ALMA observations of N83C in the early stage of star formation in the small Magellanic cloud

We have performed Atacama Large Millimeter/submillimeter Array (ALMA) observations in the¹²CO(J = 2 - 1),¹³CO(J = 2 - 1), C¹⁸O(J = 2 - 1),¹²CO(J = 3 - 2),¹³CO(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 about one-fifth of the Milky Way (MW). The ALMA observations first reveal subparsec-scale molecular structures in¹²CO(J = 2 - 1) and¹³CO(J = 2 - 1) emissions. We found strong CO peaks associated with young stellar objects (YSOs) identified by the Spitzer Space Telescope, and we also found that overall molecular gas is distributed along the edge of the neighboring H II region. We derived a gas density of ~10⁴ cm⁻³ in molecular clouds associated with YSOs based on the virial mass estimated from the¹²CO(J = 2 - 1) emission. This high gas density is presumably due to the effect of the H II region under the low-metallicity (and accordingly small-dust content) environment in the SMC; far-UV radiation from the H II region can easily penetrate and photodissociate the outer layer of¹²CO molecules in the molecular clouds, and thus only the innermost parts of the molecular clouds are observed even in¹²CO emission. We obtained the CO-to-H₂ conversion factor X_CO of 7.5 ´ 10²⁰ cm⁻² (K km s⁻¹)⁻¹ in N83C based on virial masses and CO luminosities, and it is four times larger than that in the MW, 2 10²⁰ 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 10³ M.