Characterizing [C II] Line Emission In Massive Star Forming Clumps

TL;DR

This study investigates [C II] line emission in four massive star-forming clumps in the Milky Way, revealing significant variability and exploring reasons for the absence of emission in one clump, which impacts the use of [C II] as a star formation indicator.

Contribution

It provides the first detailed [C II] and far-infrared observations of nearby Galactic massive star-forming clumps, highlighting variability and potential limitations of [C II] as a universal star formation tracer.

Findings

[C II]/FIR ratio varies by a factor of 140 among the four clumps.

One clump shows no detectable [C II] emission despite high FIR luminosity.

Possible explanations include evolutionary stage, dust attenuation, or line absorption effects.

Abstract

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)