Multiple populations in globular clusters: the possible contributions of stellar collisions

Sills, Alison; Glebbeek, Evert

doi:10.1111/j.1365-2966.2010.16876.x

Cited by 47 publications

(43 citation statements)

References 54 publications

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“…This simulates the results of detailed stellar evolution models of low-mass merged stars by the use of molecular weight as a proxy for entropy (Gaburov et al 2008;Sills & Glebbeek 2010;Ivanova et al 2013). When a binary star enters a common envelope phase both stellar envelopes are homogenized in the process under the assumption that the orbital energy and angular momentum deposited in the common envelope mixes it completely prior to ejection or merging.…”

Section: Stellar Mass Loss and Gain In _mentioning

confidence: 95%

Binary stars in the Galactic thick disc

Izzard

Preece

Jofré

et al. 2017

Monthly Notices of the Royal Astronomical Society

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The combination of asteroseismologically-measured masses with abundances from detailed analyses of stellar atmospheres challenges our fundamental knowledge of stars and our ability to model them. Ancient red-giant stars in the Galactic thick disc are proving to be most troublesome in this regard. They are older than 5 Gyr, a lifetime corresponding to an initial stellar mass of about 1.2 M . So why do the masses of a sizeable fraction of thick-disc stars exceed 1.3 M , with some as massive as 2.3 M ? We answer this question by considering duplicity in the thick-disc stellar population using a binary population-nucleosynthesis model. We examine how mass transfer and merging affect the stellar mass distribution and surface abundances of carbon and nitrogen. We show that a few per cent of thick-disc stars can interact in binary star systems and become more massive than 1.3 M . Of these stars, most are single because they are merged binaries. Some stars more massive than 1.3 M form in binaries by wind mass transfer. We compare our results to a sample of the APOKASC data set and find reasonable agreement except in the number of these thick-disc stars more massive than 1.3 M . This problem is resolved by the use of a logarithmically-flat orbital-period distribution and a large binary fraction.

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Section: Stellar Mass Loss and Gain In _mentioning

confidence: 95%

Binary stars in the Galactic thick disc

Izzard

Preece

Jofré

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…O-rich and Na-poor stars form the first population, whereas the O-poor and Narich stars constitute the second one. Intermediate-mass asymptotic giant branch (AGB) stars (≈6 M -11 M Ventura et al 2001Ventura et al , 2013Ventura & D'Antona 2009D'Ercole et al 2010) and fast-rotating massive stars (FRMS;25 M Maeder & Meynet 2006;Prantzos & Charbonnel 2006;Decressin et al 2007;Krause et al 2013a;Charbonnel et al 2014), variants and combinations of these object classes (de Mink et al 2009;Sills & Glebbeek 2010;Bastian et al 2013;Cassisi & Salaris 2014), and recently also supermassive stars (Denissenkov & Hartwick 2014) have received attention as possible sources of the enriched material. From the nucleosynthesis point of view, FRMS, or massive stars in binary systems (de Mink et al 2009), are perhaps the most obvious polluter candidates: they produce the Na-O anti-correlation directly (Decressin et al 2007), as opposed to AGB stars, in which the direct correlation in their ejecta has to be turned into an anti-correlation by a precisely prescribed mixing procedure (Ventura et al 2013).…”

Section: Light-element Anti-correlations In Globular Clustersmentioning

confidence: 99%

Gas expulsion in massive star clusters?

Krause

Charbonnel

Bastian

et al. 2016

A&A

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Context. Gas expulsion is a central concept in some of the models for multiple populations and the light-element anti-correlations in globular clusters. If the star formation efficiency was around 30 per cent and the gas expulsion happened on the crossing timescale, this process could preferentially expel stars born with the chemical composition of the proto-cluster gas, while stars with special composition born in the centre would remain bound. Recently, a sample of extragalactic, gas-free, young massive clusters has been identified that has the potential to test the conditions for gas expulsion. Aims. We investigate the conditions required for residual gas expulsion on the crossing timescale. We consider a standard initial mass function and different models for the energy production in the cluster: metallicity-dependent stellar winds, radiation, supernovae and more energetic events, such as hypernovae, which are related to gamma ray bursts. The latter may be more energetic than supernovae by up to two orders of magnitude. Methods. We computed a large number of thin-shell models for the gas dynamics, and calculated whether the Rayleigh-Taylor instability is able to disrupt the shell before it reaches the escape speed. Results. We show that the success of gas expulsion depends on the compactness index of a star cluster C 5 ≡ (M * /10 5 M )/(r h /pc), with initial stellar mass M * and half-mass radius r h . For given C 5 , a certain critical, local star formation efficiency is required to remove the rest of the gas. Common stellar feedback processes may not lead to gas expulsion with significant loss of stars above C 5 ≈ 1. Considering pulsar winds and hypernovae, the limit increases to C 5 ≈ 30. If successful, gas expulsion generally takes place on the crossing timescale. Some observed young massive clusters have 1 < C 5 < 10 and are gas-free at ≈10 Myr. This suggests that gas expulsion does not affect their stellar mass significantly, unless powerful pulsar winds and hypernovae are common in such objects. By comparison to observations, we show that C 5 is a better predictor for the expression of multiple populations than stellar mass. The best separation between star clusters with and without multiple populations is achieved by a stellar winds-based gas expulsion model, where gas expulsion would occur exclusively in star clusters without multiple populations. Single and multiple population clusters also have little overlap in metallicity and age. Conclusions. Globular clusters should initially have C 5 100, if the gas expulsion paradigm was correct. Early gas expulsion, which is suggested by the young massive cluster observations, hence would require special circumstances, and is excluded for several objects. Most likely, the stellar masses did not change significantly at the removal of the primordial gas. Instead, the predictive power of the C 5 index for the expression of multiple populations is consistent with the idea that gas expulsion may prevent the expression of multiple populations. On this bas...

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“…However, the exact processes governing the formation of multiple populations in GCs are far from being understood and are strongly debated (e.g., Bastian & Lardo 2015;Renzini et al 2015;Krause et al 2016). Competing scenarios invoke different types of possible 1P polluters, namely asymptotic giant branch stars (AGB; Ventura et al 2001;D'Ercole et al 2010;Ventura & D'Antona 2011;Ventura et al 2013), fast-rotating massive stars (FRMS; Prantzos & Charbonnel 2006;Decressin et al 2007a,b;Krause et al 2013;Chantereau et al 2015), massive binary stars (de Mink et al 2009), supermassive stars (Denissenkov & Hartwick 2014), or a combination of some of the above-mentioned polluters (Sills & Glebbeek 2010;Bastian et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Evolution of long-lived globular cluster stars

Chantereau

Charbonnel

Meynet

2016

A&A

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Context. Globular clusters host multiple populations of long-lived low-mass stars whose origin remains an open question. Several scenarios have been proposed to explain the associated photometric and spectroscopic peculiarities. They differ, for instance, in the maximum helium enrichment they predict for stars of the second population, which these stars can inherit at birth as the result of the internal pollution of the cluster by different types of stars of the first population. Aims. We present the distribution of helium-rich stars in present-day globular clusters as it is expected in the original framework of the fast-rotating massive stars scenario (FRMS) as first-population polluters. We focus on NGC 6752. Methods. We completed a grid of 330 stellar evolution models for globular cluster low-mass stars computed with different initial chemical compositions corresponding to the predictions of the original FRMS scenario for [Fe/H] = −1.75. Starting from the initial helium-sodium relation that allows reproducing the currently observed distribution of sodium in NGC 6752, we deduce the helium distribution expected in that cluster at ages equal to 9 and 13 Gyr. We distinguish the stars that are moderately enriched in helium from those that are very helium-rich (initial helium mass fraction below and above 0.4, respectively), and compare the predictions of the FRMS framework with other scenarios for globular cluster enrichment. Results. The effect of helium enrichment on the stellar lifetime and evolution reduces the total number of very helium-rich stars that remain in the cluster at 9 and 13 Gyr to only 12% and 10%, respectively, from an initial fraction of 21%. Within this age range, most of the stars still burn their hydrogen in their core, which widens the MS band significantly in effective temperature. The fraction of very helium-rich stars drops in the more advanced evolution phases, where the associated spread in effective temperature strongly decreases. These stars even disappear from the horizontal branch and the asymptotic giant branch at 13 Gyr. Conclusions. The helium constraint is no suitable criterion for clearly distinguishing between the scenarios for GC self-enrichment because only few very helium-rich stars are predicted in the investigated framework and because it is difficult to derive the helium content of GC stars observationally. However, the helium constraint indicates some difficulties of the original FRMS scenario that require the exploration of alternatives.

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Multiple populations in globular clusters: the possible contributions of stellar collisions

Cited by 47 publications

References 54 publications

Binary stars in the Galactic thick disc

Binary stars in the Galactic thick disc

Gas expulsion in massive star clusters?

Evolution of long-lived globular cluster stars

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