We present an unbiased census of deeply embedded protostars in Perseus , Serpens , and Ophiuchus , assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys .
We identify protostellar candidates based on their mid-infrared properties , correlate their positions with 1.1 mm core positions from Enoch et al .
( 18 ) , Young et al .
( 78 ) , and Enoch et al .
( 19 ) , and construct well-sampled SEDs using our extensive wavelength coverage ( \lambda = 1.25 - 1100 ~ { } \micron ) .
Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three cloud sample .
We compare to protostellar evolutionary models using the bolometric temperature-luminosity diagram , finding a population of low luminosity Class I sources that are inconsistent with constant or monotonically decreasing mass accretion rates .
This result argues strongly for episodic accretion during the Class I phase , with more than 50 % of sources in a “ sub-Shu ” ( dM / dt < 10 ^ { -6 } M _ { \odot } yr ^ { -1 } ) accretion state .
Average spectra are compared to protostellar radiative transfer models , which match the observed spectra fairly well in Stage 0 , but predict too much near-IR and too little mid-IR flux in Stage I .
Finally , the relative number of Class 0 and Class I sources are used to estimate the lifetime of the Class 0 phase ; the three cloud average yields a Class 0 lifetime of 1.7 \pm 0.3 \times 10 ^ { 5 } yr , ruling out an extremely rapid early accretion phase .
Correcting photometry for extinction results in a somewhat shorter lifetime ( 1.1 \times 10 ^ { 5 } yr ) .
In Ophiuchus , however , we find very few Class 0 sources ( N _ { \mathrm { Class~ { } 0 } } / N _ { \mathrm { Class~ { } I } } \sim 0.1 - 0.2 ) , similar to previous studies of that cloud .
The observations suggest a consistent picture of nearly constant average accretion rate through the entire embedded phase , with accretion becoming episodic by at least the Class I stage , and possibly earlier .