The effect of gas loss on the formation of bound stellar clusters

Abstract

The effect of gas ejection on the structure and binding energy of newly formed stellar clusters is investigated. The star formation efficiency (SFE), necessary for forming a gravitationally bound stellar cluster, is determined. Two sets of numerical N‐body simulations are presented. As a first simplified approach we treat the residual gas as an external potential. The gas expulsion is approximated by reducing the gas mass to zero on a given time‐scale, which is treated as a free parameter. In a second set of simulations we use smoothed particle hydrodynamics (SPH) to follow the dynamics of the outflowing residual gas self‐consistently. We investigate cases where gas outflow is induced by an outwards propagating shock front and where the whole gas cloud is heated homogeneously, leading to ejection. If the stars are in virial equilibrium with the gaseous environment initially, bound clusters only form in regions where the local SFE is larger than 50 per cent or where the gas expulsion time‐scale is long compared with the dynamical time‐scale. A small initial velocity dispersion of the stars leads to a compaction of the cluster during the expulsion phase and reduces the SFE needed to form bound clusters to less than 10 per cent.

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

“…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.…”

“…Later numerical modelling (Lada et al 1984;Geyer & Burkert 2001;Boily & Kroupa 2003a,b) showed that the actual limit for bound star 1 A brief comment on terminology: Some authors use the term "cluster" to refer only to stellar groups that remain bound after gas expulsion, while other authors use it for any significant stellar over-density regardless of its dynamical state. Here we use the word "cluster" in the latter sense.…”

The effect of the dynamical state of clusters on gas expulsion and infant mortality

“…Hills 1980;Mathieu 1983;Elmegreen 1983;Elmegreen & Clemens 1985;Pinto 1987;Verschueren & David 1989;, and increasingly numerically (e.g. Lada et al 1984;Goodwin 1997a,b;Gyer & Burkert 2001;Boily & Kroupa 2003b;Baumgardt & Kroupa 2007).…”

Role of Gas Dynamical Friction in the Evolution of Embedded Stellar Clusters

“…Also, at the same time, some clusters like Taurus and Trumpler 16 do not show mass segregation Wolk et al 2011;Hasan & Hasan 2011). Numerical investigations of the dependence of the survivability of a cluster on the Star Formation Efficiency has shown that, a significant fraction of the stars can remain bound, after rapid gas loss, for an SFE 0.3 and, for slow gas removal, up to 50% of the cluster can remain bound for SFE values down to 0.15 − 0.2 (Geyer & Burkert 2001;Baumgardt & Kroupa 2007;Goodwin 2009). However, if most open clusters are remnants of embedded clusters that have -consequent to gas removal -lost one half or more of their natal members , the similarity in the mass functions of embedded and open clusters and, the similarity in the IMF's of embedded and open clusters become difficult to understand (Lada & Lada 2003).…”

“…Most stars seem to form in clusters, and hence, understanding clusters becomes an essential requirement for understanding the formation and evolution of galaxies. Two puzzles associated with open clusters that have attracted a lot of attention are -their formation, with densities and velocity dispersions that are not too different from those of the star forming regions in the Galaxy, given that the observed Star Formation Efficiencies (SFE) are low (Tutukov 1978;Hills 1980;Mathieu 1983;Elmegreen 1983;Lada, Margulis & Dearborn 1984;Adams 2000;Geyer & Burkert 2001;Baumgardt & Kroupa 2007;Goodwin 2009) and, the mass segregation observed / inferred in some of them (de Grijs et al 2002;Sirianni et al 2002;Gouliermis et al 2004;Chen, de Grijs & Zhao 2007;Converse & Stahler 2010;Hasan & Hasan 2011), at ages significantly less than their dynamical relaxation times (Bonnell & Davies 1998;McMillan, Vesperini & Portegies Zwart 2007;Allison et al 2009Allison et al , 2010Maschberger & Clarke 2011;Olczak, Spurzem & Henning 2011). In this paper, by making analytical approximations, we explore the efficacy of gas dynamical friction, operating during the embedded phase of stellar clusters, in solving these two puzzles.…”