1990AJ 100. . 933J a) 933 Astron.J.100 (3),September1990 0004-6256/90/030933-12$00.90 © 1990Am. Astron. Soc.933 THE ASTRONOMICALJOURNAL Johnson e/tf/.1987). Astronomy Observatories,operatedby theAssociationofUniversitiesfor VisitingAstronomer,KittPeakNational Observatory,NationalOptical nuclei wouldbenaturallyexplainedifpolymerizedorganic mantle. Thelowalbedosandreddishreflectionspectraofthe P/Halley suggestacarbon-richcomposition(Kisseletal. concentrated inahandfulofactivevents(Kelleretal covered byadark,refractorymantle,andthatoutgassingis the nucleusofP/Halleyconfirmthatmuchsurfaceis which cometaryvolatilesescapeeitherbydiffusionor to anucleus-widemantleofrefractorymatter,through albedo andreddish.Thesepropertiesarethoughttobedue properties, thenucleimaybeeffectivelysummarizedaslow Jewitt andLuu1989;1990b).Intheiroptical Foundation. Research inAstronomyInc.,undercontract withtheNationalScience compounds wereabundantconstituentsofthismantle(e.g., through discretevents.High-resolutionspacecraftimagesof nuclei intheopticalwavelengthrange(4000Trojan asteroidsarecharacterizedbyabroadrangeoflinearcontinuumgradientsS'inthe cometary nucleusspectra.TheTrojanCCDspectracoverthewavelengthrange4000 ABSTRACT J.X. Luu jans, forexample,populate twodynamicallydistinct lowing) Lagrangian pointsofJupiter.Temperatures inthe thermal environmentsinwhich theseobjectsexist.TheTro- teroids mightbeproductsofthe distinctivecollisionaland similarities betweenthecometary nucleiandtheTrojanas- “clouds” associatedwiththeL4 (preceding)andL5(fol- of cometarynuclei.Ifreal,theapparent opticalandphysical the Trojanasteroidsresemblecorrespondingproperties asteroids. ferent collisionalenvironmentsofthemain-beltandTrojan compared tothoseinmain-beltasteroids(Hartmannetal. tional photometricvariationsinTrojanasteroidsarelarge rect evidencefororganicmaterialinasteroidshasyetbeen main-belt asteroids.Thismightbeaconsequenceofthedif- produced (c.f.,Cruikshank1987;Cloutis1989).Therota- Cruikshank, andGaffey1985).However,nocompellingdi- sumed toindicateasurfacerichinorganiccompounds rest inclassesCandP(GradieTedesco1982;c.f.,Bell et al.1989).Thelowalbedoandredcoloraregenerallyas- with 60%ormorefallingintothetaxonomicclassDand shank 1977)onaveragethantypicalmain-beltasteroids, properties ofthebrighterTrojansareknownwithreasonable confidence. Theseobjectsareredderanddarker(e.g.,Cruik- with cometnuclei(Hartmann1986).Selectedphysical are amongthemostattractivecandidatesforacomparison known outersolarsystempopulation,theTrojanasteroids ies ofpossiblyrelatedbodiesintheoutersolarsystem.Of now appearingintheliteraturemotivatecomparativestud- 1988), suggestingthattheTrojansareelongatedrelativeto (Gradie andVeverka1980;seealsothediscussioninBell, The visualcolors,albedos,andelongatedbodyshapesof SEPTEMBER 1990 1990AJ 100. . 933J bulk physicalpropertiesisexpectedtohavebeenminimal. 934 D.C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS ing thattheTrojansmaybelessevolvedasaresultofmutual oids inthecloudsaresmallerthanmainbelt,suggest- jan asteroidsaredynamicallyisolatedfromthemain-belt quency issmall,sothatrecentcollisionalevolutionoftheir suggest originsindistantlocationswherethecollisionfre- collisions thanaretheirmain-beltasteroidalcounterparts. vian satellitesandcaptureofshort-periodcomets(Rabe Suggested sourcesincludeinsituaccretion,captureofJo- tures ataccretion.TheoriginoftheTrojansisnotknown. ganic compoundsmightalsoresultfromthelowtempera- protoplanetary cloudneartheorbitofJupiterarethoughtto tion spectraoftheTrojanasteroids?Itisknown(mostly High volatileabundancesincometarynucleiindependently tle ofnonvolatilematerial.Ahighsurfaceabundanceor- abundance, ifshieldedfromdirectsolarradiationbyaman- could bepresentintheinteriorsofTrojanasteroidslarge have beensufficientlylowthatwaterandpossiblyother creted there(PrinnandFegley1989).Evennow,waterice volatiles couldbedirectlyincorporatedwithinobjectsac- Trojans andwhatdoesthisrangeimplyforthecomposi- dish objects.Whatistherangeofcolorspresentamong from thelow-resolutionfilterdata)thatTrojansarered- mostly onfilterphotometryhavingpoorspectralresolution. rent knowledgeofthephysicalpropertiesTrojansisbased tional diversityofthesebodies? vide answerstothefollowingquestions: In thisstudyweseektouseauniformsetofspectrapro- and Jewitt1989a,hereafterreferredtoasPaperI).Ourcur- vational programofCCDspectroscopyasteroids(Luu Trojan asteroids?Discreteabsorptionsareknowninthe published? reported inthespectraofTrojans,butisthisduetopau- oids (VilasandMcFadden1987).Nofeatureshaveyetbeen spectra ofsomenear-Earth(PaperI)andmain-beltaster- 1972; c.f.,Yoder1979).Thenumberdensitiesoftheaster- expect thetwogroupstohavecommoncolordistributions. from oneanother.Iftheyshareacommonsourcewewould cloud? Asteroidsinthetwocloudsaredynamicallyisolated the precedingcloudconsistentwiththoseinfollowing city ofspectra(asopposedtofilterdata)thathavebeen compare withthereflectionspectraofcometarynuclei?To ferent colordistributionsforthevariousgroups. ent source.Chemicaldifferencesshouldbereflectedindif- and near-Earthpopulations,presumablyhaveadiffer- spectra ofthenear-Earthandmain-beltasteroids?TheTro- Is thisobserved? cussed above,thelowalbedosofbothTrojanasteroidsand what extentarethespectralpropertiescomparable?Asdis- bulk ofthispaper,wealsopresent thespectrumofasteroid as spectralanalogsofthecometarynuclei? compounds. TowhatextentcantheTrojanasteroidsbeused cometary nucleimaybeproducedbypolymerizedcarbon oids. asteroid inthe4:3resonancewith Jupiter.Wewillshowhbw 279 Thulecomparesspectroscopically withtheTrojanaster- 279 Thule.Thuleisunusual inthatitistheonlydistant These observationsarethenaturaloutgrowthofanobser- ( 1)Whatarethebasiccharacteristicsofvisualreflec- © American Astronomical Society • Provided by the NASA Astrophysics Data System In additiontotheTrojanobservations whichformthe (2) Doabsorptionfeaturesexistinthevisualspectraof ( 3)ArethestatisticalspectralpropertiesofTrojansin (4) Howdothespectralcharacteristicscomparewith (5) HowdothereflectionspectraofTrojanasteroids 3 3 The imagescalewas0.78"perpixel.atmosphericseeing and coversthespectralrangefrom3600to7400Â,although line/mm gratingblazedatwavelengthÀ=5000Âwasused was near1.5"FWHMoneachnightofobservation. prove chargetransferefficiencyatthelowestlightlevels. flashed witha50electronequivalentsignalinordertoim- width athalfmaximum(FWHM).TheCCDwaspre- entrance slit,withaneffectivespectralresolutionof20Afull ysis. Allspectraweretakenusinga2.8"widespectrograph for thesespectra.Thegratinggivesadispersion4.9Â/pixel, set ofinterchangeablegrismstoprovidedispersion.A150 houses aTexasInstruments800XpixelCCDanduses the guiderTVwasusedtopositionasteroidinspec- the asteroidviaitsmotionwithrespecttofixedstars,and only thewavelengthrange4000-7000Âisusedinthisanal- spectrograph wasusedforallobservations.Thisinstrument here wepresentonlyabriefaccount.The“GoldCam”CCD vational circumstancesaredescribedindetailPaperI— of theKittPeakNationalObservatoryinArizona,concur- light lossattheslitjaws.Theaccuracyofguidingwas the slitwidth).Errorsinguidingpotentiallycausevariable the pointingtobetterthan+0.5"(i.e.,asmallfractionof during theacquisitionofspectra,wewereabletocontrol an intensifiedTVsystem.Thefinderwasusedtoidentify from theasteroidspectrabyusingportionsof dent spectraweretakenforeachasteroid).Generally,the demonstrated byacomparisonofthefluxrecordedininde- Since thepointingoftelescopewasmonitoredatalltimes trograph slit.Thetelescopewastrackedatasteroidalrates. rently withtheobservationspresentedinPaperI.Theobser- stant atthe±20%level.Inafewcases,largerfluxvaria- measured absolutefluxdensityatanywavelengthwascon- pendent spectraofthesameasteroid(atleasttwoindepen- due to[Oi]at5577,6300,and6363Á. can bejudgedfromtheremovalofstrongnightskylines the spectrumofnightsky.Thesuccessthisoperation but thefaintestasteroidshaveslopeerrorsof±1%/10A. than byphotonnoiseorothersourcesofnoise.Therefore,all by usingawideslit.Theeffectiveabsenceofatmospheric asteroid betweenintegrationsand/orvariableatmospheric tions weredetected,probablyresultingfromrotationofthe the slitoneithersideofasteroidtodefineandsubtract slope uncertaintyisdeterminedbysystematiceffects,rather hour angle.Exceptforthefaintestasteroids,spectral erally consistentto±1%/10À,independentofairmassor and hourangles(seePaperI).Thespectralslopesweregen- paring spectraofagivenobjecttakenatrangeairmasses dispersion inourspectrawasempiricallyconfirmedbycom- our databyobservingatsmallairmassesandhourangles seeing. the sameZ.Forpracticalpurposes weusedthesolaranalog flux densityatwavelengthÀto thefluxdensityofSunat cellation errors<0.02mag(extinction isapproximately0.1 oids. Typicalairmassdifferencesbetweenasteroidsandstan- mag/airmass intheredand0.2 mag/airmassintheblue). dard starswereA^<0.1,leading topossibleextinctioncan- using standardstarsatairmassesclosetothoseoftheaster- The presentobservationsweretakenatthe2.1mtelescope Finding andguidingatthe2.1mwereaccomplishedusing Atmospheric emissionlinesandbandswereremoved The effectsofatmosphericdispersionwereminimizedin We definethereflectivityS(À) as theratioofasteroid Atmospheric extinctionwasremovedfromthespectraby II. OBSERVATIONS 934 935 D. C. JEWITT AND J. X. LUU: CCD SPECTRA OF ASTEROIDS 935 stars 16 Cyg B, Hyades 64, and Hyades 106 (Hardorp 1978, extinction errors in the multichannel data, or to some combi- 1980) to define the spectrum of the Sun and to compute nation of all three. The systematic discrepancies shown in SCA ). We also observed, but rejected, the Hardorp solar ana- Fig. 3 highlight the importance of uniform spectral data log Hyades 142. Figure 1 shows that Hyades 142 is redder when making comparisons among asteroids. Three Trojans than the other solar analogs by 1.5%/103 Á causing us to from our current sample [asteroids 1583 Antilochus, 1867 doubt its usefulness as a solar analog. Oscillations near Deiphobus and 2241 ( 1979WM) ] were also independently k ~ 6800 A in the ratios of standard stars plotted in Fig. 1 are observed by Vilas and Smith (1985). Unfortunately, the an artifact caused by imperfect removal of interference small scale of the figures in their paper prevents a quantita- fringes in the CCD data. tive comparison with the present data. Figure 2 shows that the reflectivities of a majority of the III. DISCUSSION observed Trojan asteroids are linearly proportional to wave- a) Spectral Properties of Trojan Asteroids length in the range 40001173 Anchises from Table II of Smith oid is a —12°, hence any phase-reddening effect is likely to be et al. renormalized to A = 6000 Â and plotted with spectra small (<1%/103 A). We therefore neglect the possible in- from this work. Comparison of the spectra in Fig. 3 shows fluence of phase reddening in our spectra (see Paper I for a approximate agreement, although systematic differences in fuller discussion of phase reddening in the near-Earth and reflectivity as large as AS—10% occur at wavelengths main-belt asteroids). k < 4500 A. These differences may be attributed to the use of A histogram of S' ' ( Fig. 4 ) emphasizes that the Trojans are a different solar analog (Smith et al. used HD28992, which spectrally diverse, with a range of slopes from nearly neutral is not one of the solar analogs championed by Hardorp 1978, (S" = 3 + 1%/103 A) to very red (S" = 25 + 1%/103 Â). 1980), to rotational spectral variations on the Trojans, to The mean value of the reflectivity gradient of the Trojans is S7 = 9.6 ± 4.7%/103 Â, where the quoted uncertainty is the standard deviation on the mean of 32 asteroids. The distribution is not Gaussian, however. Two thirds of the asteroids have S ' within ¡f 2%/ 103 A of the mean. There are no blue asteroids (S" <0) in our Trojan sample. Examination of the slope-ordered spectra in Fig. 2 sug- gests that the redder Trojans are also the fainter ones in the present sample. This suspicion is confirmed by Fig. 5(a), in which we plot S ' versus the apparent magnitude of each as- teroid at the time of observation (since the Trojans are all at essentially same geocentric distance, the apparent magni- tude distribution differs from the absolute magnitude distri- bution only by a constant). There is a clear trend towards increasing S ' with increasing magnitude. The linear correla- tion coefficient computed from the data in Fig. 5(a) is rcorr r.u = 0.51 (iV=32). The chance that this correlation or a larg- 4000 4500 5000 5500 6000 6500 7000 7500 er one could arise from random, uncorrelated data is Wavelength [Â] P(r^rcorr ) = 0.005 (Bevington 1969), showing that the col- Fig. 1. Normalized ratios of solar analog spectra; both spectra were or-magnitude trend is statistically significant at this level. The slope of a linear least-squares fit to the data in Fig. 5(a) taken on the same night. The top ratio is the spectrum of Hyades 142 3 divided by that of Hyades 64, the bottom ratio is the spectrum of Hyades is 2.7 + 0.8%/10 Á/mag.To test the possibility that the 106 divided by Hyades 64. Hyades 142 is reddened with respect to other color-magnitude trend might be an artifact of a few atypical- solar analogs by 1.5%/103 Â (compare the top ratio with the bottom ly red asteroids, we arbitrarily omitted the two reddest aster- ratio). oids in Fig. 5(a) (1870 Glaukos and 1872 Helenos) and

© American Astronomical Society • Provided by the NASA Astrophysics Data System 936 D. C. JEWITT AND J. X. LUU: CCD SPECTRA OF ASTEROIDS 936

fô) Wavelength [Á] (d) Wavelength [Â]

(b) Wavelength [Á] (e) Wavelength [Â]

(c) Wavelength [Â] (f) Wavelength [À]

Fig. 2. Reflectivity spectra of 32 Trojan asteroids [(a)-(k)] and of 279 Thule (1). The asteroids are plotted on a common scale to facilitate easy comparison. Spectra of Trojan asteroids within each panel are displaced in normalized reflectivity by AS ' = — 0.5,0.0, and 0.5, respectively, for clarity of presentation. The Trojan spectra are presented in order of increasing reflectivity gradient S'.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 937 ^ Wavelength[Â] © American Astronomical Society • Provided by the NASA Astrophysics Data System NonnaUzed Reflectivity 2 Normalized Reflectivity D. C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS 4000 4500500055006000650070007500 Wavelength [Â] Wavelength [À] Fig. 2.(continued) (I) Wavelength[Á] Wavelength [À] Wavelength [Â] 937 1990AJ 100. . 933J 938 D.C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS © American Astronomical Society • Provided by the NASA Astrophysics Data System a EstimatedfromV(1,1,0) 279 3793 659 624 N 1988BY1 1988BX1 1987WR3 3451 3391 2920 2797 2759 2674 2456 2363 2357 2260 2241 2207 884 (1) 3240 1871 1870 1867 1749 1647 1583 1437 1208 1173 1873 1872 1172 Thule unnamed unnamed Leonteus Glaukos Deiphobus Telamon Antilochus Troilus Hektor Automedon Phereclos Antenor Agenor Helenos Astyanax Menelaus Diomedes Anchises Aneas Priamus Nestor Name 1979HA2 Sinon Laocoon Teucer Idomeneus Pandarus Palamedes Neoptolemus Cebriones 1984HA1 1979WM (2) 1989/Feb/07 1989/Feb/07 1989/Feb/03 1989/Feb/04 1989/Feb/03 1989/Feb/04 1989/Feb/08 1988/Oct/07 1988/Oct/09 1988/Oct/10 1988/Oct/08 1988/Oct/09 1988/Sep/09 1989/Feb/03 1989/Feb/07 1989/Feb/04 1989/Feb/04 1988/Oct/10 1988/Oct/10 1988/Oct/09 1988/Oct/08 1989/Feb/07 1989/Feb/03 1988/Oct/07 1989/Feb/04 1988/Oct/10 1989/Feb/03 1989/Feb/03 1988/Oct/08 1989/Feb/04 1988/Oct/09 1988/Oct/08 1988/Oct/07 UTDate (3) Table I.Observedasteroids. a Airmass IntegrationVS'Fig. 1.0400 1.1200 1.0700 1.1600 1.3100 1.1800 1.0900 1.4800 1.1100 1.5400 1.0500 1.2500 1.1800 1.0100 1.0600 1.1300 1.1800 1.4000 1.3700 1.0800 1.2200 1.0600 1.4400 1.0000 1.0900 1.3500 1.3500 1.1400 1.0600 1.2200 1.3100 1.0000 1.0700 (4) (5)(6)(7)(8) 2000 2400 2000 2300 2100 3000 1400 1900 1800 1800 2000 2000 1400 1200 1600 1800 1600 1300 1000 1200 1400 1800 1200 1800 1800 1800 1350 1800 1800 1800 1900 1800 1400 [s] [mag.][%/1000Â] 14.5 7.1+121 17.1 16.7 16.4 15.5 17.2 18.1 16.9 18.4 17.8 15.6 16.8 15.8 15.1 15.7 15.5 15.1 15.8 16.3 14.3 16.2 14.9 16.1 16.0 16.3 17.2 16.5 16.1 15.4 16.8 16.4 16.5 15.8 24.7 ±1 23.1 ±2 16.2 ±2 10.2 ±1 11.0+1 10.0 ±1 13.6 ±2 11.2 ±1 11.5 ±1 10.2 ±1 11.7 ±1 11.0± 1 4.4 ±1 9.0 ±1 4.8 ±1 9.4 ±1 8.6 ±1 3.1 ±1 3.8 ±1 3.4 ±1 8.8 ±1 8.6 ±1 8.2 ±1 3.5 ±1 5.8 ±2 7.1 ±1 9.8 ±1 9.4 ±1 9.0 ±1 8.9 ±1 8.2 ±1 8.1 ±1 2j 2h 2b 2i 2f 2f 2e 2i 2e 2d 2a 2b 2b 2d 2a 2g 2k 2j 2k 2h 2j 21 2a 2c 2c 2g 2e 2g 2c 2f 2d 2h 938 00 O'!00 939 D. C. JEWITT AND J. X. LUU: CCD SPECTRA OF ASTEROIDS 939 o tribution of the Trojan asteroids. An overabundance of ^0 Trojan red D fragments at small sizes might be expected if oc the dark, red material is more prone to collisional fragmen- O'! tation than its more neutral counterpart. This, in turn, would be qualitatively compatible with the expected low de- gree of metamorphism experienced by the Trojans (Bell et al. 1989). Some evidence exists for differences in the size distributions of main-belt asteroids belonging to different spectral classes (Chapman 1989).

b) Preceding Cloud Versus Following Cloud In Fig. 6 we distinguish the Trojans of the preceding cloud from those of the following cloud ( see column 4 of Table II ). Our sample includes 16 asteroids from each cloud. The fig- Wavelength [À] ure shows that the reflectivity gradients of the asteroids within the two clouds are similar, so that no evidence exists Fig. 3. Reflectivity spectra of Trojan asteroids 884 Priamus, 1172 An- for a compositional difference between the two clouds. The eas, and 1173 Anchises compared with multichannel data from Smith et mean gradient in the preceding cloud is_5" = 8.6 + 3.2%/ al. (1981). The multichannel data have been renormalized at 103 A while that in the following cloud is A" = 10.6 + 5.7%/ À = 6000 A. Spectra of 884 Priamus and 1173 Anchises are displaced in 103 A. Within the uncertainties, the difference between the normalized reflectivity by AS" = — 0.5 and 0.5, respectively, for clarity of presentation. Formal error bars in the multichannel data approach two values is zero. Therefore, we conclude that the two Tro- + 0.03-0.05 in the blue. jan clouds are spectrally indistinguishable. c) Comparison with Other Asteroids In Fig. 7 we plot the reflectivity gradient S' versus the recomputed rcorr. The correlation coefficient decreased to r semimajor axis a (AU) for near-Earth and main-belt aster- corr =0.46, which is significant at the P(r^rcorT) =0.01 level (i.e., the correlation retains 1% significance). oids (from Paper I) and Trojan asteroids (this work). Also What might be the cause of this unexpected color-magni- plotted is S' for the asteroid 279 Thule (a = 4.27 AU). As tude trend? The obvious possibility that the trend is an arti- noted earlier, the comparison of data from this paper with fact of an unknown but magnitude-dependent error in the data from Paper I is completely legitimate, since all objects data reduction is effectively eliminated by Fig. 5(b). The were observed and reduced under standardized conditions, figure shows S' versus magnitude for main-belt and near- and systematic errors are likely to be small. The figure shows Earth asteroids (data taken from Paper I). The main-belt that, from the main belt outward to the Trojans, the values of and near-Earth asteroid spectra were taken on the same S ' exhibit a clear trend towards increasing redness with in- dates as the Trojan spectra, using the same telescope and creasing heliocentric distance. The significance of the linear detector, and reduced and measured following the same pro- correlation between S' and a is better than 0.001 (i.e., the cedures. These three datasets should thus suffer identically correlation has high statistical significance). The figure also from any magnitude-dependent error which might be pres- emphasizes that the Trojan population is distinguished from ent. However, Fig. 5(b) gives no evidence for a color-mag- asteroids at smaller distances by the complete absence of nitude trend in either the main-belt or the near-Earth aster- blue asteroids. Specifically, 0 out of 32 Trojans have S'<0 oid data. We conclude that the color-magnitude trend is while 3 out of 19 NEAs and 5 out of 23 main-belt asteroids specific to the Trojans, and is not of instrumental or spectral- fall into this category. What is not clear from Fig. 7 is reduction origin. whether 5" is a smooth function of semimajor axis, or The color-magnitude trend [Fig. 5(a) ] may be plausibly whether the Trojans are simply redder as a group than all interpreted as a color-diameter trend among the Trojans. other asteroids. The former case has been advocated by Vilas The fainter Trojans in Table I have generally smaller diame- and Smith ( 1985). ters than the brighter Trojans, since the geometric albedos vary by only a factor of 2-3 while the apparent magnitudes IV. SPECTRAL FEATURES differ by a factor of 40 (Am—4). This diameter-magnitude Although the reflectivity spectra of most Trojan asteroids correlation may be seen directly from a comparison of the are well fitted by a straight line, deviations from linearity magnitudes (column 6 of Table I) and IRAS diameters exist and fall into two broad categories. (column 6 of Table II) of the Trojans. Therefore, the color- ( 1 ) Several spectra show a downturn in A" at short wave- magnitude trend in Fig. 5(a) may be interpreted as a color- lengths Z, <5000 A relative to a linear extrapolation from diameter trend, with the smaller Trojans being optically red- longer wavelengths. Good (but subtle) examples may be der than their larger counterparts. seen in asteroids 3451 ( 1984 HA1 ), 659 Nestor, 1437 Dio- We postulate that, among the Trojans, the size distribu- medes [all in Fig. 2(a) ], 1208Troilus [Fig. 2(b)] and 2207 tion of the very red D asteroids is peaked towards smaller Antenor [Fig. 2(i) ]. This downturn is seen in the spectra of diameters than are the size distributions of the (less red) C many main-belt asteroids (e.g., Zellner, Tholen, and Tesesco and P asteroids. The size distributions of the main-belt aster- 1985; Paper I) where it is attributed to the presence of an oids are thought to be controlled (at least at diameters ultraviolet charge transfer absorption in a transition metal < 100-200 km) by mutual collisions between asteroids (Da- silicate (Gaffey and McCord 1977). The downturn is stron- vis ei a/. 1979; Dermott, Harris, and Murray 1984). Presum- gest in those Trojan asteroids having relatively neutral re- ably, collisions are also important in controlling the size dis- flectivities, suggesting that the surface abundance of the UV

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1990AJ 100. . 933J 940 © American Astronomical Society • Provided by the NASA Astrophysics Data System D. C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS a b ^ IRASAlbedo,fromAsteroidsIIdatabase. * IRASdiameter,fromAsteroidsIIdatabase. p =preceedingTrojancloud,ffollowing Trojancloud. Spectralclass,fromAsteroidsIIdatabase. 279 1988BY1 1988BX1 1987WR3 3793 3451 3391 2920 2357 2241 3240 2797 2759 2674 2363 2456 2260 2207 659 624 884 o) 1873 1872 1871 1870 1867 1749 1647 1583 1437 1208 1173 1172 N Thule unnamed unnamed Leonteus 1979HA2 Sinon Laocoon Automedon Teucer Idomeneus Pandarus Palamedes Phereclos Neoptolemus Antenor Agenor Helenos Astyanax Glaukos Deiphobus Telamon Troilus Aneas Cebriones Menelaus Antilochus Diomedes Anchises Priamus Nestor Hektor 1984HA1 Name 1979WM (2) 5 4.2694 5.2244 5.2637 5.2634 5.2567 5.1667 5.0936 5.2622 5.1792 5.1665 5.1618 5.1786 5.1871 5.1323 5.1781 5.1905 5.2432 5.1383 5.2671 5.1937 5.3346 5.2459 5.1630 5.2230 5.2493 5.1066 5.2009 5.1036 5.3128 5.1646 5.1576 5.2456 5.1788 a [AU]p/f*Class'DiameterGeometric Table II.Asteroidphysicaldata. (3) (4)(5)(6)(7) Notes toTableII P n/a P P P f P f P P P P P P P P P f f P f f f f f f f f f f P f f D D D D D D D C P D U C 135 73 d 72 92 93 65 85 123 123 115 102 103 103 123 131 109 171 111 135 151 115 [km] Albedo 0.03 0.034 0.024 0.037 0.012 0.028 0.051 0.029 0.036 0.046 0.041 0.041 0.035 0.066 0.042 0.064 0.040 0.058 0.026 0.038 0.040 940 1990AJ 100. . 933J 941 Trojan asteroidsarediverse. against thevalueof5'.Thehistogramemphasizesthatspectra Fig. 4.ThenumberofTrojanshaving5'inaspecifiedrangeisplotted oids (dataarefrom PaperI).Nocolor-magnitudetrendis apparent. line isaleast-squaresfittothedata.The dataaretakenfromTableI. trend towardsreddercolorsissignificant atthe0.005level.Thesolid Fig. 5.(a)Colorvsapparentmagnitude fortheTrojanasteroids.The (b) Colorvsapparentmagnitudeformain-belt andnear-Earthaster- © American Astronomical Society • Provided by the NASA Astrophysics Data System D. C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS 0 24681012141618202224262830 S' [%/1000Â] A ~5100A,whileaweakerfeatureat5900ispartially more prominentamongthelargerTrojans,providinganin- to bethelargestinoursample.Thus,UVabsorberis material. Asnotedabove,themoreneutralTrojansalsotend absorber isanticorrelatedwiththeabundanceofdarkred tral classesmaydifferfromoneanother(seeSec.Ilia). dependent hintthatthesizedistributionsofvariousspec- feature near5900Acanbediscerned.Theestimatedabsorp- obscured byanoisespikeinthedata.In1870Glaukosonly 2 (k)].In1988BY1themostprominentabsorptionoccursat tra ofTrojans1988BY1[Fig.2(j)]and1870Glaukos in bothcasescomparabletothecontinuumpeak-to-peak tainty duetothecontinuumnoise.Theabsorptiondepthsare tion bandwidthsare300-500A,withconsiderableuncer- ^ 10 following clouds.ThefigureshowsthatthedistributionofS''issimilarin the twoclouds. Fig. 6.ColorvssemimajoraxisforTrojanasteroidsintheprecedingand Thule isalsoplotted(denotedbytheX symbol).Thefigureempha- sizes thenetcolordifference betweentheTrojansandother asteroids. Fig. 7.Colorvssemimajoraxisfornear-Earth andmain-beltasteroids (from PaperI)andTrojanasteroids from thiswork.Asteroid279 (2) Broad,shallowabsorptionsareapparentinthespec- -10 20 - 30 o # Semi-major Axisa[AU] a [AU] 3 O NEA(Paper1) * Main-Bell(PaperI) X Thule • Trojan(ThisWork) _L Í 941 1990AJ 100. . 933J 3 3 942 D.CJEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS features isobtained.Significantly,porphyrinsarecarbon- lengths oftheobservedbands. features, eitherinemissionorabsorptionnearthewave- the terrestrialatmospherepossessesnoremarkablespectral star usedtocomputethespectralflux,anddonotdependon and 1870Glaukos. asteroids observedimmediatelybeforeandafter1988BY1 noise, sothatweregardtherealityoffeaturesastenta- oids forcomparisonwithexistingand,especially,future work wasthedesiretoobtainasetofspectradistantaster- asteroids. suspicions regardingthecarbonabundanceofTrojan rich compoundsand,ifpresent,wouldsatisfylong-standing tailed comparisonuntilobservationalconfirmationofthe noted (HoldenandGafFey1987),butwewilldelayade- ing. Asimilaritytothereflectionspectraofporphyrinsis that thefailuretofindotherexamplesisperhapsnotsurpris- to-noise comparabletothatofthepresentdataissmall,so published reflectancespectrahavingresolutionandsignal- tures inthespectraofmain-beltandnear-Earthasteroids the methodofextinctioncorrectionemployed.Inanyevent, tra, andtheyarespecificallyabsentfromthespectraofthose Glaukos spectrainFig.2. grations whichwereaddedtoformthe1988BY1and1870 porting theirrealityincludes: tive, andinneedofindependentconfirmation.Evidencesup- clei havebeenidentifiedtodate,sothatanycomparison tra reportedandinPaperIareavailableonlyforP/Encke with thenewasteroidspectrareportedinthispaper. must beregardedaspreliminaryinnature.Inaddition,the spectra ofcometarynuclei.Onlyahandfulnu- nuclei comparableinspectralresolutiontotheasteroidspec- Encke, P/HalleyandP/Tempel2;seeTableIinJewitt nuclei offivecomets[P/Arend-Rigaux,P/Neujmin1,P/ compare theavailablereflectancedataoncometarynuclei ship amongtheTrojanshasalreadybeendescribedinSec. ered. Infact,empiricalevidenceforasizeversusS'relation- possibility ofsizedependentselectioneffectsmustbeconsid- smaller thanthetypicalTrojanasteroidsstudied,sothat typical cometarynucleiareaboutoneorderofmagnitude ple. and oftheTrojansaresimilar, withintheverysubstantial the twotypesofobject.Evidently, thecolorsofnuclei than anyofthenear-Earthormain-beltasteroidssampledin and Keller1989),toP/Tempel2beingsubstantiallyredder with respecttotheSun(-S^H^y=6+3%/10Â;Thomas varied, rangingfromP/Halleybeingonlyslightlyreddened III. Despitethispotentialcomplication,itisinterestingto uncertainties imposedbythesmall sizeofthenucleussam- of theTrojans,wepresentinFig. 8thecolorhistogramsof Paper I(S^empeiz=20+3%/10 Â;JewittandLuu (Paper I)provedunsuccessful.However,thenumberof 1989). Thevisualcolorsofthenucleiasagrouparequite (Luu andJewitt1990b)P/Tempel2(JewittLuu ( 1990a)forthecolorsofindividualnuclei].CCDspectra 1989). Tocomparethenucleus colordistributionwiththat An attempttoidentifytheasteroidalabsorptionswithfea- As notedintheIntroduction,amajormotivationforthis Broadband visualcolorsarecurrentlyavailableforthe (c) Theabsorptionsarenotdependentonthestandard (b) Comparablefeaturesareabsentfromallotherspec- (a) TheyexistineachofthetwoindependentCCDinte- © American Astronomical Society • Provided by the NASA Astrophysics Data System V. COMPARISONWITHCOMETARYNUCLEI ted intheP/Tempel2spectrum.Figure9showsthat that oftheTrojanasteroid1988BY1isgiveninFig.9.Only cleus ofcometP/Tempel2(fromJewittandLuu1989)with wavelengths believedtobefreeofgaseousemissionareplot- spectra. Jewitt andLuu(1989).Aclosecorrespondence existsbetweenthetwo Tempel 2[blackdotsinFig.9(b)].The P/Tempel2spectrumisfrom Fig. 9.Comparisonofthereflection spectrumofTrojanasteroid 1988BY1 [Fig.9(a)]withthespectrum ofthenucleuscometP/ (b) A moredetailedcomparisonofthespectrumnu- value of5". nuclei (white)having5'inaspecifiedrangeareplottedagainstthe Fig. 8.ThenumbersofTrojans(shadedhistogram)andcometary 0 24681012141618202224262830 S' [%/1000Â] Wavelength [A] Wavelength [Â] 942 1990AJ 100. . 933J of thenucleiandthoseTrojan asteroidscouldpoten- cometary nucleiandtheTrojan asteroids. Table IIIsummarizestheaverage physicalpropertiesofthe cient evidencetodistinguish among theseexplanations. ing suchalignment.Unfortunately, wedonotpossesssuffi- 943 D.C.JEWITTANDJ.X.LUU:CCDSPECTRAOFASTEROIDS plitudes, althoughnoobviousmechanismexistsforproduc- by sublimationtorques(seeJewittandMeech1988).Ifthe the Trojanscouldberesponsibleforlargerotationalam- etal. (1988)haveremarkedthatspinaxisalignmentamong same wayasforthecometarynuclei.Inaddition,Hartmann previous outgassinghasmodifiedthebodyshapesinjust Trojans possesshighwaterabundances,itispossiblethat explanations forthecometsincluderetentionofprimor- bofsky etal.1988).Thephysicalsignificanceoftheaspheri- projection effectduetononprincipalaxisrotationinduced dial bodyshapes,theeffectofanisotropicmasslossanda cal shapesoftheTrojansandnucleiisobscure.Proffered The mantlesmayconsistofcomplexorganicmolecules albedo variations(HartmannandCruikshank1980;Le- curve amplitudeisindicativeofshaperatherthanazimuthal postulated forthemantlesofcomets(Johnsonetal.1987). the Trojans,butsimilarlyhighcarbonabundanceshavebeen scenarios forthecometsareevenlesscertainthanthose thermal-visual photometryprovidesevidencethatthelight- light curvesindicativeofasphericity.Again,simultaneous of theabove-mentionedcometarynucleiareinferredtobe ets andtheTrojansisextendedtophysicalpropertiesby erally reddishreflectionspectraofthesebodies.Formation in carbonabundance,givingrisetothelowalbedosandgen- curves (JewittandMeech1988;JewittLuu1989; highly elongatedfromobservationsoftherotationallight- a comparisonofthegrossshapesthesebodies.The Cruikshank 1987;Johnson^a/.1987). modified byexposuretocosmicrays(Andronicoetal.1987; perienced littlemetamorphisminthenearsurfaceregions well-studied Trojansarecomparable,/>—0.02-0.05(c.f., et al.(1988)foundthattheTrojansdisplaylargerotational pel 2(cf:referencescitedabove).Independently,Hartmann and Jewitt1990b).Evidencethatthelightcurvesarecon- average geometricalbedodeterminedforthenucleiofcom- nuclei shareverylowvaluesofthegeometricalbedo.The hypothesis isclearlynon-unique. are derivedfromasingle-sourcepopulation,althoughthis patible withthehypothesisthatcometsandTrojans spectra ofthenucleusP/Tempel2and1988BY1iscom- material onTrojan1988BY1.Thesimilaritybetweenthe photometry ofP/Arend-Rigaux,P/Neujmin1,andP/Tem- tions) isprovidedbysimultaneousthermalinfrared-visual trolled byshape(ratherthanazimuthalalbedovaria- are primitivebodies,formednearR—5AU,andhavingex- column 7ofTableII).OnepopularbeliefisthattheTrojans Campins 1988;A’Hearnetal.1989).Geometricalbedosof McFadden 1987;KelleretalMillis,A’Hearn,and pel 2is-0.03±0.01(cf.Campins,A’Hearn,and ets P/Arend-Rigaux,P/Neujmin1,P/Halley,andP/Tem- mantle coveringthesurfaceofP/Tempel2and prove, acompositionalsimilaritybetweentherefractory surements. Thisobservationisconsistentwith,butdoesnot with thatof1988BY1,withintheuncertaintiesmea- reflection spectrumofthenucleusP/Tempel2isidentical (Bell etal.1989).Thesurfaceregionsarethoughttobehigh v The similaritiesobservedbetween thephysicalproperties The similaritybetweentheopticalpropertiesofcom- It isalsonoteworthythattheTrojansandcometary © American Astronomical Society • Provided by the NASA Astrophysics Data System 3 3 d 0 a 3 b b0 d cant atthe0.005level,andmay reflectasizedependenceof tude mintherange14Comets,MeteorsHI,UppsalaAstro- Jewitt, D.C.(1990a).InCometsinthe Post-HalleyEra,editedbyR.New- Hartmann, W.K.,Tholen,D.J.,Goguen,Binzel,R.P.,andCruikshank, Hartmann, W.K.,andCruikshank,D.P.(1980).Science207,976. Hartmann, W.K.(1986).InAsteroids,Comets,MeteorsII,editedbyC.-I. Hardorp, J.(1980).Astron.Astrophys.91,221. Holden, P.N.,andGaffey,M.J.(1987). Meteoritics22,412. (i.e., thereisnoevidenceforacolorimetricorcompositional Asteroids II,editedbyR.P.Binzel,T.Gehrels,andM.Matthews(Uni- Astrophys. J.347,1155. Astron. Astrophys.184,333. Arizona, Tucson),pp.528-557. 316, 847. versity ofArizona,Tucson). edited byA.H.Delsemme(UniversityofToledo,Toledo),pp.199-218. University Press,Uppsala),pp.191-193. Harris, A.W.(1979).InAsteroids,editedbyT.Gehrels(Universityof nomical ObservatoryReportNo.48,(UppsalaUniversity,Uppsala). ical Sciences(McGraw-Hill,NewYork). burn (Kluwer,Dordrecht). Lagerkvist, B.A.Lindblad,H.Lundstedt,andRickman(Uppsala D. P.(1988).Icarus73,487. © American Astronomical Society • Provided by the NASA Astrophysics Data System (5) AnumberofTrojanreflectivityspectraexhibita (6) TherangeofS'exhibitedbytheTrojanasteroidsen- (4) TheTrojanasteroidsare,ontheaverage,redderthan ( 3)Thereisnosignificantdifferencebetweenthedistri- REFERENCES jan 1988BY1.Thesimilarityisfurtherenhancedbythelarge grateful forpartialfinancialsupportfromtheNational and DaveChamberlainfortheireffortsatKittPeak.Weare uate StudentResearcher’sAwardProgramtoJ.Luu. Science FoundationtoD.JewittandfromtheNASAGrad- date. Inthissense,theTrojanasteroidsandcometary servational challenge. cant differencesbetweentheseobjectsasthenextmajorob- nuclei aresogreatthatweregardtheidentificationofsignifi- vational similaritiesbetweentheTrojansandcometary very lowgeometricalbedoswhichtheybothpossess.Obser- light-curve amplitudesseeninbothsetsofbodies,andbythe the spectrumofnucleusP/Tempel2existsinTro- nuclei arespectrallysimilar.Anessentiallyexactmatchof Thule mightbeofvalue. Johnson, R.E.,Cooper,J.F.,Lanzerotti,L.J.,andStrazzulla,G.(1987). Jewitt, D.C.(1990b).Astrophys.J.351,277. either time-variable,orassociatedwiththesideofThulenot trum ofthisobject(seeFig.21).Ifreal,thefeaturemustbe vist, Williams,Fitzsimmons,andDahlgren(IAUCircular has beenreportedinthespectrumof279ThulebyLagerk- Jewitt, D.C,andMeech,K.J.(1988).Astrophys.328,974. Jewitt, D.C,andLuu,J.X.(1989).Astron.97,1766. sampled inourspectrum,orboth.Additionalspectraof No. 4992).Nocomparablefeatureispresentinourspec- Keller, H.U.,Delamere,W.A.,Huebner,F.,Reitsema,J.,Schmidt, Millis, R.L.,A’Hearn,M.F.,andCampins,H.(1988).Astrophy.J.324, Luu, J.X.,andJewitt,D.C.(1990b).Icarus(inpress). Kissel, J.,Sagdeev,R.Z.,Bertaux,J.L.,Angarov,V.N.,Audouze, Rabe, E.(1972).InTheMotion,EvolutionofOrbits,andOriginComets, Prinn, R.G.,andFegley,B.,Jr.(1989).InOriginEvolutionofPlan- Luu, J.X.,andJewitt,D.C.(1990a).Astron.(inpress)(PaperI). Lebofsky, L.A.,Greenberg,R.,Tedesco,E.F.,andVeeder,G.J.(1988). Shoemaker, E.M.,C.S.,andWolfe,R.F.(1989).InAsteroids Zellner, B.,Tholen,D.J.,andTedesco, E.F.(1985).Icarus61,355. Yoder, C.F.(1979).Icarus40,341. Vilas, F.,andMcFadden,L.A.(1987). Bull.Am.Astron.Soc.19,825. Vilas, F.,andSmith,B.A.(1985).Icarus 64,503. Smith, D.W.,Johnson,P.E.,andShorthill,R.W.(1981).Icarus46,108. Arpigny, C,Barbiéri,Bonnet,R.M.,Cazes,S.,Coradini,Cos- Astron. Astrophys.187,889. and Zubkov,B.V.(1986).Nature321,280. Schmidt, W.K.H.,andSeige,P.(1987).Astron.Astrophys.187,807. movici, C.B.,Hughes,D.W.,Jamar,C,Malaise,D.,Schmidt,K., Blamont, J.E.,Büchler,K.,Evlanov,E.N.,Fechtig,H.,Fomenkova,M. H. U.,Whipple,F.L.,Wilhelm,K.,Curdt,W.,Kramm,R.,Thomas,N., II, editedbyR.P.Binzel,T.Gehrels,andM.S.Matthews(Univeristyof M. S.Matthews(UniversityofArizona,Tucson),pp.78-136. etary andSatelliteAtmospheres,editedbyS.K.Atreya,J.B.Pollack, Icarus 75,518. Managadze, G.G.,Podkolzin,S.N.,Shapiro,V.D.,Tabaldyev,R., Krueger, F.R.,Langevin,Y.,Leonas,V.B.,Levasseur-Regourd,A.C, N., vonHoerner,H.,Inogamov,N.A.,Khromov,V.Knabe,W., Arizona, Tucson),pp.487-523. Polonskaya, andB.G.Marsden(Springer,NewYork),pp.55-60. IAU SymposiumNo.45,editedbyG.A.Chebotarev,E.I.Kazimirchak- 1194. We thanktelescopeoperatorsGeorgeWill,DeanHudek, Note addedinproof:Anabsorptionfeaturenear5170Â 944