Characterizing [CII] 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 farinfrared luminosities, the [C II] to far-infrared luminosity ratio, L[C II]/LFIR, 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⊙. AGAL313.576+0.324 lies a factor of more than 100 below the empirical correlation curve between L[C II]/LFIR and Sn (63 mm) Sn (158 mm) 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 [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 (ΔV ∼ 250 km s-1).