Velocity-resolved high-J CO emission from massive star-forming clumps

Abstract

Context. Massive star formation is associated with energetic processes, which result in significant gas cooling via far-infrared (IR) lines. Velocity-resolved observations can constrain the kinematics of the gas, allowing the identification of the physical mechanisms responsible for gas heating. Aims. Our aim is to quantify far-IR CO line emission towards high-mass star-forming regions, identify the high-velocity gas component associated with outflows, and estimate the physical conditions required for the excitation of the observed lines. Methods. Velocity-resolved SOFIA/GREAT spectra of 13 high-mass star-forming clumps of various luminosities and evolutionary stages are studied in highly excited rotational lines of CO. For most targets, the spectra are from frequency intervals covering the CO 11−10 and 16−15 lines towards two sources, also the CO 13−12 line was observed with SOFIA/4GREAT. Angular resolutions at the line frequencies range from 14″ to 20″, corresponding to spatial scales of ~0.1–0.8 pc. Radiative transfer models were used to determine the physical conditions giving rise to the emission in the line wings. Results. All targets in our sample show strong high-J CO emission in the far-IR, characterised by broad line wings associated with outflows, thereby significantly increasing the sample of high-mass objects with velocity-resolved high-J CO spectra. Twelve sources show emission in the line wings of the CO 11−10 line (Eu/kB=365 K), and eight sources in the CO 16−15 line (Eu/kB =752 K). The contribution of the emission in the line wings to the total emission ranges from ~28% to 76%, and does not correlate with the envelope mass or evolutionary stage. Gas excitation temperatures cover a narrow range of 120–220 K for the line wings, and 110–200 K for the velocity-integrated line emission, assuming local thermodynamics equilibrium (LTE). For the two additional sources with the CO 13−12 line (Eu/kB=503 K) data, wing emission rotational temperatures of ~130 K and 165 K were obtained using Boltzmann diagrams. The corresponding non-LTE radiative transfer models indicate gas densities of 105−107 cm−3 and CO column densities of 1017−1018 cm-2 in the line wings, similar to physical conditions in deeply embedded low- and high-mass protostars. The velocity-integrated CO line fluxes correlate with the bolometric luminosity over 7 orders of magnitude, including data on the low-mass protostars from the literature. This suggests that similar processes are responsible for the high-J CO excitation over a significant range of physical scales. Conclusions. Velocity-resolved line profiles allow the detection of outflows towards massive star-forming clumps spanning a broad range of evolutionary stages. The lack of clear evolutionary trends suggest that mass accretion and ejection prevail during the entire lifetime of star-forming clumps.