Fragmentation of a Protostellar Core : The Case of CB 230

The Bok globule CB 230 (L 1177) contains an active, low-mass star-forming core which is associated with a double NIR reflection nebula, a collimated bipolar molecular outflow, and strong mm continuum emission. The morphology of the NIR nebula suggests the presence of a deeply embedded, wide binary protostellar system. High-angular resolution observations now reveal the presence of two sub-cores, two distinct outflow centers, and an embedded accretion disk associated with the western bipolar NIR nebula. In terms of separation and specific angular momentum, the CB 230 double protostar system probably results from core fragmentation and can be placed at the upper end of the pre-main sequence binary distribution. 1. Motivation and observations Binary systems have been observed in all pre-main-sequence stages of evolution and there is growing evidence for proto-binary systems, although the numbers are still small (Mundy, this volume). Both theory and observations support the hypothesis that most binary systems form by fragmentation during the gravi-tational collapse of molecular cloud cores. We have started a program of high angular resolution observations to study the early formation process of binary stars in detail, using the Owens Valley Radio Observatory (OVRO) millimeter array. This paper presents new observations of the protostellar core in the Bok globule CB 230 (D = 450 pc). CB 230 contains a dense core (Launhardt et al. 1997, 1998, 2000) and two associated NIR reflection nebulae separated by ∼10 ′′ The western nebula is bipolar, with a bright northern lobe perfectly aligned with the blue lobe of a collimated CO outflow (cf. Yun & Clemens 1994, Fig.3). The eastern nebula is much fainter and redder and displays no bipolar structure. No associated stars are visible. CB 230 was observed at 1.2 and 3 mm with the OVRO mm array in spring 2000. The mm continuum emission traced the optically thin thermal dust emission , while molecular gas was traced by the N 2 H + (1–0) and 13 CO(1–0) lines at 93 and at 110 GHz, respectively. N 2 H + (1–0) comprises seven hyperfine components and, compared to other molecules, depletes later and more slowly onto grains (Bergin & Langer 1997). It is a reliable indicator of the morphology and kinematics of protostellar cores. 13 CO traces the small-scale structure of the outflow near the driving sources. Beam sizes for the different observations vary from 1 ′′ to 10 ′′ , and the spectral resolution was …