Massive stars, globular clusters and elliptical galaxies

0.47 MB·English

by  Georges Meynet

Mem.S.A.It.Vol.79,3
(cid:13)c SAIt 2008 Memoriedella
Massive stars, globular
lusters and ellipti
al
galaxies
G. Meynet1 T. Decressin1 C. Charbonnel1,2
8
1 GenevaObservatory–UniversityofGeneva,CH-1290Sauverny,Switzerland
0
0 2 Laboratoired’AstrophysiquedeToulouseetTarbes,CNRSUMR5572,OMP,Universit
2 PaulSabatier3,14Av.E.Belin,31400Toulouse,France

n
a
J Abstract.GlobularclustersasωCenandNGC2808appeartohaveapopulationofvery
He-richstars.Fromatheoretical pointof view,oneexpectsthepresenceof He-richstars
0
inallglobularclustersshowinganoxygen-sodium(O-Na)anticorrelation.Inthispaper,we
1
brieflyrecallhowfastrotatingmassivestarscouldbethemainsourceofthematerialfrom

whichHe-richstarshaveformed.WespeculatethattheUV-upturnphenomenonobserved
]
h inallellipticalgalaxiesmightbeduetoHe-richstars.Ifthishypothesisiscorrectthenfast
p rotatingmassivestarsmayhaveplayedintheearlyevolutionaryphasesofthesesystemsa
- similarroleastheonetheyplayedinthenascentglobularclusters.
o
r
Keywords.Stars:massive–Stars:rotation–Galaxy:globularclusters–Galaxies:ellipti-
t
s cals
a
[

1 1. Chemicalanomaliesinglobular HIIregions.Avalueof4–5isconsistentwith
v clusters the chemicalyields fromsupernovae(Maeder
4
1992)formingblackholesaboveabout20–25
7 The findingof a doublesequencein the glob- M .Maeder&Meynet(2006)showedthatthe
5 ular cluster ω Cen by Anderson (1997) and wi⊙nd contributionsof low Z rotating stars are
1
thefurtherinterpretationofthebluersequence able to produce the high ∆Y/∆Z observed in
.
1 by a strong excess of helium constitutes a theblueMainSequenceinωCen.
0 major enigma for stellar and galactic evolu-
8 tion (Norris2004). Thebluersequencewith a Recent observations show that He-rich
0 metallicity [Fe/H]= -1.2 or Z = 2 · 10−3 im- stars in globular clusters might be a common
:
v pliesanHe–contentY = 0.38(0.35-0.45),i.e. feature.Forinstance,clusterslikeNGC 2808,
i an helium enrichment ∆Y = 0.14 (see also M13andNGC6441(Caloi&D’Antona2007)
X
Norris 2004). In turn, this demands a relative showawell-developedbluehorizontalbranch,
r helium to metal enrichment∆Y/∆Z of the or- andastrongslopeupwardfromtheredclump
a
derof70(Piottoetal.2005).Thisisenormous totheblueoftheRRLyraeregions.Bothfea-
and more than one order of magnitude larger tures could be explained if He-rich stars are
than the current value of ∆Y/∆Z = 4 − 5 present in these clusters. Kavirajetal. (2007)
(Pageletal.1992)obtainedfromextragalactic studied the UV and optical properties of 38
massive globular clusters in M87 (elliptical
Sendoffprintrequeststo:G.Meynet galaxy in virgo cluster). A majority of these
4 Meynet:Massiverotatingstars
clustersappearextremelybrightinthefar-UV. higher He, Na and Al abundances than their
Canonicalmodels(i.e.modelswithoutanysu- firstgenerationcounterparts.
per He-rich stars) would imply ages for these Any model aiming at explaining the
clusters about 3 - 5 Gyr larger than the age chemical properties of globular cluster stars
oftheUniverseaccordingtotheWMAPdata! should give an answer to the following ques-
This difficulty is removed when a super-He- tions: (1) Which type of stars did produce
rich(∆Y/∆Z >∼ 90)stellarcomponentissup- the material enriched in H-burning prod-
posedtobepresentintheseclusters. ucts? (2) What is the physical mechanism
It has also long been known that globu- responsible for selecting only material bear-
lar cluster stars present some striking anoma- ing the signatures of H-processing? (3) Why
lies in their content in other light elements1: does this process occur only in globular
whileinalltheGalacticglobularclustersstud- clusters?2 Prantzos&Charbonnel (2006) and
ied so far one finds “normal” stars with de- Decressinetal. (2007b,a) propose answers to
tailed chemical composition similar to that theabovethreequestions.Themainlineofthe
of field stars of same metallicity (i.e., same reasoningisrecalledbelow.
[Fe/H]),onealsoobservesnumerous“anoma-
lous”mainsequenceandredgiantstarsthatare
2. Slowwindsoffastrotating
simultaneously deficient (to various degrees)
massivestars
in C, O, and Mg, and enriched in N, Na, and
Al (forrecentreviewssee Grattonetal. 2004; Fast rotating massive stars, with an initial
Charbonnel 2005). Additionally, the abun- metallicity Z = 0.0005, with typical time av-
danceofthefragileLiwasfoundtobeanticor- eragedvelocityof500kms−1 onthemainse-
relatedwiththatofNainturnoffstarsinacou- quenceeasilyreachthecriticalvelocity3atthe
ple of globular clusters (Pasquinietal. 2005; beginning of their evolution and remain near
Bonifacioetal.2007). the critical limit during the rest of the main
Itisclearnowthatthesechemicalpeculiar- sequence and part of the central He-burning
ities have been inherited at birth by the low- phase (Decressinetal. 2007b; Ekstro¨metal.
mass stars we observe today in globular clus- 2007).Asaconsequence,duringthosephases
ters, and that their root cause is H-burning they lose large amounts of material througha
through the CNO-cycle and the NeNa- and mechanicalwind4,whichprobablyleadstothe
MgAl-chainsthatoccurredinanearlygenera- formationofaslowoutflowingkeplerianequa-
tionofmoremassiveandfasterevolvingglob- torialdisk.Thisisthekindofprocessbelieved
ular cluster stars. In other words, compelling tooccurinBestars(Porter&Rivinius2003).
evidence leads us to believe that at least two Thematerialejectedinthediskhastwoin-
generations of stars succeeded in all Galactic teresting characteristics: (1) due to rotational
globularclustersduringtheirinfancy.Thefirst mixingthattransportstheproductsoftheCNO
one correspondsto the bulk of “normal”stars cycleandoftheNe-NaandMg-Alchainsfrom
bornwith the pristinecompositionofthe pro- the coreto thestellar surface,it bearsthe sig-
tocluster gas; these objects are those with the natures of H-burning and presents abundance
highest O and Mg and the lowest He and Na patternssimilartothechemicalanomaliesob-
and Al abundances also found in their field served in the second generation stars (see for
contemporaries. The second generation con-
tainsthe starsbornoutof materialpollutedto 2 Indeed up to now the peculiar chemical pat-
variousdegreesbytheejectaofmoremassive terns observed in globular clusters, i.e., the O-Na
stars, and which presentlower O and Mg and andMg-Alanticorrelations,havenotbeenfoundin
fieldstars.
1 On the contrary, the content in heavy ele- 3 Thecriticalvelocityisthevelocityattheequa-
ments (i.e., Fe-group, α-elements) is fairly con- tor such that the centrifugal acceleration balances
stantfromstartostarinanywell-studiedindividual thegravity.
Galacticglobularcluster(withthenotableexception 4 Forexample,our60M modelloses24.3M in
⊙ ⊙
ofωCen). itsslowwind.
Meynet:Massiverotatingstars 5
more details Decressinetal. 2007b)5; (2) it
is released into the circumstellar environment
withaverylowvelocityandthuscaneasilybe
retained into the shallow potentialwell of the
globularcluster.Inthefollowingweshallcall
ittheslowwind.
Thedisk-starconfigurationlastsduringthe
whole main sequence for stars close to or at
criticalvelocity;forthemostmassivestarsthat
are at the ΩΓ-limit (i.e., when the surface lu-
minosity is near the Eddington limit), it per-
sistsontheLBVphase.Whenthestarevolves
awayfromthecriticallimitduetoheavymass
loss,theradiatively-drivenfastwindtakesover
and the disk is not supplied by the star any-
more. This happens during the central He-
burningphase,beforetheHe-burningproducts
reach the stellar surface and contaminate the
slowwindcomponent(bottompanelofFig.1).
Fromthatmomentonthehigh-speedmaterial
ejected by the star and by the potentialsuper-
nova will escape the globular cluster. We pro-
posed that this filtering mechanism, which re-
tainsinthepotentialwelloftheglobularclus-
ter only the slow stellar ejecta, is the phys-
ical mechanism responsible for supplying the
requiredH-processedmaterialforformingthe
Fig.1. Schematicview oftheevolutionoffastro-
starsofsecondgeneration.
tatingmassivestars.Thecoloursreflectthechemical
The large star-to-star spread in light ele-
compositionofthevariousstellarregionsandofthe
mentsobservedtodayindicatesthatallthema-
disk. (top) During the main sequence, a slow out-
terial ejected by the first generation massive flowingequatorialdiskformsanddominatesmatter
stars did not have the time to be fully mixed ejectionwithrespecttoradiativewinds.Earlyonthe
beforebeingrecycledinthesecondgeneration. main sequence, the [O/Na] value in the slow wind
Intheframeofourmodel,itimposes(seealso is the pristine one. (middle) As mixing proceeds,
Sects. 3 and4.3),thatstar formationoccurred matter with [O/Na] typical of H-burning (i.e., ∼ -
in the vicinity of individual massive rotating 2)isejectedintheslowwind(bottom)Duetoheavy
massloss,thestarmovesawayfromcriticalvelocity
star,fromclumpsmadeofslowwindmaterial
and does not supply its disk anymore; radiatively-
andofpristineinterstellargas.Thisstarforma-
driven fast wind takes over before the products of
tionmaybetriggeredforinstancebytheioni-
He-burning reach the stellar surface. Figure taken
sationfront.
fromDecressinetal.(2007a).
The material ejected in the slow wind
presents various degrees of enrichment in H-
burningproducts.Atthebeginningofthedisk-
star phase, pristine material is ejected giving
birthtostarswithacompositionsimilartothat of the first generation. As time proceeds, the
slow wind becomes more and more polluted
5 Weremind however that in order toreproduce in H-burning products (see Fig. 1 and 2) and
thelowestMgvaluesobservedwehadtopostulate forms stars that are more and more “anoma-
an increase of the 24Mg(p,γ)25Al nuclear reaction lous”. The slow wind phase lasts only a few
ratewithrespecttothepublishedvalues. millionyears(e.g.,4.5Myrfora60M⊙ star).
6 Meynet:Massiverotatingstars
tion (see for details Decressinetal. 2007b).
As already discussed in § 2, there is com-
pelling evidence that the slow wind of an in-
dividual star pollutes only on a small scale
around its stellar progenitor, and that it is di-
lutedlocallywithpristineISM.Inordertocon-
strainquantitativelythislocaldilutionprocess
weusetheLivariationdetectedinNGC6752
by Pasquinietal. (2005). It is important to
recall that the massive star ejecta are totally
“Li-free”, as this fragile element is destroyed
in these objects. We suppose that the mat-
ter ejected byan individualmassive star early
on the main sequence encounters more pris-
tine gas and is more diluted than the matter
ejectedlater.ThisproducesaLi-Naanticorre-
lation.Wechosedthedilutionfactorinorderto
reproduce the Li-Na anticorrelation observed
by Pasquinietal. (2005). Doing so we obtain
that the total mass of second generation stars
amountstoabouttwicethetotalmassreleased
undertheformofslowwindsbymassivestars.
Given a dilution factor, we can compute
Fig.2.Evolutionofthesurfaceabundancesinmass
howmanysmallmassstarscanbeformedfrom
fractionasafunctionoftheremainingmassfora60
theslowwindofagivenfastrotatingmassive
M stellar model at Z = 0.0005 and an initial ro-
tat⊙ionalvelocityontheZAMSof 800kms−1.The star. With an InitialMass Function(IMF), we
canestimatethetotalnumberofsecondgener-
horizontal line labeled “Slow Winds” in the lower
panel extends over the part of the evolution dur- ation low mass stars as well as the numberof
ing which the rotating star releases slow equato- normal first generation low mass stars. Thus,
rialwinds.Secondgenerationstarsareformedfrom thenumberratioofanomaloustonormalstars
clumps made of thismaterial and of pristineinter- at the time of birth of the second generation
stellar medium. We can see how as a function of stars can be predicted.Duringits evolution,a
timetheabundanceofoxygenintheslowwindde- globularclustermaylosestars,forinstanceby
creaseswhilethatofsodiumincreases.
tidalstripping.Theratioofanomaloustonor-
malstarin thatcase willchangeasa function
oftime.Therefore,thenumberratioofanoma-
3. Constraintsondilutionfactor,IMF
lousto normalstarsinpresentdayclusterde-
slopeanddegreeofevaporation
pendsonthreeparameters:thedilutionfactor,
How many massive fast rotators are needed the slope of the IMF and the number of stars
in order to reproduce the observed popu- whichhaveescapedthecluster(evaporation).
lation of anomalous stars in globular clus- Asmentionedabove,thedilutionfactorhas
ters? This question is first addressed by been calibrated in order to reproduce the Li-
Prantzos&Charbonnel(2006) for NGC 2808 Na anticorrelation observed by Pasquinietal.
andinmoredetailsinDecressinetal.(2007a) (2005). Its value only depends on the stellar
on the basis of theoretical stellar yields for models used. To constrain the IMF and the
NGC 6752. Here we just recall the method degree of evaporation we have only one ob-
usedaswellasthemainresultsforNGC6752. servable, the present day observedfraction of
Fromrotatingmassivestarmodelswecan anomaloustonormalstarsinglobularclusters
compute the amount of material ejected by (this value is equal to about 6 in NGC 6752,
slow winds with a given chemical composi- Carrettaetal. 2007). Thus we need to make
Meynet:Massiverotatingstars 7
some additionalhypothesesto constrain these
twoquantities.
InscenarioIweassumethattheclusterun-
derwent no evaporation of stars. In that case
the present day observed fraction of anoma-
loustonormalstarsgivesdirectlyaccesstothe
fraction at the time when the second genera-
tionwasborn.InthisscenariotheIMFcanbe
constrainedandthedegreeofevaporationisset
equaltozero.
InourScenarioII,weassumethatthemas-
sive polluters of first generation were born in
thecenteroftheclusterorhavemigratedvery
rapidly toward this region. Then the second
generation stars are created only in the cen-
tralregionwhiletheexternalpartofthecluster
hostsonlyfirstgenerationlong-livedlow-mass
stars.Thisstrongradialdistributionisassumed
to stay in place until the moment where su-
pernovaesweepawaytheresidualintra-cluster
gasandthusstronglymodifytheclusterpoten-
tial well. Evaporation will then preferentially
removenormalstars in the outer layersof the
cluster. In the Scenario II, we impose a slope
of the IMF equal to 1.35 and the degree of
evaporationof normalstarsis constrained(no
anomalousstarsaresupposedtohaveescaped
thecluster).
Toproduceahighnumberofpollutedstars
(typically5.7timesthenumberofnormalstars
as observedin NGC 6752)we need either (a)
inScenarioI,aflatIMFwithaslopeformas-
sivestarsaround0.55or(b)inScenarioII,that
about96%oftheunpollutednormallow-mass Fig.3. Upper panel: Expected anticorrelation be-
tween helium and [O/Na] for second generation
starshasbeenlostbythecluster.
stars(atbirth!)born frommaterialejectedthrough
Using the two sets of free parameters
slowwindsofdifferentinitialmassmodels.Lower
obtained in Scenarios I and II, we then
panel: Same as upper panel for the correlation
explored various consequences which are between 12C/13C and [O/Na]. Figure taken from
briefly recalled below (see for more details Decressinetal.(2007a)
Decressinetal.2007a).
1) In both scenarios, we can produce an
[O/Na] anticorrelation in the range −1 ≤ 2) In both scenarios, around 12% of the
[O/Na] ≤ 0.62. Quite remarkably, this range low-mass stars are expected to display a He
correspondstotheoneobservedinNGC6752 contenthigherthan0.4(inmassfraction).Note
ifoneconsidersonlythesetofstarsforwhich that in the presentmodel, He-enrichmentsare
bothmeasurementsofoxygenandsodiumare obtainedkeepingthesumC+N+Oequaltoits
available. Super-O-poor stars whose presence initial value as requested by the observations
isindirectlyinferredinsomeglobularclusters (Cohen&Mele´ndez2005;Carretta2006).
need lower dilution coefficient or some addi- 3)An anticorrelationispredictedbetween
tionalmixingoccurringinthestaritself. theheliumabundanceandthevalueoftheratio
8 Meynet:Massiverotatingstars
[O/Na]atthesurfaceofturnoffstars(seeupper 4. Discussion
panelofFig.3).
4.1.Whynoanomalousstarsinthefield
4)Morethan30%ofstarsare expectedto
ofthehalo?
havesurface12C/13Cratiosbetween10and30.
5)Theratios12C/13Cand[O/Na]arecorre- Globular clusters contain a significant popu-
lated(seelowerpanelofFig.3)6.Allthestars lation of anomalous stars. In the field, at the
with [O/Na] ratios below about -0.6 are pre- moment, only normal stars are observed at
dictedtoshowvaluesofthe12C/13Cratiosbe- the corresponding metallicity (no anticorrela-
tween15and20. tionfound).Thisindicatesthatsomethingspe-
cialoccursinclusterswhichdoesnotoccurin
6) Only a few percents of stars present at
the field. What might it be? There are many
birth16O/17O ratiosoftheorderofafewhun-
possibilities: 1) stars in the field were born in
dreds.
much shallower clusters which were rapidly
7) A correlation with a very large disper-
destroyedwhenthefirstsupernovaeappeared.
sion is predicted between the ratios 16O/17O
Thisdisruptionprocesspreventsasecondgen-
and [O/Na]. If only stars more massive than
eration of stars to appear; 2) in small easily
about60 M would contributeto the building
⊙ destroyed clusters, too few massive stars are
upofthechemicalanomaliesinglobularclus-
present, thusthe source of CNO-rich material
ters,thenonlyvaluesabove2000wouldbeex-
istooweakforallowingmanyanomalousstars
pectedwouldbeexpected.
to be formed;3) another possibility would be
8)Acorrelationbetween[C/N]and[O/Na] thatthe rotationrate of massive stars depends
is expected, whose dispersion increases at somewhatonthestrengthofthestarformation
lower[O/Na]values. event. This is of course quite speculative, but
9) In the case of Scenario I, the present itisinterestingtonotethattherotationrateof
day mass cluster represents about half of the B-typestarsinclustersseemstobehigherthan
mass which has been used to form stars. The thatofB-typestarsinthefield(Huang&Gies
restconsistsinmatterejectedbystarsandlost 2006).
by thecluster. Themasslockedintoremnants Evenifanomalousstarsareformedexclu-
represents19%ofthemassofthepresentday sively in clusters, one can still wonder why
cluster,thefirstandsecondgenerationstarsre- someofthemcouldnothaveescapedtheclus-
spectively12and69%. terandpassedtherestoftheirlifeinthefield.
It is reasonable to think that during their life-
10) In the case of Scenario II, the present
timetheglobularclusterslosepartoftheirstel-
day mass cluster represents only about9% of
larcontentby“evaporation”processeslikefor
the mass which has been used to form stars.
instancetidaldisruption.Thus,whynoanoma-
Therestconsistsinmatterejectedbystarsand
lousstarsarelostinthatway?Asexplainedin
of stars and stellar remnants lost by the clus-
our scenario II, this may arise due to the fact
ter. The mass locked into remnantsrepresents
thatanomalousstarsarepreferentiallyformed
8% ofthe mass ofthe presentdaycluster,the
inthecentralpartsoftheclusterandarestillin
first and second generation stars respectively
thatregionwhenthemajorevaporatingevents,
16and76%.
affectingmostlythestellarpopulationlyingin
Theaboveestimatesarebasedonobserved
theouterlayersofthecluster,occur.
caracteristicsof NGC6752.Usingotherclus-
It might be also that anomalous stars are
ters for calibrating the free parameters would
actuallypresentinthefield,butinsuchasmall
ofcourseproducedifferentresults.
proportionthattheyhaveuntilnowescapedde-
tection. Let us estimate an upper limit to the
6 Thesecorrelationsareobtainedfornon-evolved fraction of anomalousstars in the field, fA. If
stars. When the star evolve along the red giant no anomalous stars are detected among Nobs
branch, internal mixing may change somewhat the field stars, it means that fANobs << 1 or that
results. f << 1/N . In case N ∼ 100, much
A obs obs
Meynet:Massiverotatingstars 9
less than one star over hundred in the field is those originating from initial masses superior
anomalous. toabout50M wouldlikelygivebirthtocol-
⊙
From N and the degree of evaporation lapsars and then to Gamma Ray Bursts due
obs
(see below), we can constrain F , the frac- their fast rotation. During about 1.5 million
A
tionofstarslostbyglobularclusterswhichare years, in the very early phases of the globu-
anomalous. Let us recall that the number of larcluster(typicallywithagesbetween3.5and
starsinthehaloisaround109.Thenumberof 5 million years) one expects a rate of GRB
stars in present-day globular cluster amounts of about 2 per 1000 years per globular clus-
toabout1%ofthetotalnumberofstarsinthe ter, or 2 per 10 years when the whole system
halo (Woltjer 1975). Let us say that the glob- ofglobularclustersofagalaxyissurveyed.In
ular clusters have lost K times the number of casetheslopeoftheIMFis1.35,theexpected
stars they contain today. This implies that the ratesareloweredbyaboutthreeordersofmag-
fraction of stars now in the field but having nitudes. Typically the GRB rate would be be-
been born in globular clusters is f = K0.01. tween0.002and0.004per1000yearsandper
If K = 10 (i.e. if globular clusters lost about globularclusters.
10 times the number of stars they contain to-
day), then one field star over 10 would have
4.3.He-richstarsandtheUV-upturnin
beenbornina globularcluster.Thenthefrac-
ellipticals
tion of anomalous stars in the field, f , can
A
be written f = FAK << 1/N . This im- Interestingly, all ellipticals show the UV-
A 100 obs
plies that F << 100 . With K = 10, and upturn phenomenon, i.e. an UV excess in
N ∼ 100,AF <<K0N.o1bs0.IncreasingN will their spectra. These excesses are caused by
obs A obs
providetighterconstraintonF (providedKis anoldpopulationofhothelium-burningstars.
A
knownfromotherconsiderations).From N , Amongthemodelswhichhavebeenproposed
obs
anIMF,aswellasanobservedaveragedfrac- to explain this UV-upturn,let us mention Lee
tion of anomalous to normal stars in globular (1994); Park&Lee (1997) who propose that
clusterssomeconstrainsonthedegreeofevap- the blue core-helium-burning stars, originate
orationoffirstandsecondgenerationstarscan from the low-metallicity tail of a stellar pop-
beobtained. ulation with a wide metallicity distribution.
Bressanetal. (1994); Yietal. (1997) on the
contrary,assumesthattheUV-upturniscaused
4.2.Supernovaratesinveryyoung bymetal-richstarsthatlosetheirenvelopenear
globularclusters thetipofthefirstgiantbranch.Morerecently,
Hanetal. (2007) propose that these stars lost
In case the chemical anomalies are linked to theirenvelopebecauseofbinaryinteractions.
a population of massive stars, what would be As seen above, He-rich stars seem to
theconsequencesontheexpectedrateofsuper- be present in globular clusters (already on
novaeintheearlyevolutionaryphasesofglob- the Main Sequence). As in elliptical galax-
ular clusters? With a slope of the IMF equal ies, some globular clusters also present an
to 0.55 as in our scenario II, one would ex- extended blue horizontal branch. In globular
pect a supernova rate of 1 to 2 per 1000 year clustersthishotbranchofhelium-burningstars
per globular cluster (containing initially one is believed to arise from the He-rich stellar
million stars with masses distributed between population. Could it be also the case in ellip-
0.01 and 120 M ). This rate would be main- ticalgalaxies?Orsaidinotherwords,doellip-
⊙
tained for a few million years (about 8 mil- ticalsalsocontainapopulationofnon-evolved
lions years). If the halo of a galaxy contains He-rich low mass stars, whose origin may be
about 100 globular clusters, then, one would linked with a previous generations of fast ro-
expect1to 2supernovaeper10yearsobserv- tatingmassivestars?
ingthesystemcomposedbyallglobularclus- Inthemonolithicformationscenario,ellip-
ters. Interestingly, part of these supernovae, tical galaxies underwent at their birth a very
10 Meynet:Massiverotatingstars
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