PROBING THE INITIAL CONDITIONS OF STAR-FORMATION - THE STRUCTURE OF THE PRESTELLAR CORE L-1689B

In a recent JCMT submillimeter study, Ward-Thompson et al. (1994) obta ined the first dust continuum maps of five low-mass dense cores among the sample of starless ammonia cores from Myers and colleagues. Here, we present the results of new 1.3 mm continuum mapping observations fo r one of these cores, L 1689B, taken with the IRAM 30-m telescope equi pped with the 7-channel and 19-channel MPIfR bolometer arrays. The new 1.3 mm data, which were obtained in the 'on-the-fly' scanning mode, h ave better angular resolution and sensitivity than the earlier 800 mu m data, reaching an rms noise level of similar to 3 mJy/13 '' beam. Ou r IRAM map resolves L 1689B as an east-west elongated core of deconvol ved size 0.045 pc x 0.067 pc (FWHM), central column density N-H2 simil ar to 1.5 x 10(22) cm(-2), and mass M(FWHM) similar to 0.6 M., in good agreement with our previous JCMT estimates. We confirm that the radia l column density profile N(theta) of L 1689B is not consistent with a single power law with angular radius theta but flattens out near its c entre. Comparison with synthetic model profiles simulating our 'on-the -fly' observations indicates that N(theta(maj)) proportional to theta( maj)(-0.2) for theta(maj) less than or equal to 25 '' and N(theta(maj) ) proportional to theta(maj)(-1) for 25 '' < theta(maj) less than or e qual to 90 '', where theta(maj) is measured along the major axis of th e core. The observed mean profile is not consistent with a simple Gaus sian source, being flatter than a Gaussian in its outer region. Howeve r, the profile measured along the minor axis of L 1689B is significant ly steeper and apparently consistent with a Gaussian 'edge' in the nor th-south direction. The mass, radius, and density of the relatively fl at central region are estimated to be similar to 0.3 M., similar to 40 00 AU, and similar to 2 x 10(5) cm(-3), respectively. The mass of L 16 89B and its large (> 30) density contrast with the surrounding molecul ar cloud indicate that it is not a transient structure but a self-grav itating pre-stellar core. The flat inner profile and other measured ch aracteristics of L 1689B are roughly consistent with theoretical predi ctions for a magnetically-supported, flattened core either on the verg e of collapse or in an early phase of dynamical contraction. In this c ase, the mean magnetic field in the central region should be less than or similar to 80 mu G, which is high but not inconsistent with existi ng observational constraints. Alternatively, the observed core structu re may also be explained by equilibrium models of primarily thermally supported, self-gravitating spheroids interacting with an external UV radiation field. The present study supports the conclusions of our pre vious JCMT survey and suggests that, in contrast with protostellar env elopes, most pre-stellar cores have flat inner density gradients which approach rho(tau) proportional to tau(-2) only beyond a few thousand AU. This implies that, in some cases at least, the initial conditions for protostellar collapse depart significantly from a singular isother mal sphere.