ALMA Observations of HCO+ and HCN Emission in a massive star forming region N55 of the Large Magellanic Cloud

Abstract: 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.