Oral Presentation

Turbulence, magnetic field, and thermodynamics: what determines the IMF?

Author(s): Yueh-Ning Lee (IPGP); Patrick Hennebelle (AIM, CEA Saclay)

Presenter: Yueh-Ning Lee (IPGP)

The initial mass function (IMF) is suggested by observations to be universal in spite of the wide variety of star-forming environments. Several major physical mechanisms, such as thermal energy, turbulence, gravity, magnetic field, and radiation, are governing the formation of the stars, or even before that, the formation of the prestellar cores. In this presentation, I will first discuss the possible impact of initial conditions on the mass spectrum of stars forming inside clusters. A series of simulations was performed with a wide range of initial density, turbulent energy level, and magnetic field strength. Different regimes of core formation are suggested according to the relative importance among various forms of energy: thermal support, turbulence, magnetic field, and gravity. There is a change of the IMF slope depending on the energy regimes. By adapting the gravo-turbulent fragmentation model of Hennebelle and Chabrier (2009), the IMF from simulations is well reproduced with a powerlaw density PDF, that is often observed in collapsing clumps. On the other hand, the simulations suggest a lower mass limit that truncates the mass spectrum in extremely dense environments, giving a peak of the distribution that is independent of the large-scale conditions. We propose a mechanism to explain the peak of the IMF using a combination of the tidal forces and the thermodynamics of the first Larson core. A characteristic mass of 10 times the mass of the first Larson core, that is, around 0.2 solar masses, is robustly predicted. These results suggest that, the universality of the IMF is probably true only under certain circumstances, or possibly it is the initial conditions and local physics that are universal and some particular conditions are not so common!