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 $^{12}$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 $X_{\rm CO}$ 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 $\times 10^3 M_{\odot}$.
