19 93MNRAS.264. . .93A 91 123 2 3 17 16 x 4 5 Mon. Not.R.Astron.Soc.264,93-105(1993) The Tauridmeteoroidcomplexcontainsobjectsatallsize intheTauridComplex the calendaryearandindaytimemid-year;seee.g. visual meteorsinwell-knownbroadshowers(atnightlate fireballs (<10g)andtheTunguskafireball~of example aswarmofmeteoroidswithindividualmassesupto detections verylikelyassociatedwiththeTaurids,for ranges -submicrondust(e.g.Roosen,Berg&Farlow1973; Accepted 1993February18.Received16;inoriginalform1992November13 D. J.Asher,S.V.M.ChibeandI.Steel Stohl &Porubcan1990).Athighermassestherehavebeen Singer &Stanley1980),andparticlesthatproduceradar Anglo-Australian Observatory,PrivateBag,Coonabarabran,NSW2357,Australia DepartmentofPhysicsandMathematicalPhysics,UniversityAdelaide,SA5001,Australia tion history,theputativeoriginalgiantcomethasalsogiven most ofthepresent-dayzodiacalcomplex(Whipple1967; has) longbeenregardedastheparentbodyandsourceof is CometP/Encke(~10g),whichhas(orwhoseprogenitor rise tokm-sized(~10g,say)asteroids(Clube&Napier the intermediateevolutionarystagebetweenmore ingly demonstratedbythehugesporadicmeteoroidstream Kresák 1980).ThesizeoftheTCisperhapsmostconvinc- 1 INTRODUCTION DepartmentofPhysics,UniversityOxford,KebleRoad,Oxford0X13RH 1978; Oberst&Nakamura1987,1991),manyordinary surrounding theTauridstream(Stohl1986)andrepresenting 1908 June30(Kresák1978;Chyba,Thomas&Zahnle meteor observationsandused meteoroidorbitalelement demonstrated thestructureofTCasdelineatedby structured Tauridstreamandthezodiacalbackground. 1993). Itisnotsurprisingthat,duringitsviolentfragmenta- past ~10yr,atime-scaleconceivably supportedbyinde- distributions tomodeltheevolution ofthecomplexover 1984). ThelargestknownobjectintheTauridComplex(TC) pendent (non-dynamical)evidence bymeansoftheproposed preferred modelwasonewhere largefragmentssplitfrom association oftheFarmington meteoritewiththeTC.Our the coreobject nearperihelion,thesethenbecoming second- ~ 10gincidentontheMoonin1975June(Dormanetal. In apreviouspaper(Steel,Asher&Clube1991)we © Royal Astronomical Society • Provided by the NASA Astrophysics Data System ABSTRACT We showthatastatisticallysignificantnumberofEarth-crossingasteroidsarepart the TauridComplexofinterplanetaryobjects.Wealsoidentifyanothergroupwhich these twocomplexes,whichmayhaveacommonorigin. Key words::individual:P/Encke-TauridComplex evolution thatsuchasteroidsundergoandconsidertheimplicationsforhistoryof appears alignedwith(2212)Hephaistos.Inaddition,wedescribethekindoforbital progenitor -meteoroidsminorplanets. been demonstratedforradarmeteorsbyOlsson-Steel intention hereistoextendthisworkbyinvestigatingEarth- phic disruptiononcolhsionsintheasteroidbelt.The the pictureofbreakuplargeparentbodiesininner crossing asteroids,whicharelikelytohavearoleassecond- asteroids witheachotherandmeteoroidstreams, Stohl &Yana(1992).Otherassociationsofnear- (1988), andforprecisephotographicmeteorsbyPorubcan, asteroids (asopposedtosimplywiththeTCasawhole)has ary parentobjects(Stohl&Porubcan1992). ary parentbodieswhichundergofurthersplittingorcatastro- also withoneortwoJupiter-familycomets,furthersupport The linkagebetweenfourobservedmeteorshowersand Solar system(Obrubov1991). Apollo asteroidsdiscoveredisincreasingrapidlythrough type asteroidswerelikewiseassociated.VariousnewTC COMPLEX (Olsson-Steel 1987a;Steel1992).However,thenumberof associations havebeensuggestedinthepastfewyears since Clube&Napier(1984)proposedthatsomeApollo- P/Enckeiswellknown,anditnowalmostadecade 2 MEMBERSHIPOFTHETAURID demonstrating theexistence of thekm-sizedasteroidcom- repeatedly toimprovethestatisticalsignificanceofresults asteroids inanever-expandingdataset,andthechance search programmes,givingusthechancetolookforTC ponent oftheTC.Apreliminary versionofteststheform (1993). discussed herehasbeengiven byAsher,Clube&Steel ences between thedefinitions.Steeletal.(1991) usedthe defined a‘D-criterion’oforbital similarity,withslightdiffer- The associationofmeteoroidswithsomeindividualTC Authors, followingSouthworth &Hawkins(1963),have 19 93MNRAS.264. . .93A 3 2 +e 2=0102 within 50°of140°;theinclinationgivenisobserved Marsden 1991). value ofa<2aubeingconceivable(Marsden,private the valueofitssemimajoraxisishighlyuncertain,even a value, notadjustedforlong-termperturbations).Appended ‘aligned withTaurids’ifthelongitudeofperihelionor is determined towithin2per cent(Scotti,Rabinowitz& Despite 1991BAbeingone of thesmallestasteroidsever et al.1984andMcFadden, Gaffrey&McCord1984]. activity inthecaseof(2201)Oljato waspresentedbyRussell known intheTC[thoughtentative evidenceforcometary communication), andCometP/Encke,theonlyactivecomet asteroids withthesmallestvaluesofD(listedincolumn asteroids completeto1992March.Table1showsthe25 formalism (qinsteadof0)isthatformeteoroidstheperi- Steel 1987a)butwhichisomittedfromthisanalysisbecause are 5025P-L,whichislikelytobeaTCmember(Olsson- full weighttheuncertaintyinmeteoroidvelocity;follow- observed, at5-10m,itsorbit isaccuratelyknown,witha helion distanceqisgenerallymoreaccuratelydetermined following definitionforTauridmeteoroids: with a=2.1auande,iasabove.TheD-criterionis than atodemonstratetheexistenceofTCasteroids.On ing this,Asher(1991)usedaD-criterioninvolvingqrather than thesemimajoraxisa,lattertendingtoreflectwith with q=0.375an,e0.82andi*!4°.Thereasonforthis ing changeinDislessthan0.01nearlyallcases. much lesssignificantwithregardtotheD-criterionthan meteor tobeproduced(seeSteeletal.1991).Thereare that theparticleorbitmustintersectEarth’sfora TC ,becauseiisconstrainedtobelowbythefact Jovian perturbations,andthereforeaconventionallongitude between twoorbits,andtheresultsarebroadlysame essentially anempiricalmethodofdefiningthedifference other hand,sinceaisknownaccuratelyforasteroids,inthis due tovaryinge;hereweincludeit,andfindthattheresult- corresponding variationsineccentricitye,buttheseare perturbation theory(computationaldetailsinAsher1991)to time-scales oforder10yr,weuseBrouwer’s(1947)secular term intheD-criterionwouldhavetoolargeacontribution whilst appropriateformany(narrow)streams,theTaurids D-(,thefirst more tractablethanmakingassumptionsaboutthea,eand/ discovery ofselectioneffects),itappearsreasonableinorder one ormorecommonprogenitors)forgroupings.Though the secondargumentofperihehon.Weassumethator these termsbeingthelongitudeofascendingnodeand {a, e,i)totheTC,turnoutbealignedwithTCon on thebasisofsimilarityorbitalsize,shapeandinclination the perihehondistance{q^issmallerthanusualamongst of cometaryratherthantypicalApollo-asteroidalorbits,and high TCvalueoftheeccentricity(ejismorerepresentative distributions. Havingwrittenthat,itisnoteworthythatthe TC, basedonthecoreofTauridmeteoroidstream/ of thebulkApolloasteroidorbits. equation (2)arenoteworthyinthemselvesasbeingatypical discovered Apolloorbits;thatis,orbitsselectedusing Taurid Complexmeteoroids;DistheD-criteriondefinedin Table 1.Asteroidswithorbitalelements{a,e,i)similartothe the first15,10of20and1125asteroids meteor showers,is140°±40°(seeSteeletal.1991).InTable equation (2). were uniformlydistributedfrom0°to360°).Applyingalimit is thusoutsidethis100°-180°range;butseelaterforthe and aprobabilityofoccurrencebychance0.030(3per complex, wewouldhavefiveoutofthefirstnineasteroids restrict thestudytocoreofmeteoroidstream (equation 1here)inSteeletal.(1991),whereweaimedto of D=0.15,aswedidwithour^-containingD-criterion (probabilities ofoccurringwithinthatspecificbandinoT'ifot' reason foritsinclusioninthefirstcolumnofTable1).These are withinthisrange.(Notethat1991BAhasar=189°and abilities 0.309,0.050,0.008,0.006and0.013respectively longitude alignmentswouldhappenbychancewithprob- 1 weseethattwoofthefirstfive,five10,eight Aligned withTaurids Next, weconsiderwhethertheseasteroids,selectedonly A reasonablerangeofartotakeascorrespondingtothe 1937 UBHermes (4341) Poseidon (4197) 1982TA (5143) 1991VL (2201) Oljato (4183) Cuno 1988 VP4 1991 TB2 5025 P-L P/Encke 1991 GO 1990 HA 1991 BA 1984 KB (2212) Hephaistos (4179) Toutatis (4486) Mithra (2101) Adonis 1991 CB1 1990 TGI 6344 P-L 1991 XA 1983 VA 1991 AQ 1991 EE 1983 LC 1990 SM Others 4.20 2.22 2.27 2.62 2.51 2.25 2.63 2.61 2.58 2.26 2.48 2.20 2.24 2.30 2.18 2.16 2.40 1.64 1.69 1.98 2.22 2.16 2.16 1.96 1.84 1.83 1.87 0.62 0.34 0.85 0.44 0.90 0.98 0.57 0.94 0.64 0.64 0.62 0.90 0.64 0.84 0.62 0.77 0.71 0.81 0.69 0.79 0.69 0.72 0.64 0.66 0.79 0.65 0.76 0.69 0.74 0.66 0.59 0.68 0.71 0.68 0.52 0.77 0.42 0.77 0.63 0.71 0.49 0.78 0.39 0.84 0.44 0.76 0.52 0.76 0.36 0.84 0.50 0.77 12 0.04 16 0.24 10 0.22 16 0.21 10 0.19 12 0.19 12 0.16 12 0.14 12 0.12 12 0.06 0 0.23 4 0.20 6 — 5 0.26 6 0.25 5 0.25 2 0.22 7 0.19 9 0.18 3 0.17 2 0.16 9 0.13 3 0.12 9 0.10 5 0.07 3 0.05 1 0.10 344 304 284 238 146 126 250 243 160 132 170 236 222 113 138 123 189 129 177 172 132 146 42 26 57 89 32 19 93MNRAS.264. . .93A o o cent). Thusitappearsthat,ataconfidencelevel>95per cent, asubsetofthediscoveredEarth-crossingasteroidsare 18 predict anincreaseinboththeconfidenceleveland aligned withtheTauridmeteoroids,andweconfidently ies arediscovered;indeed,wemightexpecttheproportionof absolute numberofalignedasteroidsasmorekm-sizedbod- but wenotethat,ratherthanbeinginacompletelyrandom minimum ofthefiveprobabilities(0.006)listedin previous paragraphandtoquoteitasthesignificancelevel, order, thefirstfouroffivenumbersdecreasemonotoni- cally beforethefifthshowsariseagain.Thisissuggestiveof asteroids foundtobemembersoftheTCevenhigher probability, perhapsbeingthevaluethatbestdefinesTC ently associatedwiththeSouthernp-Geminids(Porubcanet asteroids. amongst newlydiscoveredsub-kmobjects. with D<0.2whicharenotedinthefirstcolumnTC the TC.Wewillassumethat1991BAandnineothers of Stohl&Porubcan(1990),1991BAwouldbeincludedin our limituponorslightlyto140°±50°,inlinewiththework al. 1992),anoutlyingstreamofthebroadTC.Bywidening cut-off valueDabout0.2,correspondingtotheminimum asteroids, andconsidertheremainingsixasteroidswith no independentreasontochoosethisrangeof40°rather , wehavenolongitudedefinedbymeteordataandso this 40°bracketbychanceisonly0.00009;since,unlikethe ‘Others’. Itisseenthattheseproduceanotheralignment:five indeed, ifweexcludetheHephaistosgroupandconsider the existenceoftwohigh-e,sub-Jovianasteroidstreamsand This isstillhighlysignificant.Wethusseemtohaveidentified within 40°atanylocationbychanceasbeing0.08percent. rendering aprobabilityoffiveoutsixhavingvaluesor than anyother,wemustmultiplythisfigureby360/40=9, ‘Hephaistos group’.Considering,say,theliberalrangelimits of thesesixhave222°