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We present observations of the $^3P_1$-$^3P_0$ fine-structure line of atomiccarbon using the ASTE 10 m sub-mm telescope towards RCW38, the youngest superstar cluster in the Milky Way. The detected [CI] emission is compared with theCO $J$ = 1-0 image cube presented in Fukui et al. (2016) which has an angularresolution of 40$^{\prime \prime}$ ($\sim$ 0.33 pc). The overall distributionof the [CI] emission in this cluster is similar to that of the $^{13}$COemission. The optical depth of the [CI] emission was found to be $\tau$ =0.1-0.6, suggesting mostly optically thin emission. An empirical conversionfactor from the [CI] integrated intensity to the H$_2$ column density wasestimated as $X_{\rm [CI]}$ = 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 columndensity ratio of the [CI] to CO ($N_{\rm [CI]}/N_{\rm CO}$) was derived as$\sim$ 0.1 for $A_V$ of 10-100 mag, which is consistent with that of the Orioncloud 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, whichreach up to $\sim$ 60 mag. Such a high [CI]/CO ratio in a high $A_V$ region isdifficult to be explained by the plane-parallel photodissociation region (PDR)model, which predicts that this ratio is close to 0 due to the heavy shieldingof the ultraviolet (UV) radiation. Our results suggest that the molecular gasin this cluster is highly clumpy, allowing deep penetration of UV radiationeven at averaged $A_V$ values of 100 mag. Recent theoretical works havepresented models consistent with such clumped gas distribution with a sub-pcclump size (e.g., Tachihara et al. 2018).

publications of the astronomical society of japan

Observations of the [CI] ($^3P_1$-$^3P_0$) emission toward the massive  star-forming region RCW38: further evidence for highly-clumped density  distribution of the molecular gas