SCYON Abstract

Received on May 1 2005

Runaway collisions in young star clusters. I. Methods and tests

AuthorsMarc Freitag (1,2), Frederic A. Rasio (2), Holger Baumgardt (3)
Affiliation(1) Astronomisches Rechen-Institut, Heidelberg, Germany
(2) Northwestern University, Evanston, USA
(3) Sternwarte, University Bonn, Germany
Submitted toMonthly Notices of the Royal Astronomical Society
Contactm-freitag@northwestern.edu
URLhttp://arxiv.org/abs/astro-ph/0503129
Links

Abstract

We study the collisional runaway scenario to form an intermediate-mass black hole (IMBH, MBH > 100 Msun) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (Mstar ~= 30-120 Msun) through mass segregation and gravothermal collapse. Previous work has established the conditions for this to happen before the massive stars evolve off the main sequence (MS). Here we investigate the next stage by implementing direct collisions between stars. As massive stars collide with each other in the high-density core, the formation of a very massive star (VMS, Mstar > 400 Msun), a possible progenitor for an IMBH, through a runaway sequence of mergers is likely to ensue. Using a Monte Carlo stellar dynamics code, we follow the core collapse and subsequent collisional phase. Collisions are treated either as ideal, ``sticky-sphere'' mergers or using realistic prescriptions derived from 3-D hydrodynamic computations. In this paper we present the runaway scenario in a general astrophysical context. We then explain the numerical methods we use to investigate it. Finally, we report on test computations carried out to ensure that our implementation of the important physics is sound. In a companion paper, we present results from more than 100 cluster simulations we performed to determine the conditions leading to the collisional formation of a VMS and the characteristics of the runaway sequences.