417-435. Univ. of Arizona Press, Tuscon. Ephemerides o f Minor Planets (1978). Soviet Academy of the Sciences, USSR. HERGET, P. (1968). Ephemerides o f Comet Schwassmann-Wachmann I and the Outer Satellites o f Jupiter. Publications of the Cincinnati Observatory No. 23.
O F JS, J9, A N D T R O J A N S
113
JOHNSON, P. E., SMITH, D. W., AND SHORTHILL, R. W. (1981). An outburst of comet Tempel-2 observed spectrophotometrically. Nature 289, 155-156. MATSON, D., JOHNSON, T. V., AND VEEDER, G. (1977). Soft.maturity and planetary regoliths: Moon, Mercury, and the asteroids. Proc. 8th Lunar Sci. Conf. 1001-1011. OKE, J. B. (1969). A multichannel photoelectric spectrometer. Proc. Astron. Soc. Pacific 81, 11-22.