ALMA Observations of HCO+ and HCN Emission in a massive star forming
  region N55 of the Large Magellanic Cloud
  We present the results of high spatial resolution HCO$^{+}$($1-0$) and
HCN($1-0$) observations of N55 south region (N55-S) in the Large Magellanic
Cloud (LMC), obtained with the Atacama Large Millimeter/submillimeter Array
(ALMA). N55-S is a relatively less extreme star-forming region of the LMC
characterized by a low radiation field. We carried out a detailed analysis of
the molecular emission to investigate the relation between dense molecular
clumps and star formation in the quiescent environment of N55-S. We detect ten
molecular clumps with significant HCO$^{+}(1-0)$ emission and eight with
significant HCN($1-0$) emission, and estimate the molecular clump masses by
virial and local thermodynamic equilibrium analysis. All identified young
stellar objects (YSOs) in the N55-S are found to be near the HCO$^{+}$ and HCN
emission peaks showing the association of these clumps with recent star
formation activity. The molecular clumps that have associated YSOs show
relatively larger linewidths and masses than those without YSOs. We compare the
clump properties of the N55-S with those of other giant molecular clouds (GMCs)
in the LMC and find that N55-S clumps possess similar size but relatively lower
linewidth and larger HCN/HCO$^{+}$(1$-$0) flux ratio. These results can be
attributed to the low radiation field in N55-S resulted by relatively low star
formation activity compared to other active star-forming regions like
30Doradus-10 and N159. The dense gas fraction of N55-S is $\sim$ 0.025, lower
compared to other GMCs of the LMC supporting the low star formation efficiency
of this region.
