The Formation of Common-Envelope, Pre-Main-Sequence Binary Stars

Abstract

Recent observational investigations of the frequency of occurrence of premain-sequence binary stars have reinforced earlier suspicions that “binary formation is the primary branch of the star-formation process” (Mathieu 1994). As Bodenheimer et al. (1993) have reviewed, a number of different theories have been proposed to explain the preponderance of binary stars. Klein et al. (1998) show how the direct fragmentation of protostellar gas clouds may occur in early phases of collapse (at cloud densities n ∼ 10³ -10¹⁰cm⁻³). But at higher densities, clouds are unable to cool efficiently upon contraction. Consequently, direct fragmentation becomes problematical. Because higher mean densities are associated with systems having shorter dynamical times, one is led to consider mechanisms other than direct cloud fragmentation for forming binary systems with orbital periods less than a few hundred years. Here we investigate whether such binaries can form by spontaneous fission of rapidly rotating protostars.