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Mon.Not.R.Astron.Soc.000,1–5(2013) Printed10January2014 (MNLATEXstylefilev2.2) A successful search for hidden Barbarians in the Watsonia ⋆ asteroid family 4 A. Cellino1†,S. Bagnulo2, P. Tanga3, B. Novaković4 and M. Delbò3 1 1INAF - Osservatorio Astrofisico di Torino, Pino Torinese 10025, Italy 0 2Armagh Observatory, College Hill, Armagh BT61 9DG, UK 2 3Universityof Nice Sophia Antipolis, CNRS, Observatoire de la Coˆte d’Azur, Laboratoire Cassiopée, France n 4Department of Astronomy, Faculty of Mathematics, University of Belgrade, 11000 Belgrade, Serbia a J 9 Accepted 2013December 20.Received2013December20;inoriginalform2013October15 ] P ABSTRACT E Barbarians,sonamedaftertheprototypeofthisclass(234)Barbara,arearareclassof h. asteroids exhibiting anomalous polarimetric properties. Their very distinctive feature p isthattheyshownegativepolarizationatrelativelylargephase-angles,whereall“nor- - mal”asteroidsshowpositivepolarization.The originofthe Barbarianphenomenonis o unclear, but it seems to be correlated with the presence of anomalous abundances of r t spinel, a mineral usually associated with the so-called Calcium Aluminum-rich inclu- s sions(CAIs)onmeteorites.SinceCAIsaresamplesoftheoldestsolidmatteridentified a [ inoursolarsystem,Barbariansareveryinterestingtargetsforinvestigations.Inspired by the fact that some of the few known Barbarians are members of, or very close to 1 the dynamical family of Watsonia, we have checked whether this family is a major v repositoryofBarbarians,inordertoobtainsomehints abouttheir possiblecollisional 1 origin. We have measured the linear polarization of a sample of nine asteroids which 3 are members of the Watsonia family within the phase-angle range 17◦ − 21◦. We 9 found that seven of them exhibit the peculiar Barbarian polarization signature, and 1 . we concludethat the Watsonia family is a repositoryofBarbarianasteroids.The new 1 Barbarians identified in our analysis will be important to confirm the possible link 0 between the Barbarian phenomenon and the presence of spinel on the surface. 4 1 Key words: asteroids: general – polarization. : v i X r a 1 INTRODUCTION polarization becomes perpendiculartothescattering plane, a situation commonly described as positive polarization, at The degree of linear polarization of sunlight scattered by phaseanglevalueslargerthantheso-calledinversion angle, anasteroid towardanobserverdependsonthephase-angle, ◦ which is generally around 20 . namelytheanglebetweentheasteroid-Sunandtheasteroid -observerdirections.Thephase-polarizationcurvesofallat- Afewyearsago,Cellino et al.(2006)discoveredaclass mospherelessbodiesofthesolarsystemexhibitqualitatively of asteroids exhibiting peculiar phase-polarization curves, similar trends, but their detailed features vary according characterized by a very unusual extent of the negative po- ◦ to the specific properties (including primarily the geomet- larizationbranch,withaninversionanglearound30 ,much ric albedo) of individual surfaces. In the phase-angle range largerthanthevaluescommonlydisplayedbymostobjects. ∼ 0◦÷20◦, asteroids exhibit the so-called branch of nega- Since the prototype of this class is the asteroid (234) Bar- tive polarization, in which, in contrast to what is expected bara, these objects have been since then commonly known from simple single Rayleigh-scattering or Fresnel-reflection as Barbarians. Only half a dozen Barbarians are known to- mechanisms, the plane of linear polarization turns out to day:(234)Barbara,(172)Baucis,(236)Honoria,(387)Aqui- be parallel to the plane of scattering (the plane including tania, (679) Pax, and (980) Anacostia (Cellino et al. 2006; the Sun, the target and the observer). The plane of linear Gil-Hutton et al. 2008; Masiero & Cellino 2009). The polarimetric properties of the Barbarians are fairly unexpected.The observed large negative polarization ⋆ Based on observations made with ESO Telescopes at the La branchisnotpredictedbytheoreticalmodelsoflightscatter- Silla-ParanalObservatoryunderprogrammeID091.C-0027 † E-mail: [email protected] (AC); [email protected] (SB); ing, but in fairly special situations, including surfaces com- [email protected](PT);[email protected](BN);[email protected] posed of very regularly-shaped particles (spheres, crystals) (MDB) orsurfaceshavingconsiderablemicroscopic opticalinhomo- 2 A. Cellino et al. geneity (Shkuratovet al. 1994). Although Barbarians are tionproperties.SuchhighCAIabundanceshaveneverbeen certainlyuncommon,theydoexist,andtheinterpretationof found in meteorite on Earth (so far, the richest CAI abun- their polarization features may lead to important advances dance, found on CV3 meteorites, is about 10%). There- in our understanding of both light-scattering phenomena, fore, Sunshineet al. (2008) conclude that spinel-rich aster- andoftheoriginandevolutionoftheseobjects.Potentialex- oids“might bemoreancient thananyknownsample inour planations range from peculiar surface composition and/or meteoritecollection,makingthemprimecandidatesforsam- texture,tothepossiblepresenceofanomalouspropertiesat plereturn” missions. macroscopic scales due the presence of large concavities as- Many interesting questions are certainly open. Which sociatedwithbigimpactcraters(Cellino et al.2006).Forin- processes are involved in the onset of physical mechanisms stance,(234)Barbarahasaverylongrotationperiod,which which produce the Barbarian polarimetric behaviour? Are mightbetheeffectofabigcollision.Delboèt al.(2009)sug- Barbarians really among the oldest objects accreted in our gestedthat(234)Barbaracouldhaveasurfacecharacterised solarsystem?Ifso,whyaretheytheonlyoneclassofprim- bylarge-scalecraters.Thisisconfirmedbyananalysisofstill itive objects being characterized by an anomalous polari- unpublishedoccultation data by one of us (PT). metric behaviour? Why are they so rare? Are they unusu- In terms of taxonomy based on spectro-photometric ally weak against collisions and fragmentation? Do space- data,allknownBarbariansareclassifiedasmembersofafew weathering phenomena affect their polarimetric properties? unusual classes, including L, Ld, and (in only one case) K. Are the taxonomic classifications of some L, Ld, K objects (234) Barbara itself is an Ld asteroid (here we use the tax- possibly wrong (in such a way that all Barbarians might onomicclassification ofBus & Binzel2002).However,there bememberofauniqueclassandnotspreadamongthreeof are L-class asteroids which are “normal” objects not ex- them)?Itisclearthatanimportantpre-requisitetoimprove hibiting the Barbarian properties. This fact seems to rule ourunderstandingof these objects, and todraw from them out a direct relationship between taxonomic class (based some possible inferences about the origin, composition and on the reflectance spectrum) and polarimetric properties. subsequent evolution of the planetesimals orbiting the Sun On the other hand, L, Ld and K classes are located, in attheepochofplanetaryformation,istofindnewmembers a Principal Component Analysis plane, along adjacent lo- of theBarbarian class, but where tolook for them? cations, which although non-overlapping,seem to represent An aid to a Barbarian search comes from the fact, re- somekindofcontinuousspectralalteration surroundingthe centlyconfirmedbyNovaković,Cellino, & Kneˇzević(2011), mostcommonS classcomplex.Thefactthatthesixknown that the spinel-bearing Barbarian (980) Anacostia belongs Barbarians identified so far belong all to one of these three to a family of high-inclination asteroids. This family is classes suggeststhatsurfacecomposition couldberesponsi- named Watsonia from its lowest-numbered member (729) ble for their polarimetric properties. Even more important, Watsonia. (980) Anacostia belongs to a small grouping two L-class Barbarians, (387) Aquitaniaand (980) Anacos- which is included in the Watsonia family and merges with tia, exhibit very similar reflectance spectra, both sharing it at larger values of mutual distances between the mem- therarepropertyofdisplayingthespectralsignature ofthe bers. In other words, (980) Anacostia belongs to a distinct spinel mineral (Burbine, Gaffey,& Bell 1992). Actually, it sub-group of the Watsonia family, but the possible inde- was exactly the fact that (980) Anacostia was found to be pendence of this sub-group from the rest of the family is a Barbarian that led Masiero & Cellino (2009) to observe highly uncertain and questionable (Novakovićet al. 2011). polarimetrically (387) Aquitania, and to discover that also AnotherknownBarbarian,(387)Aquitania,thoughnotbe- this object shares thesame polarimetric behaviour. ingamemberoftheWatsoniafamily,isalsolocatedinclose Spinel([Fe,Mg]Al2O4)isamineralcharacterizedbyin- vicinity in the space of proper orbital elements (see Sec- distinct cleavage and conchoidal, or uneven fracture prop- tion3). Finally, a member of the Watsonia family, asteroid erties. In terms of optical properties, the MgAl2O4 form of (599)Luisa,isnotaknownBarbarian(nopublishedpolari- spinel has a fairly high refractive index (n = 1.72), which metricmeasurementsareavailableforit),butitisknownto becomesevenhigherinthespinelvarietyhavingahighiron beoneofthefewspinel-richasteroidsidentifiedsofar(thus content(hercynite)(∼1.8,i.e.,muchabovethevalueschar- sharing some common properties with both Anacostia and acterizing the most common silicates present on asteroid Aquitania). surfaces, Sunshineet al. 2008).Spinelisan importantcom- Oneshouldalsonotethatthedistributionofalbedosof ponentofCalciumAluminum-richinclusions(CAI)foundin themembersoftheWatsoniafamilyobservedatthermalIR all kindsof chondritic meteorites. CAIs are refractory com- wavelengths by the WISE satellite (Masiero et al. 2011) is pounds which are thought to be among the first minerals strongly peaked around values between 0.10 and 0.15 with to have condensed in the proto-solar nebula. They are the onlyafewlikelyinterlopers,suggestingthatthefamilyisnot oldest samples of solid matter known in our solar system, a statistical fluke1. A possible common collisional origin of and they are used to establish the epoch of its formation theseasteroidsopensnewperspectivesforthesearchofnew (Sunshineet al.2008).Intermsofspectroscopicproperties, Barbariansandfortheinterpretationoftheirproperties.As- spinel is characterized by the absence (or extreme weak- teroid families are the outcomes of fragmentation of single ness)ofabsorptionbandsaround1µm,andbythepresence parent bodies disrupted by catastrophic collisions. There- of a strong absorption band at 2µm. Sunshineet al. (2008) fore, if (980) Anacostia, and possibly also (387) Aquitania, concluded that, to model the available near-IR spectra of spinel-rich asteroids, it is necessary to assume abundances of the order of up to 30% of CAI material on the surface. 1 We examined the WISE albedo distribution of the Watso- This extreme abundance,which causes a high refractive in- nia family by using the MP3C web facility available at URL dex, might also be responsible for the anomalous polariza- http://mp3c.oca.eu/ A successful search for Barbarians in the Watsonia family 3 We observed our targets using the VLT FORS2 in- strument (Appenzelleret al. 1998) in imaging polarimetric mode, and obtained 17 broadband linear polarization measurements in the R special filter from April to September 2013. The choice of an R filter (instead of the standard V filter traditionally used in many asteroid polarimetric observations) was dictated by the need of improving the S/N ratio. Polarimetric measurements were performed with the ◦ ◦ retarder wave-plate at all positions between 0 and 157.5 , at 22.5◦ steps. For each observation, the exposure time cu- mulated over all exposures varied from 8min (for 5492) to 1h and 20min (for 320971). Data were then treated as ex- plainedin Bagnulo et al.(2011),andourmeasurementsare reportedadoptingasareferencedirectiontheperpendicular to the great circle passing through the object and the Sun. Thisway,P representsthefluxperpendiculartotheplane Q Sun-Object-Earth(thescatteringplane)minusthefluxpar- allel to that plane, divided by the sum of these fluxes. It is therefore identical to the parameter indicated as P in r many asteroid polarimetric studies. In these conditions, for Figure1.Phase-polarizationdata(inRlight)forthetargetsof symmetry reasons, PU values are expected to be zero, and thepresentinvestigation, comparedwiththepolarizationcurves inspection of their values confirms that this is the case for (in V light) of the (234) Barbara, and of (12) Victoria, a large ourobservations. L-classasteroidwhichdoesnotexhibittheBarbarianbehaviour. Atsmallphaseangles(≪20◦),allasteroidsexhibitneg- Black symbols: the seven targets exhibiting the Barbarian po- ativepolarization,whereasatphaseangles&20◦,nearlyall larimetric behaviour; red symbols: our two targets that display asteroidsexhibitpositivepolarization.Bycontrast,atphase a”normal”polarimetricbehaviour;smallbluesymbolsandblue angle≃20◦,Barbariansstillexhibitarelativelystrongneg- curve:availabledatafor(234)Barbara(Masiero&Cellino2009), ative polarization (∼−1%). Therefore, to identify Barbar- andthecorrespondingbest-fitcurveusingthelinear-exponential rineladteigorneePs;rd=ashAe[de,αg/Bree−n1c]u+rveC:t·hαe,bwehste-rfietαcuirsvethfeorpthhaeseL-acnlagsles igalness,>we17d◦e,ciadneddetsotambeliasshurwehtehtheeprotlahreizmateiaosnuraetdpphoalsaeriazna-- asteroid(12)Victoria(forthesakeofclearness,individualobser- tion plane is found to be parallel or perpendicular to the vationsofthisasteroidarenotshown). scattering plane. were issued from the disruption of a common parent body exhibitingthe rare properties which producetheBarbarian 3 RESULTS polarizationphenomenon,itwouldbenaturaltoexpectthat The results of our polarimetric measurements are given in also the other, still not observed members of the Watsonia Table1andshowninFigure1,inwhichacomparisonisalso family should befound to be Barbarians. Moreover, among made with the phase-polarization curve of (234) Barbara the Watsonia family members at least one, (599) Luisa, is and (12) Victoria, a big, non-Barbarian L-class asteroid. In a known spinel-rich asteroid, like Anacostia and Aquitania. theFigure each target is represented bya different symbol, Finally, WISE albedo values in the same range of those of and some targets have more than one measurement. In the Watsonia family members have also been derived for most observedphase-anglerange,sevenofourtargetsshowapo- Barbarianasteroidsknownsofar,namely234,172,236,679 larizationvalue∼−1%,consistentwiththevalueexhibited and 980. by(234)Barbara.Infact,(234)Barbaraexhibitsmarginally higher polarization values (in absolute value) than our targets, possibly due to the different filter in which the obser- 2 NEW OBSERVATIONS vations were performed. The striking result of our observing campaign is that Our target list includes nine objects that are members of seven out of nine asteroids of our target list are Barbar- the Watsonia family (not limited to the Anacostia sub- ians. The two exceptions are 247356 and 320971. How- group) according to Novakovićet al. (2011). These objects ever, we know a priori that all family member lists are are listed in Table 1. We note that a more recent family expected to include some fraction of random interlopers search whose results are publicly available at the AstDys website2alsoidentifiedaWatsoniafamily,butusingamore (Migliorini et al. 1995), then we conclude that both non- Barbarian objects in our target list may befamily interlop- conservative value of the critical level of mutual distances ers. For instance, asteroid 320971 is listed as a Watsonian betweenfamilymemberstobeadoptedtodefinethefamily. member by Novakovićet al. (2011), but it is not included Asaconsequence,theAstDysversion oftheWatsoniafam- amongtheWatsoniamembersidentifiedinthenewAstDys ily has a smaller membership list, and does not includeour list ofasteroid families. Ontheotherhand,threeothertar- target objects 42365, 144854, 247356 and 320971. gets not included in the AstDys member list, but present in the Novakovićet al. (2011) member list, are found to be 2 http://hamilton.dm.unipi.it/astdys/index.php?pc=5 Barbarians. We conclude therefore that, apart from some 4 A. Cellino et al. Table1.PolarimetryofnineasteroidsoftheWatsoniafamilyinthespecialBessellRband.PQandPU arethereducedStokesparameters measuredinareference system suchthat PQ isthefluxperpendicular tothe planeSun-Object-Earth(the scattering plane) minusthe fluxparalleltothatplane,dividedbythesumofthetwofluxes.ItisthereforeidenticaltotheparameteroftenindicatedasPrinasteroid polarimetrystudies.Thelastcolumnlists,whenavailable,thealbedovalueaccordingtoWISE thermalIRmeasurements Date Time(UT) Exp Object Phaseangle PQ PU WISE (yyyymmdd) (hh:mm) (sec) (DEG) (%) (%) albedo 20130705 23:41 480 5492 18.79 −1.14±0.10 0.05±0.10 0.14±0.02 20130729 01:44 960 18.31 −1.01±0.09 0.00±0.09 20130603 09:40 2000 42365 23.30 −0.83±0.15 −0.06±0.15 0.17±0.02 20130803 09:08 960 18.55 −1.73±0.12 −0.06±0.12 20130712 23:50 960 56233 17.83 −1.07±0.16 −0.09±0.16 0.19±0.03 20130805 01:03 2200 19.31 −1.09±0.12 −0.07±0.12 20130730 00:38 2000 106059 18.30 −1.06±0.30 −0.15±0.31 20130804 01:09 4000 18.82 −0.94±0.14 0.00±0.14 20130828 01:24 4800 19.64 −0.84±0.20 −0.09±0.20 20130706 01:33 1400 106061 20.21 −0.94±0.11 0.06±0.11 20130809 02:46 4000 23.97 −0.57±0.12 −0.05±0.12 20130706 02:05 960 144854 21.30 −0.81±0.12 0.13±0.12 20130805 02:06 4000 24.19 −0.15±0.12 −0.13±0.13 20130813 06:28 4000 236408 18.31 −0.97±0.15 0.22±0.15 0.14±0.04 20130707 02:40 3440 247356 19.97 0.10±0.15 −0.33±0.15 20130417 08:58 4800 320971 23.78 0.11±0.31 −0.06±0.31 20130603 07:08 3440 20.53 −0.03±0.22 −0.06±0.21 details concerning family membership,theWatsonia family oid (599 Luisa) (Sunshineet al. 2008), but we clearly need is an important repository of Barbarian asteroids. This is confirmation from new observations. the immediate and most important result of our investiga- Similarly, we need to search for the Barbarian polari- tion.Wealso notethatthenumberofWatsoniaBarbarians metricfeatureinotherspinel-richasteroids.Inparticular,we identifiedinourobservingcampaignislargerthanthewhole are interested not only in 599 Luisa, but also in the Henan sample of Barbarians previously known. We also note that, family, which is known to include at least three spinel-rich exceptincaseof236408and247356(onlyasinglemeasure- asteroids (Sunshineet al. 2008). menteach)theresultingP valuesarealwayswellconsistent U Next,weneedtounderstandwhytheBarbarianandthe with zero. anomalousspinelabundancephenomenaaresorare.Appar- ently,onlyahandfulofBarbarianparentbodiesexisted,and only their disruption into many fragments made it possible toidentify todaylarger numbersof theseobjects. Inprinci- 4 DISCUSSION AND FUTURE WORK ple, one might wonder whether the unusual properties dis- playedbytheseobjectsmightalsobeinsome(stillobscure) Our results confirm once again that asteroid polarimetry way a consequence of the collisional eventsthemselves. canprovideanimportantcontributiontoasteroidtaxonomy, as noted in the past by several authors (see, for instance, Anotheropenproblemistheorigin of(387) Aquitania. Penttila¨ et al. 2005). This asteroid is not included in any family list, including Several problems, however, are now open, and deserve both the most recent classification available at the AstDys furthertheoretical and observational efforts. site, and the Novakovićet al. (2011) classification of high-I The first issue to be addressed is the relationship be- families. Yet, this object is quite close, in terms of orbital tween the Barbarian polarimetric features and the unusual elements, to (980) Anacostia and the rest of the Watsonia amount of spinel measured via spectroscopy in some of the family. The values of proper elements for (387) Aquitania knownBarbarians.Inotherwords,thenewBarbariansthat and(980) Anacostia (availablein theAstDysdatabase) are wehavefoundinourinvestigationmustbespectroscopically a = 2.73916, e = 0.23025, sinI = 0.28245, for (387) Aqui- observed in the visible and near-IR wavelengths in order to tania, and a = 2.74102, e = 0.13972, sinI = 0.29805 for checkwhethertheyexhibitthespinelfeatures.Wedobelieve (980)Anacostia,whereaisthepropersemi-majoraxis,ethe that this will be the case, since we already know three ob- propereccentricity,andI istheproperinclination.Theonly jectsthatarebothspinel-richandBarbarians(234Barbara, onerelevantdifference,which preventsanyfamily search to 387Aquitaniaand980Anacostia),adweknowalsothatthe includetheminanacceptablegroup,isthepropereccentric- Watsoniafamilyincludesatleastoneotherspinel-richaster- ity. One should conclude that there is no relation between A successful search for Barbarians in the Watsonia family 5 (387) Aquitania and (980) Anacostia, and that the similar- fore at disposal for futurephysicalinvestigations (including ityoftheirorbitalsemi-majoraxesandinclinationsarejust photometric, spectroscopic, and polarimetric campaigns.) a coincidence. Moreover, both asteroids display stable or- At the same time, our results definitively confirm the fam- bitsintermsofcharacteristicLyapunovexponents,andany ily of Watsonia as a sizeable group of Barbarians having a possible evolution due to non-gravitational forces, (e.g. the common collisional origin, and open new perspectives for Yarkovsky effect) seem also very unlikely, because both as- the development of models in order to try and sort out in teroidsarefairlylarge,withsizesoftheorderof80÷100km a coherent scenario several pieces of observational evidence according to thermal radiometry data. Finally, thecollision which appear now to form a complicated puzzle. whichcouldhaveproducedafamilyincludingtwofragments of this size had to be extremely energetic, and should have producedahugefamily,ofwhichthereisnoevidencetoday. ACKNOWLEDGMENTS On the other hand, Aquitania and Anacostia exhibit Part of this work was supported by the COST Action too many relevant similarities in their physical properties, MP1104 Polarization as a tool to study the Solar System which suggests that they may come from a unique parent and beyond. Inparticular, fundingforShortTermScientific bodyfragmented byacollision. Averytentativescenario is MissionCOST-STSM-MP1104-12750ofACattheArmagh that,inaveryearlyepoch,aviolentcollisiondestroyedabig Observatory is kindly acknowledged. The work of B.N. has parentbodywhich wastheprogenitor oftheBarbarians we been supported by the Ministry of Education and Science seetodayinthiszone.Mostofthefirst-generationfragments ofSerbia,undertheProject 176011. Acareful reviewbyK. producedbytheeventarenowgoneduetogravitationalper- Muinonen helped toimprove thepaper. turbations and Yarkovsky-driven drift in semi-major axis. ThecurrentWatsoniafamilycouldbetheproductofamore recentdisruptionofoneoftheoriginalfragmentsofthefirst REFERENCES event.Thisregioncouldstillincludeseveralfirst-generation fragments,preferentiallythebiggestoneswhich,duetotheir Appenzeller I.et al., 1998, TheMessenger, 94, 1 size, experienced no or extremely weak Yarkovsky evolu- Bagnulo S., Belskaya I., Boehnhardt H., Kolokolova L., tion, like (387) Aquitania and (980) Anacostia. Some grav- MuinonenK.,SterzikM.,TozziG.-P.,2011,JQSRT,112, itational perturbations could have also played some role. If 2059 one looks at the a - e or a - sinI plots making use of the Burbine T. H., Gaffey M. J., Bell J. F., 1992, Metic, 27, facility available at the AstDys site3, it is easy to see that 424 the Watsonia family is crossed, in semi-major axis, by a Bus S. J., Binzel R.P., 2002, Icarus, 158, 146 couple of thin resonances, one of which is located close to CellinoA.,BelskayaI.N.,BendjoyaP.,diMartinoM.,Gil- thevalueofaofboth(387)Aquitaniaand(980)Anacostia. HuttonR.,MuinonenK.,TedescoE.F.,2006,Icarus,180, So, we cannot rule out entirely the idea that some limited 565 evolution in eccentricity of these objects could be due to Delbo` M., Ligori S., Matter A., Cellino A., Berthier J., these perturbation effects and the two objects might have 2009, ApJ, 694, 1228 beenoriginallycloserintheproperelementspace.Westress Gil-HuttonR.,MesaV.,CellinoA.,BendjoyaP.,Penãloza againthatthiskindofscenarioisonlyeminentlyspeculative L., & Lovos F. 2008, A&A,482, 309 and deservesfurtheranalysis before it can beaccepted as a Masiero J., Cellino A., 2009, Icarus, 199, 333 plausible one. Masiero J., Mainzer A. K., Gray T. and other 15 authors, Wepointoutthatasolargerelativefractionofobserved 2011, Ap.J., 741:68 members of the Watsonia family turning out to be Barbar- Migliorini F.,ZappalaV.,VioR.,Cellino A.,1995, Icarus, ians has a further implications to be taken into account. If 118, 271 theparentbodywas differentiated,orhadsomegradient of Novaković B., Cellino A., Kneˇzević Z., 2011, Icarus, 216, compositionasafunctionofdepth,withthepresenceofCAI 69 materiallimitedtosomethinshell,thenthespinelsignature PenttilaÄ.,Lumme,K.,Hadamcik,E.,Levasseur-Regourd, would bepresentonlyin aminorityofthefamily members. A.-C., 2005, A&A,432, 1081 This is in disagreement with what we observe, indicating Shkuratov Y. G., Muinonen, K., Bowell, E. and other 7 thattheanomalouscompositionoftheparentbodywasnot authors, 1994, EM&P, 65, 201 limited to some thin region. In conclusion, we are not look- Sunshine J. M., Connolly H. C., McCoy T. J., Bus S. J., ing simply at some kind of surface process (like alteration LaCroix L. M., 2008, Science, 320, 514 by space weathering or impacts), but at the properties dis- played by asteroids originated from different depths inside a single parent body which was anomalously spinel-rich in mostofitsvolume(thiscanbeevenmoretrueiftheWatso- niafamilyisasecondaryeventderivedbyafirst-generation destruction of a larger body.) TheissuesofSection1arestillopenanddeservecareful attention. The immediate result of our investigation is that we have now a much larger sample of Barbarians than be- 3 http://hamilton.dm.unipi.it/astdys2/Plot/