THE EVOLUTION OF INTERMEDIATE-MASS PROTOSTARS .1. BASIC RESULTS

We present a numerical calculation of the structure and evolution of protostars in the mass range from 1 to 8 M.. In this first communication, we model the protostar as being built up from a spherically symmetric accretion flow, with a constant mass accretion rate of 10(-5) M. yr-1. The protostar is initially fully convective due to the steady state burning of deuterium near its center. We derive analytically the mass-radius relation during this phase. At a mass of 2.4 M., there appears a radiative barrier, which prevents the convective transport of deuterium to the center. The region inside the barrier exhausts its deuterium and becomes radiatively stable, while the deuterium further out ignites in a subsurface shell. The shell burning swells the protostar, with a maximum radius of 10.5 R. being attained at 3.6 M.. Further mass addition results in rapid gravitational contraction and the ignition of hydrogen via the CN cycle. The star joins the zero-age main sequence by a mass of 8 M.. Over a significant mass range, we find that protostars contain a substantial amount of deuterium. We speculate that the ignition of this fuel during subsequent pre-main-sequence contraction could drive the surface convection that is apparently present in many Herbig Ae and Be stars.