We present the results of a search for all embedded protostars with internal luminosities \leq 1.0 L _ { \odot } 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 } \geq 4 \times 10 ^ { -3 } ( d / 140 pc ) ^ { 2 } L _ { \odot } , 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 } \leq 1.0 L _ { \odot } ; 15 have L _ { int } \leq 0.1 L _ { \odot } .
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 _ { \odot } .
Between 75 - 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 \mbox { $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 .