ALMA reveals molecular cloud N55 in the Large Magellanic Cloud as a site
  of massive star formation
  We present the molecular cloud properties of N55 in the Large Magellanic
Cloud using $^{12}$CO(1-0) and $^{13}$CO(1-0) observations obtained with
Atacama Large Millimeter Array. We have done a detailed study of molecular gas
properties, to understand how the cloud properties of N55 differ from Galactic
clouds. Most CO emission appears clumpy in N55, and molecular cores that have
YSOs show larger linewidths and masses. The massive clumps are associated with
high and intermediate mass YSOs. The clump masses are determined by local
thermodynamic equilibrium and virial analysis of the $^{12}$CO and $^{13}$CO
emissions. These mass estimates lead to the conclusion that, (a) the clumps are
in self-gravitational virial equilibrium, and (b) the $^{12}$CO(1-0)-to-H$_2$
conversion factor, X$_{\rm CO}$, is 6.5$\times$10$^{20}$cm$^{-2}$(K km
s$^{-1}$)$^{-1}$. This CO-to-H$_2$ conversion factor for N55 clumps is measured
at a spatial scale of $\sim$0.67 pc, which is about two times higher than the
X$_{\rm CO}$ value of Orion cloud at a similar spatial scale. The core mass
function of N55 clearly show a turnover below 200M$_{\odot}$, separating the
low-mass end from the high-mass end. The low-mass end of the $^{12}$CO mass
spectrum is fitted with a power law of index 0.5$\pm$0.1, while for $^{13}$CO
it is fitted with a power law index 0.6$\pm$0.2. In the high-mass end, the core
mass spectrum is fitted with a power index of 2.0$\pm$0.3 for $^{12}$CO, and
with 2.5$\pm$0.4 for $^{13}$CO. This power-law behavior of the core mass
function in N55 is consistent with many Galactic clouds.
