Massive Binary Stars and Self-Enrichment of Globular Clusters
Robert G. Izzard, Selma E. de Mink, Onno R. Pols, Norbert Langer, Hugues Sana, Alex de Koter
aa r X i v : . [ a s t r o - ph . S R ] F e b Mem. S.A.It. Vol. 1, 1 c (cid:13) SAIt 2013
Memorie della
Massive Binary Stars and Self-Enrichment ofGlobular Clusters
Robert G. Izzard , Selma E. de Mink ⋆ , , Onno R. Pols , Norbert Langer ,Hugues Sana and Alex de Koter Argelander Institut für Astronomy, Universität Bonn, Germany. Space Telescope Science Institute, Baltimore, Maryland, U.S.A. Johns Hopkins University, Baltimore, Maryland, U.S.A. Department of Astrophysics / IMAPP, Radboud University Nijmegen, The Netherlands. Astronomical Institute Anton Pannekoek, University of Amsterdam, The Netherlands.
Abstract
Globular clusters contain many stars with surface abundance patterns indicat-ing contributions from hydrogen burning products, as seen in the anti-correlated elementalabundances of e.g. sodium and oxygen, and magnesium and aluminium. Multiple gener-ations of stars can explain this phenomenon, with the second generation forming from amixture of pristine gas and ejecta from the first generation. We show that massive binarystars may be a source of much of the material that makes this second generation of stars.Mass transfer in binaries is often non-conservative and the ejected matter moves slowlyenough that it can remain inside a globular cluster and remain available for subsequentstar formation. Recent studies show that there are more short-period massive binaries thanpreviously thought, hence also more stars that interact and eject nuclear-processed material.
1. The mass budget and enrichmentcandidates
The abundance correlations and helium enrich-ment observed in globular cluster stars implythat proton-burning reactions are responsible(Prantzos et al. 2007, and many contributionsto this volume). Hot hydrogen burning makeshelium, nitrogen and aluminium, while des-troying oxygen, carbon and magnesium, as re-quired in models of self-enrichment in globu-lar clusters. However, the number of stars ina second, or further, generation is often sim-ilar to or exceeds the number in the first gen-eration (Carretta et al. 2009), and the amountof nuclear-processed material currently in theiratmospheres is similar to, or larger than, that ⋆ Hubble fellow. present in the atmospheres of the first stellargeneration. It is not clear how so much nuclear-processed mass can end up in the second gen-eration of stars. Four main channels have beeninvestigated to date:1.
Massive Asymptotic Giant Branch(AGB) stars are the canonically accepted primecandidates for self-enrichment (Ventura et al.2001). During their thermally-pulsing AGB(TPAGB) phase, hot-bottom burning e ff ect-ively cycles the whole stellar envelope througha hot hydrogen burning shell. A star of mass4 M ⊙ . M .
10 M ⊙ ejects about ( M −
1) M ⊙ of nuclear-processed material, which isabout 10% of the mass of the whole stellargeneration. This does not take into accountbinary interaction which reduces the nuclear-processed TPAGB mass yield (Izzard 2004) R.G. Izzard et al.: Massive Binaries and GC Self-Enrichment while allowing for significant helium enrich-ment (Vanbeveren et al. 2012).2.
Rapidly rotating massive stars also ejecthydrogen-burned material if they spin fastenough (Decressin et al. 2007). Rotationalmixing transports material from the hot stel-lar core to the surface where it is ejected ifthe star exceeds its critical rotation rate. Thisis predicted to happen in some stars (de Minket al. 2013) although the number of rapidly ro-tating stars is such that only 3% of the mass ofall massive stars is ejected in this manner (deMink et al. 2009b).3.
Stellar mergers in dense cores of globu-lar clusters may also contribute to the reservoirof nuclear processed material (Glebbeek et al.2009) although this channel probably does notcontribute enough mass to make the secondgeneration of stars (Sills & Glebbeek 2010).4.
Massive binary stars are another sourceof nuclear processed material, as we explore inthe following.
2. Massive binary stars
While there is some doubt about whethermost stars are in multiple stellar systems,we can be sure that most stars with massesexceeding about 2 M ⊙ live with a compan-ion star (Kouwenhoven et al. 2007; Raghavanet al. 2010; Fuhrmann & Chini 2012). Justas importantly, the latest estimate of the O-type binary-period distribution in young, openclusters shows that more of them are close , i.e.liable to interact by mass transfer, than previ-ously thought (Sana et al. 2012). Only about29% of O-type stars evolve as single stars: therest either have their envelope stripped (33%),merge (24%) or accrete mass (14%).Because stars expand as they age, in a closebinary the initially more massive (primary)star overflows its Roche lobe first, transfer-ring mass onto the (initially less massive) sec-ondary (Fig. 1). Material flows through thefirst Lagrange point onto the companion, car-rying with it both the chemical signature ofthe primary star and angular momentum. Thetransferred mass settles onto the surface of thesecondary, spinning it up, but – at least ini-tially – not greatly altering its chemical abund- ance because material near the surface of theprimary is never hot enough for nuclear reac-tions to be e ffi cient.Accretion and spin up continues until themass of the secondary increases by about 10%,at which point it rotates so fast that materialat its equator is unbound (Packet 1981). Anyfurther mass transferred by Roche-lobe over-flow is ejected from the binary system at avelocity which is low compared to the proto-globular cluster ejection speed. This mater-ial may be retained in the cluster for furtherstar formation. As the primary continues totransfer mass, it loses its unburned envelopeand material originally deep inside the star,which has undergone nuclear burning, is ex-posed at the stellar surface. First, layers burnedby the CN cycle, then CNO, and later NeNaand MgAl cycles, are transferred through theLagrange point and ejected from the binarysystem. Detailed binary evolution models sug-gest that about three quarters of the transferredmass is ejected from a close binary system, i.e.an accretion e ffi ciency less than about 0 .
25 (deMink et al. 2009b), the binary-star physics re-mains highly uncertain and its study continues(e.g. van Rensbergen et al. 2011; de Mink et al.2013).While the binary-star scenario has not yetbeen explored in detail, it is observed innature. The binary star RY Scuti is eject-ing material rich in helium and nitrogen,and poor in oxygen and carbon, at a ve-locity of about 50 km s − (Smith, Gehrz, &Goss 2001) i.e. more slowly than a stellarwind or the escape speed of a young globularcluster. Further examples of binary mass trans-fer include the Algol systems (van Rensbergenet al. 2011), X-ray binaries (Flannery & Ulrich1977) and Wolf-Rayet binaries (Petrovic et al.2005) which must also be products of non-conservative mass transfer.It is clear that a copious amount of materialis ejected from interacting binary stars, muchof which has been processed by nuclear burn-ing. We estimate that as much as 13% of themass of a generation of stars can be ejectedin massive binaries, an amount similar to thatejected from rapidly rotating massive stars andAGB stars combined (de Mink et al. 2009b). .G. Izzard et al.: Massive Binaries and GC Self-Enrichment 3 Hydrogen-burned ashes: ejected ...new star formation? ab Figure 1.
Schematic view of Roche-lobe overflow in a massive binary system. (a)
At the start ofRoche-lobe overflow, the primary star (left) overflows its Roche lobe and transfers material to thesecondary (right). (b)
By the end of Roche-lobe overflow, the secondary has accreted unburnedmaterial while hydrogen-burned material from deep inside the primary has been ejected fromthe binary system and may mix with other sources of interstellar gas from which a subsequentgeneration of stars may form.
3. Frascati-fuelled Perspective
It is unlikely that anyone would bet more thana bottle of Frascati’s finest white wine onany single one of the proposed scenarios forglobular cluster self-pollution being the only source of mass for a second generation of stars.Massive AGB stars are generally considered the best candidate because they can processmaterial through hot hydrogen-burning prior toits ejection in a slow wind, although if thirddredge up happens in these stars they maynot be responsible (although see Yong et al.2008). The mass range which contributes toclusters is unclear also, are super-AGB stars
R.G. Izzard et al.: Massive Binaries and GC Self-Enrichment candidates (D’Ercole et al. 2012)? Rapidly ro-tating massive stars certainly exist, but theirtotal ejected mass is not enough even assum-ing – realistically? – that they are all rapid ro-tators (de Mink et al. 2009b). Binary stars mayeject enough mass to satisfy the requirementsof a second stellar generation, but quite howconservative is binary mass loss is not cleareven after many decades of study (e.g. de Minket al. 2007, and references therein). The com-petition between star formation and cluster gasejection is also relevant because massive starsevolve quickly relative to AGB stars. It may bethat massive-star ejecta escapes from the glob-ular cluster before forming any new stars (seee.g. Charbonnel et al. and other contributionsto this volume).Uncertainties in stellar physics, e.g. mass-loss rates, mixing rates and nuclear reactionrates, a ff ect stellar yield predictions consider-ably (e.g. Ventura & D’Antona 2005; Izzardet al. 2007; Stancli ff e & Je ff ery 2007; de Minket al. 2009a; Meynet et al. 2013; and many oth-ers). The magnesium-aluminium negative cor-relation is particularly di ffi cult to reproduce be-cause it requires proton capture at temperatureswhich massive stars are unable to reach, whilesuch burning is possible in massive AGB stars(Ventura et al. 2011). Still, the massive-binarychannel remains relatively unexplored and aserious contributor to the mass that makes thesecond generation of stars in globular clusters. Acknowledgements.
RGI thanks the conference or-ganisers, the Alexander von Humboldt Foundationfor supporting his work and Richard Stancli ff e for acritical reading of the manuscript. References
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