The contribution of halo red giant mass loss to the high-vel

The origin of gas falling from the halo toward the disk of the Milky Way is still largely unclear. Here the amount of gas shed by the (older) halo red giants is estimated. The distribution of red giants (RGs) in the halo is not known but that of a subset o

Astronomy&Astrophysicsmanuscriptno.0947

(DOI:willbeinsertedbyhandlater)February2,2008

Thecontributionofhaloredgiantmassloss

tothehigh-velocitygasfallingontotheMilkyWaydisk

KlaasS.deBoer

Sternwarte,Universit¨atBonn,AufdemH¨ugel71,D-53121Bonn,Germany

arXiv:astro-ph/0402630v1 26 Feb 2004Received24December2003/Accepted10February2004Abstract.TheoriginofgasfallingfromthehalotowardthediskoftheMilkyWayisstilllargelyunclear.Heretheamountofgasshedbythe(older)haloredgiantsisestimated.Thedistributionofredgiants(RGs)inthehaloisnotknownbutthatofasubsetofstarsinthepostRGphase,thesdBstarsofthehorizontal-branch(HB),http://doc.guandang.netingthemid-planedensityandz-distributionofsdBstars,theratioofsdBstarstoallHBstars,andtheRGmassloss,theinfallduetototalmasslostbyallhaloRGstarsatz>1kpciscalculated.Forthe˙haloRGs 1.4·10 5M⊙kpc 2yr 1whilethethickdiskcomponentRGscontributeextendedhalocomponentM˙thickdiskRGs 5.4·10 5M⊙kpc 2yr 1,eachwithanuncertaintyofafactor4.ThetotalrateofinfallM˙RGsatz>1kpc 7·10 5M⊙kpc 2yr 1,asizeablefractionoftheequallyuncertainduetoRGmass-lossisMobservedrateofinfallofmaterial.SincemostoftheRGstarsintheextendedhaloareold,theirmasslossispredominantlymetal-poor,whilethatofthediskRGsismoremetal-rich.Thegalacticfountain owprovidesadditionalmetal-richinfallandsmallgalaxiesbeingaccretedcontributetotheinfallofgasaswell.Keywords.Galaxy:halo-Galaxy:structure-Galaxy:evolution-ISM:high-velocityclouds1.TheinfallproblemThedetectionathighgalacticlatitudeofcomplexesofneutralhydrogengashavingtheirvelocitydirectedtowardtheSolarvicinity(Mulleretal.1963)showedthatneu-tralgascloudsarefallingtowardtheMilkyWaydisk.Itwasspeculatedthatthegasmightcomefromintergalacticspace(Oort1966),inwhichcaseitshouldbealmostfreeofmetals.Thediscoveryofthisin uxofpossiblypristinematterwasembracedbymodellersoftheevolutionofthe

MilkyWay.The“closedbox”evolutionmodelspredicted

manymoremetal-poorG-dwarfsthanactuallyareknown

toexisttoday(seereviewsbyAudouze&Tinsley1976or

Pagel1997).Sincethattimetheamountofinfallneeded

intheevolutionmodelswastakeneitherasafreeparame-

terorsetequaltotheamountofinfallascanbeestimated

fromtheobservedhalohigh-velocityclouds(HVCs).

Thatrateofinfallobservedwasratheruncertainbe-

causeinthe1970sonlypartoftheskyhadbeensurveyed

forHi.Inaddition,thedistancetotheHVCsaswellas

intermediatevelocityclouds(IVCs)wasunknown(andis

stilllargelyunknowntoday;seereviewbyWakker&van

Woerden1997).Knowledgeofsuchdistancesis,ofcourse,

crucialforthecalculationofthemassinHVCsandIVCs

andthusofaninfallrate.EssentiallyallgasseenathighgalacticlatitudescanberegardedasfallingtowardtheMilkyWaydisk;toseethatonehastocorrecteachobservedvelocityforthevelocityoftheSun

initsorbitaroundtheMilkyWay.Moreover,anygasatlarge

z-distance(thusbeinghalogas)havingzeroverticalvelocity

wouldblendinvelocitywithgasofthediskneartheSun.For

moreontheproblemsee,e.g.,Kaelbleetal.(1985).1

The origin of gas falling from the halo toward the disk of the Milky Way is still largely unclear. Here the amount of gas shed by the (older) halo red giants is estimated. The distribution of red giants (RGs) in the halo is not known but that of a subset o

2KlaasS.deBoer:HaloredgiantmasslossandHVCinfallratetheMilkyWayhalo,whichfallstowardthedisk.Alsogas

fromothercaughtsatellitedwarfgalaxies(c.f.,Helmi&

White1999)maybepresent.

AlsomolecularhydrogenwasdetectedinHVCsand

IVCs(seeRichteretal.1999;Gringeletal.2000;Bluhm

etal.2001;Richteretal.2001),whichwouldrequirethe

presenceofdust.Recently,Evansetal.(2003)discovered

dustintheglobularclusterNGC7078.Thismaterialwill

stayintheclusteruntilitcanbestrippedwhentheglob-

ularclusteronitsorbitzippesthroughtheMilkyWay

disk.HaloRGstars,however,wouldshedtheirgasand

dustrightintothehalo.

Theamountofgasshedbyred-giant(RG)starsinthe

halooftheMilkyWay(MW)isproposedtobeanessential

componentcontributingtotheinfall.Itsproductionrateis

assessed,tobecomparedtotheobservationalfactsabout

infallavailabletoday.

2.Age,mass,andmetallicityofhalostars

TheMWhaloiscomposedofstarsbelongingtotheMW

abinitio,mostlikelysupplementedbystarsaccretedfrom

satelliteobjects(e.g.,theSagittariusdwarfgalaxy).

Abinitiohalostarsareasoldastheglobularclusters

( 13Gyr).Theylikelyhaveachemicalcompositionsim-

ilartothatofglobularclusterstars.Themain-sequence

massMinitofthosestarsbeingnowRGwas,assuming

theyaremetal-poor(Z=0.001),between0.92and0.82

M⊙(seetheevolutiontracksbySchalleretal.1992).

RGstarsbelongingtotheMWthickdiskwillinpart

beyoungerandmoremetal-richthantheabinitohalo

stars.TheirMinitislarger,upto1.5M⊙(orjustupto

1.1M⊙forsolarmetallicity,Z=0.02;Schalleretal.1992).

Ithasbeenspeculatedthatsomeyoungstarsare

presentinthehalo.Inhighlatitudebluestarsurveysstars

werefoundthatseemedtohave(fromBalmerline ts)

main-sequence-likegravities.Ifthesewereindeedmain-

sequencestars,thenonewouldhavestarformationin

thehalo.However,mostsuchclaimsdidnotstandup

toscrutiny.Theirnumberissmallanywayandtheire ect

onoverallhalostarmasslossisnegligible.

Accretionofstarsfromsatellitegalaxiestookplacein

theearlyphasesoftheMW(seethereviewbyFreeman

&Bland-Hawthorne2002).Thoseaccretedsatelliteswere

rathermetal-poor.Duetotheirdisruptionnonewstars

haveformedinthedebrissinceaccretionsothattheir

RGshaveMinitinthesamerangeasthoseproperofthe

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