Evolution of massive stars at very low metallicity including rotation and binary interactions

0.19 MB·English

by  S. -C. Yoon

Evolution of massive stars at very low metallicity including
rotation and binary interactions
S.-C Yoon∗, M. Cantiello† and N. Langer†
∗DepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064,USA
†AstronomicalInstitute,UtrechtUniversity,Princetonplein5,3584CC,Utrecht,TheNetherlands
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0 Abstract. Wediscussrecentmodelsontheevolutionofmassivestarsatverylowmetallicityincludingtheeffectsofrotation,
2 magnetic fieldsandbinarity. Verymetal poor starsloseverylittlemassand angular momentum duringthemainsequence
evolution, and rotation plays a dominant role in their evolution. In rapidly rotating massive stars, the rotationally induced
n
mixing time scale can be even shorter than the nuclear time scale throughout the main sequence. The consequent quasi-
a
chemically homogeneous evolution greatly differs from the standard massive star evolution that leads to formation of red
J
giants with strong chemical stratification. Interesting outcomes of such a new mode of evolution include the formation of

8 rapidlyrotatingmassiveWolf-Rayetstarsthatemitlargeamountsof ionizingphotons, theformationofalonggamma-ray
2 bursts and a hypernovae, and the production of large amounts of primary nitrogen. We show that binary interactions can
furtherenhancetheeffectsofrotation,asmassaccretioninaclosebinaryspinsupthesecondary.

]
h Keywords: Stars:evolution,Stars:rotation,Stars:binary
p PACS: 43.35.Ei,78.60.Mq
-
o
r 1. INTRODUCTION large amount of angular momentum at lower metallic-
t
s ity,resultingintheabundantproductionofenergeticsu-
a Massive stars affect the evolution of the early universe pernovaeand gamma-raybursts (Yoon& Langer 2005;
[
in a number of ways. They are believed to be the main Woosley&Heger2006;Yoon,Langer&Norman2006).

1 source of reionizing photons at high redshift, and their Detailed numerical simulations of the evolution of
v explosionsprovidelargeamountsofenergyintothesur- massive stars including the effects of rotation involve
3
rounding medium. Most elements heavier than helium manyuncertainphysicalprocessessuchasthetransport
7
begintoexistintheearlyuniverseduetonucleosynthe- of angular momentum and chemical species due to ro-
3
sis in massive stars. The history of star formation and tationally induced instabilities. Studies by Langer et al.
4
. theevolutionofgalaxiesintheearlyuniversearethere- (1999), Heger, Langer & Woosley (2000) and Hirschi,
1
forecloselyrelatedtosuchfeedbackfrommassivestars. Meynet & Maeder (2004) indicate that their adopted
0
Thishasmotivatedmanytheoreticalstudiesontheevolu- angular momentum transport mechanisms (Eddington
8
0 tionofmassivestarsofthefirstandsecondgenerations, Sweet circulations, shear instability and baroclinic in-
: whicharecharacterizedbyzero/lowmetallicity. stability)aretooinefficienttoexplaintheobservedspin
v
Mass loss due to stellar winds from massive stars rates of white dwarfs and young neutron stars: their
i
X is mainly driven by metal lines (Castor, Abbott & models predict one or two orders of magnitude higher
r Klein 1975; Pauldrach, Puls & Kudritzki R.P. 1986; spin rates than the observed values. More recent mod-
a
Vink,deKoter&Lamers2001).Asaresult,metalpoor els adopting the prescription by Spruit (2000) for the
massive stars are expected to lose only a small amount Tayler-Spruitdynamoin differentiallyrotatingradiative
of massand angularmomentumandto be keptrotating layers are shown to be more consistent with observa-
rapidly,duringtheirlifetimes(seeEkströmandMeynet, tionsintermsofthespinratesofstellarremnants(Heger,
however,inthisvolume).Rotation–oneoftheprimary Woosley&Spruit2005;Suijsetal.inprep.).Thevalid-
factors to determine the evolution of massive stars – ity of the Talyer-Spruitdynamohas recentlybeen chal-
may thus play an even more important role in the evo- lenged by several authors (Denissenkov, Pavel, & Pin-
lution of massive stars in the early universe, compared sonneault 2007; Zahn, Brun & Mathis, S. 2007) but it
to the case of galactic metal-rich massive stars. For in- seems clear that an efficient braking mechanism that is
stance, the high ratio of nitrogen to carbon abundance comparabletowhatSpruitsuggestsisneededtoexplain
observed in extremely metal poor stars may be related observations.
tochemicalmixingdueto rotationallyinducedinstabil- Here we present recent models of both single and
ities in massive stars in the early universe(Chiappiniet binary massive stars at very low metallicity including
al. 2006). Stellar cores could also more easily retain a the Tayler-Spruit dynamo as well as other effects of
chemical mixing by Eddington Sweet circulations and
the classical core-envelope structure is not developed
(Maeder1987;Langer1992).Suchastarwouldbegrad-
uallytransformedintoarapidlyrotatingmassivehelium
star(i.e.,WRstar)bytheendofthecorehydrogenburn-
ing.Thismodeofevolutionissupposedtobeparticularly
important for metal poor massive stars. At high metal-
licity, strong winds significantly spin down a massive
star early on the main sequence, rendering rotationally
inducedmixingtooinefficientto lead to the chemically
homogeneousevolution(seeFig.1).
In Fig. 2, we summarize how the evolution and fi-
nal fate of massive stars differ according to the initial
mass and rotational velocity, at two different metallici-
FIGURE 1. Total angular momentum in 20 M⊙ models as ties,basedonourstellarevolutionmodels(Yoon,Langer
afunctionofevolutionarytime.Thesolidlineandthedashed & Norman2006).Slowly rotatingstars follow the clas-
linesgivetheresultswithZ=10−5and0.02respectively. sical evolutionary path such that they evolve redwards
on the HR diagram, and end their life as a (super) gi-
antwithathickhydrogenenvelope.Inthiscase,thecore
rotation such as rotationally induced chemical mixing, is strongly braked down during the giant phase by the
enhancedmasslossnearthebreak-upvelocityandtidal heavy envelope,resulting in a specific angular momen-
interactions.We suggestthatrotationcouldleadtovery tum of the order of 1014 cm2s−1 in the pre-collapsing
different types of massive star evolution at very low core.TypeIIsupernovaearetheexpectedoutcome,leav-
metallicity, compared to the case of metal rich stars. ing millisecond pulsars or black holes as remnants de-
Implicationsforsupernovaeand gamma-raybursts, and pendingontheinitialmass.
massive star feedback in the early universe are briefly Ontheotherhand,thequasi-chemicallyhomogeneous
discussed. evolutionofrapidlyrotatingstarshaveseveralinteresting
consequences. Avoiding the giant phase, some of them
can retain a large amount of angular momentum in the
2. EVOLUTIONOFSINGLE STARS AT core (j ≥∼1016 cm2s−1) that may lead to produc-
CO
VERY LOWMETALLICITY tion of a long gamma-rayburst (GRB) or a jet-induced
energetic supernova/hypernova (Yoon & Langer 2005;
It is well knownthat manymassive stars in our Galaxy Woosley&Heger2006).AhigherratioofGRBs/HNeto
as well as in the Small/Large Magellanic Cloud are SNe is predictedfor lowermetallicity (Yoon,Langer&
rapid rotators (e.g. Maeder & Meynet 2000; Mokiem Norman2006),andtheroleoftheGRBs/HNefeedback
et al. 2006; Mokiem et al. 2007). One of the most im- maybesignificantintheearlyuniverse.E.g.,suchener-
portant effects of rotation on the evolution of massive geticsupernovaemighthaveleftuniquenucleosynthetic
stars is rotationally induced mixing of chemical ele- signaturesintheearlyuniverseasdiscussedbyNomoto
ments(Maeder&Meynet2000;Heger&Langer2000). (inthisvolume;Tominaga,Umeda&Nomoto2007),and
In magnetic models, stars tend to rotate rigidly due couldhaveaffectedthehistoryofstarformationathigh
to magnetic torques, and thus it is Eddington-Sweet redshift(Kobayashi,Spiringel&White2007).Thevery
circulations that mainly induce chemical mixing in- efficientrotationallyinducedchemicalmixingalsoleads
side stars (Maeder & Meynet 2005; Heger, Woosley to abundant production of primary nitrogen as shown
& Spruit 2005; Yoon & Langer 2005), rather than the in Fig. 3. It is also remarkable that massive WR stars
shearinstabilitythatisimportantinnon-magneticmod- couldbeproducedevenatverylow/zerometallicitydue
els(Maeder&Meynet2000;Heger,Langer&Woosley tothequasi-chemicallyhomogeneousevolution,without
2000;Meynet&Maeder2002).Efficientchemicalmix- theneedofstellarwinds.Suchmassivehelumstarspro-
ingacrossthe boundarybetweenthecoreandtheenve- duce a few times more hydrogen ionizing photons and
lopeinastarbyEddington-Sweetcirculationsisusually about100times morehelium ionizingphotonsthan the
prohibited due to the chemical gradient built up by nu- correspondingslowly rotatingstars(Fig. 4). Thismight
clear burning in the convective core (Zahn 1992). If a have consequences in the history of reionization in the
star is born with a sufficiently large amount of angular earlyuniverse,dependingontheinitialrotationalveloc-
momentum, however, mixing could occur on a shorter ityfunctionofthefirstandsecondgenerationsofstars.
time scale than the nuclear burning time scale. I.e., the
star becomes chemically homogeneous due to efficient
FIGURE2. Finalfateofourrotatingmassivestarmodelsattwodifferentmetallicities(Z=0.001&0.00001),intheplaneof
initialmassandinitialfractionoftheKeplerianvalueoftheequatorial rotationalvelocity.TobothsidesoftheGRBproduction
regionforZ=0.001,blackholesareexpectedtoforminsideWRstars,butthecorespinisinsufficienttoallowGRBproduction.
ForZ=0.00001,thepair-instabilitymightoccurtotherightsideoftheGRBproductionregion(seeHegeretal2003),althoughthe
rapidrotationmayshiftthepairinstabilityregiontolargermasses.Thedashedlineintheregionofnon-homogeneous evolution
separates TypeII supernovae (SN II;left) and blackhole (BH;right) formation, wherethe minimummass for BHformation is
simplyassumedtobe30M⊙.FromYoon,Langer&Norman(2006).
Langer1999;Wellstein, Langer& Braun2001).Binary
starevolutioniscloselyrelatedtothechangeintheradii
of the stellar components throughout the evolutionary
stages.Atsolarmetallicity,theenvelopeofamassivestar
drastically expands during the helium core contraction,
and the case B mass transfer is most likely. As metal-
licity decreases, the rapid expansion of the envelope of
amassivestarisdelayedtoalaterevolutionarystageas
revealed in Fig. 5, and the Case C mass transfer would
become more common (see de Mink et al. in this vol-
ume).
In the stellar models at very low metallicity (Z ≤
10−5),massivestarsremainfairlycompactwiththemax-
imumradiuslessthanafewhundredssolarradiithrough-
outtheevolutionifovershootingisignored.Thisimplies
thata veryclose orbitmaybe requiredfora verymetal
poor star to undergo Roche-lobe overflows in a binary
system.Ourmodelsalsoindicatethat,atverylowmetal-
licity,masstransferisusuallydynamicallystableregard-
less of the evolutionarystages of the primary,since the
FIGURE 3. CNO elements yields by stellar winds from 3 enveloperemainsradiative.However,rapidexpansionof
2(dfi0iflflaeenrddecnZitrec=vleo1sl)0u,−tMi5oinn(aoitrp=yens4e0sqquauenandrceZess)=.w1it0h−M5(inaistt=eri4s0ksa)n,danZd=Mi1n0it−=8 tthioeneinsvoebloseprevuepdtion∼the10m3oRd⊙eldsuwriinthgZth=eC1O0−c5owrehceonnotvraecr--
shooting is considered (de Mink et al. in this volume),
and it remains uncertain whether very metal poor stars
3. EVOLUTION OFBINARYSTARS AT wouldbecomesuper-giants(seealsoWoosleyinthisvol-
ume).
VERY LOWMETALLICITY
A high mass transfer rate in a massive close binary
usuallyleadstothermalexpansionoftheenvelopeofthe
Massivestarevolutionbecomesmuchcomplicatedbybi-
mass-accreting star. This effect becomes, however, less
naryinteractions(e.g.de Greve& deLoore1992;Pod-
significantforlower metallicity andbinarysystems can
siadlowski, Joss & Hsu 1992; Pols 1995; Wellstein &
avoidcontactorcommonenvelopephasemoreeasily(de
FIGURE4. Totalnumberofhydrogen(leftpanel)andhelium(rightpanel)ionizingphotonsemittedthroughouttheevolutionof
massivestarmodelsatZ=10−5,asafunctionoftheinitialmass.Thelinesconnectingasterisksandfilledtrianglesgivetheresults
from rapidly rotating stellar models that undergo the quasi-chemically homogeneous evolution (see the text), and non rotating
models,respectively.
mass accretion should experiencestrong chemicalmix-
ing during the rest of its life, unless the orbit remains
close enough for the star to be spun down by tidal in-
teraction on a short time scale, after the mass transfer
phase. If the secondary is rejuvenated sufficiently, even
thequasi-chemicallyhomogeneousevolutioncanbe in-
duced by mass accretion, eventually leading to produc-
tion of a long GRB (Cantiello et al. 2007). This binary
star scenario for long GRB progenitors suggests that a
significant fraction of long GRBs should occur in run-
away stars (see Fig. 6 and Cantiello et al. 2007) as im-
plied by a recent observational study of Hammer et al
(2006).
In short, a large fraction of massive binaries at very
lowmetallicitymayundergodynamicallystableCaseC
FIGURE 5. Change in the radii of 20 M⊙ models with
Vrot,init=0.3VKepleratfourdifferentmetallicities,asafunction masstransfer.Therefore,mostprimariesinclosebinaries
ofcentraltemperature. would explode as type Ibc supernovae,while most sec-
ondariesastypeIIsupernovae,butsomeofthemaslong
GRBs/hypernovae.Theeffectsofchemicalmixingsuch
Minketal.,inthisvolume).Therefore,itisexpectedthat as productionof primarynitrogenmay be significantin
formationofcompactbinarysystemssuchasblackhole massaccretingsecondarystars.
X-ray binaries via common envelope phases should be
lesscommonatlowermetallicity.
The role of rotation could be enhanced by binary 4. CONCLUDINGREMARKS
interaction. A slowly rotating star could be spun up
to the break-up velocity by mass accretion (Langer et We still do not fully understand the details about the
al.2003;Petrovic,Langer&vanderHucht2005).Mod- effects of rotation such as transport processes of angu-
els with fast semi-convection show that rapid mass ac- lar momentum and chemical species due to magnetic
cretion leads to rejuvenation in mass-accreting stars, torques,EddingtonSweetcirculationsandotherpossible
which weakens the chemical gradient between the core rotationallyinducedhydrodynamicinstabilities in stars,
and the envelope(Braun & Langer 1995). All these ef- which are important ingredients in recent stellar evolu-
fects of mass accretion, i.e., increase in mass and an- tionmodels.Improvedtreatmentsoftherotation-related
gularmomentumandrejuvenation,favorchemicalmix- physicsarethusneededforfuturestudies,inconnection
ing by Eddington Sweet circulations in mass-accreting with observational tests and multi-dimensional simula-
stars.Therefore,sucharapidlyrotatingstarproducedby tions(e.g.Brottetal.inthisvolume;Talon2007).How-
4. Castor,J.I.,Abbott,D.C.,&Klein,R.I.,1975,ApJ,195,
157
5. Chiappini, C., Hirschi, R., Meynet, G., Ekström, S.,
Primary Secondary Maeder,A.,&Matteucci,F.,2006,A&A,449,27
6. Denissenkov,P.A.,&Pinsonneault,M.,2007,ApJ,655,
1157
7. deGreve,J.P.,&deLoore,C.,1992,A&AS,96,653
8. Hammer,F.,Flores,H.,Schaerer,D.,Dessauges-Zavadsky,
M.,LeFloch,E.,&Puech,M.,2006,A&A,454,103
9. Heger,A.,&Langer,N.,2000,ApJ,544,1016
10. Heger,A.,Langer,N.,&Woosley,S.E.,2000,ApJ,528,
368
11. Heger,A.,Woosley,S.E.,&Spruit,H.C.,2005,ApJ,626,
350
WR star 12. Hirschi,R.,Meynet,G.,&Maeder,A.,2004,A&A,425,
Secondary 649
13. Kobayashi, C., Springel,V., &White,S.D.M.,2007,
MNRAS,376,1465
14. Langer,N.,1992,A&A,265,17
16. Langer,N.,Heger,A.,Wellstein,S.,&Herwig,F.,1999,
A&A,346,37
16. Langer,N.,Yoon,S.-C.,Petrovic,J.,&Heger,A.,2003,in
StellarRotation,Proc.IAU-Symp.215,ASP,SanFrancisco,
SN explosion A.Maeder,P.Eenens,eds.
(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1) Secondary Run away 111789... MMMaaaeeedddeeerrr,,,AAA...,,,1&&98MM7ee,yyAnn&eettA,,GG,1..,,722800,001055,,9AAR&AA&,4A4,03,81,014413
20. Meynet,G.,&Maeder,A.,2003,A&A,390,561
SN Kick 21. Mokiem,M.R.,deKoter,A.,&Evans,C.J.etal.,2006,
A&A,456,1131
FIGURE 6. Runaway star scenario for long GRB progeni-
22. Mokiem,M.R.,deKoter,A.,&Evans,C.J.etal.,2007,
tors.Inaclosemetalpoorbinarysystem,thesecondarygains
A&A,465,1003
mass and angular momentum from the primary. If the sec-
23. Spruit,H.C.,2002,A&A,381,923
ondary is rejuvenated and spun up sufficiently as a result, it
24. Pauldrach, A.W.A.,Puls,J.,&Kudritzki, R.P.,1986,
undergoes the quasi-chemically homogeneous evolution. The
A&A,164,86
primarybecomesaWRstarandexplodeasatypeIb/csuper-
25. Petrovic,J.,Langer,N.,&vanderHucht,K.A.,2005,
nova.Ifthesupernovakickunboundsthesystem,thesecondary
A&A,435,1013
runsawayatavelocityof∼10...100km/s,andexplodeasa
26. Petrivic,J.,Langer,N.,Yoon,S.-C.,&Heger,A.,2005,
longGRB/hypernovaaftertravelingafewtoseveralhundreds
A&A,435,247
PCsaway.SeeCantielloetal.(2007)fordetaileddiscussions.
27. Podsiadlowski,Ph.,Joss,P.C.,&Hsu,J.J.L.,1992,ApJ,
391,246
28. Pols,O.R.,1994,A&A,290,119
ever,abovediscussionsclearlyindicatethattheevolution 29. Talon,S,2007,[astro-ph/0708.1499]
of metal poor massive stars much depends on rotation. 30. Tominaga,N.,Umeda,H.,&Nomoto,K.,2007,ApJ,660,
516
Addressingtheroleofthemassivestar feedbackin the
31. Vink,J.S.,deKoter,A.,&Lamers,H.J.G.L.M.,2001,
earlyuniverseincludingtheeffectsofrotationandbina-
A&A,369,574
rity will be an exciting but challenging subject for the
32. Wellstein,S.,&Langer,N.,1999,A&A,350,148
next decade. In particular, future observational studies 33. Wellstein,S.,Langer,N.,&Braun,H.,2001,A&A,369,
on massive star populations (e.g. WR/O ratio) in metal 939
poorgalaxiesandsupernovae/GRBsathighredshiftmay 34. Woosley,S.E.,&Heger,A.,2006,ApJ,637,914
35. Yoon,S.-C.,&Langer,N.,2005,A&A,443,643
providestrongconstraintsfortheoreticalmodels,asdis-
36. Yoon,S.-C.,Langer,N.,&Norman,C.,2006,A&A,460,
cussedinYoon,Langer&Norman(2006)ingreaterde-
199
tail.
37. Zahn,J.-P.,1992,A&A,265,115
38. Zahn,J.-P.,Brun,A.S.,&Mathis,S.,2007, A&A,in
press,[astro-ph/0707.3287]
REFERENCES
1. Braithwaite,J.,2006,A&A,449,451
2. Braun,H.,&Langer,N.,1995,A&A,297,483
3. Cantiello,M.,Yoon,S.-C.,Langer,N.&Livio,M.,2007,
A&A,465,L29-L33