INNER STRUCTURE OF STARLESS CORE L694-2 DERIVED FROM MILLIMETER-WAVE INTERFEROMETRY

摘要

We study the density structure of the candidate contracting starless core L694-2 using 1.3 mm dust continuum observations from the IRAM Plateau de Bure Interferometer (PdBI) and the Berkeley-Illinois-Maryland Association (BIMA) array, which probe spatial scales from 10,000 to 500 AU. The long-baseline PdBI observations detect no emission from the core and limit the maximum contamination from a compact component: F_c < 2.7 mJy. The flux limit corresponds to a very small disk mass, M_(disk) approx< 5 x 10~(-4) solar mass (60 K/T_(disk)), and bolsters the "starless" interpretation of the L694-2 core. The shorter baseline BIMA data are compared to a series of density models using a physically motivated temperature distribution with a central minimum. This analysis provides clear evidence for a turnover from the steep density profile observed in the outer regions in dust extinction to substantially more shallow behavior in the inner regions (<7500 AU). The best-fit Bonnor-Ebert, Plummer-like, broken power law, and end-on cylinder models produce very similar flattened profiles and cannot be distinguished. We quantify the sensitivity of the inferred structure to various uncertainties, including the temperature distribution, the accuracy of the central position, and the presence of a weak, unresolved central component. The largest uncertainty comes from the temperature assumption; an isothermal model modifies the best-fit parameters by ～2 σ, with the inferred density profiles more shallow. Dust emission and extinction profiles are reproduced by an embedded isothermal cylinder with scale height H = 13″.5 inclined at a small angle to the line of sight. The turnover observed in the L694-2 density distribution suggests that pressure forces still support the core and that it has not fully relaxed, as in the inside-out collapse model, despite the extended inward motions inferred from molecular line observations by Lee, Myers, & Tafalla. In the context of the cylindrical density model, these inward motions may represent the contraction of a prolate core along its major axis.