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

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 12 CO (J = 1−0) and 13 CO (J = 1 − 0) emission lines obtained with the Nobeyama 45m telescope. Differences in clump properties such as mass, size, and gravitational boundedness reflect the different evolutionary stages of the M17-HII and M17-IRDC clouds. Clumps in the M17-HII cloud are denser, more compact, and more gravitationally bound than those in M17-IRDC. While M17-HII hosts a large fraction of very dense gas (27%) that has 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 TRAO 14m telescope, trace all gas with column density higher than 3 × 10 22 cm −2 , confirm the deficiency of high density ( 10 5 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.

ATOMS: ALMA three-millimeter observations of massive star-forming regions – II. Compact objects in ACA observations and star formation scaling relations

“…Increasing HCN-to-HCO + ratios with increasing infrared luminosity have been found in LIRGs (Papadopoulos 2007). In the Galaxy, more evolved HII regions also show higher HCN-to-HCO + ratios than infrared dark clouds (Nguyen-Luong et al 2020). However, in our data, we do not see any trend of HCN-to-HCO + ratios against the L bol spanning four orders of magnitude.…”

“…The line ratio of HCN-to-HCO + seems to be sensitive to environments (Pety et al 2017;Shimajiri et al 2017). High HCN-to-HCO + ratios have been found in far-UV irradiated environments such as evolved Galactic HII regions (Nguyen-Luong et al 2020), AGNs (Aladro et al 2015) or luminous infrared galaxies (LIRGs; Papadopoulos 2007). This implies that HCO + abundance is sensitive to the ionization degree of molecular gas.…”

Cloud structures in M17 SWex : Possible cloud-cloud collision

“…In contrast, Povich et al (2016) interpreted that this core might be a foreground object and the main structure of M17 SWex may be located to 2 kpc. Recently we analyzed the cloud structures in this area using the wide-field 12 CO, 13 CO, and C 18 O observations (Shimoikura et al 2019;Nguyen-Luong et al 2020), and found that the molecular gas in the M17 and M17 SWex regions is continuous in the position-position velocity space, which is consistent with the argument of Povich et al (2016). Thus, in this paper, we adopt a distance to M17 SWex of 2 kpc.…”

“…The details of the observations are described by Shimoikura et al (2019), Nakamura et al (2019) and Nguyen-Luong et al (2020). In brief, we carried out mapping observations in 12 CO (J = 1 − 0) and 13 CO (J = 1 − 0) toward the M17 region using the FOREST receiver (Minamidani et al 2016) installed on the 45 m telescope of the Nobeyama Radio Observatory.…”

“…In addition, the clumps in the M17 HII region are more massive than those in M17 SWex, but have similar sizes and therefore higher densities. Thus, Nguyen-Luong et al (2020) concluded that the deficiency in the high-mass stellar population in M17 SWex presumably comes from the extremelylow dense gas fraction. A similar trend is observed in other nearby star-forming regions such as Orion A and Aquila Rift (Nakamura et al 2019).…”