Paul Clark (Cardiff University)

The IMF: a fight between accretion and fragmentation?

The “competitive accretion” model of the initial mass function (IMF) appears to play an important role in essentially all the numerical simulations conducted to date. However, we highlight in this talk that competitive accretion requires that the accretion onto existing protostars is balanced by the formation of new stars — a phenomenon not discussed in the original model. As such, we present the idea that the formation of the IMF is actually the balance accretion and fragmentation, highlighting that the classic Salpeter slope is likely the result of a balancing act, and we discuss why this balance may have the result that we see in nature. The results of recent simulations by various groups can also be explained by this picture. In addition, we discuss how the conditions in the galactic centre, where the gas heating is dominated by cosmic rays (as opposed to photoelectric heating, which dominates in the solar neighbourhood), may lead to a top-heavy protostellar IMF. This effect arises due to the increase in the accretion rate from the hotter gas in the galactic centre, coupled with the lack of fragmentation due to the longer cooling times.