Thermal and Dynamical Properties of a Protostar and Its Contraction to the Stage of Quasi-Static Equilibrium

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Abstract
The thermal and dynamical evolution of a protostar of one solar mass has been investigated from the stage transparent to radiation to the stage of the beginning of quasi-hydrostatic equilibrium. The evolution in the transparent stage has been studied in the ρ-T diagram by comparing the time-scale of cooling or heating with the time-scale of free contraction or free expansion. We have taken into account the emission of radiation by H2 molecules, grains and ions (C+, Si+ and Fe+) as cooling processes, and interstellar star-light and cosmic ray particles as heating sources. It has been shown that when ρ≲×10−18 g/cm3 the gas cloud of one solar mass expands and returns to the interstellar gas, while the cloud of ρ≳×10−18 g/cm3 contracts to become a star. In the subsequent opaque stage, the energy loss from the star is determined by the diffusion of photons, and the contraction becomes more and more adiabatic. When the contraction proceeds to a certain extent, the growth of a pressure gradient in the central region gives rise to a bounce of this region. The bouncing core and the contracting envelope build up a new density distribution, and the protostar contracts again as a whole. The protostar repeats this process a few times. When the release of gravitational energy becomes large enough to ionize hydrogen atoms completely, the bounce of the core gives rise to a strong shock wave. The arrival of the shock wave at the surface gives rise to a sudden increase in the luminosity accompanied probably by explosive phenomena such as the production of high energy particles and light elements Li, Be and B. Afterward the star is in quasi-hydrostatic equilibrium, and in a short period of about 103 years the star becomes wholly convective. The subsequent evolution is a quasi-static contraction toward the main sequence.

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