Effect of optical depth on study of chemical properties of massive star forming clumps†.

Here we present the study on chemical properties of massive star forming clumps using N₂H⁺(1-0), H¹³CO⁺(1-0), HCN(1-0) and HN¹³C(1-0) data from the literature [Astron. Astrophys. 563, A97 (2014)]. We found that abundances of H¹³CO⁺ and HN¹³C are affected by H₂ column densities. As the median values of these two abundances increase by nearly 10 times from stages A to B, H¹³CO⁺ and HN¹³C are suitable for tracing the evolution of massive star forming clumps. The order of rapidity in growth of abundances of all the four studied molecules from stages A to B, is H¹³CO⁺, HCN, HN¹³C, and N₂H⁺, from the highest to the lowest. Our results suggest that the observing optically thin molecular lines with high angular resolution are necessary to study the chemical evolution of massive star forming clumps. [ABSTRACT FROM AUTHOR]