Optical and Dynamical Characterization of Comet-Like Main-Belt Asteroid (596) Scheila
SSubmitted, 2011-05-09; Accepted, 2011-09-14
Preprint typeset using L A TEX style emulateapj v. 11/10/09
OPTICAL AND DYNAMICAL CHARACTERIZATION OF COMET-LIKE MAIN-BELT ASTEROID (596)SCHEILA
Henry H. Hsieh a,b , Bin Yang a , Nader Haghighipour aa Institute for Astronomy, Univ. of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA b Hubble Fellow
Submitted, 2011-05-09; Accepted, 2011-09-14
ABSTRACTWe present observations and a dynamical analysis of the comet-like main-belt object, (596) Scheila. V -band photometry obtained on UT 2010 December 12 indicates that Scheila’s dust cloud has ascattering cross-section ∼ . M d ∼ × kg. V − R color measurements indicate that both the nucleus and dust are redder thanthe Sun, with no significant color differences between the dust cloud’s northern and southern plumes.We also undertake an ultimately unsuccessful search for CN emission, where we find CN and H Oproduction rates of Q CN < × s − and Q H O < s − . Numerical simulations indicate thatScheila is dynamically stable for >
100 Myr, suggesting that it is likely native to its current location.We also find that it does not belong to a dynamical asteroid family of any significance. We considersublimation-driven scenarios that could produce the appearance of multiple plumes of dust emission,but reject them as being physically implausible. Instead, we concur with previous studies that theunusual morphology of Scheila’s dust cloud is most simply explained by a single oblique impact,meaning this object is likely not a main-belt comet, but is instead the second disrupted asteroid afterP/2010 A2 (LINEAR) to be discovered.
Subject headings: comets: general — minor planets, asteroids INTRODUCTION
Discovered on 1906 February 21, main-belt asteroid(596) Scheila has a diameter of d = 113 .
34 km (Tedesco etal. P orb = 5 .
01 years, a semi-major axis of a = 2 .
927 AU, an eccentricity of e = 0 . i = 14 . ◦ , and a Tisserand parameter(with respect to Jupiter) of T J = 3 . e.g. , dust emission) due to thesublimation of volatile ices, but occupy stable orbits en-tirely confined to the main asteroid belt (Fig. 1). MBCsare unlikely to originate in the outer solar system likeother comets, and are probably native to the main belt(Fern´andez et al. etal. (2011b) proposed the alternate possibility that activ-ity is in fact primarily dependent on heliocentric distance,and only peaks during the post-perihelion portion of each [email protected], [email protected],[email protected] MBC’s orbit due to the finite time required for solar ther-mal waves to propagate through insulating surface ma-terial before reach the subsurface reservoirs of volatilematerial below. Currently, both hypotheses are consis-tent with the available evidence, though we note the firsthypothesis could be ruled out (at least as a universallyapplicable explanation for MBC activity modulation) bythe determination of a pole orientation for an MBC thatis inconsistent with seasonal modulation of activity ( i.e. ,where peak activity does not occur near a solstice posi-tion, as required by the seasonal modulation hypothesis,but near an equinox position). Likewise, the second hy-pothesis could be ruled out as a universal explanation forMBC activity modulation by the discovery of an MBCwhich exhibits peak activity prior to reaching perihelion.To date, gas emission has never been directly detectedfor an MBC ( e.g. , Jewitt et al. et al. a r X i v : . [ a s t r o - ph . E P ] S e p sistent only with sublimation-driven dust ejection (Hsieh et al. et al. (2010) initially re-ported that numerical dust modeling indicated that theobject’s apparent activity was due to emission that per-sisted over several months, implying that it was likely dueto ice sublimation. However, follow-up modeling basedon high-resolution Wide Field Camera 3 images from theHubble Space Telescope (Jewitt et al. et al. (2010),who employed both ground-based data and data fromthe OSIRIS Narrow Angle Camera aboard the EuropeanSpace Agency’s Rosetta spacecraft, and thus were able tostudy the dust tail from two different viewing perspec-tives, providing additional constraints on their model.Thus, while P/2010 A2 may have appeared comet-like,it is likely not a true comet ( i.e. , an object that exhibitssublimation-driven activity) and is better characterizedas a disrupted asteroid, that is, an object that exhibitsdust emission as a consequence of an impact (or impacts),and not through the action of sublimating ice.All studies of Scheila’s 2010 outburst published to date(Jewitt et al. et al. et al. et al. OBSERVATIONS
We observed Scheila on multiple occasions shortly afterthe discovery of its comet-like activity. Optical imagingwas performed in photometric conditions on UT 2010December 12 using the University of Hawaii (UH) 2.2 mtelescope, while optical spectroscopy was obtained usingthe 10 m Keck I telescope on UT 2010 December 17. De-tails of these observations are listed in Table 1. Results ofadditional near-infrared observations conducted as partof this observational campaign are reported in Yang &Hsieh (2011).Imaging observations employed a Tektronix2048 × . (cid:48)(cid:48)
219 pixel − ) behind Kron-Cousins filters. Spectroscopic observations were madeusing Keck’s Low Resolution Imaging Spectrometer(LRIS; Oke et al. × . (cid:48)(cid:48)
135 pixel − ). We used a 1 . (cid:48)(cid:48) − and resolution of ∼ . (cid:48)(cid:48) (cid:48)(cid:48) ) wereemployed to sample the sky background. We obtainedtwo 900s exposures for a total integration time of 1800s.Bias subtraction, flat-fielding, and wavelength calibra-tion were performed using Low-Redux software. Objectidentification, extraction, and flux calibration were per-formed using the Image Reduction and Analysis Facility(IRAF) software package. RESULTS & ANALYSIS
Photometric Analysis
We find a mean V -band magnitude for Scheila’s nu-cleus of m V = 14 . ± .
01 mag inside a photometryaperture 3 . (cid:48)(cid:48) m V = 0 .
37 magfrom the quiescent magnitude ( m V = 14 .
59 mag) pre-dicted by Scheila’s IAU phase law ( H V = 8 . ± . G = 0 . ± .
06; Warner et al. ∼
40% of the nucleus scattering cross-section inside thephotometry aperture.In a rectangular aperture enclosing the entire dustcloud (“A” in Figure 2), we find a total magnitude of m V = 13 . ± .
02 mag, corresponding to a photometricexcess of ∆ m V = 0 .
86 mag. The size of this rectangularaperture is chosen to encompass the entire dust cloud asobserved in V -band before it becomes visibly indistin-guishable from the background sky. However, we see inFigure 2 that the dust cloud extends somewhat fartherwhen observed in R -band. Measuring the dust cloud’s R -band flux using a larger aperture and comparing tothe flux measured using the same aperture used for our V -band data, we determine that the V -band flux of thedust cloud may be underestimated by ∼ m V = 13 . ± .
02 mag and ∆ m V = 0 .
96 mag.Using the nucleus for calibration, we find a total dustscattering cross section of A d = (1 . ± . × km ( ∼ ρ = 1640 kg m − (cf. the Tagish Lakemeteorite; Hildebrand et al. a d = 1 × − m, we can estimate the totaldust mass, M d , of the cloud using M d = 43 A d ρa d (1)Inserting the scattering cross-section we measure forScheila’s dust cloud, we thus obtain an approximate dustmass of M d ∼ × kg. This estimate is of course ∼ xavier/LowRedux/ based on numerous assumptions, particularly averagedust grain size and bulk density, as specified above, andis only computed to provide approximate physical con-text to the photometric excess that was measured. Forreference, employing the different grain size distributionand density assumptions made by Jewitt et al. (2011)( ρ = 2000 kg m − ; a d = 1 × − m) and Bodewits et al. (2011) ( ρ = 2500 kg m − ; a d = 1 × − m), we wouldinstead obtain M d ∼ × kg and M d ∼ × kg,respectively.For comparison, Larson (2010) measured ∆ m V =1 .
24 mag on 2010 December 3, while Bodewits et al. (2011) measured ∆ m V = 0 .
66 mag on 2010 December 14-15 and Jewitt et al. (2011) measured ∆ m V = 1 .
26 magon 2010 December 28 and ∆ m V = 1 .
00 mag on 2011 Jan-uary 5. However, given the different observational andinstrumental circumstances involved (especially giventhe technical difficulty of observing the extremely brightnucleus and comparatively extremely faint dust cloud si-multaneously), with the exception of the decline betweenthe two measurements by Jewitt et al. (2011), we do notconsider any dust mass fluctuations implied by compar-ing these disparate data sets to be reliable. V − R color measurements (Table 2) of the dust-contaminated nucleus indicate that it is redder than theSun, with the entire dust cloud with nucleus flux sub-tracted (“Dust A ” in Table 2) having an effectively iden-tical red color, similar to colors measured for other activecomets and active Centaurs (Bodewits et al. V − R ≈ .
44 mag,consistent with a D-type spectral classification (Fornasier et al. B ” and “Dust C ”), but within estimated uncertain-ties, find no significant color differences. Spectroscopic Analysis
Gas Emission Search
The most sensitive probe of sublimating gas in a cometis CN emission at 3880˚A. We show the spectral image ofScheila from 3700˚A to 4100˚A in Figure 3a. In this im-age, the horizontal continuum corresponds to reflectedlight from the nucleus. OH sky emission lines are visibleas vertical bands, and dark Fraunhofer lines in the Solarspectrum and prominent Ca H (3933˚A) and K (3966˚A)absorption lines are also visible. In a two-dimensionalspectral image, the intensity, I e , of a cometary emissionlines should be highest near the center of the continuumand gradually decrease with increasing distance from thecontinuum, moving in the spatial direction. No spectralfeatures near 3880˚A exhibit such behavior, and we there-fore conclude that no gas is detected.We also search for CN emission in a one-dimensionalspectrum extracted from the spectral image using a 1 . (cid:48)(cid:48) × . (cid:48)(cid:48) (cid:48)(cid:48) to 16 (cid:48)(cid:48) from the nucleus. Calibration wasperformed using a nearby flux standard star and a solar analog star (Fig. 3b). Shaded regions in Figures 3b and3c indicate where CN emission is expected, but we againfind no evidence of emission, consistent with work byBodewits et al. (2011), Howell & Lovell (2011), and Jehin et al. (2011). Gas Production Rate Computations
To estimate Scheila’s CN production rate, we re-move the continuum using a scaled solar analog spec-trum (Fig. 3c), which should then leave only gasemission. Standard errors in three wavelength re-gions in the residual spectrum (3760˚A–3830˚A; 3830˚A–3900˚A, where CN emission is expected; and 3980˚A–4050˚A; each 70˚A in width) are 2 . × − , 2 . × − , and 1 . × − erg cm − s − ˚A − , respectively(Fig. 3c). We choose 2 . × − erg cm − s − ˚A − as a conservative estimate of the uncertainty in theCN band. Since 9P/Tempel 1 observations (Meech et al. × (2 . × − erg cm − s − ˚A − ), or ∼ × − erg cm − s − ˚A − .We calculate the integrated CN band flux, f CN , bysumming the emission flux in the shaded region, obtain-ing f CN = 8 . × − erg cm − s − . We then convert f CN to the total number of CN molecules, N CN , using L CN = 4 π ∆ f CN (2)log N CN = log L CN + 2 . r h − log g ( l ) (3)where ∆ and r h are in cm and AU, respectively, and g ( l ) is the resonance fluorescence efficiency, which de-scribes the number of photons scattered per second perradical, in erg s − molecule − . During our observations,Scheila had a radial velocity of ˙ r h = 2 . − , forwhich g ( l, r h ) = 2 . × − erg s − molecule − whenthe Swings effect is taken into account (Schleicher 2010).Substituting f CN = 8 . × − erg cm − s − , we obtain N CN = 5 . × .A simple Haser (1957) model is used to derive the CNproduction rate, Q CN , from N CN , assuming isotropic out-gassing, constant radial expansion of the gas coma, and a2-step exponential decay process. We use l p = 1 . × and l p = 2 . × as the effective Haser scale lengthsat r h = 1 AU (A’Hearn et al. v = 1 . · r − . h km s − (4)(Biver et al. 1997). Integrating the computed spatial col-umn density model over a rectangle 1800 km × Q CN < × s − . Taking average ratios ofspecies in previously observed comets (log[ Q CN /Q OH ] = − . Q OH /Q H O = 90%) (A’Hearn et al. Q H O < s − . How-ever, given the uncertainties involved in assuming ratiosof comet species measured at much closer heliocentricdistances remain unchanged for an object in the mainasteroid belt, we regard this estimate to be precise, atbest, to an order of magnitude. Dynamical Analysis
To gain a more complete understanding of the circum-stances surrounding Scheila’s unusual outburst, we alsoconsider various aspects of the object’s dynamical nature.Specifically, we consider its likely origin and whether itbelongs to an asteroid family.To address the first issue, an effort motivated by thepossibility that cometary objects in the asteroid belt maynot necessarily originate where they are currently seen(cf. P/2008 R1 (Garradd); Jewitt et al. σ values equal to1 × and 100 × the JPL-tabulated uncertainties for eachorbital element (Fig. 4a). We then use the N-body inte-gration package, Mercury (Chambers 1999), to integratethe orbit of each test particle forward in time for 100Myr.In three runs using different randomly generated sets oftest particles, no objects escape from the asteroid belt,indicating that Scheila is dynamically stable over thistime period, and is likely not a recent arrival from else-where in the main belt or the outer solar system. Wenote that despite the 100-fold difference in their initialdispersions, objects from both the 1- σ and 100- σ sets oftest particles diverge to occupy similar regions of orbitalelement space (Fig. 4b). This divergence occurs quickly(within 10 years) and then remains approximately con-stant for the 100 Myr test period (Fig. 4c), with all ob-jects remaining roughly confined to 2 . < a (AU) < . . < e < .
28, and 11 . ◦ < i < . ◦ . Objectsas large as Scheila in the main asteroid belt are oftensimply assumed to be dynamically stable, of course, butthe example of the Centaur (2060) Chiron shows thatit is possible for similarly large objects to occupy dy-namically unstable orbits ( e.g. , Nakamura & Yoshikawa1993). As such, given Scheila’s unusual comet-like out-burst, it is useful to have explicit confirmation (providedby our simulations) that the object is in fact dynamicallystable, and is therefore unlikely to be a recently-arrivedinterloper from elsewhere in the solar system.Next, given the suggestion that MBCs might bepreferentially found among members of young aster-oid families due to their increased likelihood of hav-ing fresh near-surface ice compared to undisrupted as-teroids (Hsieh 2009), we perform a hierarchical cluster-ing analysis (Zappal`a et al. ∼ δv (cid:48) = 120 m s − , where commonly recognized familieshave tens to thousands of dynamically related members,usually within much smaller cutoff values, as shown byNesvorn´y et al. (2005), despite the fact that their anal-ysis was performed when far fewer asteroids were knowncompared to the present day. Such a small number ofrelated asteroids indicates that Scheila likely does notbelong to a family.This conclusion is consistent with Scheila’s size: ob-jects ∼
100 km in diameter are predicted to have colli- sional destruction timescales longer than the age of thesolar system (Farinella et al. et al. et al. i.e. , several km or more below its surface)is low. DISCUSSION
Scheila’s Physical Nature
Yang & Hsieh (2011) found that Scheila’s near-infraredspectrum (from 0.8 to 4.0 µ m) exhibits a consistent redslope, has no apparent absorption features, and generallyresembles spectra of D-type asteroids. Using an intimatemixing model incorporating water ice, amorphous car-bon, and iron-rich pyroxene, they were able to reproduceScheila’s spectrum except for a clearly visible water ab-sorption feature that is present in the synthetic spectrumbut not the observed spectrum. Despite the absenceof unambiguous evidence of water ice in these observa-tions, given the signal-to-noise ratio of the data, Yang &Hsieh (2011) concluded that the presence of water ice onScheila’s surface could not be excluded to a level of a fewpercent. They further noted that the similarity betweenScheila and D-type asteroids, which in turn have beennoted for their similarities to classical cometary nucleifrom the outer solar system (Fitzsimmons et al. 1994),suggests that Scheila could contain preserved ice deepwithin its interior ( e.g. , Jones et al. et al. et al. etal. et al. et al. et al. et al. e.g. ,Jewitt et al. et al. e.g. , Rousselot et al. et al. ∼ or better.While this sensitivity level is beyond the reach of cur-rent Earth-bound facilities for the km-scale (and smaller)MBCs at the distance of the asteroid belt, Rivkin &Emery (2010) and Campins et al. (2010) have reportedwater ice detections (corresponding to much larger sur-face coverage than predicted for the MBCs) for the 100-km-scale asteroid (24) Themis, which belongs to the sameThemis asteroid family that also contains 133P and 176P(Hsieh & Jewitt 2006). A similar absorption feature hasalso been detected on outer belt asteroid (65) Cybele(Licandro et al. et al. (2011) though, who sug-gest that the absorption feature can be equally well ex-plained by the non-volatile mineral goethite. Rivkin &Emery (2010) and Campins et al. (2010) acknowledgethe thermal instability of water ice that they claim todetect and propose various scenarios how such surfaceice could be maintained. However, none of these scenar-ios has yet been confirmed to actually plausibly accountfor a widespread, long-lived surface layer of water ice asimplied by their observations. We further note that nooutgassing or dust emission has ever been observed for(24) Themis (also noted by Rivkin & Emery 2010), andas such, the connection between ice on main-belt aster-oids and the activity of MBCs remains unsubstantiated.In summary, while the hypothesis that activity inMBCs is sublimation-driven is supported by indirect ev-idence such as numerical modeling results and observa-tions showing recurrent activity ( e.g. , Hsieh et al. et al. et al. Morphology Analysis and Outburst Scenarios
The most obvious aspect of Scheila’s dust cloud thatshould bear clues as to its origin is its unusual morphol-ogy. Unlike many other comets which typically exhibit a single tail, often pointed in the antisolar direction,and a coma, Scheila exhibits two distinct curved dustplumes extending to the North and the South (Figure 2).Deeper and higher resolution imagery also shows a faintwestward-pointing dust “spike” (Jewitt et al. cf . Finson &Probstein 1968), and therefore suggest the action of amore unusual ejection mechanism.Ishiguro et al. (2011b) present numerical modeling re-sults that show that the cloud’s morphology can be ac-counted for by a hollow cone of dust (as expected froman impact; e.g. , Richardson et al. i.e. , Scheila’s northern and southernplumes), consistent with the greater optical depth ex-pected of such a structure along its edges. A solid coneof ejected dust (as expected from a sublimation-drivenjet) similarly pushed back would instead exhibit centralbrightening due to greater optical depth in the jet’s core,and not appear to exhibit multiple plumes. The dis-parate strengths of the northern and southern plumesmay indicate an oblique angle of incidence for the im-pact, with more dust expected downrange of the inboundimpactor (cf. Ishiguro et al. et al. (2011b)plausibly accounts for the appearance of multiple dustplumes with a single impact, if Scheila’s dust cloud isproduced by a sublimation-driven process, any scenariothat similarly accounts for multiple observed dust plumeslikely requires multiple active sites. If these active sitesare collisionally excavated, multiple impacts would haveto have occurred on timescales shorter than their deple-tion timescales. This scenario is unlikely, however, giventhe typically low rate of impacts on any single body inthe asteroid belt (e.g., Farinella & Davis 1992) and theexpected short depletion timescales for surface volatileson main-belt asteroids (Hsieh 2009). Multiple active sitescould be possible if near-surface ice is abundant, trigger-ing sublimation at multiple points via thermal stresses.This scenario is contradicted though by Scheila’s historyof observed inactivity until 2010, and the fact that it firstexhibited a comet-like dust cloud at ν ∼ ◦ , well beforeperihelion, when the surface was receiving only ∼ et al. (2011b). Asymmetry in the density of material formingthe cone could then be caused by diurnal effects wheresublimation, and therefore dust emission, increases in in-tensity when the active site is at its closest to the subsolarpoint and receives maximum Solar heating, and decreaseswhen the active site is farthest from the subsolar pointand it receives somewhat diminished Solar heating, as-suming the rotational pole is not directed exactly at theSun.Given Scheila’s rotation period of 15.848 hr (Warner2006; Ishiguro et al. et al. (2011b) is far simpler than one involving arotating, diurnally-varying cometary jet oriented exactlyin such a way that it mimics the asymmetric Sunward-directed hollow cone of ejected material that otherwisearises naturally in the impact scenario. As such, in theabsence of compelling evidence that sublimation must betaking place, we favor that impact scenario as the mostplausible explanation for Scheila’s unusual dust emission,and therefore consider it to be the second disrupted as-teroid to be discovered, after P/2010 A2. MBCs vs. Disrupted Asteroids
New and upcoming all-sky survey facilities like thePanoramic Survey Telescope And Rapid Response Sys-tem (Pan-STARRS; Kaiser et al. et al. et al. (2011) havetaken the approach of designating all comet-like objectswith main-belt orbits as MBCs, based on their appear-ance alone ( i.e. , using the observational definition of acomet as any solar system object exhibiting mass loss inthe form of extended surface brightness features such as acoma or tail). Using this definition, they classify Scheilaas an “impact-activated MBC”, agreeing with this workin terms of physical conclusions, if not in terminology.We prefer, however, to retain the physical meaning of a“comet” as an active body whose gas or dust emission isdue to the sublimation of volatile ices. As such, we chooseto define MBCs as only those comet-like objects withmain-belt orbits whose activity is sublimation-driven (ac- cording to our best judgement), as originally specified inHsieh & Jewitt (2006), and objects like P/2010 A2 andScheila, where dust emission is the direct consequence ofan impact, as disrupted asteroids.Of course, imposing the requirement that activity mustbe sublimation-driven for an object to be considered anMBC implies that we know the underlying cause of ap-parent activity for every comet-like main-belt object,which is not necessarily always the case (or indeed everthe case, at least not until the first successful gas detec-tion is made).From a practical standpoint, the development of an un-ambiguous method of distinguishing sublimation-drivendust emission and impact-driven dust emission will re-quire the discovery and thorough individual investiga-tions of many more examples of both types of objectsbefore a set of reliable criteria can be developed for each,meaning that significant additional work is required be-fore such a classification system can be achieved.In the meantime, we can draw the following prelimi-nary conclusions and observations regarding the differ-ences between MBCs and disrupted asteroids, based onthe currently extremely limited populations of each:1.
Long-lived activity is not a reliable indicator of adust emission event’s source.
All five currentlyrecognized MBCs exhibited activity persisting overweeks or months (Hsieh et al. et al. et al. et al. et al. > (cid:29) µ m) particles,which simply take longer to dissipate than smallerparticles (Jewitt et al. et al. e.g. , for238P; Hsieh et al. et al. et al. (2011) arguedthat observations of rapid fading of the dust cloud(approximately one month after its discovery) wasan indication of impact-driven emission. While theshort time between the appearance of Scheila’s dustcloud (in December 2010) and this rapid fading(observed in January 2011) means that this inter-pretation is likely correct, we note that decreasingactivity (with no earlier observations of increas-ing activity) was also observed for 133P in 2002(Hsieh et al. et al. Dust clouds with unusual morphologies may in-dicate unusual formation circumstances.
As dis-cussed in Section 4.2 (and references within) forScheila and by Jewitt et al. (2010) and Snodgrass et al. (2010) for P/2010 A2, the unusual structuresof their respective dust clouds appear to be indica-tions of non-sublimation-driven origins. In the caseof P/2010 A2, an observed gap between the nucleusand the dust tail strongly suggested that a dustcloud had been ejected in a single impulsive eventand was slowly drifting away from the nucleus, con-sistent with being an ejecta cloud produced by animpact, and inconsistent with cometary dust emis-sion. The implications of Scheila’s pair of dustplumes are considered at length in Section 4.2.While such interpretations can be used to drawpreliminary conclusions, we caution against rely-ing solely on morphology-based determinations ofa dust emission event’s source. Multi-tail struc-tures have been observed for comets believed toexhibit sublimation-driven activity ( e.g. , 238P andP/La Sagra; Hsieh et al. et al. e.g. , shadowing of iso-lated active sites). Detailed numerical dust model-ing must be employed to ensure that any unusualmorphological structures cannot be plausibly ex-plained by both sublimation-driven and impact-driven dust emission.3.
Recurrent activity, separated by intervening periodsof inactivity, is extremely difficult to explain as theconsequence of impacts.
Upon the initial discov-ery of a comet-like main-belt asteroid, conclusionsabout the nature of its activity have generally beenthe result of numerical dust modeling (Boehnhardt et al. et al. et al. cf .133P and 238P; Hsieh et al. ∼ e.g. , Farinella &Davis 1992). One might envision a scenario inwhich a main-belt object routinely encounters anoverdensity of impactors at certain parts of its or- bit, perhaps due to an intersecting cometary debrisstream, for example. The contrived nature of sucha scenario (in which debris streams dense enough toreliably impact 133P and 238P on each orbital pas-sage only intersect these specific objects and do notcause comparable events on other asteroids withsimilar orbits) argues strongly against its plausi-bility though. On the other hand, repeated ac-tive episodes are routinely observed for comets forwhich activity is driven by sublimation, and canbe naturally explained for the MBCs as the con-sequence of seasonal variations in solar heating ofisolated active sites, or perhaps simply from the in-crease in solar heating experienced by each objectnear perihelion (Hsieh et al. i.e. , bothMBCs and disrupted asteroids) have been observed to ex-hibit recurrent activity to date, we note that the remain-ing five objects were discovered as comet-like bodies re-cently enough that they have not actually yet completedfull orbits since their respective discoveries. As such,continued monitoring of all of these objects to search forrecurrent activity will be important for validating theiridentification as MBCs or disrupted asteroids. SUMMARY
We have conducted a photometric, spectroscopic, anddynamical study of comet-like main-belt asteroid (596)Scheila, and report the following findings: • Scheila’s dust cloud comprises a V -band photomet-ric excess of ∆ = 0 .
96 mag over the expected qui-escent brightness of the nucleus, giving it a scat-tering cross-section ∼ M D ∼ × kg. Separate V − R color mea-surements of each of the two curved plumes of thedust cloud show that they are both redder than theSun, with no signficant color differences betweenthe plumes themselves. • We find no evidence of CN emission in Scheila’sdust cloud down to the sensitivity level of ourspectroscopic observations, and place upper lim-its to CN and H O production rates of Q CN =8 . × s − and Q H O ∼ s − . • An analysis of Scheila and its dynamical vicinityusing numerical simulations indicate that it is dy-namically stable for >
100 Myr, suggesting that itis likely native to its current location. We also findthat it does not belong to a dynamical asteroidfamily of any significance, meaning that it is un-likely to be the product of a recent fragmentationor cratering event that could have exposed deeplyburied ice.Considering the available evidence obtained by us andothers, we conclude that Scheila is most likely a disruptedasteroid, and not an MBC (where dust emission is theresult of the sublimation of ice, as in other comets), inagreement with previous work. This conclusion is largelyfounded on consideration of the unusual morphology ofScheila’s dust cloud, coupled with the simplicity of theimpact scenario required to reproduce it and the inabil-ity of any physically plausible sublimation-driven scenar-ios to do likewise. While the currently known examplesof MBCs and disrupted asteroids suggest that morpho-logical analyses can provide strong indications as to thenature of comet-like activity in main-belt objects discov-ered in the future, we caution against relying on suchevidence on its own. We suggest instead that recurrence(or absence of recurrence) of comet-like activity is thebest indicator available to date of whether an object islikely to be a MBC or a disrupted asteroid, underscoring the importance of long-term monitoring observations ofthese objects.H.H.H. is supported by NASA through Hubble Fellow-ship grant HF-51274.01, awarded by the Space TelescopeScience Institute, operated by the Association of Univer-sities for Research in Astronomy, Inc., for NASA, undercontract NAS 5-26555. B.Y. and N.H. acknowledge sup-port through the NASA Astrobiology Institute under Co-operative Agreement No. NNA08DA77A issued throughthe Office of Space Science. We thank Carl Hergenrotherfor first alerting us to Scheila’s unusual activity, AnthonyReadhead, Roger W. Romani, and Joseph L. Richardsfor obtaining Keck observations for us, Marc Kassisand Richard Morriarty for technical assistance, DavidNesvorn´y for providing hierarchical clustering analysissoftware, David Jewitt, Masateru Ishiguro, and IwanWilliams for valuable discussions, and an anonymous ref-eree for helpful comments on this manuscript.
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TABLE 1Observation Log
UT Date Telescope Obs. a N b t c Filters θ s d ν e r h f ∆ g α h α pl i V . (cid:48)(cid:48) R . (cid:48)(cid:48) . (cid:48)(cid:48) a Type of observation (Im: imaging; Sp: spectroscopy) b Number of images c Total effective exposure time in seconds d FWHM seeing in arcsec e True anomaly in arcsec f Heliocentric distance in AU g Geocentric distance in AU h Solar phase angle in degrees i Orbit plane angle (between the observer and object orbit plane as seen from the object) in degrees
TABLE 2Nucleus and Dust Photometry m V m R V − R Nucleus a ± ± ± Ab ± ± ± Ac ± ± ± Bd ± ± ± Ce ± ± ± a Photometry of nucleus (including unresolved near-nucleus coma) inside a 3 . (cid:48)(cid:48) b Photometry of nucleus and dust inside aperture “A”as marked in Figure 2 c Nucleus-subtracted photometry of dust inside aper-ture “A” (Fig. 2) d Photometry of dust inside aperture “B” (Fig. 2) e Photometry of dust inside aperture “C” (Fig. 2)Oke, J. B., Cohen, J. G., Carr, M., Cromer, J., Dingizian, A.,Harris, F. H., Labrecque, S., Lucinio, R., Schaal, W., Epps, H.,& Miller, J. 1995, PASP, 107, 375Prialnik, D., & Rosenberg, E. D. 2009, MNRAS, 399, L79Read, M. T., Bressi, T.H., Gehrels, T., Scotti, J. V., &Christensen, E. J. 2005, IAU Circ., 8624, 1Richardson, J. E., Melosh, H. J., Lisse, C. M., & Carcich, B.2007, Icarus, 190, 357Rousselot, P., Dumas, C., & Merlin, F. 2011, Icarus, 211, 553-558.Rivkin, A. S., & Emery, J. P. 2010, Nature, 464, 1322-1323.Schleicher, D. G. 2010, AJ, 140, 973Schorghofer, N. 2008, ApJ, 682, 697Snodgrass, C., Tubiana, C., Vincent, J.-B., Sierks, H., Hviid, S.,Moissi, R., Boehnhardt, H., Barbieri, C., Koschny, D., Lamy,P., Rickman, H., Rodrigo, R., Carry, B., Lowry, S. C., Laird,R. J. M., Weissman, P. R., Fitzsimmons, A., Marchi, S., & TheOsiris Team. 2010, Nature, 467, 814 Tedesco, E. F., Noah, P. V., Noah, M., & Price, S. D. 2004, IRASMinor Planet Survey, IRAS-A-FPA-3-RDR-IMPS-V6.0. NASAPlanetary Data System.Thomas, P. C., Binzel, R. P., Gaffey, M. J., Storrs, A. D., Wells,E. N., Zellner, B. H. 1997, Science, 277, 1492Warner, B. D. 2006, Minor Planet Bulletin, 33, 58Warner, B., Harris, A. W., Nakano, S., Yoshimoto, K., Guido, E.,Nevski, V., Yusa, T., Foglia, S., Buzzi, L., Concari, P., Galli,G., Tombelli, M., Bryssinck, E., & Gonzalez, J. J. 2010,Central Bureau Electronic Telegrams, 2590, 1Yang, B., & Hsieh, H. H. 2011, ApJ, 737, L39Zappal`a, V., Cellino, A., Farinella, P., & Kne˘zevi´c, Z. 1990, AJ,100, 2030Zappal`a, V., Cellino, A., Farinella, P., & Milani, A. 1994, AJ,107, 772 Fig. 1.—
Plots of eccentricity (top) and inclination (bottom) versus semimajor axis showing the distributions in orbital elementspace of main-belt asteroids (black dots), main-belt comets (red circles), and likely disrupted asteroids P/2010 A2 and Scheila(blue circles). Also marked with dotted lines are the semimajor axes of Mars ( a Mars ) and Jupiter ( a Jup ), the semimajor axis ofthe 2:1 mean-motion resonance with Jupiter, and the loci of Mars-crossing orbits and Jupiter-crossing orbits. Fig. 2.—
Composite R -band image of Scheila constructed from data obtained using the UH 2.2 m telescope on 2010 December12 and comprising data equivalent to 2230 s in total exposure time. Rectangular photometry apertures used to measure dustfluxes are marked, where aperture “A” is 53 (cid:48)(cid:48) × (cid:48)(cid:48) in size, “B” is 17 . (cid:48)(cid:48) × . (cid:48)(cid:48)
0, and “C” is 9 . (cid:48)(cid:48) × . (cid:48)(cid:48) Fig. 3.— (a) Long slit spectral image of Scheila, taken on UT 2010 December 17 using Keck I. The wavelength range (3830˚A < λ < σ uncertainties in thethree wavelength regions discussed in the text. Fig. 4.—
Plots of semimajor axis versus eccentricity (top) and inclination (bottom) for (a) the initial orbital elements and (b) final orbitalelements after a 100 Myr dynamical integration of 100 1- σ Scheila test particles (red dots) and 100 100- σ test particles (blue dots), and (c)the orbital elements of the same 100 1 σ test particles shown at the start of our simulation (overlapping black dots at the center of eachpanel) and 100 1- σσ