CO Spectral Line Energy Distributions in Galactic Sources: Empirical Interpretation of Extragalactic Observations

• Published in: Astrophysical journal. Letters 2017-02, Vol.836 (1), p.117
• Main Authors: Indriolo, Nick, Bergin, E. A., Goicoechea, J. R., Cernicharo, J., Gerin, M., Gusdorf, A., Lis, D. C., Schilke, P.
• Format: Article
• Language: English
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BEAMS; CARBON; CARBON MONOXIDE; Cores; EMISSION; Emission lines; ENERGY SPECTRA; EXCITATION; Fluxes; GALAXIES; HEAT EXCHANGERS; Interstellar gas; Interstellar matter; Interstellar medium; Line spectra; Molecular gases; MOLECULES; MONOCLINIC LATTICES; Orion nebula; PERTURBED ANGULAR CORRELATION; Physics; Star formation; STARS
• ISSN: 0004-637X; 2041-8205; 1538-4357; 2041-8213
• DOI: 10.3847/1538-4357/836/1/117

The relative populations in rotational transitions of CO can be useful for inferring gas conditions and excitation mechanisms at work in the interstellar medium. We present CO emission lines from rotational transitions observed with Herschel /HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N. Integrated line fluxes from these observations are combined with those from Herschel /PACS observations of the same sources to construct CO spectral line energy distributions (SLEDs) from 5 ≤  J u  ≤ 48. These CO SLEDs are compared to those reported in other galaxies, with the intention of empirically determining which mechanisms dominate excitation in such systems. We find that CO SLEDs in Galactic star-forming cores cannot be used to reproduce those observed in other galaxies, although the discrepancies arise primarily as a result of beam filling factors. The much larger regions sampled by the Herschel beams at distances of several megaparsecs contain significant amounts of cooler gas, which dominate the extragalactic CO SLEDs, in contrast to observations of Galactic star-forming regions, which are focused specifically on cores containing primarily hot molecular gas.