The chemical evolution of Omega Centauri's progenitor system

0.35 MB·English

by  D. Romano

Mon.Not.R.Astron.Soc.000,1–11(2006) Printed5February2008 (MNLATEXstylefilev1.4)
The chemical evolution of Omega Centauri’s progenitor system
⋆
1 2 1 1
Donatella Romano, Francesca Matteucci, Monica Tosi, Elena Pancino,
1 3 4 3
Michele Bellazzini, Francesco R. Ferraro, Marco Limongi and Antonio Sollima
1INAF–OsservatorioAstronomicodiBologna,ViaRanzani1,I-40127Bologna,Italy
2DipartimentodiAstronomia,Universita`diTrieste,ViaTiepolo11,I-34131Trieste,Italy
3DipartimentodiAstronomia,Universita`diBologna,ViaRanzani1,I-40127Bologna,Italy
4INAF–OsservatorioAstronomicodiRoma,ViaFrascati33,I-00040MonteporzioCatone,Italy
7
0
Accepted2006December21.Received2006December20;inoriginalform2006November27
0
2

n ABSTRACT
a ChemicalevolutionmodelsarepresentedfortheanomalousglobularclusterωCentauri.After
J demonstratingthat the chemical features of ωCen can not be reproducedin the framework

7 of the closed-box self-enrichmentscenario, we discuss a model in which this cluster is the
remnantofadwarfspheroidalgalaxyevolvedinisolationandthenswallowedbytheMilky

1 Way.Bothinfallofprimordialmatterandmetal-enrichedgasoutflowshavetobeconsidered
v
inordertoreproducethestellarmetallicitydistributionfunction,theage-metallicityrelation
2
andseveralabundanceratios.Yet, aslongasan ordinarystellar massfunctionandstandard
6
stellar yieldsare assumed,we failby far to getthe enormousheliumenhancementrequired
1
toexplainthebluemainsequence(and,perhaps,theextremehorizontalbranch)stellardata.
1
0 Rotatingmodelsof massivestars producingstellar windswith largeheliumexcessesatlow
7 metallicitieshavebeenputforwardaspromisingcandidatestosolvethe‘heliumenigma’of
0 ωCen(Maeder&Meynet,2006,A&A,448,L37).However,weshowthatforanyreasonable
/ choice of the initial mass function the helium-to-metal enrichmentof the integrated stellar
h
population is unavoidably much lower than 70 and conclude that the issue of the helium
p
enhancement in ωCen still waits for a satisfactory explanation. We briefly speculate upon
-
o possiblesolutions.
r
t Keywords: galaxies:dwarf–galaxies:evolution–globularclusters:individual(ωCentauri)
s
a –stars:abundances–stars:chemicallypeculiar.
:
v
i
X
r
a 1 INTRODUCTION sodium,magnesiumandaluminumcouldbeduetoproton-capture
fusion reactionsoccurring during quiescent hydrogen and helium
The globular cluster (GC) ωCen (NGC5139) is a unique win-
burningwithinthegiantsthemselves,variationsintheabundances
dow intoastrophysics (see Smith2004, for arecent review). The
of heavier species – such as iron-peak and neutron-capture ele-
estimated mass, between (2.5 ± 0.3) × 106 M⊙ (van de Ven ments–shouldbeimmunetosuchprocessesand,therefore,likely
et al. 2006) and 5.1 × 106 M⊙ (Meylan et al. 1995) – or even reflectpatternsimprintedontheobservedstarsbypreviousstellar
7.1 × 106 M⊙ (Richer et al. 1991), makes it the most massive generations(LloydEvans1983;Cohen&Bell1986;Norris&Da
GC of the Milky Way. Its total mass better compares with that
Costa1995;seealsoGratton,Sneden&Carretta2004,foracom-
of a small dwarf spheroidal galaxy such as Sculptor (M ≃
Sculptor prehensivereviewonabundancevariationswithinGCs).
6.4 × 106 M⊙; Mateo 1998) rather than with that of a typical
Galactic GC (MGGC ∼ 105 M⊙; Harris 1996). The large degree ThefirstindicationthatωCenwaschemicallyinhomogeneous
ofchemicalself-enrichmentobservedinωCengiantandsubgiant camefromthelargecolorwidthoftheredgiantbranch(RGB)es-
membersalsosetsitapartfromalltheotherGalacticglobulars. tablishedinanearlierphotometricworkbyWoolley(1966).Inthe
Indeed,ωCenistheonlyknownGGCtoexhibitalargedegree last decade, both extensive spectroscopic surveys and wide-field
ofchemicalself-enrichmentinalltheelementsstudied(Freeman& photometricstudies–mostlyofgiantωCenmembers–havedefi-
Rodgers 1975; Cohen 1981; Mallia&Pagel 1981; Gratton1982; nitelyconfirmedtheexistenceofseveral(uptofive)discretestellar
Norris & Da Costa 1995; Smith, Cunha & Lambert 1995; Smith populationscoveringalargerangeinmetallicity(Norris,Freeman
et al. 2000; Pancino et al. 2002). While star-to-star variations in & Mighell 1996; Suntzeff & Kraft 1996; Lee et al. 1999; Hilker
theabundancesoflightelementssuchascarbon,nitrogen,oxygen, &Richtler2000;Hughes&Wallerstein2000;Pancinoetal.2000;
Reyetal.2004;Sollimaetal.2005a).Themetallicitydistributions
of (sub)giant starsand main sequence turnoff (MSTO)starslook
⋆ E-mail:[email protected] pretty much the same (Stanford et al. 2006). From their spectro-
(cid:13)c 2006RAS
2 D. Romanoet al.
scopicandphotometricMSTOdata,Stanfordetal.(2006)findthat (e.g.Carraro&Lia2000;Bekki&Freeman2003;Tsuchiya,Kor-
theformationofωCentookplacemostlikelyover2–4Gyr;both chagin&Dinescu2004).Thetotalmassoftheparentobjectranges
anullagerangeandagerangeshigherthan6Gyraredeemedun- from108 M⊙ tosome109 M⊙ inthose models. Alternativesce-
likely (cf. previous works by Norris & Da Costa 1995; Hilker & nariosenvisageanoff-centrestellarsuperclusterseed,whichwould
Richtler2000;Hughes&Wallerstein2000;Smithetal.2000;Pan- havetrappedoldergalacticfieldstarsduringitsformationprocess
cino et al. 2002; Rey et al. 2004; Sollima et al. 2005a; see also (Fellhauer,Kroupa&Evans2006).Otherpossibleexplanationsfor
Kayseretal.2006). Ithasalsobeenfoundthatthefaintmainse- theclusterorigin–namely,mergingbetweentwoormoresmaller
quenceofωCensplitsintoatleasttwodistinctbranches(Ander- globularsorbetweenadwarfgalaxyandacluster(Icke&Alcaino
son1997;Bedinetal.2004;Sollimaetal.2006b).Thebluemain 1988; Norris et al. 1997), or formation triggered by cloud-cloud
sequence (bMS) contains ∼25 per cent of the stars and is 0.3 ± collisions (Tsujimoto & Shigeyama 2003) – have been put for-
0.2dexmoremetal-richthantheredone(rMS;Piottoetal.2005). ward, but seem less likely on the basis of the continuous trends
These observations are most likely explained by an anomalously of heavy elements-to-iron and lanthanum-to-iron observed for S
highheliumabundanceofbMSstarsofY >0.38(∆Y/∆Z >70; stars in ωCen (Vanture, Wallerstein & Suntzeff 2002, and refer-
Norris2004;Piottoetal.2005).Suchanintriguingpossibilityhas encestherein).Clear evidence againsttheparent systemevolving
beenquantitativelyinvestigatedintheframeworkofsomewhatide- asaclosedboxhasbeenputforwardbyIkuta&Arimoto(2000).
alizedscenariosfortheformationandevolutionofωCen(Bekki& InthispaperwedealwiththechemicalevolutionofωCen.We
Norris2006). discusstwopossiblescenariosofformation:inthefirstone,thepre-
Ithasbeenshown(Norrisetal.1997)thatthemetal-richcom- cursorofωCenisasmallsystemwhichevolveseitherasaclosed-
ponent of thecluster,whichismorecentrallyconcentrated, hasa boxorwithsomeexchangeofmatterwiththesurroundings;inthe
smaller line-of-sight velocitydispersion and alower systemicro- secondone,theclusteristheleftoverofanancientnucleateddwarf
tation about the cluster’s minor axis than the metal-poor one. A galaxyswallowedbyourGalaxysome10Gyrago.Wepayspecial
morerecent analysisbySollimaetal.(2005b) confirmsthetrend attentiontotheintriguingsubjectofheliumenhancement inbMS
ofdecreasingvelocitydispersionwithincreasingmetalabundance stars.Forthefirsttime,thiscriticalissueisfacedintheframework
inthemetallicityrange−2.0<[Fe/H]<−1.0,butshowsalsothat ofacomplete,self-consistentchemicalevolutionmodel,takingall
intheextrememetal-richextensionofthestellarpopulation, now therelevantphysicsintoaccount.InSection2welisttheobserva-
bettersampledthankstonewgenerationinstrumentation(Pancino tionsthatweusetoconstrain our chemical evolution model. The
et al. 2000), this decreasing trend is reversed (see fig. 9b of Sol- modelispresentedinSection3.Section4summarizesthemodel
limaetal.2005b).Asymmetriesinthedistributionandvelocityof results,thatarediscussedandcomparedtopreviousinvestigations
thestarscouldtestifypastaccretioneventswithinωCen(Ferraro, inSection5.InSection5wealsodrawourconclusions.
Bellazzini&Pancino2002;Pancinoetal.2003),butevidencefor
theseisnotdefinitiveyet (Plataisetal.2003). Tofurther compli-
catetheoverallpicture,theorbitofωCenisfoundtobestrongly
2 THENATUREOFTHECHEMICALENRICHMENTIN
retrograde,almostcoplanarwiththeMilkyWaydisc,andtohave
OMEGACENTAURI
smallapogalacticon,unlikeanyknownGalacticGC(e.g.Majewski
etal.2000).Atvariancewithmostglobulars,therelaxationtimefor It was recognized several years ago that simple models for clus-
ωCenisverylong,uptoafewtimes109 yearsinthecoreanda ter enrichment can not reproduce the number of metal-rich stars
fewtimes1010 yearsathalf-massradius(Meylanetal.1995;van observed in the metallicitydistribution function (MDF) of ωCen
deVenetal.2006).Indeed, recentresultssuggest thatthecluster giants(Norrisetal.1996;Suntzeff&Kraft1996;Ikuta&Arimoto
isnotyetrelaxedeveninthecentralregions(Ferraroetal.2006). 2000).BasedonCaabundancesobtainedfromlow-resolutionspec-
Thisclustercouldthenmantainforafairlylongtimetheimprinting tra,Norrisetal.(1996)foundabimodaldistributionwithametal-
ofitsinitialconditions,thusallowingonetousethecurrentlyob- poor component, peaking at [Ca/H] ≃ −1.4 dex and comprising
serveddistributionofstarsofdifferentpopulationsinordertotrace nearly 80 per cent of the stars, and a metal-rich one, comprising
back the cluster formation and evolution (e.g. Merritt, Meylan & nearly20percentofthestarsat[Ca/H]≃−0.9dex(Fig.1,lower
Mayor1997). rightpanel).Suntzeff&Kraft(1996)alsofoundasharpriseatlow
Thelargemass,spreadinelementabundances,flattenedshape metallicitiesandahigh-metallicitytail,butnoevidenceforasec-
androtationallcomeclosetothepicturewhereωCenisthesur- ondaryhumpathighermetallicities(Fig.1,upperrightpanel).
viving remnant of alargersystem. Thechemical and kinematical Thesharprisetoameanof[Fe/H]≃−1.6dexandthelong
segregationsdetectedinωCenaddevenmorerelevancetothepic- tailathighermetallicitieshavebeenconfirmedbyanumberofsub-
tureofadwarfgalaxyprogenitorbeingsubjecttoaccretionevents, sequent studies, usingboth higher resolutionspectroscopy and/or
since such gradients are present in nearly all of the Local Group highqualityphotometry(e.g.Hilker&Richtler2000;Frinchaboy
dwarf spheroidals (e.g. Harbeck et al. 2001; Tolstoy et al. 2004; et al. 2002; Sollima et al. 2005a; Stanford et al. 2006; Kayser et
Kochetal.2006,tonameafew).Severalauthorshavespeculated al. 2006). Some of these works also show that the MDF is more
onthepossibilitythatωCenisthenakednucleusofadwarfsatel- complexthanpreviouslythought,withseveralseparatepeaksiden-
litegalaxy captured into a retrograde Galactic orbit many billion tifiedintheobserveddistribution(e.g.Sollimaetal.2005a;Fig.1,
years ago (Dinescu, Girard & van Altena 1999; Majewski et al. left panel). According to subgiant branch (SGB) data, the differ-
2000;Smithetal.2000;Gnedinetal.2002;Bekki&Norris2006), entpopulationshaveagescomparablewithin2Gyr;actually,they
followingmoregeneralideasonanaccretedoriginforGCsinour mightbeevencoeval(Sollimaetal.2005b;seealsoFerraroetal.
ownaswellasexternal galaxies(Zinnecker et al.1988; Freeman 2004).Awideragerange,∆t ≃2–4Gyr,isinferredfromMSTO
1993).Self-consistentdynamicalmodelsaswellasN-bodyhydro- data(Stanfordetal.2006;seealsoHilkeretal.2004andKayseret
dynamicalsimulationscanbefoundintheliteraturewhichsucceed al.2006).
to reproduce the main features of the cluster by assuming that it Theabundanceratiosofdifferentchemicalspeciesprovidean-
formedinisolationandthenfellinsidetheGalacticpotentialwell other independent constraint on the evolutionary time-scales. As
(cid:13)c 2006RAS,MNRAS000,1–11
The chemicalevolutionofω Cen 3
Piottoetal.2005)cannotbeexplainedaslongastheyoriginated
fromtheejectaofrMSstarsinitiallywithinωCen.Rather,mostof
thehelium-richgasnecessarytoformthebMShadtocomefrom
fieldstellarpopulations surrounding ωCenwhenitwasthecom-
pact nucleusof aGalacticdwarf satellite(Bekki&Norris2006).
Inotherwords,theoriginaltotalmassoftherMSpopulationmust
havebeenlargerthanthepresent-dayoneandmostoftherMSstars
must have been removed from the proto-ωCen after their ejecta
were used to form the bMS population. Searches for tidal debris
from ωCen’s hypothetical parent galaxy in the solar neighbour-
hood are possible in principle (Dinescu 2002). Indeed, a distinct
populationofstarswithωCen-likephase-spacecharacteristicsand
metallicitiesconsistentwiththoseofωCenmembersemergesfrom
catalogsofmetal-deficientstarsinthevicinityoftheSun(Dinescu
2002;Mezaetal.2005).
Thoughgasfuelingfromanancienthostgalaxystandsasan
attractive hypothesis in many respects, to the best of our knowl-
edgethispossibilityhasneverbeenstudiedbymeansoffullyself-
consistentchemicalevoutionmodelsbefore.
3 THECHEMICALEVOLUTIONMODEL
Figure 1.Shown aresomeofthe observed metallicity distribution func-
We follow the evolution of the abundances of several chemical
tionsforωCengiants.Generalfeaturesofalltheobserveddistributionsare
species in the gaseous medium out of which ωCen’s stars form,
thesteepriseatlowmetallicities andthehigh-metallicitytail(seetextfor
details). in the case of either a GC precursor or a dwarf spheroidal pro-
genitor. The adopted chemical evolution model isone zone, with
†
instantaneous andcompletemixingofgasinsideitandnoinstan-
firstdiscussedbyLloydEvans(1983),theenrichmentofs-process taneousrecyclingapproximation(i.e.,thestellarlifetimesaretaken
elements (e.g. Ba, La) in the metal-rich ωCen stars is likely to into account in detail). The GC precursor has an initial mass of
be that of the gasclouds out of which thestarsformed. Smithet the same order of magnitude of the present one (MωCen = 2.5–
al.(2000)suggestedpreviousgenerationsoflow-massasymptotic 5 × 106 M⊙). Both a closed-box model and models where ex-
giant branch (AGB) stars as polluters and argued in favour of a changesofmatterwiththesurroundingsareallowedareanalyzed.
protracted period of star formation in ωCen, of the order of 2– For the dwarf galaxy precursor, when the computation starts the
3Gyr.Shortertime-scalesareinferredfromtheobserved kneein baryonicmatter(coolinggas)isembeddedinarelativelymassive
the[α/Fe]versus[Fe/H]relation,ifTypeIasupernovae(SNeIa)in (Mdark/Mbar =10),diffuse(Rdark/Reff =10),virializeddarkmat-
ωCenrestorethebulkoftheirirontotheinterstellarmedium(ISM) terhalo.Initially,thebaryonicmassinsidethedarkmatterpoten-
onthesametime-scaleasinthesolarneighbourhood(∼1Gyr;Pan- tial well is two orders of magnitude higher than the present one.
cinoetal.2002).However,onemustbeawarethatthetime-scale Assoonasthestarformationbegins,thethermalenergyofthegas
for the maximum enrichment by SNeIa in a specific system de- startstoincreaseasaconsequenceofmultipleSNexplosions,even-
pendsstronglyontheassumptionsabouttheSNprogenitors,stel- tuallyexceedingitsbindingenergy‡.Whenthisconditionismet,
larlifetimes,initialmassfunction(IMF),starformationrate(SFR) partofthegasescapesthegalacticpotentialwell;weassumethat
and, last but not least, possible metal-enriched gasoutflowsfrom thisgasisdefinitivelylostfromthesystem.
the system. It has been demonstrated (Matteucci &Recchi 2001,
and referencestherein) thattime-scalesaslong as∼4–5 Gyr can
beobtainedwithsuitableassumptions.Wewillcomebacktothis 3.1 Basicequations
issuelateron,whendiscussingmodelresultsinSection5.
Weusethefollowingbasicequation(Tinsley1980)
Gnedin et al. (2002) used simple dynamical modeling to
demonstrate that if ωCen had always evolved in isolation on its dG (t) dGin(t) dGout(t)
i =−X (t)ψ(t)+R (t)+ i − i (1)
presentorbitintheMilkyWay,itwouldhavebeenunabletoretain dt i i dt dt
thes-process-richwindmaterialfromAGBstarsbecauseofstrip-
to track the evolution of the fractional gas mass in the form
ping from ram pressure during many passages through the disc. of element i normalized to the initial gaseous mass, G (t) =
SubsequentnumericalsimulationsdemonstratedthatanωCen-like X (t)M (t)/M .ThequantityX (t)representstheabuindance
objectcanoriginatefromanucleateddwarfgalaxyintrudinginto i gas bar i
theMilkyWay.Boththeorbitalparametersandtheobservedsur-
facebrightnessprofileofthepresent-dayωCenarereproducedby † Noticethat‘instantaneous’actuallymeans‘ontime-scalesshorterthan
atidaldisruptionscenariowherethefallingdwarfhasitsouterstel-
theadoptedtimestepforintegrationoftheequations’.
larenvelopealmostcompletelystripped,whereasacentral,dense ‡ Wecomputethebindingandthermalenergiesofthegasaccordingtothe
nucleusstillsurvivesowingtoitscompactness(Bekki&Freeman
recipesofBradamante,Matteucci&D’Ercole(1998).However,weadopt
2003; Tsuchiya et al. 2004; Ideta & Makino 2004). Interestingly atypicalefficiencyofthermalizationfrombothTypeIIandTypeIaSNeof
enough, also the number fraction (25 per cent) of the bMS stars ηSNII=ηSNIa=0.20,ratherthan0.03(seeRomano,Tosi&Matteucci2006,
withextremeheliumenhancement(∆Y ≈0.12–0.14;Norris2004; andreferencestherein).
(cid:13)c 2006RAS,MNRAS000,1–11
4 D. Romanoet al.
by mass of the element i at the time t; by definition, the sum- up with a stellar mass Mstars(t = 3) ≃ 5 × 107–108 M⊙. Given
mation over all the elements inthe gasmixture isequal tounity. thesimilarityoftheresults,inthecaseofthedwarfgalaxyparent
Thefirsttermontherighthandsideaccountsforgasconsumption wewillshowonlythepredictionsforourmostmassivemodel(ν=
by star formation; the second term on the right hand side refers 0.35Gyr−1,wmax≃5Gyr−1).
i
togasreturnbydyingstars.Allthecomplicateddependencieson Thoughinourmodelsthestarformationactivitylasts∼3Gyr,
the adopted stellar initial mass function and lifetimes, SNIa pro- itisworthnotingthatmostofthestars(nearly75percentofthe
genitors and stellarnucleosynthesis products hidden intheR (t) clusterpopulation)actuallyformduringthefirst1Gyr.Inthedwarf
i
termarenot madeexplicit here; theinterestedreader can findall progenitorscenario,whilethestarformationproceeds,thegalaxy
of them discussed in considerable detail in Matteucci & Greggio getsalmostcompletelydepletedofitsgas,partlyowingtogascon-
(1986).Sufficeitheretosaythatinthisworkweuseanextrapo- sumption by the adopted long-lasting star formation activity, but
latedSalpeter(1955)IMForaScalo(1986)IMF,bothnormalized mostofallbecauseofefficientgasremovalthroughthelarge-scale
tounityoverthe0.1–100M⊙ stellarmassrange,toshowthepre- galacticoutflows.
dictionsfromboth steepand flat,although ‘standard’, IMFs.The ThechemicalpropertiesofourωCenprogenitorsystemsare
adoptedstellarnucleosynthesisprescriptionsarediscussedinSec- discussed and compared with the available observations in Sec-
tion3.3. tion4.
InourmodelstheSFRisasimpleSchmidt’s(1963)law:
ψ(t)=νGk(t). (2)
3.3 Nucleosynthesisprescriptions
Thequantityν isthestarformationefficiency,namelytheinverse
Inthisworkweadoptthemetallicity-dependentyieldsofvanden
ofthetypicaltime-scaleforstarformation,andisexpressedinunits
Hoek & Groenewegen (1997) for single low- and intermediate-
ofGyr−1.Theexponentkissettobe1. massstars(LIMSs;0.96 m/M⊙ 6 8)andtheyieldsofNomoto
ThelasttwotermsinEquation(1)accountforanygasinflow
etal.(1997)formassivestars(136m/M⊙670).Theselatterare
and/oroutflow.Fortheclosedsystem,allthegasavailableforstar
computedforasolar chemicalcomposition ofthestars.Thestel-
formationisinsituwhenthestarformationbeginsatt=0,andno
lar yields are then scaled to the current metallicity of the model
infallfromoutsideisconsidered.Fortheopensystems,therateof
by means of the production matrix formalism (Talbot & Arnett
gasinfallisparametrizedas
1973).We(arbitrarily)uselinearinterpolationsandextrapolations
dGin(t) Xine−t/τ to cover the 1–100 M⊙ stellar mass range. The effect of adopt-
dit = τ(1−ie−tnow/τ), (3) ingdifferentyieldsetswillbethoroughlyanalyzedinaforthcom-
ingpaper(Romanoetal.,inpreparation;noticethatchangingthe
withτ,theinfalltime-scale,settobe0.5Gyr,andXin,theabun- adopted yieldsetsisnot expected toaffect significantly themain
i
dances of the infalling gas, set totheir primordial values. In par- resultspresentedinthispaper).
ticular,weassume YP =0.248, inagreement withthepredictions For stars evolving in binary systems which will give rise
of the standard big bang nucleosynthesis (SBBN) theory and the to SNIa explosions, the nucleosynthesis prescriptions are from
constraintsfromthecosmicmicrowavebackground(CMB;seeRo- Iwamotoetal.(1999).Inourmodels,asubstantialfractionofCu
manoetal.2003,andreferencestherein). and Zn is produced by SNeIa, following the suggestions of Mat-
TherateofgaslossviaSN-drivenlarge-scaleoutflowsisdif- teuccietal.(1993).
ferentfromzeroonlyfortheopenmodels,andsimplyproportional
totheamountofgaspresentatthetimet:
dGiout(t) =w X (t)G(t). (4) 4 RESULTS
dt i i
4.1 Theclosed-boxpicture
The quantity w is a free parameter which describes the effi-
i
ciencyofthegalacticwind;itisexpressedinGyr−1andmayhave Inthissectionwebrieflydiscusstheresultsobtainedintheframe-
different values for different elements (e.g. Recchi, Matteucci & workoftheclosed-boxself-enrichmentscenario,whereanymatter
D’Ercole2001). exchange betweentheproto-ωCenanditsenvironment isstrictly
forbidden.
InFig.2,thetheoreticalmetallicitydistributionasafunction
3.2 Theevolutivecontext of[Fe/H]or[Ca/H](thicksolidlines)iscomparedwiththephoto-
metricandspectroscopic empirical ones(thindashed histograms;
AccordingtoBekki&Freeman(2003,andreferencestherein),in
thedwarfprogenitorscenariotheinitialstellarmassofωCen’shost Norriset al. 1996; Suntzeff & Kraft 1996; Sollimaet al. 2005a).
was significantly higher than that currently observed and can be This comparison shows the shortcomings of this simple picture.
Inthecontextofaclosed-boxevolution,metallicitiesmuchhigher
estimatedas
thanobserved arequicklyattained,sothatmostof thestarsform
M
M = ωCen , (5) frommatterwithachemicalcompositionfromonetenthofsolarto
dwarf
(1−f )f
lost n supersolar,atvariancewiththeobservations.
where f = 0.05 is the mass fraction of the compact nucleus TheresultsdisplayedinFig.2arefortheScalo(1986)IMF,
n
and flost = 0.2 is the stellar mass fraction that gets lost through that – owing to its steeper slope for m > 2 M⊙ with respect to
long-term (∼10 Gyr) tidal interaction with the Milky Way. If Salpeter’s, x = 1.7 rather than 1.35 – allows a lower fraction of
MωCen ≃ 5 × 106 M⊙ (Meylan et al. 1995), one gets Mdwarf = high-massstarstopollutetheISMwiththeirmetal-richejecta.Ob-
1.25×108M⊙.SinceMωCenmightbeafactoroftwolower(van viously,byadoptingaSalpeterIMFthepredicteddistributiongoes
deVenetal.2006),werunseveralmodels,startingwithaninitial towards even higher metallicities. Flattening the IMF in the very
gaseousmassMgas(t=0)=Mbar=5×108–109M⊙andending lowstellarmassdomain,asinaKroupaetal.’s(1993) IMF,does
(cid:13)c 2006RAS,MNRAS000,1–11
The chemicalevolutionofω Cen 5
Figure2.Comparisonofobserved(thindashedhistograms)andpredicted Figure3.Comparisonofobserved(thindashedhistograms)andpredicted
(thicksolidlines)metallicitydistributionfunctionsofωCenstars.Theoret- (thicksolidlines)metallicitydistributionfunctionsofωCenstars.Theoreti-
icalpredictionsrefertotheclosed-boxself-enrichmentscenario.Thethe- calpredictionsrefertotheevolutivepicturewhereωCenistheremnantofa
oreticaldistributionsareconvolvedwithaGaussianofdispersionσ=0.2 largersystemevolvedinisolationandthenaccretedandpartiallydisrupted
dexinorderto(generously)takeobservationalerrorsintoaccount. bytheMilkyWay.ThetheoreticaldistributionsareconvolvedwithaGaus-
sianofdispersionσ=0.2dexinorderto(generously)takeobservational
errorsintoaccount.
notpreventthesystemfromsuddenlyreachingtoomuchhighmetal
abundances.Evenintheframeworkofextremescenarioswherethe
formation of SNIa progenitors is suppressed and lower Fe yields metallicitysystem.Thus,theprogenitorofωCenmusthavebeena
fromcorecollapseSNeareassumed(Chieffi&Limongi,inprepa- massiveenoughsystemtoallowdynamicalfrictiontodragittothe
ration),westillfailtoreproducetheobservedMDF.Sincewithin innerGalacticregions(Bekki &Freeman2003). Inlightof these
theschemeofaclosed-boxself-enrichmentwecannotfulfileven considerations,inthefollowingwediscusstheresultsforanucle-
such a basic observational constraint, we deem it meaningless to ateddwarfhostingωCenwithinitialmassMbar=109M⊙.Aftera
analyzefurtherthemodelresultsandswitchtothe‘open’scenar- 3Gyrevolution,theparentsystemhaslostmostofitsgasthrough
ios. galacticwindsandendedupwithastellarmassMstars ∼108M⊙,
Indeed,onlyifasubstantialfractionofSNejectaescapesfrom whichisconsistentwiththatofωCen’sparentgalaxyaccordingto
thecluster,theobservedMDFcanbereproduced. Ourresultsup- thecomputationsofBekki&Freeman(2003).
portsthefindingsofIkuta&Arimoto(2000) that significantout-
flowfromtheclusterisneededinordertoreducetheeffectiveyield
perstellargeneration.Ikuta&Arimoto(2000)obtainedthebestfit 4.2.1 Stellarmetallicitydistributionandage-metallicityrelation
to the observed MDF with a model involving gas outflow, infall
TheobservedMDFoflong-livedstarsinagalaxyisanimportant
atthevery earlystage ofchemical evolutionand abimodal IMF.
record of its past evolution. In fact, the relative numbers of stars
However,thedurationofstarformationfortheirbest-fitmodelwas
whichformedatanymetallicitytestifytheinterplayoffundamental
only0.28Gyr,muchshorterthanthatinferredfromcurrentobser-
processessuchasstarformation, infallofgasfromthesurround-
vationsandassumedhere.
ingsandmetal-enrichedgasoutflowsatanytime.Reproducingthe
currentlyobservedMDFofωCen’sstarssignificantlyrestrainsthe
freeparameterspaceofourdwarfgalaxymodel.Wefindthatafast
4.2 Astrippeddwarfgalaxy?
earlycollapsecoupledwithanintense(perunitmass)starforma-
Anopen,small-massmodelhenceoffersaviablesolution,butthe tionactivity,hψi≃0.1M⊙ yr−1 duringthefirst1Gyrevolution,
questionable assumption has to be made that, while SN products givesrisetoadistributionpeakedat[Fe/H]∼ −1.6,asobserved
easily leavethe cluster, thestellar wind ejecta arecompletely re- (Fig.3, leftpanel), thanks to thestrong galactic wind whicheffi-
tainedandthesurroundingmediumremainsunperturbed.Thecur- cientlyremovesthemetalsfromtheproto-ωCen.Inourmodel,the
rentmassofωCenisunlikelytogeneratesufficientdynamicalfric- thermal energy of the gas exceeds its binding energy nearly 200
tion to modify its orbit to its present small size (Majewski et al. millionyearsaftertheonsetofthestarformation,owingtomulti-
2000).Dynamicalmodeling,however,pointsoutthatthelongand pleSNexplosions:thegasisthensweptawaybyastronggalactic
complex star formation history of ωCen is inconsistent with the windandthestarformationslowlyfades.Weimposethatthewind
cluster originating on itspresent orbit: withaperiod of only 120 isdifferential,namelythattheSNejectaleavethegalaxymoreeas-
Myr(Dinescuetal.1999),thefrequentdisccrossingswouldhave ily than the stellar wind ejecta (see also Recchi et al. 2001). In
swept out all the intracluster gas very soon, leading to a mono- particular, by assuming a wind efficiency w ≃ 5 Gyr−1 for the
i
(cid:13)c 2006RAS,MNRAS000,1–11
6 D. Romanoet al.
Figure4.PredictedAMRforωCen(thicksolidline)comparedto(i)the
AMRinferredfromasampleof∼250SGBstarswith[Fe/H]errorsless
than0.2dexandageerrors lessthan 2Gyr,fortwodifferent choices of
distance modulus and reddening (stars; Hilker et al. 2004); (ii) the age-
metallicity diagram fortheMSTOsampleofStanfordetal. (2006;dots,
only stars with V < 18 are considered). Notice, however, that the most
metal-richstarsat[Fe/H]> −1.0mighthaveanaccretedoriginandages
comparabletothatofthemainclusterpopulation(seetextforreferences).
SNejecta,nostarswith[Fe/H]> −0.4dexareformed,inagree-
mentwiththeobservations(Fig.3,leftpanel).Suchhighoutflow
rates–morethantentimestheSFR–areoftenrequiredtorepro-
ducethehigh-qualitydataofnearbydwarfspheroidals(Lanfranchi
&Matteucci2003,2004).InFig.3wecomparethepredictionsof
thismodel(thicksolidlines)withthesameobserveddistributions
ofFigs.1and2.Boththesteepriseatlowmetallicitiesandtheex-
tendedmetallicitytailarewellreproduced;inparticular,theagree-
ment with the up-to-date distribution of Sollima et al. (2005a) is
strikinglygood.Thisisencouraging,butthemodelhastobetested
against many more observational constraints inorder toprove its
validity.
InarecentstudybyHilkeretal.(2004;seealsoKayseretal.
2006), newly derived spectroscopic abundances of ironfor ∼400
ωCenmembers havebeenused incombination withthelocation
ofthestarsintheCMDtoinfertheage-metallicityrelation(AMR)
ofthesystem.Thesuggestedagespreadisabout 3Gyrandthere
is some indication that the AMR could level off above [Fe/H] ≃
Figure 5. Predicted (solid lines) versus observed (filled circles; see text
−1.0dex. Thisisclearly seen inFig.4, wherethe starswiththe
forreferences) abundance ratios ofseveral chemical species to iron as a
errorbarsrepresenttheAMRforthesubsampleofstarswithmost
function of[Fe/H].Theαelements oxygen(panel a),magnesium (panel
reliableageandmetallicitydeterminations,fortwochoicesofthe b),silicon(panelc)andcalcium(paneld)areshown,aswellastheiron-
reddeninganddistancemodulusvalues(seeHilkeretal.2004;their peakelementcopper(panele).Conservativeerrorsareshownintheright
table 1). Evidence for the most metal-rich stellar populations of handcornerofeachpanel. Thecrossesinpanelemarkthetimeelapsed
ωCenbeingyoungerby2–4Gyrthanthemostmetal-pooronehas sincethebeginningofstarformation(0.1,0.3,1,2,2.6Gyr);theyarenot
been found also by Stanford et al. (2006) from extensive Monte superimposed on the track in all panels in order to avoid confusion and
§ overcrowdedplots.
CarlosimulationsontheirMSTOdata(Fig.4,dots) .Althougha
largedispersionispresentinthedata,ourmodelclearlypredictsthe
correctrunofmetallicitywithage(Fig.4,thicksolidline).Notice
thatinourmodeltheSFRgoestozeroaftera3Gyrevolution,when 4.2.2 Abundanceratios
[Fe/H]≃−0.4dex,becauseofthestronggalacticoutflowswhich InFig.5wedisplayourpredictionsforseveralabundanceratiosas
efficientlyremoveanyresidualgasfromthegalaxy(seeprevious
afunctionof[Fe/H](thicksolidlines).ThedatadisplayedinFig.5
paragraph).
(circles) have been collected fromthe literature. Data from high-
resolution optical spectra (Franc¸ois, Spite & Spite 1988; Brown
&Wallerstein1993; Norris&DaCosta1995; Smithet al. 1995,
§ Notethatsuchanagedifferenceseemsinconsistentwiththeresultsob- 2000;Cunhaetal.2002;Pancinoetal.2002;Vantureetal.2002)
tainedbyFerraroetal.(2004)andSollimaetal.(2005b)fromthemagni- areshownassmallcircles.Datafromlow-andmedium-resolution
tudelevel,shapeandextensionoftheturnoffregion. infraredspectra(Origliaetal.2003)areshownasbigcircles.
(cid:13)c 2006RAS,MNRAS000,1–11
The chemicalevolutionofω Cen 7
Wehavecheckedthatallliteratureabundanceratiosareinrea-
sonableagreementwitheachother(weconsidertwostudiesinrea-
sonableagreementwhentheabundancesofthestarsincommondo
not differ,onaverage,bymorethan0.1dex, whichisthetypical
uncertainty of the measurements). Asfar as [Fe/H]is concerned,
mostpapersagreewitheachotherwiththeexceptionsofFranc¸ois
etal.(1988),Pancinoetal.(2002),Vantureetal.(2002)andOriglia
etal.(2003),allhaving[Fe/H]∼0.2dexlower,whichisamarginal
(∼2σ)discrepancy.Wethereforecomparedatmosphericparame-
ters(T ,logg andv ),atomicdata(loggf)andequivalentwidth
eff t
(EW)measurementsamongtheabovestudies,tofindpossibleex-
planationsforthediscrepancies.InthecaseofFranc¸oisetal.,the
turbolent velocities are significantly lower (∼ 0.5 km s−1) and
theadoptedsolarcompositionissignificantlyhigher,logε(Fe)⊙=
7.67.Vantureetal.haveinsteadamuchlowerv (by0.4kms−1)
t
than average, and loggf slightlylower (∼ 0.06 dex). In the case
Figure6.TheoreticalTypeII(solidline)andTypeIa(dashedline)SNrates
ofPancinoetal.,allparametersappearingoodagreementwiththe (numberpercentury)fortheωCenprogenitor.TheSNIarateismultiplied
otherliteraturesources,butfortheonlystarincommon(ROA371) byafactoroftentomakeitclearlyvisible.
thereisanaveragedifferenceinEWof∼6.5mA˚.Allthesefactors
togethersuggestthatweshouldrevisetheFranc¸oisetal.,Pancino
etal.andVantureetal.[Fe/H]valuesupwardsby0.2dex.Origliaet seenfromfig.6ofCunhaetal.(2002).However,ifweadoptthe
al.haveonlystarsincommonwithPancinoetal.,withdifferences Grevesse&Sauval(1998)solarcompositionforCunhaetal.,asin
inTeff(∼150K),logg(∼0.3dex)andvt(∼0.3kms−1),butvery Fig.5,thediscrepancyrisesto0.3dex([Cu/Fe]=−0.61inCunha
similarabundances. Evenifwedonot havethetoolstofullyun- et al.). We choose to use the same solar abundance (Grevesse &
derstandthecauseofthediscrepancies(themethod,resolutionand Sauval1998),toputintoevidencetherealdiscrepancybetweenthe
spectralrangeemployedbyOrigliaetal.areverydifferentfromthe twostudies,whichreflectstheactualuncertaintyinthederivation
restofthepapers),wechoosetorevisethe[Fe/H]valuesupwards ofCuabundances.
by0.2dex,inconformitywithwhatdoneforFranc¸oisetal.(1988), Theoretical values are normalized to the solar ones of
Pancinoetal.(2002)andVantureetal.(2002). Grevesse&Sauval(1998).Elementssuchascarbonandnitrogen
Concerningtheα-elements,thereisagoodagreementamong arenotconsidered,astheiroriginalabundancescouldbeeasilyal-
various studies, except for a few notable exceptions. Oxygen is teredintheatmospheresofthesampledgiantstars¶.Theneutron
a very difficult element to measure, since only one line, [OI] at capture elements Y, Ba, La and Eu deserve special attention and
6300 A˚,isusedbymost authors, thatliesinaregionplagued by willbetreatedindetailelsewhere(Romanoetal.,inpreparation).
atmospheric O2 absorption and is blended with a NiI and a ScII It can be seen that the model reproduces satisfactorily the
line.However,someauthorsalsoincludetheweak6363A˚ linein decreasing trend of various [α/Fe] ratios with time (metallicity),
their analysis, whileonlypart of theauthors performafull spec- with the possible exception of Mg, which is underestimated for
tralsynthesis ontheregion. Inspiteof this,theabundance deter- [Fe/H]>−1.0.Appropriateadjustmentoftheadoptednucleosyn-
minations are in reasonable agreement with each other, with the thesisprescriptionsandIMFslopewouldmakethemodelpredic-
marginalexceptionofVantureetal.(2002),thatwechoosetoleave tionsfitthedataevenbetter.Indeed,Franc¸oisetal.(2004)findthat
asitis,sincetheoverallscatterin[O/Fe]issignificantlylargerthan theNomoto etal.(1997) yieldsofMg weareusingneedsignifi-
forotherelements(seeFig.5).ForMgI,Smithetal.(2000)have cantcorrectionstobest fittheabundance dataofverymetal-poor
anabundance ∼ 0.2 dexhigher than average, which ispartlyex- Galactichalostars,andthestellarmassfunctionisknowntoflat-
plainedbythelowerloggf (∼0.1dex)oftheemployedlines.We teninthe very-low stellar massdomain (e.g.Kroupa et al. 1993;
therefore lower the Smith et al. [Mg/Fe] abundances by 0.1 dex, Chabrier 2003). However, what really matters here is to provide
bringingtheminreasonableagreementwithotherdeterminations. an overall, coherent interpretative framework for the whole body
Silicon abundances are a tricky business, since there isa general ofdataforωCen,ratherthantoattempttopreciselyfitaspecific
disagreementamongauthors(onlyFranc¸oisetal.1988andSmith observationalconstraint.
etal.2000appeartobeonthesamescale),butnoclearcausefor The high ratios of [O/Fe], [Mg/Fe], [Si/Fe] and [Ca/Fe] for
thediscrepanciesemergesfromouranalysis,sowechoosetoapply [Fe/H]< −1.2clearlypoint toSNeIIasthemajordriversof the
nocorrectionforthisparticularelement.TheCaIdeterminationby chemical enrichment. Yet, the indication of a decrease of these
Norris&DaCosta(1995)is∼0.2dexhigherthanallotherstudies abundanceratiosintheintermediateandmetal-richsubpopulations
consideredhere,thatisfullytakenintoaccount bythe∼0.2dex (Pancinoetal.2002;Origliaetal.2003)aretheunmistakablesign
lowerloggf valuesadoptedbythoseauthors,sowereviseNorris ofsignificantTypeIaSNpollution,occuringatlatertimesbutbe-
&DaCosta’s[Ca/Fe]downwardsby∼0.2dex. forestarformationstops.
Finally, only two papers have studied Cu in ωCen so far, Asfarascopperisconcerned,itisworthstressingthattheflat
namelyPancinoetal.(2002)andCunhaetal.(2002),havingonly behaviourofthe[Cu/Fe]dataindicatesnoevolutioninthecopper-
onestarincommon,ROA371.Thelinelistforthesynthesisofthe
CuI5782A˚ lineusedisdifferentinthetwopapers. Theadopted
atmosphericparametersareverysimilar,butthesolarcopperabun- ¶ Actually, a large scatter is present also in the abundances of oxygen.
dances differ by 0.09 dex. All this together makes an ∼ 0.2 dex AlthoughcommonlyexplainedasthesignatureofprocessingintheCNO
differencebetweenthetwostudiesofROA371([Cu/Fe]=−0.33 cycle(e.g.Norris&DaCosta1995),itisworthstressingthatitcouldbeat
inPancino et al.and [Cu/Fe]= −0.52 inCunha et al.)as can be leastpartlyduetothewealthofobservationalproblemsdiscussedabove.
(cid:13)c 2006RAS,MNRAS000,1–11
8 D. Romanoet al.
to-iron yields over much of the chemical evolution within ωCen
(see Fig. 5, panel e, circles, and Cunha et al. 2002, their fig. 6).
Yet, from Fig. 5, panel e, it is seen that our chemical evolution
model(thicksolidcurve)overpredictsthecopper-to-ironratioover
the whole metallicity range. The predicted behaviour of [Cu/Fe]
results from the assumption that the major astrophysical site for
the synthesis of Cu are SNeIa (Matteucci et al. 1993), and these
stellar factories happen to contribute to the chemical enrichment
ofωCenalreadyatthelowestmetallicities.Thisisapparentfrom
Fig.6,whereweshowtheTypeII(solidline)andTypeIa(dashed
line)SNratespredictedbyourmodelforωCen.TheSNIaratehas
beenmultipliedbyafactoroftentomakeitclearlyvisible.SNeII
appear soon (a few million years) after their massive progenitors
are born, thus their rate closely follows the SFR. SNeIa, instead,
comefromintermediate-tolow-massstellarprogenitorsinbinary
systems (Matteucci & Greggio 1986), which explode on varying
Figure7.RelativeHeabundanceasafunctionofmetallicity.Thetheoret-
time-scales: they start to contribute significantly to the chemical ical relation (thick line) is compared tothe empirical one obtained from
enrichment of the proto-ωCen ∼400 Myr after the beginning of RRLyræstardata(filledcircles;seeSollimaetal.2006a,andreferences
starformation,whentheISMhasattainedametallicityof[Fe/H]≃ therein, fordetails about the derivation of the relative Heabundances in
−1.6(Fig.6,upperxaxis),whilethemaximumenrichmenttakes thesevariablestarsviathemass-luminosityparameterA).Theboxrepre-
k sentsthelevelofHeenhancementrequiredinordertoexplainthebMSdata
placeonlyseveralMyrlater,att≃1.1Gyr,when[Fe/H]≃−1.2 .
(Norris2004;Piottoetal.2005).
Lateron, att ≃2.6Gyr([Fe/H]≃ −0.6),thecontributionfrom
SNeIanearlyequalsthatfromSNeII,butonlyafewstarsformfrom
this SNIa-enriched material before the star formation stops. It is
thelevelofHeenhancementrequiredtoexplainthebMSandhor-
worthemphasizingherethatthemetal-enrichedoutflowlengthens
izontal branch (HB) data (∆Y = 0.12–0.14; Norris 2004; Piotto
theenrichmenttime-scale,i.e.thetimethatittakesfortheISMto
et al. 2005). Yet, our model predictions are fully consistent with
reachagivenmetallicity:intheabsenceofsuchanenrichedwind,
thetinychangeinheliumabundance(∆Y =0.035±0.066)over
it would take only 0.7 Gyr for the ISM to reach a metallicity of
awidemetallicityrange(−2.26[Fe/H]6 −1.1)impliedbythe
[Fe/H]≃−1.2and1.8Gyrtoreach[Fe/H]≃−0.6.
observations of 74 RR Lyræ variables by Sollima et al. (2006a).
In their 1993 paper, in order to fit the solar neighbourhood
Thefactthatthereisapopulationofmetal-intermediateRRLyræ
data, Matteucci and coworkers adopted Cu yields from SNeIa a
starsinωCen withluminosity and pulsational properties that are
factorof100higherthanpredictedbycurrentSNIamodels.When
incompatible with a significant helium overabundance adds new
adopting the lower Cu yields from SNeIa predicted by Iwamoto
complexity to the peculiar chemical features of ωCen, as it en-
etal.(1999),wefindaflatbehaviourof[Cu/Fe]versus[Fe/H]in
tailsthe coexistence of twopopulations withsimilar metallicities
ωCen (Fig. 5, panel e, thin solid curve). While this is consistent
butverydifferentheliumabundancesinthecluster(Sollimaetal.
withthedataofCunhaetal.(2002),itdoesnotaccountfortherise
2006a).
for[Fe/H]>−1.0pointedoutbyPancinoetal.(2002).Clearly,the
ThestarsonthebluesideoftheMSshownospreadinmetal-
issueofcopperproductioninstarsneedtobefurtherinvestigated.
licity(Piottoetal.2005),thussuggestingthattheyformedfroma
Apartfromtheflatness,anevenmorestrikingfeatureistheoverall
well-homogenizedmedium.Ontheotherhand,inordertoelevate
Cudeficiencyinconjunctionwithastrongs-processenhancement.
Y fromitsprimordialvalue0.248to∼0.40onemustassumethat
ThesamechemicalsignatureappearstocharacterizetheSagittar-
the material from which the bMS stars formed was made up al-
iusdwarf spheroidal galaxy, thus lending support totheideathat
mostentirelyofpureejectafrompreviousstellargenerations(Nor-
ωCenistheremainingnucleusofanaccreteddwarfGalacticsatel-
ris2004).Infact,aslongasthefreshejectaofdyingstarsaredi-
lite(McWilliam&Smecker-Hane2005).
lutedandmixedupwithpre-existinggasintheframeworkofaho-
mogeneouschemicalevolutionmodel,thereisnowayofattaining
asignificantheliumenrichmentoftheISMbythetimethemetal-
4.3 HeliumenrichmentinωCen
licity increases from [Fe/H] ∼ −2.0 to ∼ −1.2 dex (see Fig. 7,
In this section, we explore the issue of the helium enrichment in solidline).Thisresultisindependentoftheadoptedstellaryields,
ωCenwithinthepicturewherethisclusteristhecompactsurvivor ascanbeimmediatelyunderstoodfromaninspectionofFigs.8and
of alargersatellitesystem ingestedby theMilkyWaymanyGyr 9.There,weshowthemassfractionofHeintheejectaofLIMSs
ago. andmassivestarsasafunctionofstellarmass,calculatedas
In Fig. 7 we compare the theoretical ∆Y versus [Fe/H] re- Y =Y + mpHe , (6)
ejecta ini
lation(thick solidline) tothe data (box and filledcircles). When m−mrem
using standard stellar yields (see Section3.3), after a 3 Gyr evo- whereY istheinitialheliumabundanceofthestar,p isthestel-
ini He
lutionweobtainanegligibleincreaseoftheHeabundanceinthe laryield,namelythefractionalmassofthestarofinitialmassm
ISM(∆Y =0.01),thusunderestimatingbyanorderofmagnitude which is restored to the ISM in the form of newly produced He,
andm isthemassoftheremnant. Thequantitiesdisplayedin
rem
Fig. 8 have been computed using the tables of van den Hoek &
k Noticethatabimodaldistributionofthedelaytimesfortheexplosion, Groenewegen (1997) for LIMSs and Nomoto et al. (1997; stars)
with‘prompt’and‘tardy’events,hasrecentlyproventobestmatchpresent andWoosley&Weaver(1995; filledcircles)formassivestars.In
SNIadata(Mannucci,DellaValle&Panagia2006). thecaseofvandenHoek&Groenewegen’syields,thebiggerthe
(cid:13)c 2006RAS,MNRAS000,1–11
The chemicalevolutionofω Cen 9
much lower Heamount (Fig.9). Since, according totheSalpeter
IMFweareusing,ineachstellargenerationabout 85percentof
themassfallsinthe0.1–8M⊙ massrange,itiseasytoguessthat
afterthe∼1Gyrevolutionneededtoachieve[Fe/H]≃−1.2inthe
ISM, the average helium abundance of the medium out of which
thebMSstarsformwillbedefinitelylowerthanrequired.Indeed,
whenincludingtheyieldsoftheGenevagroupinourhomogeneous
chemicalevolutionmodel,wealwaysfindY <0.30(∆Y <0.05).
WewillcomebacktotheissueoftheheliumenrichmentinωCen
inafuturepaper(Romanoetal.,inpreparation).
5 DISCUSSIONANDCONCLUSIONS
Inthisworkwestudytheformationandevolutionoftheanomalous
Figure8.MassfractionofHeintheejectaofLIMSsaswellasmassive globular cluster ωCen. The constraints established by the wealth
starsasafunctionofstellarmass.Yejectavalueswerecalculatedusingthe ofverygood-qualityabundancedataforitsmemberstarscollected
tables byvandenHoek&Groenewegen (1997)forLIMSsandWoosley overthelastdecadesignificantlyrestraintheevolutivepicture.We
&Weaver(1995;filledcircles)andNomotoetal.(1997;stars)formassive showthatintheframeworkoftheclosed-boxself-enrichmentsce-
stars.InthecaseofvandenHoek&Groenewegen’syields,thebiggerthe nario(stillanoftenusedapproximation)themetallicitydistribution
sizeofthesymbol,thehighertheinitialmetallicityofthestar(Z=0.001,
functionofωCen’sstarscannotbereproduced.Ontheotherhand,
0.004,0.008).Forcomparison,theY valuesuggestedforthebMSstarsof
themainchemicalpropertiesofωCenarenicelyreproducedifitis
ωCenisalsodisplayedasadashedline.Theadoptedstellarlifetimesare
thecompactremnantofadwarfspheroidalgalaxyevolvediniso-
reported(forafewobjects)ontheupperxaxis.
lationandthenaccreted–andpartlydisrupted–bytheMilkyWay.
Thisevolutivepicturehasalreadyalloweddifferentauthorstoex-
plainthepresent-daypeculiarlocationneartheGalacticcentreand
surface brightness profile of ωCen (see, e.g., Bekki & Freeman
2003; Tsuchiya et al. 2004; Ideta & Makino 2004); our analysis
givesfurtherstrengthtotheaccreteddwarfpicturebyprovingthat
theingestedsatellitewouldalsohavesimilarchemicalpropertiesto
ωCen.Indeed,byassumingarelativelylong-lastingstarformation
activity(thoughwithmostofthestarsformingwithin1Gyr),stan-
dardIMFandstandardstellaryields,ourmodelssatisfactorilyre-
produceseveralobservedabundanceratiosasafunctionof[Fe/H].
Regarding the helium abundances of ωCen, a great deal of
work has been recently devoted to the enigma of the anomalous
helium-to-metalenrichmentofbMSandhotHBstars(Norris2004;
Leeetal.2005;Piottoetal.2005;Bekki&Norris2006;Maeder&
Meynet2006).Starswhichpopulatethe‘bluehook’regioncould
havesufferedanunusuallylargemasslossontheRGBand,con-
sequently,experiencedtheheliumcoreflashwhiledescendingthe
Figure9.MassfractionofHeintheejectaofrotatingstarsasafunction
white dwarf cooling curve (Moehler et al. 2002). Therefore, it is
ofstellarmass.Yejecta values werecalculated usingthetablesbyMeynet not clear which fraction of the He enhancement truly reflectsthe
& Maeder (2002), Hirschi (2006) and Meynet et al. (2006) fordifferent
pristine abundance. Stars on the bMS, instead, likely give more
initial metallicities and/or rotational velocities of the stars. Upside-down
triangles:Z=10−8;stars:Z=10−5;circles:Z=0.004.Emptysymbols: trustworthyindication:theypointto∆Y =0.12–0.14(∆Y/∆Z>
vini=800kms−1;filledsymbols:vini=300kms−1,exceptforthe9M⊙ 70).Recent observations ofRRLyræstarssuggest thatapopula-
and40M⊙ starsatZ =10−8,forwhichvini=500kms−1and700km tionwithanormal(i.e.nearlyprimordial)heliumcontentinhabits
s−1,respectively.Forcomparison,theY valuesuggestedforthebMSstars theclusteraswell,thuscomplicating theoverallpicture(Sollima
ofωCenisalsodisplayedasadashedline.Theadoptedstellarlifetimesare etal.2006a). Ourhomogeneous chemical evolutionmodel,when
reported(forafewobjects)ontheupperxaxis. adopting a3Gyr long starformation history, standard yieldsand
Salpeter’sIMF,predictsthattheheliumabundanceintheISMal-
mostdoesnotchangeduringthecluster’sprogenitorevolution,in
sizeofthesymbol,thehighertheinitialmetallicityofthestar(Z= agreement withtheRRLyræstardata,butinsharpdisagreement
0.001,0.004,0.008).ThequantitiesdisplayedinFig.9havebeen withtheabundancesinferredforthebMS.Wecompareexistinghe-
computed using the tables of Meynet & Maeder (2002), Hirschi liumyieldsforlow-metallicityrotatingstarstotheoutputsofstan-
(2006)andMeynetetal.(2006)forrotatingstars,fordifferentini- dardstellarevolutionwhichdoesnottakerotationintoaccount,and
tial metallicities and/or rotational velocities of the stars (see fig- findthatthewindsofmassivestarsofintermediaterotationveloc-
urecaption).Werefertothoseauthorsfordetailsaboutthestellar ities do indeed eject significant amounts of He (see also Maeder
modelassumptions.Inthecaseofthestandardyields(Fig.8),Y &Meynet2006).However,onceweightedwithanormalIMF,the
ejecta
isalwaysbelowthequantityneededtoexplainthebMSdata.Ro- heliumyieldofastellargenerationturnsouttobelowerthanthat
tatingstars, instead, doactually (slightly) exceed Y =0.38in requiredtoexplainthebMSanomalies(seeSection4.3).Itisworth
ejecta
thehigh-mass domain, but below 8 M⊙ theyalways contributea noticingatthispointthateveniftheescapeofSNejectathrough
(cid:13)c 2006RAS,MNRAS000,1–11
10 D. Romanoet al.
galacticwindshelpstokeeptheglobalmetalcontentofthegalaxy FreemanK.C.,1993,inSmithG.H.,BrodieJ.B.,eds,ASPConf. Ser.
low, thus favouring high ∆Y/∆Z values, it also leads to longer Vol.48,Theglobularclusters-galaxyconnection.Astron.Soc.Pac.,San
enrichmenttime-scales(seeSection4.2.2),thusallowinglow-and Francisco,p.608
intermediate-massstarstospreadtheirrelativelyhelium-poor gas FreemanK.C.,RodgersA.W.,1975,ApJ,201,L71
FrinchaboyP.M.,RheeJ.,OstheimerJ.C.,MajewskiS.R.,PattersonR.
throughtheISM.Adhocscenariosinwhichthewindsofmassive
J.,JohnsonW.Y.,Dinescu D.I.,PalmaC.,Westfall K.B.,2002, in
stars remain confined and pollute only the central regions where
vanLeeuwenF.,HughesJ.D.,PiottoG.,eds,ASPConf.Ser.Vol.265,
theHe-richMSstarsareactuallymoreconcentratedmayrepresent
OmegaCentauri: AUnique Window into Astrophysics. Astron. Soc.
awayoutoftheproblem.Heliumdiffusionhasrecentlybeenpro-
Pac.,SanFrancisco,p.143
posed asa viable mechanism tosignificantly increase the helium GnedinO.Y.,ZhaoH.,PringleJ.E.,FallS.M.,LivioM.,MeylanG.,2002,
contentofprotostellarclouds(Chuzhoy2006). ApJ,568,L23
AssuminganIMFstronglybiasedtowardsmassivestarscould GrattonR.G.,1982,A&A,115,336
alsosolvetheproblem,butcurrentobservationalevidenceseemsto GrattonR.,SnedenC.,CarrettaE.,2004,ARA&A,42,385
favouraSalpeter-likeIMFindwarfspheroidalsaswellasGalac- GrevesseN.,SauvalA.J.,1998,SpaceSci.Rev.,85,161
ticglobulars(Wyse2005,andreferencestherein).Anotheroption HarbeckD.,etal.,2001,AJ,122,3092
wouldbethatthebMSwasformedapartasastarclusterandthen Harris,W.E.,1996,AJ,112,1487
HilkerM.,RichtlerT.,2000,A&A,362,895
mergedwiththerMSinthecentralregionofωCen’shostgalaxy,
HilkerM.,KayserA.,RichtlerT.,WillemsenP.,2004,A&A,422,L9
apossibilityfirstpointedout byBekki &Norris(2006). Wenote
HirschiR.,2006,A&A,inpress(astro-ph/0608170)
that if the star cluster originated mostly from matter ejected by
HughesJ.,WallersteinG.,2000,AJ,119,1225
slowwindsfromrotatingmassivestarswith[Fe/H]∼ −1.2,then
IckeV.,AlcainoG.,1988,A&A,204,115
itwouldhaveboththehighheliumabundanceanduniformmetal- IdetaM.,MakinoJ.,2004,ApJ,616,L107
licityinferredfrombMSobservations.Measuringtheheliumabun- IkutaC.,ArimotoN.,2000,A&A,358,535
dancesofstarsmoremetal-richthan[Fe/H]∼−1.0wouldhelpus IwamotoK.,BrachwitzF.,NomotoK.,KishimotoN.,UmedaH.,HixW.
todiscriminatebetweenascenariowheretheheliumabundancein R.,ThielemannF.-K.,1999,ApJS,125,439
theISMmonotonically increaseswithtimeand ascenario where KayserA.,HilkerM.,RichtlerT.,WillemsenP.G.,2006,A&A,458,777
theformationofahelium-richpopulationisafortuitouseventoc- KochA.,GrebelE.K.,WyseR.F.G.,KleynaJ.T.,WilkinsonM.I.,Har-
beckD.R.,GilmoreG.F.,EvansN.W.,2006,AJ,131,895
curredonashorttime-scaleunderveryspecialconditions.Wewill
KroupaP.,ToutC.A.,GilmoreG.,1993,MNRAS,262,545
examinethesescenariosindetailinaforthcomingpaper,together
LanfranchiG.A.,MatteucciF.,2003,MNRAS,345,71
withthemostupdated(evenadhoc)nucleosynthesisprescriptions.
LanfranchiG.A.,MatteucciF.,2004,MNRAS,351,1338
LeeY.-W.,JooJ.-M.,SohnY.-J.,ReyS.-C.,LeeH.-C.,WalkerA.R.,1999,
Nat,402,55
ACKNOWLEDGMENTS LeeY.-W.,eta.,2005,ApJ,621,L57
LloydEvansT.,1983,MNRAS,204,975
We thank L. Stanford for kindly providing her data in machine- MaederA.,MeynetG.,2006,A&A,448,L37
readableformat. MajewskiS.R.,PattersonR.J.,DinescuD.I.,JohnsonW.Y.,OstheimerJ.
C.,KunkelW.E.,PalmaC.,2000,inNoelsA.,MagainP.,CaroD.,Je-
hinE.,ParmentierG.,ThoulA.A.,eds,TheGalacticHalo:FromGlob-
ularClustertoFieldStars.Institutd’AstrophysiqueetdeGe´ophysique,
REFERENCES
Lie`ge,p.619
AndersonA.J.,1997,Ph.D.Thesis,Univ.California,Berkeley MalliaE.A.,PagelB.E.J.,1981,MNRAS,194,421
Bedin L. R.,Piotto G.,Anderson J.,Cassisi S.,KingI. R.,Momany Y., MannucciF.,DellaValleM.,PanagiaN.,2006,MNRAS,370,773
CarraroG.,2004,ApJ,605,L125 MateoM.L.,1998,ARA&A,36,435
BekkiK.,FreemanK.C.,2003,MNRAS,346,L11 MatteucciF.,GreggioL.,1986,A&A,154,279
BekkiK.,NorrisJ.E.,2006,ApJ,637,L109 MatteucciF.,RecchiS.,2001,ApJ,558,351
BradamanteF.,MatteucciF.,D’ErcoleA.,1998,A&A,337,338 MatteucciF.,RaiteriC.M.,BussoM.,GallinoR.,GrattonR.,1993,A&A,
BrownJ.A.,WallersteinG.,1993,AJ,106,133 272,421
CarraroG.,LiaC.,2000,A&A,357,977 McWilliamA.,Smecker-HaneT.A.,2005,ApJ,622,L29
ChabrierG.,2003,PASP,115,763 MerrittD.,MeylanG.,MayorM.,1997,AJ,114,1074
ChuzhoyL.,2006,MNRAS,369,L52 MezaA.,NavarroJ.F.,AbadiM.G.,SteinmetzM.,2005,MNRAS,359,
CohenJ.G.,1981,ApJ,247,869 93
CohenJ.G.,BellR.A.,1986,ApJ,305,698 MeylanG.,MayorM.,DuquennoyA.,DubathP.,1995,A&A,303,761
CunhaK.,SmithV.V.,SuntzeffN.B.,NorrisJ.E.,DaCostaG.S.,PlezB., MeynetG.,MaederA.,2002,A&A,390,561
2002,AJ,124,379 MeynetG.,Ekstro¨mS.,MaederA.,2006,A&A,447,623
DinescuD.I.,2002,invanLeeuwenF.,HughesJ.D.,PiottoG.,eds,ASP MoehlerS.,SweigartA.V.,LandsmanW.B.,DreizlerS.,2002,A&A,395,
Conf.Ser.Vol.265,OmegaCentauri: AUniqueWindowintoAstro- 37
physics.Astron.Soc.Pac.,SanFrancisco,p.365 NomotoK.,HashimotoM.,TsujimotoT.,ThielemannF.-K.,KishimotoN.,
DinescuD.I.,GirardT.M.,vanAltenaW.F.,1999,AJ,117,1792 KuboY.,NakasatoN.,1997,Nucl.Phys.A,616,79c
FellhauerM.,KroupaP.,EvansN.W.,2006,MNRAS,372,338 NorrisJ.E.,2004,ApJ,612,L25
FerraroF.R.,BellazziniM.,PancinoE.,2002,ApJ,573,L95 NorrisJ.E.,DaCostaG.S.,1995,ApJ,447,680
FerraroF.R.,SollimaA.,PancinoE.,BellazziniM.,StranieroO.,Origlia NorrisJ.E.,FreemanK.C.,MighellK.J.,1996,ApJ,462,241
L.,CoolA.M.,2004,ApJ,603,L81 NorrisJ.E.,FreemanK.C.,MayorM.,SeitzerP.,1997,ApJ,487,L187
FerraroF.R.,SollimaA.,RoodR.T.,OrigliaL.,PancinoE.,BellazziniM., OrigliaL.,FerraroF.R.,BellazziniM.,PancinoE.,2003,ApJ,591,916
2006,ApJ,638,433 PancinoE.,FerraroF.R.,BellazziniM.,PiottoG.,ZoccaliM.,2000,ApJ,
Franc¸oisP.,SpiteM.,SpiteF.,1988,A&A,191,267 534,L83
Franc¸oisP.,MatteucciF.,CayrelR.,SpiteM.,SpiteF.,ChiappiniC.,2004, PancinoE.,PasquiniL.,HillV.,FerraroF.R.,Bellazzini M.,2002,ApJ,
A&A,421,613 568,L101
(cid:13)c 2006RAS,MNRAS000,1–11