ALMA observations of N83C in the early stage of star formation in the Small Magellanic Cloud

Abstract

We have performed Atacama Large Millimeter/submillimeter Array (ALMA) observations in ${}^{12}$CO($J=2-1$), ${}^{13}$CO($J=2-1$), C${}^{18}$O($J=2-1$), ${}^{12}$CO($J=3-2$), ${}^{13}$CO($J=3-2$), and CS($J=7-6$) lines toward the active star-forming region N83C in the Small Magellanic Cloud (SMC), whose metallicity is $\sim$ 1/5 of the Milky Way (MW). The ALMA observations first reveal sub-pc scale molecular structures in ${}^{12}$CO($J=2-1$) and ${}^{13}$CO($J=2-1$) emission. We found strong CO peaks associated with young stellar objects (YSOs) identified by the $Spitzer$ Space Telescope, and also found that overall molecular gas is distributed along the edge of the neighboring HII region. We derived a gas density of $\sim 10^4$ cm${}^{-3}$ in molecular clouds associated with YSOs based on the virial mass estimated from ${}^{12}$CO($J=2-1$) emission. This high gas density is presumably due to the effect of the HII region under the low-metallicity (accordingly small-dust content) environment in the SMC; far-UV radiation from the HII region can easily penetrate and photo-dissociate the outer layer of ${}^{12}$CO molecules in the molecular clouds, and thus only the innermost parts of the molecular clouds are observed even in ${}^{12}$CO emission. We obtained the CO-to-H${}_2$ conversion factor $X_{\mathrm{CO}}$ of $7.5\times 10^{20}$ cm${}^{-2}$ (K km s${}^{-1}$)${}^{-1}$ in N83C based on virial masses and CO luminosities, which is four times larger than that in the MW, 2 $\times 10^{20}$ cm${}^{-2}$ (K km s${}^{-1}$)${}^{-1}$. We also discuss the difference in the nature between two high-mass YSOs, each of which is associated with a molecular clump with a mass of about a few $\times 10^3{M}_{\odot }$.

arXiv comment12 pages, 4 figures, 2 tables, accepted for publication in ApJ

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