SCYON Abstract

Received on June 13 2017

The impact of a star-formation efficiency profile on the evolution of open clusters

AuthorsB. Shukirgaliyev (1,2), G. Parmentier (1), P. Berczik (3,4,5), and A. Just (1)
Affiliation(1) Astronomisches Rechen-Institut, Zentrum für Astronomie der Universit&aauml;t Heidelberg
(2) Fesenkov Astrophysical Institute;
(3) The International Center of Future Science of the Jilin University
(4) National Astronomical Observatories of China and Key Laboratory for Computational Astrophysics, Chinese Academy of Sciences
(5) Main Astronomical Observatory, National Academy of Sciences of Ukraine
Accepted byAstronomy & Astrophysics


Aims. We study the effect of the instantaneous expulsion of the residual star-forming gas on star clusters wherein the residual gas has a density profile shallower than that of the embedded cluster. This configuration is expected if star formation proceeds with a given star-formation efficiency per free-fall time in a centrally-concentrated molecular gas clump. Methods. We perform direct N-body simulations whose initial conditions are generated by the program "mkhalo" from the package "falcON" (McMillan & Dehnen (2007)) adapted for our models. Our model clusters initially have a Plummer profile and are in virial equilibrium with the gravitational potential of the cluster-forming clump. The residual gas contribution is computed based on the model of Parmentier & Pfalzner (2013). Our simulations include mass loss by stellar evolution and the tidal field of a host galaxy. Results. We find that a star cluster with a minimum global SFE of 15 percent is able to survive instantaneous gas expulsion and to produce a bound cluster. Its violent relaxation lasts no longer than 20 Myr, independently of its global SFE and initial stellar mass. At the end of violent relaxation the bound fractions of surviving clusters with the same global SFEs are similar regardless of their initial stellar mass. Their subsequent lifetime in the gravitational field of the Galaxy depends on their bound stellar masses. Conclusions. We therefore conclude that the critical SFE needed to produce a bound cluster is 15 percent, which is roughly twice smaller than earlier estimates of 33 percent. Thus we have improved the survival likelihood of young clusters after instantaneous gas expulsion. Those can now survive instantaneous gas expulsion with global SFEs as low as those observed for embedded clusters of Solar Neighbourhood (15-30 percent). This is the consequence of the star cluster having a density profile steeper than that of the residual gas. However, in terms of the effective SFE, measured by the virial ratio of the cluster at gas expulsion, our results are in agreement with previous studies.