Table Of ContentMem.S.A.It.Vol.1,1
(cid:13)c SAIt 2013 Memoriedella
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The e ect of intermediate mass close binaries on the
chemical evolution of Globular Clusters
NickiMennekens, DanyVanbeverenandJean-PierreDeGreve
3
1
0 AstrophysicalInstitute,VrijeUniversiteitBrussel,Pleinlaan2,1050Brussels,Belgium
2 e-mail:[email protected]
n
a
J Abstract. ThechemicalprocessesduringtheAsymptoticGiantBranch(AGB)evolution
ofintermediatemasssinglestarspredictmostoftheobservationsofthedifferentpopula-
1
tionsinGlobularClustersalthoughsomeimportantissuesstillneedtobefurtherclarified.
1
In particular, to reproduce the observed anticorrelations of Na-O and Al-Mg, chemically
enrichedgaslost duringtheAGBphase ofintermediatemasssinglestarsmust bemixed
]
A withmatterwithapristinechemicalcomposition.Thesourceofthismatterisstillamatter
of debate. Furthermore, observations reveal that a significant fraction of the intermediate
G
massandmassivestarsarebornascomponents ofclosebinaries.Wewillinvestigatethe
. effectsofbinariesonthechemicalevolutionofGlobularClustersandontheoriginofmat-
h
ter with a pristine chemical composition that is needed for the single star AGB scenario
p
towork. Weuse apopulation synthesis code that accounts for binary physics inorderto
-
o estimatetheamountandthecompositionofthematterreturnedtotheinterstellarmedium
r ofapopulationofbinaries.Wedemonstratethatthemasslostbyasignificantpopulationof
t
s intermediatemassclosebinariesincombinationwiththesinglestarAGBpollutionscenario
a mayhelptoexplainthechemicalpropertiesofthedifferentpopulationsofstarsinGlobular
[ Clusters.
1
Keywords.Binaries:close-Galaxies:clusters:general
v
3
2
4 1. Introduction that is still observed today. In Vanbeveren et
2 al.(2012)itwasshownthatintermediatemass
1. SeveralGlobularCluster(GC)self-enrichment close binaries (IMCBs) can help to achieve
0 scenarios have recently been proposed, such this.Thepresentpaperisasummaryofthelat-
3 as the AGB-scenario (D’Ercole et al. 2010), terwork.
1 the winds of fast rotating massive stars sce-
v: nario (Decressin et al. 2007) and the massive 2. IMCBevolution
i close binary scenario (de Mink et al. 2009).
X
The mainchallengein allthese is thatthe en- IMCBs are those with an initial primary (the
r richedmaterialliberatedbythefirstgeneration initiallymostmassivestar)massbetween3and
a
ofstarsneedstobemixedwithpristinematter, 10M ,andaninitialorbitalperiodbetween1
⊙
also present in the vicinity, before the forma- dayand10years,thelatterbeingthemaximum
tionofasecondgeneration.Thelatterthenalso for most binaries to be interacting. There are
needstobesufficientlylarge,i.e.asleastasnu- nowadaysmanyindicationsthatthebinarystar
merous as the fraction of the first generation frequency f is very high (close to 100%) in
b
2 Mennekensetal.:TheeffectofIMCBsonthechemicalevolutionofGCs
normalfieldstarsinthismassrange.Thisisnot mergingiftoomuchangularmomentum(AM)
only observed directly, but also inferred indi- is lost. Otherwise, eventuallya double WD is
rectly,e.g.bythefactthatintheoreticaltypeIa obtained,whichmaystillresultinaSNIabut
supernova(SN Ia)studiessuchasMennekens notinasourceofslowwinds.Wewillinvesti-
et al. (2010), a very high f needs to be as- gatetheamountandcompositionofmassthat
b
sumed in order to match the observed rate of is lost naturally in a slow way through these
suchevents.Thecurrentlyobserved f in(old) RLOFandCEprocesses.
b
GCs,however,seemstobemuchlower.There For this, a population number synthesis
are two reasons though why the IMCB fre- (PNS) code including the results of detailed
quency in young GCs may have been much binary evolution is used. This code is exten-
higher. Firstly, the IMCB frequencymay well sively described in De Donder & Vanbeveren
be higher than the low mass close binary fre- (2004). Both the RLOF and CE process are
quency,withthelatterrelevantfortheGCscur- in PNS codes subject to a number of param-
rently seen. Secondly, many of the low mass eterizations. Not only the function and val-
singlestarsseeninoldGCstodaymayhavein ues of these parameterswill be discussed, but
fact originally been part of a binary, that has later also their influence on the results of this
howeverbeendestroyedthroughtheGyrsasa study. In the case of stable RLOF, a first pa-
resultofdynamicalinteractions. rameteristheMT efficiencyβ, thefractionof
WhenevertheprimaryinanIMCBfillsits matter lost by the donor that is actually ac-
Roche lobe, this will initiate a mass transfer creted by the gainer. If β < 1, mass is lost
(MT)event.Dependingonwhenthishappens, into the interstellar medium (ISM) and an as-
its naturewillbedifferent.We distinguishbe- sumption needs to be made about how much
tweencaseA (duringcoreHburning),caseB AM this mass carries away from the system.
(duringshellHburning)andcaseC(aftercore This is done by a parameter η, proportional
He burning). Case A or early case B (named to the AM loss caused by a fixed amount of
Br,i.e.atatimewhenthedonor’souterlayers massloss.MostPNScodesassumethatmassis
are still radiative) will result in the expansion lostwiththespecificorbitalAMofthegainer,
ofthestarbeingslowedandgiverisetoastable but this is only true in the case of a process
streamofmatterfromdonortogainer,termed thatremovesmasssymmetricallywithrespect
Roche lobe overflow (RLOF). For larger or- to the equatorialplaneof this star. Asthere is
bital periods however, the donor’s outer lay- limited observational evidence for significant
ers are already deeply convective by the time masslossinsucha wayinintermediatemass,
MT (case Bc or C) starts, and thus mass loss non-degeneratestars,ourstandardassumption
willleadtothisstarexpandingevenfurther.It is that mass is lost into a circumbinary disk
will eventually engulf the otherstar, resulting after passing through the second Lagrangian
in two stellar cores rotating within one com- point. It thus removes a much larger specific
mon envelope(CE). Some time afterthis first orbital AM, which will lead to more systems
MT event, when the primary has already be- merging. When an unstable CE phase is en-
comeawhitedwarf(WD),alsothesecondary countered,therotationalenergyofthetworo-
will fill its Roche lobe, initiating a MT event tating stellar cores needs to be converted into
intheoppositedirection.Here,alsocaseA,B kinetic energy used to expel the envelope in
andCcanbedistinguished.However,because time,beforethesystemmerges.Intheformal-
the gainer is now a WD with a small surface ism by Webbink (1984), this conversion effi-
area and the mass ratio in such systems is of- ciencyisproportionaltotheparameterα,with
ten verylarge,itis assumedthatthis MTwill largervaluesthusreducingthechanceofmerg-
alwaysbecomeunstableandthusresultinaCE ing.
phase. The only exceptionare SN Ia progeni- Itisobviousfromthenon-conservativeex-
tors,whicharenotconsideredinthisstudyas ample in Fig. 1 that the initial orbital period
theydonotproduceslowwinds.Duringevery critically determines the types of MT phases,
RLOForCEphase,thesystemhasachanceof andthustheamountoflostmass.Toillustrate,
Mennekensetal.:TheeffectofIMCBsonthechemicalevolutionofGCs 3
Fig.1. Different types of MT en-
counteredasafunctionofinitialor-
bitalperiodfora6.0+5.4M binary
⊙
and a non-conservative assumption
(β = 0.5). The first line at the top
indicatesthenature of thefirstMT
phase, the second line (if any) that
of the second MT phase. Those in
grayitalicsmergeduringthepartic-
ular MT episode. In the plot itself,
blacklinesshowtheamountofmass
lossduringthefirstMTphase,gray
linesduringthesecond.Thisisdone
for the total mass loss (solid), He
massloss(dashed)andTP-AGBen-
richedmassloss(dotted).
fortherangeinvolvingastablefirstMTphase centrated at the time of the first peak, and the
(logP<1.9),somemassislostduringthisfirst secondpeakdisappears.
phase,andsomeduringthesecond.Ifthecon-
servative assumptionwere used instead, obvi- Table1showstheobtainedresultsapplied
ously no mass would be lost during the first to the context relevant for GCs, for four dif-
MT phase, but also not during the second, as ferentsets of parameters:the conservativeas-
thegainerwouldthenbecomesufficientlymas- sumption,thesamebutwithlowCE-efficiency,
sivetoexplodeasaSNII.Withaslightlylower thenon-conservativeassumption,andagainthe
masscompanion(3.6M )andthusmassratio, same but with the specific gainer orbital AM
⊙
the totalamountof mass loss is similar in the loss,resultinginan(onaverage)muchsmaller
conservativeandnon-conservativeassumption. value of the AM loss parameter η. The table
While in the latter case it is divided between showsthefraction∆Moftheinitialmasscon-
the first and second MT phase, in the former tainedinIMCBsthatisreturnedtotheISMin
itisconcentratedduringthesecondMTphase, a slow way throughRLOF and CE processes.
asthesecondarybecomesmuchmoremassive Thisfractionisbetween20and50%.Asignif-
there. icant part ∆Mpris of this mass is still pristine,
i.e.hasnotbeenaffectedbyanynuclearreac-
tioninthesestars.Asecondfraction∆Mpris+He
is enriched only in He, while a third frac-
3. Results
tion ∆M is also enriched by early AGB
EAGB
Figure2showsthatmasslosscausedbyRLOF processes. Only a minorfraction ∆M of
TPAGB
and CE processes terminates within 1 Gyr, about 10% has been enriched in TP-AGB el-
confirming the previous assertion that this is ements, i.e. those produced by Hot Bottom
beforedynamicalinteractionshavethechance Burning.TheHeabundanceafterthefirstgen-
to destroy many binaries. It is obvious that erationis typically ∼ 0.3,havingstarted from
thetwoMTphasesshowtwopeakswhichare Y = 0.24. Extrapolating, after a second gen-
population-wisestilldistinctintime.Thegen- eration Y ∼ 0.36 will be obtained, which is
eral shape of the distribution is not affected in line with what is neededobservationally.It
when the CE-efficiency parameter α is set to is important to note that of the matter that is
0.1 instead of 1.0, a value that according to not ejected during RLOF and CE processes,
recent observations is more realistic. If one the majority will eventually still be returned
considerstheextremelynon-conservativecase totheISM.Onlyaminorfractionwillremain
wherethegainerdoesnotaccreteanymaterial locked in forever as remnant masses and an
(β = 0), then the mass loss is obviously con- even smaller fraction will be ejected by SNe
4 Mennekensetal.:TheeffectofIMCBsonthechemicalevolutionofGCs
Fig.2.Amountsofmasslossforan
entire population of IMCBs under
the conservative assumption (β =
1), as a function of time after star-
burst. The figure shows the total
mass loss (solid), He mass loss
(dashed)andTP-AGBaffectedmass
loss (dotted), again separated for
first (black) and second (gray) MT
phase.
Name β η α ∆M ∆Mpris ∆Mpris+He ∆MEAGB ∆MTPAGB Y Mergers
∆M ∆M ∆M ∆M
conservative 1.0 2.3 1.0 40% 23% 22% 47% 8% 0.30 76%
lowCE-efficiency 1.0 2.3 0.1 20% 14% 11% 62% 14% 0.31 83%
non-conservative 0.5 2.3 1.0 34% 43% 7% 40% 9% 0.28 86%
lowAM-loss 0.5 0.038* 1.0 53% 35% 18% 41% 6% 0.29 70%
Table 1. Results obtained with the PNS code for a population of 100% IMCBs. See text for
definitionofsymbols.Asteriskdenotesweightedaverage.
(andthusnottakenintoaccounthere).Therea- material needs to be mixed before the forma-
sonisthattheabsolutemajorityofIMCBswill tionofasecondgenerationofstars.
at some point in their evolution merge (as is
alsoindicatedinthetable).Fromthispointon,
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