Stellar density distribution in the NIR on the Galactic plan

Garzon et al. (1997), Lopez-Corredoira et al. (1999) and Hammersley et al. (2000) have identified in TMGS and DENIS data a large excess of stars at l=27 deg and b=0 deg which might correspond to an in-plane bar. We compared near infrared CAIN star counts a

Astronomy&Astrophysicsmanuscriptno.(willbeinsertedbyhandlater)

arXiv:astro-ph/0306623v1 30 Jun 2003

StellardensitydistributionintheNIRontheGalacticplaneat

longitudes15-27deg.CluesfortheGalacticbar?

S.Picaud1,A.Cabrera-Lavers2andF.Garz´on2,3

123

CNRSUMR6091,ObservatoiredeBesan¸con,BP1615,F-25010Besan¸conCedex,France

InstitutodeAstrof´ sicadeCanarias,38200LaLaguna,Tenerife,SpainDepartamentodeAstrof´ sica,UniversidaddeLaLaguna,Tenerife,Spain

Received6March2003/Accepted28May2003

Abstract.Garz´onetal.(1997),L´opez-Corredoiraetal.(1999)andHammersleyetal.(2000)haveidenti edinTMGSandDENISdataalargeexcessofstarsatl=27 andb=0 whichmightcorrespondtoanin-planebar.WecomparednearinfraredCAINstarcountsandsimulationsfromtheBesan¸conModelofGalaxyon15 elds

http://doc.guandang.netparisonscon rmtheexistenceofanoverdensityatlongitudeslowerthan27 whichisinhomogeneousanddecreasesverysteeplyo theGalacticplane.Theobservedexcessinthestardistributionoverthepredicteddensityisevenhigherthan100%.Itsdistancefromthesunisestimatedtobelowerthan6kpc.Ifthisoverdensitycorrespondstothestellarpopulationofthebar,weestimateitshalf-lengthto3.9±0.4kpcanditsanglefromtheSun-centerdirectionto45±9degrees.Keywords.Galaxy:structure–Galaxy:stellarcontent–Galaxy:disk

1.Introduction

Starcountshavebeenusedforyearstoexamineatvari-ouslevelsofdetailthestellarcontentsintheGalaxy(seePaul1993),whosestructuralparametersofthevariousmorphologicalcomponentsarestillfarfrombeingcom-pletelyknown.Inthelasttwodecadestherehasbeenacombinede ortinthisdirectionwiththeuseofde-tailedmodelsofstellargalacticdistribution(Bahcall&Soneira1980;Robin&Cr´ez´e1986;Wainscoatetal.1992;L´opez–Corredoiraetal.2002)alongwithlargearea,highsensitivityandmulticolourstarcountsurveys.Thenearinfrared(NIR)membersofthesesurveys(Eatonetal.1984,Garz´onetal.1993,Hammersleyetal.1994,L´opez–Corredoiraetal.2000,Epchtein1997,Skrutskieetal.1997)arenotablyusefulfortheanalysisofthegalacticstructurebecauseoflessinterstellarextinctioncomparedtotheopticalbands,inparticularinthehiddenon-planeareasoftheinnerGalaxy.Itisinthesezonesthatthemorphologicalstructuresarelesswellstudied.

Thee ectofthehighextinctiontowardstheinnerGalaxyalongtheGalacticplane,addedtothepositionoftheSunveryneartotheplaneitself(Hammersleyetal.1995),makesitparticularlydi culttodetectlargescalestellarstructuresinthecentralpartoftheGalaxy.

Garzon et al. (1997), Lopez-Corredoira et al. (1999) and Hammersley et al. (2000) have identified in TMGS and DENIS data a large excess of stars at l=27 deg and b=0 deg which might correspond to an in-plane bar. We compared near infrared CAIN star counts a

2S.Picaudetal.:Stellardensityatlongitudes15-27deg.

densitycomparedtothemodelpredictionmaythenbeusedtoanalyzethemorphologicalextentofsuchanextrapopulation.

Thispaperisorganizedasfollows.Webrie ydescribethestarcountdataandtheBesan¸conmodelusedinthiswork.Wethendiscussthemethodofderivingtheextinctiondistributionalongthelineofsightfromthedataalone,whichisthenusedinthemodelforthepre-dictions.Thosepredictionsarethencomparedwiththedataintwodi erentplanes:magnitudeandcolourcounts,whichbothservefortheanalysis.Weprovideconclusionsdescribingthemodelitselfandthederivedgeometryforthecomponentresponsiblefortheexcesses.

2.TheBesan¸conModeloftheGalaxy

TheapproachoftheBesan¸conmodelisslightlydi erentfromtheonesusedpreviouslytostudythebar.Here,wegiveabriefdescriptionofthemodel,dealingparticularlywiththethindiscpopulationmodel.MoredetailscanbefoundinRobin&Cr´ez´e(1986),Bienaym´eetal.(1987)

andRobinetal.(2003)1

.

2.1.Globaldescription

TheBesan¸conModelofStellarPopulationSynthesis,de-velopedinthevisible,nearandmidinfrared,aimsatgiv-ingaglobal3-dimensionaldescriptionoftheMilkyWay,includingstellarpopulationssuchasthethindisc,outerbulge,thickdiscandspheroid,aswellasdarkhaloandinterstellarmatter.

Itsapproachissemi-empirical:boththeoreticalschemes(stellarevolution,galacticevolution,galacticdy-namics)andempiricallawsfoundintheliteratureareused.BoltzmannandPoissonequationsareusedtomakethemodeldynamicallyself-consistentinthedirectionper-pendiculartotheplane.Arti cialcataloguesofstarsareproduced,givingboththefundamentalcharacteristicsofstars(age,distance,absolutemagnitudes,kinematics)andobservableones(apparentmagnitudes,colours,propermotion,radialvelocities)directlycomparablewithobser-vations.Absorption,photometricerrorsandPoissonnoisearealsoaddedtomakesimulationsascloseaspossibletoobservations.TheBesan¸conmodelhasbeenusedtoconstrainthedensitylawortheluminosityfunctionofthestellarpop-ulations:thindisc(Ruphyetal.1996;Haywoodetal.1997),thickdisc(Robinetal.1996;Reyl´e&Robin2001),spheroid(Robinetal.2000)andbulge(Picaud&Robin,inpreparation).

2.2.Thindisc

Inthestudiedregion,thickdiscandspheroidpopulationsgiveanegligiblecontribution,andthosefromthebulge

ang.html

age(Gyr)

Table1.Axisratiosofthe6agecomponentsoftheoldthindisc.

likelyonlycontaminatestarcountsattheinnermost eld(l=15 ).Thus,theonlydominantstellarpopulationtakenintoaccountbythemodelisthethindisc.ItsdensityismodeledastheEinastolaw(1979):thethindiscispidedin7agecomponentsandeachcomponentdistributionisdescribedbyanaxisymmetricellipsoid.The rstcompo-nent(0-0.15Gyr)iscalledtheyoungdiscandthe6otheronesformtheolddisc,whichonlyispresentedherebe-causeoftheverysmallnumberofstarsbelongingtotheyoungone.Thedensitylawofeachellipsoidoftheolddiscisthesubtractionoftwomodi edexponentials,thesecondonerepresentingthecentr …… 此处隐藏：27789字，全部文档内容请下载后查看。喜欢就下载吧 ……