Models of Star Formation

Abstract

The collapse of spherically symmetric protostars has been reinvestigated on the basis of new numerical tools and techniques that have been recently developed. In addition, a much more consistent set of thermodynamic functions (particularly for molecular hydrogen) and a substantially improved version of the equation of state have been used. Protostellar cores are found to be always on the verge of dynamical instability due to the fact that the gas is cool and dense there, and the adiabatic exponent ${\Gamma _1} = {(\partial \ln P/\partial \ln \rho)_{ad}}$ p varies around the critical number 4/3. During the overall collapse of a Jeans-unstable cloud fragment, which lasts 3–4 initial free-fall times, many cores form and undergo subsequent explosive reexpansion in a quasi-periodic way, the cycles becoming shorter and shorter. The final contraction to a (pre-) main-sequence star takes place on time scales of a few thousand years. It starts out with much higher mean densities, yielding an optical depth for the fragment of the order of unity and, as a consequence, higher temperatures in the core which now stabilizes. The main accretion phase appears to be nothing but a last-minute- effect, the probability to observe ‘classical’ protostars Is thus very low.