Protostellar collapse models of prolate molecular cloud cores

Abstract

The continued detection of binary systems among pre-main-sequence stars suggests that fragmenta- tion is a very frequent process during the early stages of star formation. However, the fragmentation hypothesis rests only upon the results of three-dimensional hydrodynamics code calculations. The validity of isothermal frag- mentation calculations was questioned by the results of Truelove et al. (1997), and more recently, of Boss et al. (2000), who found, working at very high spatial resolution, that a particular Gaussian cloud model collapsed isothermally to form a singular lament rather than a binary or quadruple protostellar system as predicted by previous calculations. Suciently high spatial resolution is necessary to resolve the Jeans length and hence avoid articial fragmentation in isothermal collapse calculations. Here we use an adaptive, spherical-coordinate hydro- dynamics code based on the \zooming coordinates to investigate the isothermal collapse of centrally condensed (Gaussian), prolate (2:1 axial ratio) cloud core models, with thermal energy 0:22 and varied rotational energy (0:246 0:00025), to discern whether they will still undergo fragmentation into a protostellar binary system, as found in most previous prolate cloud collapse calculations, or condense all the way into a thin lament, as suggested by the linear analysis of Inutsuka & Miyama (1992) and the ndings of Truelove et al. and Boss et al. for the spherical, Gaussian cloud model. The prolate clouds all collapsed self-similarly to produce an intermediate barlike core, which then shrank indenetely into a singular lament without fragmenting. Collapse of the bar into a thin lament also occurred self-similarly, with the forming laments being much longer than the Jeans length. Since the laments form at maximum densities that are typical of the transition from the isothermal to the nonisothermal phase, gradual heating may retard the collapse and allow fragmentation of the lament into a binary or multiple protostellar core, as required to explain the high frequency of binary stars.