The thermodynamics of molecular cloud fragmentation

Star formation under non-Milky Way conditions

Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: [seyit;spaans]@astro.rug.nl

Received: 31 March 2009
Accepted: 23 November 2009

Abstract

Context. Properties of candidate stars, forming out of molecular clouds, depend on the ambient conditions of the parent cloud. We present a series of 2D and 3D simulations of fragmentation of molecular clouds in starburst regions, as well as of clouds under conditions in dwarf galaxies, leading to the formation of protostellar cores.

Aims. We explore in particular the metallicity dependence of molecular cloud fragmentation and the possible variations in the dense core mass function, as the expression of a multi-phase ISM, due to dynamic and thermodynamic effects in starburst and metal-poor environments.

Methods. The adaptive mesh refinement code FLASH is used to study the level of fragmentation during the collapse. With this code, including self-gravity, thermal balance, turbulence, and shocks, collapse is simulated with four different metallicity-dependent cooling functions. Turbulent and rotational energies are considered as well. During the simulations, number densities of 10 are reached. The influences of dust and cosmic ray heating are investigated and compared to isothermal cases.

Results. The results indicate that fragmentation increases with metallicity, while cosmic ray and gas-grain collisional heating counteract this. We also find that modest rotation and turbulence can affect the cloud evolution as far as fragmentation is concerned. In this light, we conclude that radiative feedback in starburst regions will inhibit fragmentation, while low-metallicity dwarfs should also enjoy a star formation mode in which fragmentation is suppressed.