Large-scale Molecular Gas Distribution in the M17 Cloud Complex: Dense Gas Conditions of Massive Star Formation?

Nguyen-Luong, Quang; Nakamura, Fumitaka; Sugitani, Koji; Shimoikura, Tomomi; Dobashi, Kazuhito; Kinoshita, Shinichi W.; Kim, Kee-Tae; Kang, Hynwoo; Sanhueza, Patricio; Evans II, Neal J. and White, Glenn J. (2020). Large-scale Molecular Gas Distribution in the M17 Cloud Complex: Dense Gas Conditions of Massive Star Formation? The Astrophysical Journal, 891(1), article no. 66.

DOI: https://doi.org/10.3847/1538-4357/ab700a

Abstract

The non-uniform distribution of gas and protostars in molecular clouds is caused by combinations of various physical processes that are difficult to separate. We explore this non-uniform distribution in the M17 molecular cloud complex that hosts massive star formation activity using the 12CO (J = 1–0) and 13CO (J = 1–0) emission lines obtained with the Nobeyama 45 m telescope. Differences in clump properties such as mass, size, and gravitational boundedness reflect the different evolutionary stages of the M17-H ii and M17-IRDC clouds. Clumps in the M17-H ii cloud are denser, more compact, and more gravitationally bound than those in M17-IRDC. While M17-H ii hosts a large fraction of very dense gas (27%) that has a column density larger than the threshold of ~1 g cm−2 theoretically predicted for massive star formation, this very dense gas is deficient in M17-IRDC (0.46%). Our HCO+ (J = 1–0) and HCN (J = 1–0) observations with the Taeduk Radio Astronomy Observatory 14 m telescope trace all gas with a column density higher than 3 × 1022 cm−2, confirming the deficiency of high-density (≳105 cm−3) gas in M17-IRDC. Although M17-IRDC is massive enough to potentially form massive stars, its deficiency of very dense gas and gravitationally bound clumps can explain the current lack of massive star formation.

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