On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

Bekki, Kenji; Mackey, A. D.

doi:10.1111/j.1365-2966.2008.14320.x

Cited by 48 publications

(51 citation statements)

References 54 publications

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“…Proposed ideas for the evolution of GCs containing multiple stellar populations include accretion of interstellar matter after the first star formation episode (Bekki & Mackey 2009), cluster mergers (van den Bergh 1996), and several self-enrichment scenarios, in which ejecta from a first stellar generation fuels a second star formation episode. Lightelement-enriched material is the result of hot H-burning mixed up to the convective zone of stars (Denisenkov & Denisenkova 1990).…”

Section: Introductionmentioning

confidence: 99%

Second-generation stars in globular clusters from rapid radiative cooling of pre-supernova massive star winds

Lochhaas¹,

Thompson²

2017

Monthly Notices of the Royal Astronomical Society

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Following work by Wünsch and collaborators, we investigate a self-enrichment scenario for second generation star formation in globular clusters wherein wind material from first generation massive stars rapidly radiatively cools. Radiative energy loss allows retention of fast winds within the central regions of clusters, where it fuels star formation. Secondary star formation occurs in ∼ 3 − 5 Myr, before supernovae, producing uniform iron abundances in both populations. We derive the critical criteria for radiative cooling of massive star winds and the second generation mass as a function of cluster mass, radius, and metallicity. We derive a critical condition on M/R, above which second generation star formation can occur. We speculate that above this threshold the strong decrease in the cluster wind energy and momentum allows ambient gas to remain from the cluster formation process. We reproduce large observed second generation fractions of ∼ 30 − 80% if wind material mixes with ambient gas. Importantly, the mass of ambient gas required is only of order the first generation's stellar mass. Second generation helium enrichment ∆Y is inversely proportional to mass fraction in the second generation; a large second generation can form with ∆Y ∼ 0.001 − 0.02, while a small second generation can reach ∆Y ∼ 0.16. Like other self-enrichment models for the second generation, we are not able to simultaneously account for both the full range of the Na-O anticorrelation and the second generation fraction.

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Section: Introductionmentioning

confidence: 99%

Second-generation stars in globular clusters from rapid radiative cooling of pre-supernova massive star winds

Lochhaas¹,

Thompson²

2017

Monthly Notices of the Royal Astronomical Society

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“…For instance, mergers of star clusters with an age difference of ∼200 Myr 1 as well as interactions of star clusters and star-forming giant molecular clouds, 32 have been suggested as the possible origin of extended turn-off regions.…”

Section: Using the Subgiant Branch To Constrain The Cluster's Maximummentioning

confidence: 99%

The exclusion of a significant range of ages in a massive star cluster

Grijs

Deng

2014

Nature

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Stars spend most of their lifetimes on the main sequence in the Hertzsprung-Russell diagram. The extended main-sequence turnoff regions -containing stars leaving the main sequence after having spent all of the hydrogen in their cores -found in massive (more than a few tens of thousands of solar masses), intermediateage (about one to three billion years old) star clusters [1][2][3][4][5][6][7][8] are usually interpreted as evidence of cluster-internal age spreads of more than 300 million years 2, 4, 5 , although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order 10 7 years 9, 10 . Here we report that the stars beyond the main sequence in the two billionyear-old cluster NGC 1651, characterized by a mass of ∼ 1.7 × 10 5 solar masses 3 , can be explained only by a single-age stellar population, even though the cluster has clearly extended main-sequence turn-off region. The most plausible explanation for the extended main-sequence turn-offs invokes the presence of a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar-population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional intermediate-age star clusters 2,3,5, 11 , suggesting that an extended main-sequence turn-off does not necessarily imply the presence of a significant cinternal age dispersion.We obtained archival Hubble Space Telescope/Wide Field Camera-3 observations of the NGC 1651 field in the F475W ("B") and F814W ("I") broad-band filters (Methods). The corresponding colourmagnitude diagram, that is, the observational counterpart of the Hertzsprung-Russell diagram, is shown in Fig. 1. When stars have exhausted their core hydrogen supply, hydrogen fusion continues in a shell outside the stellar core. At this stage, stars leave the main sequence and evolve onto the subgiant branch. The colour-magnitude diagram of NGC 1651 exhibits a clearly extended main-sequence turnoff and a very narrow subgiant branch. This is surprising, given the corresponding, far-reaching implications for our interpretation of such extended turn-offs in the context of star cluster evolution.Star clusters more massive than a few tens thousands of solar masses were, until recently, considered single-generation ("simple") stellar populations. It was thought that all of their member stars had formed approximately simultaneously from molecular gas originally confined to a small volume of space. As a consequence, all cluster stars would thus have similar ages, a very narrow range in chemical composition and individual stellar masses that followed the initial mass function, that is, the stellar mass distribution at the time of star birth. In the past decade, however, consensus has emerged that massive star clusters are no longer ideal simple stellar populations [12][13][...

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“…The contribution of field AGB stars is very minor in the present study. Bekki & Mackey (2009) and Pflamm-Altenburg & Kroupa (2009) investigated how star clusters can capture cold molecular gas or accrete ISM in galaxies using idealized modeling of the accretion process. The present work and theirs are therefore complementary to each other.…”

Section: Ejection and Accretion Of Gas From Agb Starsmentioning

confidence: 99%

Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

Bekki¹

2017

Mon. Not. R. Astron. Soc.

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Most old globular clusters (GCs) in the Galaxy are observed to have internal chemical abundance spreads in light elements. We discuss a new GC formation scenario based on hierarchical star formation within fractal molecular clouds. In the new scenario, a cluster of bound and unbound star clusters ('star cluster complex', SCC) that have a power-law cluster mass function with a slope (β) of 2 is first formed from a massive gas clump developed in a dwarf galaxy. Such cluster complexes and β = 2 are observed and expected from hierarchical star formation. The most massive star cluster ('main cluster'), which is the progenitor of a GC, can accrete gas ejected from asymptotic giant branch (AGB) stars initially in the cluster and other low-mass clusters before the clusters are tidally stripped or destroyed to become field stars in the dwarf. The SCC is initially embedded in a giant gas hole created by numerous supernovae of the SCC so that cold gas outside the hole can be accreted onto the main cluster later. New stars formed from the accreted gas have chemical abundances that are different from those of the original SCC. Using hydrodynamical simulations of GC formation based on this scenario, we show that the main cluster with the initial mass as large as [2 − 5] × 10 5 M ⊙ can accrete more than 10 5 M ⊙ gas from AGB stars of the SCC. We suggest that merging of hierarchical star cluster complexes can play key roles in stellar halo formation around GCs and self-enrichment processes in the early phase of GC formation.

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On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

Cited by 48 publications

References 54 publications

Second-generation stars in globular clusters from rapid radiative cooling of pre-supernova massive star winds

Second-generation stars in globular clusters from rapid radiative cooling of pre-supernova massive star winds

The exclusion of a significant range of ages in a massive star cluster

Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

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