Spectral Line Survey toward Molecular Clouds in the Large Magellanic
  Cloud
  Spectral line survey observations of 7 molecular clouds in the Large
Magellanic Cloud (LMC) have been conducted in the 3 mm band with the Mopra 22 m
telescope to reveal chemical compositions in low metallicity conditions.
Spectral lines of fundamental species such as CS, SO, CCH, HCN, HCO+, and HNC
are detected in addition to those of CO and 13CO, while CH3OH is not detected
in any source and N2H+ is marginally detected in two sources. The
molecular-cloud scale (10 pc scale) chemical composition is found to be similar
among the 7 sources regardless of different star formation activities, and
hence, it represents the chemical composition characteristic to the LMC without
influences of star formation activities. In comparison with chemical
compositions of Galactic sources, the characteristic features are (1) deficient
N-bearing molecules, (2) abundant CCH, and (3) deficient CH3OH. The feature (1)
is due to a lower elemental abundance of nitrogen in the LMC, whereas the
features (2) and (3) seem to originate from extended photodissociation regions
and warmer temperature in cloud peripheries due to a lower abundance of dust
grains in the low metallicity condition. In spite of general resemblance of
chemical abundances among the seven sources, the CS/HCO+ and SO/HCO+ ratios are
found to be slightly higher in a quiescent molecular cloud. An origin of this
trend is discussed in relation to possible depletion of sulfur along molecular
cloud formation.
