IDENTIFYING THE LOW-LUMINOSITY POPULATION OF EMBEDDED PROTOSTARS IN THE c2d OBSERVATIONS OF CLOUDS AND CORES

We present the results of a search for all embedded protostars with internal luminosities <= 1.0 L-circle dot in the full sample of nearby, low-mass star-forming regions surveyed by the Spitzer Space Telescope Legacy Project From Molecular Cores to Planet Forming Disks'' (c2d). The internal luminosity of a source, L-int, is the luminosity of the central source and excludes luminosity arising from external heating. On average, the Spitzer c2d data are sensitive to embedded protostars with L-int >= 4 x 10(-3)(d/140 pc)(2) L-circle dot, a factor of 25 better than the sensitivity of the Infrared Astronomical Satellite (IRAS) to such objects. We present a set of selection criteria used to identify candidates from the Spitzer data and examine complementary data to decide whether each candidate is truly an embedded protostar. We find a tight correlation between the 70 mu m flux and internal luminosity of a protostar, an empirical result based on both observations and detailed two-dimensional radiative transfer models of protostars. We identify 50 embedded protostars with L-int <= 1: 0 L-circle dot; 15 have L-int <= 0: 1 L-circle dot. The intrinsic distribution of source luminosities increases to lower luminosities. While we find sources down to the above sensitivity limit, indicating that the distribution may extend to luminosities lower than probed by these observations, we are able to rule out a continued rise in the distribution below L-int = 0.1 L-circle dot. Between 75% and 85% of cores classified as starless prior to being observed by Spitzer remain starless to our luminosity sensitivity; the remaining 15%-25% harbor low-luminosity, embedded protostars. We compile complete spectral energy distributions for all 50 objects and calculate standard evolutionary signatures (L-bol, T-bol, and L-bol/L-smm) and argue that these objects are inconsistent with the simplest picture of star formation, wherein mass accretes from the core onto the protostar at a constant rate.