Oral Presentation
Universality of the Initial Mass Function
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 present during the formation of the stars, or even before that, the formation of the prestellar cores. I will first discuss the possible impact of initial conditions on the mass spectrum of stars forming inside clusters. The observations suggest a high-mass end slope of -1.3 for the IMF, while this does not seem to have been readily reproduced by any of the present day simulations. A series of numerical experiments was performed with a wide range of initial density, turbulent energy level, and magnetic field strength. We compare the stellar mass spectrum outcome to the prediction adapted from the gravo-turbulent fragmentation model of Hennebelle and Chabrier (2009). The IMF from simulations is well reproduced only when a powerlaw density PDF, which is often observed in collapsing clumps, is applied. Nonetheless, a shallower slope is suggested. On the other hand, the simulations exhibit 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, regardless of the bulk temperature of the diffuse gas and the presence of the magnetic field. 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 self-regulated and universal.
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