ROTATING PROTOSTARS AND PROTOSTELLAR DISKS .1. EQUILIBRIUM-MODELS

This paper is part of a series dealing with the structure and dynamic stability of rotating protostellar cores. As a first step in our study , we have generated numerical equilibrium models for isentropic, axisy mmetric protostellar cores in rapid rotation. These models represent e ndstates for collapse from two different types of initial precollapse cloud conditions, chosen to be reasonable cases for the formation of l ow- or intermediate-mass stars on the basis of other theoretical or ob servational work. Specifically, we consider the equilibrium cores whic h would form from the collapse of uniformly rotating clouds with the d ensity distributions of singular isothermal spheres and of truncated G aussian spheres. The major structural differences between the two sequ ences are largely due to their distinct angular momentum distributions . The protostellar cores which result from singular isothermal initial conditions can be readily interpreted as slowly rotating stars surrou nded by massive, rotationally supported disks. A ''star'' and a ''disk '' are not easily distinguished for the Gaussian cases, but the outer regions of these models are typically in rapid, nearly Keplerian rotat ion. For reasonable assumptions about parameters, the most rapidly rot ating protostellar cores that we can calculate accurately correspond t o highly flattened disk or star/disk systems of roughly solar mass wit h equatorial radii of a few AUs or less. For the protostellar cores th at result from the collapse of singular isothermal spheres, we find th at a significant range in parameter space exists in which protostellar disks are much smaller than the typical dimensions usually considered for the solar nebula. These conditions may be conducive to the format ion of relatively compact planetary systems such as 51 Pegasi. Our axi symmetric star/disk models are fully two-dimensional in the sense that both the vertical disk structure and the central starlike regions are resolved. These models will be used as a numerical laboratory for stu dies of various dynamic processes in protostellar disks. (C) 1997 Acad emic Press.