Characterizing [C II] Line Emission in Massive Star-forming Clumps

Because the 157.74 μm [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 Stratospheric Observatory For Infrared Astronomy (SOFIA) airborne observatory toward four dense, high-mass, Milky Way clumps. Despite similar far-infrared luminosities, the [C II] to far-infrared luminosity ratio, ${L}_{[{\rm{C}}{\rm\small{II}}]}$ /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 ${L}_{\odot }$ . AGAL313.576+0.324 lies a factor of more than 100 below the empirical correlation curve between ${L}_{[{\rm{C}}{\rm\small{II}}]}$ /L_FIR and ${S}_{\nu }(63\,\mu {\rm{m}})/{S}_{\nu }(158\,\mu {\rm{m}})$ 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" ${\rm{H}}\,{\rm\small{II}}$ region phase where dust attenuation of UV flux limits the region of ionized carbon to undetectably small volumes. Alternatively, its apparent lack of [C II] emission may arise from deep absorption of the [C II] line against the 158 μm 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 ( ${\rm{\Delta }}V\sim 250$ km s^-1).