Characterizing [C II] Line Emission In Massive Star Forming Clumps
  Because the 157.74 micron [C II] line is the dominant coolant of star-forming
regions, it is often used to infer the global star-formation rates of galaxies.
By characterizing the [C II] and far-infrared emission from nearby Galactic
star-forming molecular clumps, it is possible to determine whether
extragalactic [C II] emission arises from a large ensemble of such clumps, and
whether [C II] is indeed a robust indicator of global star formation. We
describe [C II] and far-infrared observations using the FIFI-LS instrument on
the SOFIA airborne observatory toward four dense, high-mass, Milky Way clumps.
Despite similar far-infrared luminosities, the [C II] to far-infrared
luminosity ratio, L([C II])/L(FIR) varies by a factor of at least 140 among
these four clumps. In particular, for AGAL313.576+0.324, no [C II] line
emission is detected despite a FIR luminosity of 24,000 L_sun.
AGAL313.576+0.324 lies a factor of more than 100 below the empirical
correlation curve between L([C II])/L(FIR) and S_\nu (63 micron)/S_\nu (158
micron) found for galaxies. AGAL313.576+0.324 may be in an early evolutionary
"protostellar" phase with insufficient ultraviolet flux to ionize carbon, or in
a deeply embedded ``hypercompact' H II region phase where dust attenuation of
UV flux limits the region of ionized carbon to undetectably small volumes.
Alternatively, its apparent lack of \cii\, emission may arise from deep
absorption of the \cii\, line against the 158 micron continuum, or
self-absorption of brighter line emission by foreground material, which might
cancel or diminish any emission within the FIFI-LS instrument's broad spectral
resolution element (~250 km/s)
