THESPITZERc2d SURVEY OF NEARBY DENSE CORES. V. DISCOVERY OF A VeLLO IN THE “STARLESS” DENSE CORE L328

Abstract

This paper reports the discovery of a Very Low Luminosity Object (VeLLO) in the "starless" dense core L328, using the Spitzer Space Telescope and ground-based observations from near-infrared (NIR) to millimeter wavelengths. The Spitzer 8 μm image indicates that L328 consists of three subcores of which the smallest one may harbor a source, L328-IRS, while two other subcores remain starless. L328-IRS is a Class 0 protostar according to its bolometric temperature (44 K) and the high fraction (∼72%) of its luminosity emitted at submillimeter wavelengths. Its inferred "internal luminosity" (0.04–0.06 L☉) using a radiative transfer model under the most plausible assumption of its distance as 200 pc is much fainter than for a typical protostar, and even fainter than other VeLLOs studied previously. Note, however, that its inferred luminosity may be uncertain by a factor of 2–3 if we consider two extreme values of the distance of L328-IRS (125 or 310 pc). Low angular resolution observations of CO do not show any clear evidence of a molecular outflow activity. But broad line widths toward L328, and Spitzer and NIR images showing nebulosity possibly tracing an outflow cavity, strongly suggest the existence of outflow activity. Provided that an envelope of at most ∼0.1 M☉ is the only mass accretion reservoir for L328-IRS, the star formation efficiency is close to the canonical value ∼30%, and L328-IRS has not yet accreted more than 0.05 M☉, at the assumed distance of 200 pc, L328-IRS is destined to be a brown dwarf.