Observations of the [CI] (${}^3{P}_1$-${}^3{P}_0$) emission toward the massive star-forming region RCW38: further evidence for highly-clumped density distribution of the molecular gas

Abstract

We present observations of the ${}^3{P}_1$-${}^3{P}_0$ fine-structure line of atomic carbon using the ASTE 10 m sub-mm telescope towards RCW38, the youngest super star cluster in the Milky Way. The detected [CI] emission is compared with the CO $J$ = 1-0 image cube presented in Fukui et al. (2016) which has an angular resolution of 40${}^{\prime \prime }$ ($\sim$ 0.33 pc). The overall distribution of the [CI] emission in this cluster is similar to that of the ${}^{13}$CO emission. The optical depth of the [CI] emission was found to be $\tau$ = 0.1-0.6, suggesting mostly optically thin emission. An empirical conversion factor from the [CI] integrated intensity to the H${}_2$ column density was estimated as $X_{[\mathrm{CI}]}$ = 6.3 $\times$ 10${}^{20}$ cm${}^{-2}$ K${}^{-1}$ km${}^{-1}$ s (for visual extinction: $A_V$ $\le$ 10 mag) and 1.4 $\times$ 10${}^{21}$ cm${}^{-2}$ K${}^{-1}$ km${}^{-1}$ s (for $A_V$ of 10-100 mag). The column density ratio of the [CI] to CO ($N_{[\mathrm{CI}]}/N_{\mathrm{CO}}$) was derived as $\sim$ 0.1 for $A_V$ of 10-100 mag, which is consistent with that of the Orion cloud presented in Ikeda et al. (2002). However, our results cover an $A_V$ regime of up to 100 mag, which is wider than the coverage found in Orion, which reach up to $\sim$ 60 mag. Such a high [CI]/CO ratio in a high $A_V$ region is difficult to be explained by the plane-parallel photodissociation region (PDR) model, which predicts that this ratio is close to 0 due to the heavy shielding of the ultraviolet (UV) radiation. Our results suggest that the molecular gas in this cluster is highly clumpy, allowing deep penetration of UV radiation even at averaged $A_V$ values of 100 mag. Recent theoretical works have presented models consistent with such clumped gas distribution with a sub-pc clump size (e.g., Tachihara et al. 2018).

arXiv comment33 pages, 21 figures, PASJ accepted for publication

References in corpus (16)

• How to Calculate Molecular Column Density
• A PDR-Code Comparison Study
• Formation of Massive Molecular Cloud Cores by Cloud-cloud Collision
• The On The Fly Imaging Technique
• On-The-Fly Observing System of the Nobeyama 45-m and ASTE 10-m Telescopes
• The two molecular clouds in RCW 38; evidence for formation of the youngest super star cluster in the Milky Way triggered by cloud-cloud collision
• Modelling [CI] emission from turbulent molecular clouds
• An Alternative Accurate Tracer of Molecular Clouds: The "$X_{\mathrm{CI}}$-Factor"
• X-ray and IR Point Source Identification and Characteristics In the Embedded, Massive Star-Forming Region RCW 38
• The ${}^{12}$CO/${}^{13}$CO ratio in turbulent molecular clouds
• Extensive [CI] Mapping toward the Orion-A Molecular Cloud
• Atomic Carbon and CO Isotope Emission in the Vicinity of DR15
• The Structure of the Star-forming Cluster RCW 38
• Extended Carbon Line Emission in the Galaxy: Searching for Dark Molecular Gas along the G328 Sightline
• A VLT/NACO Study of Star Formation in the Massive Embedded Cluster RCW 38
• An ALMA view of the Galactic super star cluster RCW38 at 270-AU resolution

Cited by in corpus (7)

• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• An ACA Survey of [CI] ${}^3{P}_1{-}^3{P}_0$, CO $J=4-3$, and Dust Continuum in Nearby U/LIRG
• CI and CO in Nearby Spiral Galaxies -- I. Line Ratio and Abundance Variations at ~ 200 pc Scales
• A Low Frequency Pilot Survey of Southern HII Regions in the Vela Constellation
• ATLASGAL-selected massive clumps in the inner Galaxy. X. Observations of atomic carbon at 492 GHz
• ALMA view of the $\rho$ Ophiuchi A PDR with a 360-au beam: the [CI] emission originates from the plane-parallel PDR and extended gas
• Cosmic-ray-driven enhancement of the C${}^0$/CO abundance ratio in W51C