Dark stars at the Galactic centre - the main seque

Mon.Not.R.Astron.Soc.000,1–22(2008)Printed10September2008(MNLATEXstylefilev2.2)DarkstarsattheGalacticcentre–themainsequencePatScott1?,MalcolmFairbairn2;3?andJoakimEdsj¨o1?1Cosmology,ParticleAstrophysicsandStringTheory,DepartmentofPhysics,StockholmUniversity&HighEnergyAstrophysicsandCosmologyCentre(HEAC),AlbaNovaUniversityCentre,SE-10691Stockholm,Sweden2TheoryDivision,CERN,CH-1211,Geneva23,Switzerland3Physics,KingsCollegeLondon,Strand,LondonWC2R2LS,UKSubmittednthSeptember2008ABSTRACTInregionsofveryhighdarkmatterdensitysuchastheGalacticcentre,thecaptureandannihilationofWIMPdarkmatterbystarshasthepotentialtosignificantlyaltertheirevolu-tion.WedescribethedarkstellarevolutioncodeDarkStars,andpresentaseriesofdetailedgridsofWIMP-influencedstellarmodelsformainsequencestars.WedescribethechangesinstellarstructureandmainsequenceevolutionwhichoccurasafunctionoftherateofenergyinjectionbyWIMPs,formassesof0.3–2.0MandmetallicitiesZ=0.0003–0.02.WeshowwhatratesofenergyinjectioncanbeobtainedusingrealisticorbitalparametersforstarsattheGalacticcentre,includingdetailedconsiderationofthevelocityanddensityprofilesofdarkmatter.Captureandannihilationratesarestronglyboostedwhenstarsfollowellipticalratherthancircularorbits.DarkstarsareunlikelytoexistonanycircularorbitsintheMilkyWay.IfthereisaspikeofdarkmatterinducedbythesupermassiveblackholeattheGalacticcentre,solar-massstarsfollowingorbitswithperiodsaslongas50yearsandeccentricitiesaslowas0.9couldbesignificantlyaffected.Theobservationoflow-massstarsonsuchorbitswouldeitherprovideadetectionofWIMPdarkmatter,orplacestringentlimitsonthecombinationoftheWIMPmass,spin-dependentnuclear-scatteringcross-section,halodensityandvelocitydistributionneartheGalacticcentre.Insomecases,thederivedlimitsontheWIMPmassandspin-dependentnuclear-scatteringcross-sectionwouldbeofcomparablesensitivitytocurrentdirect-detectionexperiments.Keywords:darkmatter,stars:evolution,stars:fundamentalparameters,stars:interiors,Galaxy:centre,elementaryparticles1INTRODUCTIONObservationscontinuetosupporttheexistenceofnon-baryonicdarkmatter(DM;Bergstr¨om2000;Bertoneetal.2005;Cloweetal.2006;Komatsuetal.2008)withacosmologicalabundancearoundfivetimesthatofbaryonicmatterbutofunknowncomposi-tion.Weakly-interactingmassiveparticles(WIMPs)areapopularandconvenientclassofdarkmattercandidatesbecausetheirweak-scalemassesandcouplingsnaturallygiverisetoanappropriatethermalrelicabundanceintheearlyuniverse.TypicalWIMPs,suchasthelightestneutralinoinsupersym-metry(Jungmanetal.1996),possesnon-zeronuclear-scatteringandself-annihilationcross-sections.Thenuclear-scatteringcross-sectionmakesitpossibleforWIMPstocollideelasticallywithnu-cleiinmassivebodiessuchasstars,obtainvelocitieslessthanthelocalescapevelocityandbecomegravitationallybound(Press&Spergel1985;Griest&Seckel1987;Gould1987b,a).Thispopula-tionofWIMPswillcontinuetoscatteroffnucleiinthestar,sinking?E-mail:[e-mail],[e-mail],[e-mail],thestructureofthehostbodymaybealteredbytheenergyproducedinWIMPannihilations,orbyenergytransportcausedbytheWIMP-nucleusscatteringeventsthemselves.Thiswasfirstrealisedover30yearsagointhecontextofheavyneutrinos(Steigmanetal.1978).Thepo-tentialfortransporteffectstomodifythestructureofstellarcoreswasinitiallydevelopedbySpergel&Press(1985)andFaulkner&Gilliland(1985).ImplicationsofannihilationforstellarevolutionwerefirstexploredbySalati&Silk(1989)andBouquet&Salati(1989a).Aseriesofsubsequentstudies(Gillilandetal.1986;Ren-zini1987;Spergel&Faulkner1988;Faulkner&Swenson1988;Bouquetetal.1989;Bouquet&Salati1989b;Delucaetal.1989;Salati1990;Dearbornetal.1990b,a;Giraud-Heraudetal.1990;Christensen-Dalsgaard1992;Faulkner&Swenson1993)focuseduponpossibleimpactsofenergytransportby‘cosmion’WIMPsdesignedtosolvethesolarneutrinoproblem.Withtheadventofneutrinooscillationsthisproblemhasofcoursedisappeared.Fur-thermore,theexistenceofmuchmorestringentlimitsuponthec2008RASarXiv:0809.1871v1[astro-ph]10Sep20082P.C.Scott,M.FairbairnandJ.Edsj¨oWIMP-nucleonscatteringcross-sections(e.g.Desaietal.2004;Angleetal.2008;CDMSCollaboration2008;Behnkeetal.2008)andanimprovedunderstandingofthedistributionofdarkmatterongalacticscales(e.g.Bertone&Merritt2005;Diemandetal.2007)meanthatthelikelihoodofseeingchangesinducedpurelybyWIMPenergytransportseemssomewhatdiminished.Indeed,latereffortstoconstrainWIMPphysicsviahelioseismologyhaveprovenfruitless(Bottinoetal.2002).RecenttimeshaveseenaresurgentinterestintheimpactsofWIMPsuponstars,nowfocussingalmostexclusivelyuponthein-fluenceofannihilation.Moskalenko&Wai(2007)andBertone&Fairbairn(2008)showedthatitcouldbepossibletoseewhitedwarfsheatedbyWIMPannihilation,attheGalacticcentreandinglobularclustersrespectively.Spolyar,Freese&Gondolo(2008)showedthatWIMPannihilationmightbeabletopartiallyinhibittheformationofPopIIIstars,resultingingiant,cool,primordialstarssupportedentirelybyannihilationenergy.Inpreviouslettersthecurrentauthorspresentedthefirstnumericalsimulationsofthestructureandevol