On the model of symbiotic stars

We examine the level of substructure and mass segregation in the massive, young cluster Westerlund 1. We find that it is relatively smooth, with little or no mass segregation, but with the massive stars in regions of significantly higher than average surface density. While an expanding or bouncing-back scenario for the evolution of Westerlund 1 cannot be ruled out, we argue that the most natural model to explain these observations is one in which Westerlund 1 formed with no primordial mass segregation and at a similar or larger size than we now observe.

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“…Tutukov 1978;Lada et al 1984;Goodwin & Bastian 2006;Baumgardt & Kroupa 2007), or by their birth environments (e.g. Kruijssen et al 2011).…”

Section: Introductionmentioning

confidence: 99%

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

Gennaro

Goodwin

Parker

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…The subsequent response of the star cluster to the impact of gas expulsion has been the subject of a large number of theoretical investigations (Tutukov 1978;Hills 1980;Lada et al 1984;Verscheuren & David 1989;Goodwin 1997;Kroupa et al 1999;Adams 2000;Geyer & Burkert 2001;Kroupa et al 2001;Boily & Kroupa 2003a,b;Fellhauer & Kroupa 2005;Banerjee & Kroupa 2013). Hills (1980) found already that the star formation efficiency SFE of the cluster is a key property in determining the fate of the cluster after gas expulsion.…”

Section: Introductionmentioning

confidence: 99%

Reaction of massive clusters to gas expulsion – The cluster density dependence

Pfalzner

Kaczmarek

2013

A&A

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Context. The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group when stellar interactions can be neglected. Aims. We investigate the circumstances in which stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. Methods. The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different star formation efficiences. Concentrating on the most massive clusters observed in the Milky Way, we test to what extent the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size. Results. We find that stellar interactions leading to ejections are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way. Therefore, ejections are the prime mass loss process in these systems. Even in the loosely bound OB associations, stellar interactions are responsible for at least ∼5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is because of the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density dependence on the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.

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“…In this paper, we aim to quantify the increase in the fraction of second-generation stars to the total number of low-mass stars by another dynamical mechanism not taken into account in the above studies, namely the effect of primordial gas expulsion (i.e., the fast ejection of the remaining gas left by star formation after the onset of supernovae, see e.g., Tutukov 1978). Gas expulsion can strongly modify the total binding energy of the cluster and can lead to an efficient loss of first-generation stars from the cluster.…”

Section: Introductionmentioning

confidence: 99%

Evolution of two stellar populations in globular clusters

Decressin¹,

Baumgardt²,

Charbonnel

et al. 2010

A&A

128

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Aims. We investigate the early evolution of two distinct populations of low-mass stars in globular clusters under the influence of primordial gas expulsion driven by supernovae and study whether this process can increase the fraction of second generation stars at the level required by observations. Methods. We analyse N-body models that take the effect of primordial gas expulsion into account. We divide the stars into two populations that mimic the chemical and dynamical properties of stars in globular clusters so that second-generation stars start with a more centrally concentrated distribution. Results. The main effect of gas expulsion is to eject mostly first-generation stars while second-generation stars remain bound to the cluster. In the most favourable cases, second-generation stars can account for 60% of the bound stars we see today. We also find that, at the end of the gas expulsion phase, the radial distribution of the two populations is still different, so that long-term evolution will further increase the fraction of second generation stars. Conclusions. The large fraction of chemically anomalous stars is readily explainable as a second generation of stars formed out of the slow winds of rapidly rotating massive stars if globular clusters suffer explosive residual gas expulsion for a star formation efficiency of about 0.33.

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On the model of symbiotic stars

Cited by 189 publications

References 9 publications

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

Reaction of massive clusters to gas expulsion – The cluster density dependence

Evolution of two stellar populations in globular clusters

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