Compositional study of asteroids in the Erigone collisional family using visible spectroscopy at the 10.4 m GTC

5.5 MB·

by  David Morate

Astronomy&Astrophysicsmanuscriptno.Morate_Erigones_LatestVersion (cid:13)cESO2017
January16,2017
Compositional study of asteroids in the Erigone collisional family
using visible spectroscopy at the 10.4m GTC
DavidMorate1,2,JuliadeLeón1,2,MárioDePrá3,JavierLicandro1,2,AntonioCabrera-Lavers1,4,Humberto
Campins5,NoemíPinilla-Alonso6,andVíctorAlí-Lagoa7
1 InstitutodeAstrofísicadeCanarias(IAC),C/VíaLácteas/n,38205LaLaguna,Tenerife,Spain
2 DepartamentodeAstrofísica,UniversidaddeLaLaguna,38205LaLaguna,Tenerife,Spain
3 ObservatórioNacional,CoordenaçãodeAstronomiaeAstrofísica,20921-400RiodeJaneiro,Brazil
4 GTCProjectOffice,38205LaLaguna,Tenerife,Spain
5 PhysicsDepartment,UniversityofCentralFlorida,P.O.Box162385,Orlando,FL32816-2385,USA
7 6 DepartmentofEarthandPlanetarySciences,UniversityofTennessee,Knoxville,37996TN,USA
1 7 LaboratoireLagrange,OCA,Boulevarddel’Observatoire,B.P.422906304NiceCedex04-France
0
2 January16,2017

n
a ABSTRACT
J
Twoprimitivenear-Earthasteroids,(101955)Bennuand(162173)Ryugu,willbevisitedbyaspacecraftwiththeaimofreturning
3
samples back to Earth. Since these objects are believed to originate in the inner main belt primitive collisional families (Erigone,
1
Polana,ClarissaandSulamitis)orinthebackgroundofasteroidsoutsidethesefamilies,thecharacterizationoftheseprimitivepop-

ulations will enhance the scientific return of the missions. The main goal of this work is to shed light on the composition of the
]
P Erigonecollisionalfamilybymeansofvisiblespectroscopy.Asteroid(163)Erigonehasbeenclassifiedasaprimitiveobject(Bus
E 1999;Bus&Binzel2002),andweexpectthemembersofthisfamilytobeconsistentwiththespectraltypeoftheparentbody.We
haveobtainedvisiblespectra(0.5-0.9µm)for101membersoftheErigonefamily,usingtheOSIRISinstrumentatthe10.4mGran
.
h TelescopioCanarias.Wefoundthat87%oftheobjectshavetypicallyprimitivevisiblespectraconsistentwiththatof(163)Erigone.
p Inaddition,wefoundthatasignificantfractionoftheseobjects(∼50%)presentevidenceofaqueousalteration.
-
o Keywords. minorplanets,asteroids:general-methods:dataanalysis-techniques:spectroscopic
r
t
s
a
1. Introduction compositions, will shed light on the evolutionary history of the
[
SolarSystem.

1Primitive asteroids are considered to be composed of the most Asteroidcollisionalfamiliesaregroupsofasteroidssharing
vpristinematerialsintheSolarSystem,beingremnantsofthepro- very similar orbital properties (Hirayama 1918), thought to be
1cesses which followed the condensation of the proto-planetary thedirectresultofenergeticcollisionalevents.Spectroscopicob-
6nebulaandtheformationoftheplanets.Thematerialsonthese servationsprovidedthefirstconfirmationofthecollisionalorigin
7objects have been altered over time due to different processes, ofafamily:theVestafamily(Binzel&Xu1993).Thespectro-
3
0suchasspaceweatheringandaqueousalteration(Fornasieretal. scopicstudyofcollisionalfamilieshavesteadilyincreasedsince
.2014).Aqueousalterationactsmainlyonprimitiveasteroids(C, then, focusing on the characterization of their mineralogy. Ad-
1B,andlowalbedoX-types,accordingtotheDeMeoetal.(2009) ditionally, considering that near-Earth asteroids (NEAs) come
0classification scheme), producing a low-temperature (< 320K) primarily from the main asteroid belt (Bottke et al. 2002), col-
7
chemicalalterationofthematerialsduetothepresenceofliquid lisional families are good sources of NEAs, as they generate
1
water.Thiswateractsasasolventandgenerateshydratedmateri- plenty of small fragments during their formation. Families lo-
:
valslikephyllosilicates,sulfates,oxides,carbonates,andhydrox- cated close to particular resonances in the belt can easily send
Xiides.Thepresenceofhydratedmaterialsthusimpliesthatliquid thesefragmentstothenear-Earthspace.Inthissense,theinner
water was present in the primordial asteroids, produced by the asteroidbelt(theregionlocatedbetweentheν resonance,near
armeltingofwatericebyaheatingsource(Fornasieretal.2014). 2.15AU,andthe3:1mean-motionresonancew6ithJupiter,at2.5
Themostunambiguousindicatorofhydrationisthe3µmhydra- AU) is considered as a primary source of near Earth asteroids
tion band observed in infrared photometry and spectroscopy of (Bottkeetal.2002).
manyprimitiveasteroids.Thisfeatureiscorrelatedwiththe0.7 NASA OSIRIS-REx (Lauretta et al. 2010) and JAXA1
µmFe2+ →Fe+3 oxidizedironabsorptionbandobservedinthe
Hayabusa2(Tsudaetal.2013)sample-returnmissionshavetar-
visiblespectraoftheseasteroids(Vilas1994;Howelletal.2011;
geted two near Earth asteroids: (101955) Bennu and (162173)
Rivkin2012).
Ryugu, respectively. These are primitive asteroids that are be-
Apartfromthisaqueousalteration,primitiveasteroidshave lieved to originate in the inner belt, where five distinct sources
undergone minimal geological or thermal evolution, experienc- havebeenidentified:fourprimitivecollisionalfamilies(Polana,
ing,onthecontrary,anintensecollisionalevolutionthataffected Erigone,SulamitisandClarissa)andapopulationoflow-albedo
their shape, size, and surface composition. Therefore, studying
the products of those collisional events and their mineralogical 1 JapanAerospaceExplorationAgency
Articlenumber,page1of19
A&Aproofs:manuscriptno.Morate_Erigones_LatestVersion
andlow-inclinationbackgroundasteroids(Campinsetal.2010, 0.25
2013;Bottkeetal.2015).Identifyingandcharacterizingthepop-
ulations from which these two NEAs might originate will en-
hancethesciencereturnofbothmissions.
0.2
Withthismainobjectiveinmindweinitiatedin2010aspec-
troscopic survey in the visible and the near-infrared to charac- y
terizetheprimitivecollisionalfamiliesintheinnerbeltandthe cit
low-albedo background population (PRIMitive Asteroid Spec- ntri0.15
troscopic Survey, PRIMASS). We started with the largest one, ce
c
the Polana family (Pinilla-Alonso et al. 2015; de León et al. E
2015), using, among other, visible spectra obtained with the
0.1
10.4mGranTelescopioCanarias,locatedattheElRoquedelos
MuchachosObservatory,intheislandofLaPalma(Spain).We
foundthat,despitethedynamicalandcollisionalcomplexityof
thePolanafamily(Walshetal.2013;Milanietal.2014;Dykhuis 0.05
&Greenberg2015),thereisaspectralhomogeneitybothinthe
6
visibleandthenear-infraredwavelengths,alltheasteroidsshow-
ingacontinuuminspectralslopesfrombluetomoderatelyred ) 5.5
s
typicalofB-andC-typeprimitiveasteroids. ee 5
To continue with the PRIMASS survey, we have observed egr 4.5
and characterized the Erigone family, the second largest one d
of the four primitive collisional families in the inner belt. We n ( 4
o
haveperformedouranalysisusingdataobtainedwiththe10.4m ati 3.5
Gran Telescopio Canarias during the semester 2014B (Septem- n 3
ber 2014 - February 2015). In Section 2 we describe the ob- cli
n 2.5
servations and the data reduction. In Section 3 we present the I
2
analysis performed on the data, including taxonomical classifi-
cation, computation of spectral slopes and analysis of aqueous 1.5
alteration. In Section 4 we discuss the obtained results, and in 1
2.2 2.25 2.3 2.35 2.4 2.45 2.5
Section5wesummarizetheconclusions.
Semimajor axis (A.U.)
2. Observationsanddatareduction Fig. 1. Proper semi major axis (a) versus proper eccentricity (e) and
properinclination(i)fortheprimitiveasteroidfamiliesintheinnermain
Thesampleofasteroidsweobservedinthisstudyhasbeense- belt.Erigonefamilyisdepictedinblack.ThePolana(Nysa-Polanacom-
lected using the Minor Planet Physical Properties Catalogue2 plexinthisplot),Sulamitis,andClarissafamiliesaredepictedingreen,
(MP3C),whichacknowledgesasadatasourcetheNASAPlan- gray,andcyan,respectively.
etary Data System (PDS). Orbital data regarding the asteroid
families are extracted from a dataset containing asteroid dy-
namical families including both analytic and synthetic proper on different nights (see details in the next section). Therefore,
elements. These families were computed by David Nesvorny from thepreviouslist of asteroids, weselected those having an
(Nesvorny2012)usinghiscodebasedontheHierarchicalClus- apparent visual magnitude in the range 18 < m < 21 to get a
V
teringMethod(HCM),asdescribedinZappalaetal.(1990)and goodcompromisebetweenthenumberofasteroidsobservedand
Zappala & Cellino (1994). The MP3C catalogue provides also the signal-to-noise of the spectra. This means that for some of
information on the absolute magnitude H, the diameter D, and thenightstherewerenovisibleasteroidsfulfillingthesecriteria.
thegeometricalbedopV.Inthecaseofthediametersandalbedos In those cases (16 in total), we first tried to find objects having
weusedthevaluesprovidedbyWISE(Wide-fieldInfraredSur- SDSS color-based taxonomical classification available (prefer-
veyExplorer,Masieroetal.2011).Familymembershipisbased entially C-types). When this information was not available, we
onvaluesofthesyntheticproperelements,i.e.,semimajoraxis selected objects having p > 0.1. Our last option was to select
V
(a),eccentricity(e)andinclination(i),andalsoontheabsolute objects with no information on their albedo. We describe this
magnitude as a function of semi major axis (a,H). According smallsub-sampleindetailinSection3.
totheseparameters,theErigonefamilycontainsatotalof1785
asteroids.
The selection criterion was quite simple. The Erigone col- 2.1. Observations
lisional family is a primitive one according to the information
we currently have: from the list of 1785 members of the fam- We obtained low-intermediate resolution visible spectroscopy
ily, 1015 have no albedo information, but from the remaining for a total of 101 asteroids using the Optical System for Imag-
770 objects, 692 have geometric albedo values p < 0.1. Be- ingandLowResolutionIntegratedSpectroscopy(OSIRIS)cam-
V
sides, 156 objects have SDSS color-based taxonomies, the ma- era spectrograph (Cepa et al. 2000; Cepa 2010) at the 10.4m
jorityofthembelongingtoprimitiveclasses(91C-types,10B- GranTelescopioCanarias(GTC),locatedattheElRoquedelos
types,7X-types,39S-types,5L-types,2K-types,1V-type,and Muchachos Observatory (ORM) in La Palma, Canary Islands,
1 A-type). Therefore, we selected those asteroids with geomet- Spain.TheOSIRISinstrumentconsistsofamosaicoftwoMar-
ric albedo p < 0.1. Observations were done in service mode coni CCD detectors, each with 2048 x 4096 pixels and a total
V
unvignetted field of view of 7.8 x 7.8 arcmin. The single pixel
2 http://mp3c.oca.eu/MP3C/ physical size is 15 µm, giving a plate scale of 0.127 "/pix. To
Articlenumber,page2of19
DavidMorateetal.:VisiblespectroscopyofErigonecollisionalfamily
increasethesignaltonoiseforourobservationsweselectedthe Table1.Equatorialcoordinatesofthesolaranaloguestarsusedtoob-
tainthereflectancespectraoftheobservedasteroids.
2x2binningmodewithareadoutspeedof200kHz(whichhas
a gain of 0.95 e-/ADU and a readout noise of 4.5 e-), as corre-
spondswiththestandardoperationmodeoftheinstrument. ID Star α δ
1 SA93-101 01:53:18.0 +00:22:25
All the spectra were obtained using the OSIRIS R300R
grism,whichproducesadispersionof7.74Å/pixfora0.6"slit 2 SA98-978 06:51:34.0 -00:11:28
3 SA102-1081 10:57:04.4 -00:13:10
(intheworstcasescenario,aseeingof3.0"wouldtranslateinto
4 SA110-361 18:42:45.0 +00:08:04
aresolutionof38.7Å/pix),withaspectralcoveragefrom4800
5 SA112-1333 20:43:11.8 +00:26:15
to 10000 Å. The R300R grism is used in combination with a 6 SA115-271 23:42:41.8 +00:45:10
second order spectral filter. However, there is still a slight con- 7 SA107-998 15:38:16.4 +00:15:23
tamination in the spectrum, with a distinguishable contribution
forwavelengthsat4800-4900Åand9600-9800Å.Then,tobe
conservative, we do not consider here data beyond 9000 Å. A subtracted,andaonedimensionalspectrumwasextractedusing
5.0"-widthslitwasusedtoaccountforpossiblevariableseeing anextractionaperturethatvarieddependingontheseeingofthe
conditions and it was oriented to the parallactic angle to mini- corresponding night. After the extraction, the one dimensional
mize loses due to atmospheric dispersion. Series of three spec- spectrawerewavelengthcalibratedusingXe+Ne+HgArlamps.
tra (whenever possible) were taken for all the targets, with ex- Asafinalstep,thethreespectraofthesameobject,whenavail-
posure times ranging from 150-600 s, depending on the target able,wereaveragedtoobtainonefinalspectrumoftheasteroid.
brightness.ObservationaldetailsarelistedinTable3.Informa- Inordertocorrectfortelluricabsorptionsandtoobtainrel-
tionincludesasteroidnumber,dateofobservation,startingUT, ative reflectance spectra, at least one solar analogue star from
airmass, exposure time, solar analogue stars and seeing at the theLandoltcatalogue(Landolt1992)wasobservedeachnight.
moment of the observations. Consecutive spectra were shifted Whenpossible,morethanonesolaranaloguestarwasobserved
in the slit direction by 10 arcsecs, in order to improve the sky inordertoimprovethequalityofthefinalspectraandtomini-
subtractionandthefringingcorrection. mizepotentialvariationsinspectralslopeintroducedbytheuse
Observations were done in service mode (within GTC pro- of one single star. These stars were observed using the same
gram GTC39-14B) on different nights along September3 2014- spectral configuration as that for the asteroids, and at a simi-
February 2015. Night conditionswere rather variable, covering larairmass.Thelistofthesolaranaloguesusedinthisstudyis
a wide range of different weather conditions. This was due to showninTable1.
the fact that the program was classified as a "filler" (C-band) The MATLAB routines in our pipeline were used to align
programwithintheGTCnightlyoperationschedule.Theaimof thespectraoftheobjectsandthecorrespondingsolaranalogue,
these type of programs is to obtain high signal to noise spectra usingthetheoreticalwavelengthpositionsofthetelluriclinesat
fortargetsthatarerelativelybrightfora10m-classtelescopeas 6867.19 and 7593.70 Å. Once aligned, the spectrum of the ob-
GTC in non-optimal weather conditions, that would include a jectwasdividedbythatofthesolaranalogue,andtheresultwas
highseeingvalue(largerthan1.5arcsec),brightmoon,orsome normalizedtounityat0.55µm.Whenmorethanonesolarana-
cirruscoverage.Becauseofthis,spectraqualitymightvaryfrom loguewasobserved,wedividedthespectrumoftheasteroidby
onenighttoanother(seeTable3,lastcolumn).Sincetheweather the spectra of the stars and checked against any possible varia-
conditions (i.e. clouds, sky brightness, etc.) are the main con- tions in spectral slopes, which were of the order of 0.6%/1000
straintalongtheobservation,thisvariationisunrelatedtothetar- Å. A variation smaller than 1%/1000 Å is typically considered
getbrightness.Forcompleteness,wehaveincludedinthisstudy
asagoodvalue.
thevisiblespectraofasteroids(163)Erigone,theparentbodyof
Oncethefinalspectrumforeachobjectwasobtained,abin-
thefamilyand(571)Dulcinea,thesecondlargestasteroidinthe
ning was applied to each spectra, taking intervals of 11 points
family, both from the SMASS II catalog4. There were no more
asthebinsize.Then,thereflectancevaluecorrespondingtothe
asteroidsfromtheErigonefamilywithpublishedvisiblespectra.
centralwavelengthofthebinsizewassubstitutedwiththeme-
All in all, our sample of asteroids from the Erigone collisional
dian reflectance value, in order to avoid spectral disturbances,
familyincludesatotalof103objects.
andthusmakingtheresultingspectrummorerobust.Inorderto
choose the binning size, we selected the worst spectrum in our
sample and applied different binning sizes, until its quality im-
2.2. Datareduction
proved.Sincethefinestspectralfeaturewewanttomeasure,the
A reduction pipeline for asteroid spectroscopic data obtained 0.7µmband,hasanapproximatewidthof2000Å,weconsider
withtheGTCwasdevelopedinordertooptimizethereduction that the selected binning is sufficient to improve the quality of
process.ThispipelinecombinesstandardIRAF5tasksandsome the spectra, and to not affect the obtained results. The spectral
MATLABfunctions. range extends from 0.5 to 0.9 µm, with a step of 0.0055 µm.
UsingtheIRAFtasksincludedinourpipeline,imageswere SpectraareshowninFig.B.1.
initially bias and flat-field corrected, using lamp flats from the
GTCInstrumentCalibrationModule.Skybackgroundwasthen
3. Analysisandresults
3 SomeoftheasteroidswereobservedduringAugust2014uponre-
Aftercomputingthefinalspectra,ataxonomicclassificationwas
quest from the GTC operations team to fill observational gaps during
particularnights. made using M4AST6, which is an online tool for modelling of
4 Availableathttp://smass.mit.edu/catalog.php asteroid spectra (Popescu et al. 2012). The method used by the
5 IRAFisdistributedbytheNationalOpticalAstronomyObservato- M4AST tool to classify asteroid spectra is the following: first,
ries,whichareoperatedbytheAssociationofUniversitiesforResearch the spectrum is fitted with a polynomial curve, and then this
inAstronomy,Inc.,undercooperativeagreementwiththeNationalSci-
enceFoundation. 6 http://m4ast.imcce.fr/
Articlenumber,page3of19
A&Aproofs:manuscriptno.Morate_Erigones_LatestVersion
Table2.Asteroidsinoursampleforwhichthereisnoinformationon
C−type (43%) Others (13%)
their visible geometric albedo or the albedo value is larger than 10%
(p >0.1).
V
T−type (7%)
Asteroid p SDSSClass. M4ASTClass.
v
38661 - S Sr
B−type (11%)
39895 - S S
56349 - C B
186446 - C Ch
18759 0.303 - Sr
24037 0.129 - L
X−type (27%)
132383 0.356 - L
38106 - - L
50068 - - L
C−type (58%) Others (30%)
66403 - - Ch
69706 - - S
70511 - - V
76922 - - Xk
85727 - - S
107070 - - L
166264 - - Xk
B−type (7%)
186446 - - Ch
X−type (5%)
in Table 2. Besides, the three asteroids having p > 0.1 corre-
V
Fig.2.DistributionofthetaxonomicalclassesfortheErigonefamily.
spond to non-primitive classes. In the case of the remaining 10
Toppanelshowsthedistributionforthesampleof103asteroidsstudied
asteroidswithnoalbedoinformationwefoundamixtureoftax-
inthispaper.Bottompanelshowsthedistributionforthecolor-based
onomies,having2C-types,2X-types,2S-types,3L-types,and
taxonomyfromtheSDSSdata(theXclassintheSDSSclassification
1V-type.Fromthetotalof86asteroidshavingp <0.1,thereis
includes the T-type from the Bus taxonomy). The "Others" class in- V
cludesallnon-primitivetaxonomies(S-types,V-types,andL-types). onlyonesingleobjectwithanon-primitiveclassification,being
anL-type.
To perform one final comparison with the taxonomical dis-
curveiscomparedtoeachoftheclassesdefinedbytheDeMeo tributionwefoundfromourvisiblespectra,wesearchedforall
et al. (2009) taxonomy at the corresponding wavelengths. The theasteroidsintheErigonefamilyhavingSDSScolor-basedtax-
tool then selects the taxonomical class producing the smallest onomy. A total of 156 objects belonging to the Erigone family
standarddeviation. wereclassifiedaccordingto(DeMeo&Carry2013).Inthelower
panelofFig.2weshowtheirtaxonomicdistribution.Thereisa
Wearedealingwithspectrainthevisiblewavelengthrange.
goodagreementbetweentheproportionofCandB-typesfrom
Thus,weareusingtheBus&Binzel(2002)taxonomy,inwhich
oursampleandtheonefromtheSDSStaxonomy,beingthepro-
most of the classes overlap the DeMeo et al. (2009) ones. We
portionofX-typeasteroidssignificantlylargerinourcase.The
havecheckedindividuallythosecasesinwhichthetaxonomical
difference between both non-primitive distributions is probably
classeswereexclusivetotheDeMeoetal.(2009)taxonomyand
duetotheselectioncriterion(p <0.1).
visuallyclassifiedthemaccordingtoBus&Binzel(2002). V
Fig.3showsthedistributioninthe(a,H)spaceofthe1785
To obtain robust results, both original and binned spectra
asteroids that have been identified as members of the Erigone
wereclassifiedusingtheM4ASTtool.Aχ2 method(Bevington
collisional family (grey circles). As described by Vokrouhlický
& Robinson 1992) was used to test how well the spectra fitted
et al. (2006), the family shows signs of having experienced
tothetemplates.Thechosenresultwastheonecorrespondingto
dynamical spreading via the Yarkovsky thermal forces. Solid
thesmalleststandarddeviation.Wheneveraspectrumwasvery
differentfromthebest-fittingtemplate,orwhenthestandardde- curvesinFig.3definetheboundariesofthefamily,alsoknown
asthe“Yarkovskycone",computedusingthefollowingexpres-
viationswereverysimilarfortwoormoretaxonomicalclasses,
sion:
we simplified the method: the binned spectrum was fitted to a
third-orderpolynomialandthentheprocedurewasrepeated.The 0.2H =log (∆a/C),
taxonomicalclassificationobtainedforeachasteroidisshownin 10
thelastcolumnofTable4. where∆a=a−a ,witha definedasthecenterofthefamily.In
c c
Theclassificationyieldedatotalof44C-typeasteroids(in- practice,a isoftencloseto,orthesameas,thesemimajoraxis
c
cluding the subclasses Cb, Cg, Ch, and Cgh), 28 X-type aster- of the largest member of the family, in this case asteroid (163)
oids(andsubclassesXcandXk),11B-types,7T-types,and13 Erigone (Vokrouhlický et al. 2006; Bottke et al. 2015). Bottke
objects with non-primitive classifications: 6 S-types (and sub- etal.(2015)showedthat,fortheErigonefamily,C =1.9x10−5.
classes), 1 V-type, and 6 L-types. These results are illustrated ThisYarkovskyconeisbasicallyanenvelopearoundthecenter
with a pie chart in Fig. 2 (upper panel). As expected from our of the family, indicating the furthest that a family member can
selection criterion (objects with p < 0.1), the majority of the driftasafunctionofitssize.Objectsoutsidethisconearelikely
V
asteroids belong to primitive taxonomical classes (C-, B-, X-, familyinterlopers.Fig.3showstheposition,withrespectofthis
and T-). As mentioned in Section 2, and due to observational cone, of the asteroids studied in this work. Different colors are
constraints, 16 asteroids from the 101 observed did not fulfill associated to different spectral classes, as it is indicated in the
the selection criteria. Their taxonomical classification is shown legendofthefigure.Itisinterestingtonotethatmostoftheas-
Articlenumber,page4of19
DavidMorateetal.:VisiblespectroscopyofErigonecollisionalfamily
ablenumberofmainbeltprimitiveasteroidspresentanabsorp-
30
tion feature around 0.7 µm, attributed to charge transfer transi-
Erigones tionsinoxidizediron(Vilas&Gaffey1989;Vilas1994;Barucci
25 Polanas et al. 1998), which is indicative of, or associated with, aque-
s ous alteration in the surface of these objects (i.e. presence of
d
steroi20 hEyridgroanteed, tmheinepraarlesn).t Fboordnyasoiefrtheet afla.m(2il0y14st)udshieodweind tthhiastw(1o6r3k),
er of a15 porfe2s.e2n±tst0h.1is%pawrtiitchulraerspaebcstortopttihoencfeoanttuinreuautm0..7Toµmse,awrcithhfaordespaitdh
b absorptionfeatureamongourErigonefamilymembers’spectra
m
Nu10 wefollowtheproceduredescribedinCarvanoetal.(2003)with
someminoradjustments.Forthoseobjectsshowingthisfeature
wecharacterizeitscentralwavelengthpositionanddepth.
5
The first step was to compute the continuum of the absorp-
tion band by fitting a straight line tangent to the spectra at two
0 positions,0.54-0.56µmand0.86-0.88µm,whicharethelimits
-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9
Spectral slope S (%/0.1µm) ofthe0.7µmabsorptionband(greenlineinthetoppanelofFig.
5).Wetestedslightlydifferentrangestocomputethecontinuum,
Fig.4.Histogramshowingthedistributionforthevaluesofthespectral findingnosignificantchangesinourresults.Then,wefittedthe
slopefortheprimitiveasteroidsintheErigonefamily(red).Thedistri- spectruminthisintervalusingaforth-orderspline(redcurvein
butionoftheslopesoftheasteroidsofthePolanafamilyfromdeLeón toppanelofFig.5).
etal.(2015)isshowninblueasacomparison.
Thefinalstepwastoremovethecontinuumbydividingthe
spline fit by the straight line we previously obtained (bottom
panel of Fig. 5). In order to compute the depth and the central
teroidshavingnon-primitivetaxonomiesfalloutsidethefamily
wavelengthpositionoftheabsorptionbandandtheircorrespond-
boundaries,confirmingtheymostlikelyareinterlopers.
ing errors we run a Monte Carlo model with 1000 iterations,
In the following sections we will perform a more detailed
randomly removing 10 points from the spectrum in the range
analysis of the asteroids of the Erigone family with a primitive
from0.54to0.88µmateachiteration,thenrepeatingtheabove
taxonomical classification, i.e., C-, X-, B-, and T-types. A total
describedprocedure.Thebanddepthiscomputedasthediffer-
of90objectshavebeenanalyzed.
ence,in%,betweenareflectancevalueof1andthereflectance
value corresponding to the central wavelength position. The fi-
3.1. Spectralslopes nal values for the band depth and central wavelength are com-
putedasthemeanvaluesobtainedforthefullMonteCarlorun,
Giventhatprimitiveasteroidshavefeatureless,linearspectra,we
andtheerrorsarethecorresponding1σstandarddeviations.The
startedbycomputingthespectralslopeS(cid:48),asdefinedby(Luu&
criterion to decide whether an object showed an aqueous alter-
Jewitt1990),between0.55and0.90µm:
ationbandwastoruleoutthoseobjectspresentingbandswitha
(cid:32)dS/dλ(cid:33) depth smaller than 1%, or relative errors in the computation of
S(cid:48) = , thedepthlargerthan15%.Thedetectionthresholdof1%corre-
S
0.55 spondtothepeak-to-peakscatterinourspectra,whichseemsto
wheredS/dλistherateofchangeofthereflectivityintheafore- beabetterindicatorofthespectrumqualitythanthecalculated
mentionedwavelengthrange,andS isthereflectivityat0.55 signaltonoiseratio.
0.55
microns. To compute it, a linear least-squares fit between 0.55 We have to note that there are some cases in which the de-
and0.90µmhasbeenappliedtoeveryprimitiveasteroidspectra. tected bands have higher than expected depths. In those cases
Wenormalize7 theslopeat0.55µm.S(cid:48) ismeasuredinunitsof (in particular, that of asteroid 210564), large depth values arise
%/1000Å.Theresultingvaluesforthecomputedspectralslopes from the fact that the absorption band is quite broad, and the
areshowninTable5.Theslopeerrorstakeintoaccountboththe limitingregionsarenarrowandnotperfectlydefined.Inthespe-
1σ uncertainty of the linear fit and the variation of 0.6%/1000 cificcaseofasteroid210564,ifwechangethecontinuum-fitting
Å, attributable to the use of different solar analogue stars dur- upper limits, the band depth might vary from ∼ 9% to ∼ 5%.
This is due to the fact that when moving the upper limit be-
ing the night (see Section 2.2 for more details). Fig. 4 shows
low0.88microns,wemightfallalreadyinsidetheband,produc-
thedistributionofthecomputedslopesforthe90primitiveob-
ingheavybanddepthshiftswithsmallerrorbars.Thesespecial
jects of the Erigone collisional family (red). As a comparison
cases might reveal the limitations of our data (if we had infor-
weshowthedistributionofthevisiblespectraslopesoftheas-
mationover0.9µm,thebandlimitswouldbeclearer).
teroids of the Polana family (blue) from de León et al. (2015),
In Table 5, we indicate for each asteroid its taxonomical
which are compatible with a B-type parent. The two distribu-
tionsaresignificantlydifferent,withtheasteroidsoftheErigone classification, if it has or not an absorption band at 0.7 µm
(YES/NO),andthecenteranddepthoftheband,whenpresent.
familyshowing,ingeneral,redderspectralslopes.
Wefoundthat52ofthe90primitiveasteroidsstudiedpresentan
absorptionbandat0.7µm,andthatthisbandispresentregard-
3.2. Aqueousalteration less the specific primitive spectral class. Fig. 6 shows the pro-
portion of asteroids showing hydration band for each primitive
Severalstudies(Vilas1994;Fornasieretal.1999;Carvanoetal.
class.InthecaseofC-typeasteroids,almostallofthempresent
2003;Rivkin2012;Fornasieretal.2014)showthataconsider-
thehydrationfeature(∼ 88%).Thisproportionisprogressively
7 ThisisthecentralwavelengthoftheJohnsonV filter,whichisusu- smaller for B-types (∼ 36%), X-types (∼ 28.5%), and T-types
allyusedasnormalizationreference. (∼ 14%). This decreasing trend in our results is in agreement
Articlenumber,page5of19
A&Aproofs:manuscriptno.Morate_Erigones_LatestVersion
19
18
17
e16
d
u
t
ni15
g
a
m
14
e
t
olu13 ALL Erigones
bs X−type
A
12 C−type
B−type
11
T−type
10 L−type
Others
9

2.3 2.32 2.34 2.36 2.38 2.4 2.42
Semimajor axis (A.U.)
Fig.3.Absolutemagnitude(H)oftheasteroidsfromtheErigonefamilystudiedinthisworkasafunctionoftheirpropersemimajoraxis.The
Erigonefamily(atotalof1785objects)isshowningrey.Thecoloredcirclescorrespondtothedifferenttaxonomicalclassesfoundforoursample
of103members:C-types(red),X-types(black),B-types(green),T-types(yellow),andtherestofnon-primitiveclasses(S-types,L-types,and
V-types,inblue).Thesolidlinesrepresenttheboundariesofthefamily(socalled"Yarkovskycone").TheobjecthavingthesmallestvalueofH
andlocatedatthebottomofthisconeistheparentbodyofthefamily,asteroid(163)Erigone.
withpreviousstudiesoftherelativeincidenceofthisfeaturein 3.3. Comparisonwithspectraof(101955)Bennuand
asteroids distributed through the main belt. Vilas (1994) found (162173)Ryugu
anincidenceof47.7%inC-typeasteroidsand33%inB-types,
whileFornasieretal.(2014)foundanincidenceof50.7%inC- As we described in Section 1, the aim of the characteriza-
typesanda9.8%inB-types.TheproportionofC-typeasteroids tion of the Erigone primitive family is that, together with the
showingthe0.7µmabsorptionbandissignificantlylargerinthe Polana,ClarissaandSulamitisfamilies,andthelow-albedoand
caseoftheErigonefamily,withabout87%oftheasteroids. low-inclination background asteroids, they are the most likely
sources of the two NEAs that are the targets of OSIRIS-REx
and Hayabusa 2 sample-return missions: (101955) Bennu and
The presence of a primitive collisional family, with all the (162173)Ryugu.Spectroscopicandphotometricobservationsof
observedobjectslocatedbetween2.3and2.4AU,andwiththe theseasteroidssuggestthattheyarecomposedofprimitivemate-
majority of its members showing the 0.7 µm hydration band is rials,pointingtoanoriginintheaforementionedpopulations,re-
ingoodagreementwiththeresultspresentedbyFornasieretal. inforcedbytheresultsofdynamicalsimulations(Campinsetal.
(2014), where it is suggested that the aqueous alteration pro- 2010,2013;Bottkeetal.2015).
cesses dominate in primitive asteroids located between 2.3 and Spectral comparison might shed light upon the origins of
3.1 AU. Moreover, it is stated that the proportion of hydrated both asteroids and so, we have compared the available visible
primitive objects in the region where the Erigone family is lo- spectra of asteroids (101955) Bennu and (162173) Ryugu with
cated is 64%, in very good agreement with the proportion of thedataobtainedinthiswork.
hydratedobjectswehavefoundinoursample,thisis,57.7%(52 Inthecaseof(162173)Ryugu,thereareseveralreferencesin
outof90). theliteratureshowingvisiblespectraofthissmall(∼800m),low
albedo (p = 0.07) near-Earth asteroid. A first spectrum from
V
Binzel et al. (2001) shows an ultraviolet drop-off in reflectance
We found no significant correlations between the band short-wards 0.65 µm and provides a classification of Cg-type.
depthsorthebandcentersandthetaxonomicalclass,theaster- Two other visible spectra were presented in Vilas (2008), ob-
oidsorbitalparameters,thealbedo,orthesizeoftheobjects.A tainedonJulyandSeptember2007.Thesetwospectraweredif-
similarlackofcorrelationsisfoundbyCarvanoetal.(2003)and ferentfromeachother,andalsodifferentfromtheonebyBinzel
Fornasier et al. (2014) in their respective studies for asteroids etal.(2001),showingnoultravioletdrop-off.Thespectrumob-
through the whole main belt. Additionally, the mean values we tained in July showed an absorption band at 0.7 µm and a red
foundforboththebanddepth,2.9±1.5%,andthebandcenter spectralslope,whiletheoneobtainedinSeptember,withamuch
position,7053±160Å,areinexcellentagreementwiththoseob- higher signal-to-noise, presented a neutral slope and showed a
tainedbyFornasieretal.(2014):2.8±1.2%forthebanddepth marginal,veryshallowabsorptioncenterednear0.6µmAccord-
and6914±148Åforthebandcenter.Theseresultssuggestthat ingtoVilas(2008),thesedifferencessuggestthatthesurfaceof
the values obtained for the parameters used to characterize the the asteroid covers the conjunction of two different geological
0.7 µm hydration band are independent of the location of the units.Acomparisonbetweenthesethreespectracanbeseenin
asteroidsinthemainbelt. theleftpanelofFig.3fromCampinsetal.(2013).Additionalro-
Articlenumber,page6of19
DavidMorateetal.:VisiblespectroscopyofErigonecollisionalfamily
1.08 45
88.6%
1.06 40
1.04 35
1.02 ds30
oi
er
1 ast25
of
0.98 er20
b
m
u
e0.96 N15
c
n
a 0.5 0.6 0.7 0.8 0.9 10 28.6%
t
c Wavelength (µm)
e 5 36.4%
fl
14.3%
e
r
0
ve1.08 C−type X−type B−type T−type
i
t
la1.06 Bandcenter: 0.7018µm Fig. 6. Percentage of asteroids showing the 0.7 µm absorption band
e (darkblue)foreachprimitivetaxonomicclass(green)
R Depth: 1.64 %
1.04 .
1.02
family (see section 3.2). This result, together with the absence
1 ofthe0.7µmabsorptionbandinallthesubsequentvisiblespec-
traofRyuguobtainedbyotherauthors,suggestthattheabsorp-
0.98 tionbandobservedintheJuly2007spectrumfromVilas(2008)
mightbeduetosomeartifact.
0.96 Regarding (101955) Bennu, a primitive (p = 0.043) and
V
rathersmall(∼500m)near-Earthasteroid,theonlyavailablevis-
0.5 0.6 0.7 0.8 0.9
ibledataarefoundinClarketal.(2011)andHergenrotheretal.
Wavelength (µm)
(2013). According to these studies, Bennu is classified as a B-
typeasteroidintheBus&Binzel(2002)taxonomy.Inaddition,
Fig. 5. Example figure of the process followed to compute the cen-
inBinzeletal.(2015),spectraobtainedintheNIRrangeshow
tralwavelengthpositionandthedepthofthe0.7µmabsorptionband
spectralvariability,pointingtowardsaC-typeclassaccordingto
on the spectra of asteroid (72384). Top panel shows the straight line
DeMeoetal.(2009).AsithasbeenshowninClarketal.(2010)
(green)usedtoremovethecontinuumfromthefittedabsorptionband
and de León et al. (2012), asteroids classified as B-types in the
(redcurve).Bottompanelshowstheresultofthiscontinuumremoval
andtheiterativeprocedureusingaMonteCarlomodeltocomputethe visiblecanpresentconsiderableslopevariationintheNIR,from
mentionedbandparameters(seetextfordetails). negative, blue slopes, to positive, redder ones. The lower panel
ofFig.7showsthevisiblespectrumofBennu(black)compared
tothemeanvisiblespectraoftheasteroidsoftheErigonefamily
tationallyresolvedvisiblespectraof(162173)Ryuguwerepre- classifiedasC-types(andsubclasses),inred,andthoseclassified
sented by Lazzaro et al. (2013), Moskovitz et al. (2013), and asB-typesinblue,observedinthiswork.Fromavisualinspec-
Sugitaetal.(2013),allofthemcompatiblewithaC-typeclassifi- tion, the visible spectrum of Bennu seems to marginally show
cationandshowingnoabsorptionfeatureat0.7µmandaspectral the presence of the hydration feature 0.7 µm. We followed the
slopesimilartotheSeptember2007spectrumfromVilas(2008), same approach as in the previous section in order to study the
making very unlikely the suggestion of two different surfaces. presenceofapossibleaqueousalterationband.Ourcalculations
Therefore,beingtheonewiththehighestsignal-to-noise,wese- yielded the presence of a band centered at 7486±76 Å, with a
lected it to perform our spectral comparison. The upper panel depthof0.96±0.02%.Thisdepthisslightlybelowthethreshold
of Fig. 7 shows the September 2007 (Vilas 2008) spectrum of (1%)establishedforapositivedetection,andthewavelengthpo-
Ryugucomparedtothemeanvisiblespectraoftheasteroidsof sitionofthebandcenterissignificantlydifferentfromthemean
theErigonefamilyclassifiedasC-types(andsubclasses),inred, valuefoundforthefamily(7053±160Å).Therefore,weruleout
andthoseclassifiedasB-typesinblue.Thevisiblespectrumof thepresenceofthishydrationbandinthespectrumofBennu.
RyuguisingoodagreementwiththemeanspectrumofC-type
asteroidsintheErigonefamily,evenifnotshowingthe0.7µm
4. Discussion
absorptionfeature.
Even if the signal-to-noise is quite poor, we have done the Nopreviousspectroscopicstudieshadbeenperformeduntilnow
exerciseofcomputingboththewavelengthcentralpositionand ontheErigoneprimitivecollisionalfamily,withonlytwoaster-
thebanddepthofthe0.7µmabsorptionbandpresentintheJuly oidsalreadyclassifiedusingvisiblespectroscopy:(163)Erigone,
2007spectrumofRyugu(Vilas2008).Ourcalculationsyielded and(571)Dulcinea.Itisclearlyshownthattheasteroidsstudied
adepthof11.7±1.3%,andabandcenterof6870±55Å.Theab- in this work are spectroscopically consistent with the hypothe-
sorptionbanddepthandbandcenterareverydifferentfromthe sisofacommonparentbody,andthatthisparentbodyis(163)
meanvaluescomputedbyFornasieretal.(2014),andalsofrom Erigone,classifiedasaC-typeasteroid.Moreover,(163)Erigone
theonescomputedinthisworkfortheasteroidsintheErigone shares the particular spectroscopic feature at 0.7 µm with most
Articlenumber,page7of19
A&Aproofs:manuscriptno.Morate_Erigones_LatestVersion
1.3
Ryugu (Vilas, September 07) C−type
1.15 Erigone mean C−type B−type
1.2
nce1.1 Erigone mean B−type ance XT−−ttyyppee
a ct1.1
t e
c1.05 fl
e e
fl r
ere 1 ative 1
tiv0.95 Rel0.9
a
el0.9
R
0.8
0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
0.85 Wavelength(microns)
0.08. 45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 Fig.8.ComputedmeanspectraoftheasteroidsintheErigonefamily
Wavelength (µm) classifiedasC-types(red),B-types(blue),X-types(black),andT-types
(green). The mean spectrum for each class is plotted as a thick line,
while the ±1σ of the mean is shown with dashed lines. Note that the
differentmeanspectraarewelldifferentiatedfromoneanother.

Bennu (Clark et al. 2011)
1.15 Erigone mean B−type b) ThemajorityoftheprimitiveasteroidsintheErigonefamily
nce1.1 Erigone mean C−type (52outof90)showevidenceofaqueousalteredmineralson
a their surfaces. We conducted the same analysis as that de-
t
c1.05 scribed in Section 3.2 for the data in de León et al. (2015),
e
fl and we found that, according to our criterion, only one ob-
re 1 ject in the Polana family (asteroid 29626), showed the 0.7
e
tiv0.95 µmabsorptionfeature,havingabanddepthof1.15±0.11%
ela0.9 and a band center of 7300 ± 211Å. This difference in the
R two families is well explained by their two parent bodies.
0.85 Asteroid (142) Polana shows no signs at all of the hydra-
tion feature at 0.7 µm. On the other hand, (163) Erigone,
0.8
0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 the parent body of the Erigone family, shows evidence of
Wavelength (µm)
aqueous alteration. Therefore, the presence of the hydra-
tion feature in the spectra of most of its family members is
Fig.7.ComparisonbetweentheSeptember2007(Vilas2008)visible somewhat expected. An interesting explanation for the dif-
spectrumofRyuguinthetoppanelandthevisiblespectrumof(101955)
ference in hydration between the two families is presented
BennufromClarketal.(2011)inthebottompanel,withthemeanspec-
by Matsuoka et al. (2015). They propose that space weath-
traoftheC-type(red)andB-type(blue)asteroidsintheErigonefamily.
eringeffectsonC-typeasteroidstendtoshallowthe0.7µm
Standarddeviationofthemean(±1σ)isshownwithverticallines.
absorption feature. Space weathering processes might have
removed the aqueous alteration band in the Polana family,
of the family members. Another primitive collisional family in since the Polana family is older than Erigone: according to
theinnerbelt,thePolanafamily,that,togetherwithErigone,Su- Bottke et al. (2015), Erigone is 130 ± 30 Myr old, while
lamitis,andClarissa,arethefourprimitivefamiliesintheinner Polana(referredtoasNewPolanaintheirpaper)andEulalia
belt, has been recently studied by several authors (Walsh et al. are1400±150Myrold,and830+370Myroldrespectively.
−100
2013; Milani et al. 2014; Dykhuis & Greenberg 2015; de León
RegardingthelikelihoodoftheErigonefamilyofbeingthe
et al. 2015; Pinilla-Alonso et al. 2015). Comparing our results
source for the asteroids (101955) Bennu and (162173) Ryugu,
on the Erigone family with those obtained from de León et al.
wehavecomparedvisiblespectraofbothobjectswiththemean
(2015)forthePolanafamily,weobservetwomaindifferences:
spectraoftheprimitiveasteroidsinthefamily.Inoursample,we
havefoundapproximately56%ofnon-interlopersshowingevi-
a) The Erigone family presents a different distribution of
dence of aqueous alteration. According to Bottke et al. (2015),
taxonomicalclassesfromthatofthePolanafamily,referred
there is little to no chance for the smaller families (Erigone,
to as the Polana-Eulalia complex in de León et al. (2015).
ClarissaandSulamitis)tobethesourceofbothasteroids:
In our sample, we found 44 C-type objects, 28 X-types,
11 B-types and 7 T-types, plus 13 interlopers (S-types - (101955) Bennu: Bennu is classified as a B-type asteroid
and L-types). The mean spectra for each class are clearly in the visible wavelength range, according to Clark et al.
differentiated, specially in the case of the X and T classes (2011) and Hergenrother et al. (2013). Even if the fraction
(seeFig.8).Onthecontrary,inthePolanafamilywefound of B-type asteroids in the Erigone family is small, and the
mainlyC-types(51%)andB-types(42%),withfewX-types probability of Bennu coming from the Erigone family is
(5%)andonlyoneS-type,withthemeanspectrafortheC-, small(Bottkeetal.2015),thespectralcomparisonbetween
B-, and X-types presenting similar values (de León et al. thespectrumofBennuandthemeanspectrumoftheB-type
2015). In addition, the slope distribution for the objects in asteroidsiscompatiblewithapossibleoriginintheErigone
the Erigone family is fairly redder than that of the Polana family.
family, as can be seen in Fig. 4, mainly due to the larger
fractionofX-typesfoundintheformer. - (162173)Ryugu:Intheirpaper,Vilas(2008)reportthepres-
enceofanaqueousalterationbandaround0.7µminoneof
Articlenumber,page8of19
DavidMorateetal.:VisiblespectroscopyofErigonecollisionalfamily
thethreevisiblespectraobtainedforRyugu(July2007,see Acknowledgements. DMgratefullyacknowledgestheSpanishMinistryofEcon-
their Fig. 4). Following the same approach we used for our omyandCompetitiveness(MINECO)forthefinancialsupportreceivedinthe
aqueousalterationstudy,thedepthofthebandwecomputed form of a Severo-Ochoa PhD fellowship, within the Severo-Ochoa Interna-
tional PhD Program. DM, JdL, JL, and VL acknowledge support from the
intheJuly2007spectrumfromVilas(2008),11.7±1.3%,is
projectAYA2012-39115-C03-03andESP2013-47816-C4-2-P(MINECO).JdL
far from the medium depth of the aqueous altered asteroids acknowledgessupportfromtheInsitutodeAstrofísicadeCanarias.HCacknowl-
inFornasieretal.(2014)(2.8±1.2%)andalsointhiswork edgessupportfromNASA’sNear-EarthObjectObservationsprogramandfrom
(2.9 ± 1.5%). However, from the spectral comparison, the theCenterforLunarandAsteroidSurfaceSciencefundedbyNASA’sSSERVI
programattheUniversityofCentralFlorida.Theauthorsgratefullyacknowledge
possibility of JU originating in the Erigone family should
3 thereviwer,SoniaFornasier,forhercommentsandsuggestions.Theresultsob-
not be discarded. Ryugu is a C-type asteroid, and the most tainedinthispaperarebasedonobservationsmadewiththeGranTelescopio
abundant spectral classes in the Erigone family are C-type Canarias(GTC),installedintheSpanishObservatoriodelRoquedelosMucha-
asteroids and subclasses (Cb,Cg,Ch and Cgh). This would chosoftheInstitutodeAstrofísicadeCanarias,intheislandofLaPalma.
point to a possible origin of Ryugu in the Erigone family,
evenifallthevisiblespectraofthisasteroidobtainedbydif-
ferentauthorsshowno0.7µmabsorptionband. References
SincethePolanafamily,suggestedinBottkeetal.(2015)as Barucci,M.A.,Doressoundiram,A.,Fulchignoni,M.,etal.1998,Icarus,132,
themostprobablesourceforbothNEAs,doesnotshowsignsof 388
Bevington,P.R.&Robinson,D.K.1992,Datareductionanderroranalysisfor
aqueousalteredasteroids(Walshetal.2013;Dykhuis&Green-
thephysicalsciences
berg 2015; de León et al. 2015), the Erigone family cannot be Binzel,R.P.,DeMeo,F.E.,Burt,B.J.,etal.2015,Icarus,256,22
discardedasapossiblesourceforbothNEAs.Furtherobserva- Binzel,R.P.,Harris,A.W.,Bus,S.J.,&Burbine,T.H.2001,Icarus,151,139
Binzel,R.P.&Xu,S.1993,Science,260,186
tions of (101955) Bennu and (162173) Ryugu should be done
Bottke,W.F.,Morbidelli,A.,Jedicke,R.,etal.2002,Icarus,156,399
inordertocheckwhetherthe3µmabsorptionfeatureispresent Bottke,W.F.,Vokrouhlický,D.,Walsh,K.J.,etal.2015,Icarus,247,191
ornot,definitelyindicatingevidenceofaqueousalteration,and Bus,S.J.1999,PhDthesis,MASSACHUSETTSINSTITUTEOFTECHNOL-
OGY
confirmingorrulingoutthechancesforErigonebeingthesource
Bus,S.J.&Binzel,R.P.2002,Icarus,158,146
oftheseobjects. Campins,H.,deLeón,J.,Morbidelli,A.,etal.2013,AJ,146,26
Campins,H.,Morbidelli,A.,Tsiganis,K.,etal.2010,ApJ,721,L53
Carvano,J.M.,Mothé-Diniz,T.,&Lazzaro,D.2003,Icarus,161,356
Cepa,J.2010,inHighlightsofSpanishAstrophysicsV,ed.J.M.Diego,L.J.
5. Conclusions
Goicoechea,J.I.González-Serrano,&J.Gorgas,15
Cepa,J.,Aguiar,M.,Escalera,V.G.,etal.2000,inSocietyofPhoto-Optical
We studied a total of 103 visible spectra of asteroids in the
InstrumentationEngineers(SPIE)ConferenceSeries,Vol.4008,Opticaland
Erigone primitive family. We observed 101 of these asteroids IRTelescopeInstrumentationandDetectors,ed.M.Iye&A.F.Moorwood,
with the 10.4m Gran Telescopio Canarias. These objects had 623–631
never been observed before via spectroscopy. We added the Clark,B.E.,Binzel,R.P.,Howell,E.S.,etal.2011,Icarus,216,462
Clark,B.E.,Ziffer,J.,Nesvorny,D.,etal.2010,JournalofGeophysicalRe-
visible spectra of (163) Erigone and (571) Dulcinea from the
search(Planets),115,6005
SMASSdatabaseforcompleteness. deLeón,J.,Pinilla-Alonso,N.,Campins,H.,Licandro,J.,&Marzo,G.A.2012,
We have seen that the slope distribution and the taxonomic Icarus,218,196
deLeón,J.,Pinilla-Alonso,N.,Delbo,M.,etal.2015,Icarus,inpress
classification of the asteroids studied are in agreement with the
DeMeo,F.E.,Binzel,R.P.,Slivan,S.M.,&Bus,S.J.2009,Icarus,202,160
assumption of a primitive family. Most of the observed objects DeMeo,F.E.&Carry,B.2013,Icarus,226,723
are C-types, highly consistent with the classification of (163) Dykhuis,M.J.&Greenberg,R.2015,Icarus,252,199
Fornasier,S.,Lantz,C.,Barucci,M.A.,&Lazzarin,M.2014,Icarus,233,163
Erigone, supporting its status as the parent body of the family.
Fornasier,S.,Lazzarin,M.,Barbieri,C.,&Barucci,M.A.1999,A&AS,135,
Intermsoftaxonomicaldistribution,42%oftheasteroidsinour
65
surveyareclassifiedasC-typeobjects,28%asX-types,11%as Hergenrother,C.W.,Nolan,M.C.,Binzel,R.P.,etal.2013,Icarus,226,663
B-types,7%asT-types,and13%oftheobjectsasnon-primitive Hirayama,K.1918,AJ,31,185
Howell,E.S.,Rivkin,A.S.,Vilas,F.,etal.2011,637
types, most likely interlopers (S-types and subclasses, and L-
Landolt,A.U.1992,AJ,104,340
types). Lauretta,D.S.,Drake,M.J.,Benzel,R.P.,etal.2010,MeteoriticsandPlanetary
In addition, the study on aqueous alteration performed on ScienceSupplement,73,5153
Lazzaro,D.,Barucci,M.A.,Perna,D.,etal.2013,A&A,549,L2
thegroupofprimitiveobjectsshowsahighnumberofhydrated
Luu,J.X.&Jewitt,D.C.1990,AJ,99,1985
asteroids. The C-type class is the one which shows the largest Masiero,J.R.,Mainzer,A.K.,Grav,T.,etal.2011,ApJ,741,68
numberofhydratedasteroids,namely∼86%.Theotherprimitive Matsuoka,M.,Nakamura,T.,Kimura,Y.,etal.2015,Icarus,254,135
Milani,A.,Cellino,A.,Kneževic´,Z.,etal.2014,Icarus,239,46
classes, also showing hydration band, present a much smaller
Moskovitz,N.A.,Abe,S.,Pan,K.-S.,etal.2013,Icarus,224,24
fraction: B-type objects, ∼36%, X-type objects, ∼28% and T- Nesvorny,D.2012,NASAPlanetaryDataSystem,189
type,∼ 14%.Thisdistributionofthehydrationasafunctionof Pinilla-Alonso,N.,deLeón,J.,Walsh,K.J.,etal.2015,Icarus,submitted
Popescu,M.,Birlan,M.,&Nedelcu,D.A.2012,A&A,544,A130
thetaxonomicalclassisinagreementwiththeoneinFornasier
Rivkin,A.S.2012,Icarus,221,744
etal.(2014)forasteroidsinthemainbelt. Sugita, S., Kuroda, D., Kameda, S., et al. 2013, in Lunar and Planetary Inst.
Based on the spectral comparison only, and as in the case TechnicalReport,Vol.44,LunarandPlanetaryScienceConference,2591
Tsuda,Y.,Yoshikawa,M.,Abe,M.,Minamino,H.,&Nakazawa,S.2013,Acta
of the Polana family, studied in de León et al. (2015), we can-
Astronautica,91,356
notdiscardthepossibilityofErigonebeingthesourcefamilyfor Vilas,F.1994,Icarus,111,456
nearEarthasteroids(101955)Bennuand(162173)Ryugu.The Vilas,F.2008,AJ,135,1101
spectralclassespresentinthefamilyarecompatiblewiththetax- Vilas,F.&Gaffey,M.J.1989,Science,246,790
Vokrouhlický,D.,Brož,M.,Bottke,W.F.,Nesvorný,D.,&Morbidelli,A.2006,
onomic classification of both asteroids. Future research should
Icarus,182,118
includefurtherspectroscopicstudy,bothinthevisibleandnear- Walsh,K.J.,Delbó,M.,Bottke,W.F.,Vokrouhlický,D.,&Lauretta,D.S.2013,
infraredregions,oftheotherprimitivefamiliesintheinnerbelt, Icarus,225,283
Zappala, V. & Cellino, A. 1994, in IAU Symposium, Vol. 160, Asteroids,
suchasSulamitisandClarissa,inordertocompletelyrulethem
Comets,Meteors1993,ed.A.Milani,M.diMartino,&A.Cellino,395
out as the possible sources of asteroids (101955) Bennu and Zappala,V.,Cellino,A.,Farinella,P.,&Knezevic,Z.1990,AJ,100,2030
(162173)Ryugu.
Articlenumber,page9of19
A&Aproofs:manuscriptno.Morate_Erigones_LatestVersion
Table3.Observationalcircumstancesoftheasteroidspresentedinthispaper.CheckTable1fortheIDnumberofeachsolaranaloguestar.
Object Date UTstart Airmass Exposuretime(s) SAs Seeing(")
10992 2014-08-16 05:46 1.117 3x200 5 0.9
11856 2015-01-12 01:10 1.482 3x250 2 2.0
18759 2014-12-17 22:35 1.149 3x250 1 2.0
19415 2014-09-18 22:23 1.310 3x300 4,6 1.0
20992 2014-12-17 03:13 1.248 3x250 2 1.9
23397 2015-01-14 03:51 1.292 3x300 1,3 1.5
24037 2014-12-18 01:27 1.305 2x200 1 2.7
25381 2014-12-18 00:22 1.352 3x500 1 2.0
37437 2014-12-17 22:04 1.335 3x500 1 2.5
38106 2014-10-14 03:43 1.033 3x400 1,2 1.0
38173 2014-12-15 23:46 1.554 3x300 1,2 2.0
38661 2014-09-13 03:04 1.257 3x200 1,6 0.9
39694 2014-09-19 00:18 1.198 3x400 4,6 0.9
39895 2014-09-15 03:38 1.507 3x300 1,6 0.8
42155 2014-09-13 03:36 1.095 3x200 1,6 0.8
42552 2014-09-15 02:50 1.513 3x250 1,6 0.9
44766 2014-12-17 03:58 1.039 3x200 2 1.3
44942 2015-01-14 03:29 1.496 3x200 1,3 1.5
45357 2015-01-14 04:22 1.416 3x300 1,3 1.5
49731 2015-01-14 04:46 1.920 3x200 1,3 1.9
49859 2015-05-04 23:37 1.300 3x180 3,7 1.5
50068 2015-05-05 00:10 1.131 3x180 3,7 1.5
52870 2015-01-19 05:57 1.124 3x500 3 2.0
52891 2015-01-15 03:53 1.197 3x500 3 1.2
56349 2015-01-15 04:29 1.163 3x500 3 1.1
65354 2014-12-13 01:05 1.048 3x500 2 1.5
66309 2014-10-14 02:34 1.456 3x250 1,2 1.4
66325 2014-10-13 04:20 1.847 3x300 1,2 1.5
66403 2014-09-14 01:35 1.110 3x200 1,6 0.9
67891 2014-09-15 02:09 1.684 3x200 1,6 1.0
67918 2014-09-13 02:39 1.214 3x200 1,6 0.9
67940 2014-09-15 03:14 1.709 3x200 1,6 0.9
68114 2014-09-17 01:22 1.142 3x250 6 1.0
68685 2014-10-12 00:30 1.406 4x250 6 0.9
69266 2014-12-18 01:52 1.218 3x250 1 3.0
69706 2014-09-15 04:56 1.023 3x500 1,6 0.8
70312 2014-09-18 03:01 1.088 3x300 1,4 1.3
70361 2014-12-17 03:38 1.180 3x200 2 1.7
70427 2015-01-14 05:06 1.606 3x250 1,3 1.7
70511 2014-09-15 04:20 1.010 3x300 1,6 0.7
71932 2014-10-13 02:22 1.176 3x200 1,2 1.2
72047 2014-11-25 22:13 1.027 3x350 1 1.3
72143 2014-09-19 23:39 1.414 3x250 1,6 1.3
72230 2014-09-19 01:40 1.308 2x300 4,6 0.9
72292 2014-12-18 00:57 1.276 3x450 1 2.0
72308 2014-08-16 04:04 1.304 3x250 5 0.7
72384 2014-11-25 21:49 1.201 3x250 1 1.4
72941 2014-10-13 03:57 1.997 3x300 1,2 1.6
73860 2014-09-14 03:27 1.177 3x250 1,6 0.9
74755 2014-11-25 22:53 1.017 3x300 1 1.2
74962 2014-12-02 04:21 1.136 3x500 2 1.4
75089 2015-01-19 04:48 1.096 3x350 3 2.5
76922 2014-10-08 00:02 1.292 6x250 1,6 0.7
77421 2014-10-10 05:44 1.040 2x500 1,2 0.8
78069 2014-12-02 05:29 1.377 3x500 2 1.7
78826 2014-09-13 04:53 1.114 3x200 1,6 0.8
78889 2014-10-12 02:05 2.199 4x150 6 1.4
79044 2014-10-10 05:05 1.078 3x400 1,2 0.7
85727 2015-05-04 23:47 1.176 3x250 3,7 0.9
96405 2014-09-17 22:15 1.294 5x400 1,4 1.3
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