The origin of the characteristic mass for star formation Paul Clark ITA/ZAH - Universitaet Heidelberg, Heidelberg, Germany
It has been suggested by Larson that the transition between the line-cooling and dust-cooling regimes sets a unique density and temperature in molecular clouds, and that this in turn sets a characteristic mass for the formation of stars. We present the first self-consistent calculations to capture the evolution of the gas between the line-cooling and dust-cooling regimes. Our code makes use of a time-dependent chemical network to track the abundance of the main atomic and molecular coolants, such as C+, C, O, and CO, and we include prescriptions for the optically thick/thin transitions of the cooling lines. The code also includes a detailed prescription for the dust heating and cooling that accounts for heating via the interstellar radiation field (including an accurate treatment of the ISRF's attenuation), cosmic ray heating, dust emission, gas-grain energy transfer, and photo-electric heating. We find that the transition between the line-cooling and dust-cooling regimes is indeed responsible for setting the characteristic mass of stars, although the process appears to be more complicated than the idea originally suggested by Larson. Not only does gas cool down to the dust temperature during fragmentation, but in some cases actually heats up *to* the dust temperature. The dust hence appears to act as a thermostat during the cloud fragmentation process. We also find that due to the excellent cooling abilities of the dust grains, the characteristic mass of stars is largely insensitive to either the strength of the local ISRF or the local cosmic-ray ionization rate. In fact, we find that for conditions akin to those in the galactic centre, the median stellar mass is consistent with what we find in the local stellar field population. Finally we show that accurate dust temperatures alone are not sufficient to determine the true energy balance of the presetllar cores, since the gas and dust temperatures can differ by a factor of two at radii of ~10,000 AU.
Collaborators: Simon C.O. Glover, ITA, Heidelberg Ralf S. Klessen, ITA, Heidelberg