ENVELOPE STRUCTURE OF STARLESS CORE L694-2 DERIVED FROM A NEAR-INFRARED EXTINCTION MAP

摘要

We present a near-infrared extinction study of the dark globule L694-2, a starless core that shows strong evidence for inward motions in the profiles of molecular spectral lines. The J-, H-, and K-band data were taken using the European Southern Observatory New Technology Telescope. The best-fit simple spherical power-law model has index p = 2.6 +- 0.2, over the ～0.036-0.1 pc range in radius sampled in extinction. This power-law slope is steeper than the value of p = 2 for a singular isothermal sphere, the initial condition of the inside-out model for protostellar collapse. Including an additional component of extinction along the line of sight further steepens the inferred profile. A fit for a Bonnor-Ebert sphere model results in a supercritical value of the dimensionless radius ξ_(max) = 25 +- 3. This unstable configuration of material in the L694-2 core may be related to the observed inward motions. The Bonnor-Ebert model matches the shape of the observed density profile but significantly underestimates the amount of extinction observed in the L694-2 core (by a factor of ～ 4). This discrepancy in normalization has also been found for the nearby protostellar core B335 (Harvey and coworkers). A cylindrical model with scale height H = 0.0164 +- 0.002 pc (13″.5 +- 5″) viewed at a small inclination to the axis of the cylinder provides an equally good radial profile as a power-law model, and it also reproduces the asymmetry of the L694-2 core remarkably well. In addition, this model provides a possible basis for understanding the discrepancy in the normalization of the Bonnor-Ebert model, namely, that L694-2 has prolate structure, with the full extent (mass) of the core being missed by analysis that assumes symmetry between the profiles of the core in the plane of the sky and along the line of sight. If the core is sufficiently magnetized, then fragmentation may be avoided, and later evolution might produce a protostar similar to B335.