The Herschel Orion Protostar Survey: Luminosity and Envelope Evolution
  The Herschel Orion Protostar Survey obtained well-sampled 1.2 - 870 micron
spectral energy distributions (SEDs) of over 300 protostars in the Orion
molecular clouds, home to most of the young stellar objects (YSOs) in the
nearest 500 pc. We plot the bolometric luminosities and temperatures for 330
Orion YSOs, 315 of which have bolometric temperatures characteristic of
protostars. The histogram of bolometric temperature is roughly flat; 29% of the
protostars are in Class 0. The median luminosity decreases by a factor of four
with increasing bolometric temperature; consequently, the Class 0 protostars
are systematically brighter than the Class I protostars, with a median
luminosity of 2.3 L_sun as opposed to 0.87 L_sun. At a given bolometric
temperature, the scatter in luminosities is three orders of magnitude. Using
fits to the SEDs, we analyze how the luminosities corrected for inclination and
foreground reddening relate to the mass in the inner 2500 AU of the best-fit
model envelopes. The histogram of envelope mass is roughly flat, while the
median corrected luminosity peaks at 15 L_sun for young envelopes and falls to
1.7 L_sun for late-stage protostars with remnant envelopes. The spread in
luminosity at each envelope mass is three orders of magnitude. Envelope masses
that decline exponentially with time explain the flat mass histogram and the
decrease in luminosity, while the formation of a range of stellar masses
explains the dispersion in luminosity.
