A Systemic Study of 14 Southern Infrared Dark Clouds with the N2H+, HNC, HCO+, and HCN Lines

TL;DR

This study analyzes 14 southern infrared dark clouds using molecular line data to understand their physical and chemical properties, evolutionary stages, and molecular abundances, revealing correlations and environmental effects on molecular ratios.

Contribution

It provides a detailed chemical and physical characterization of IRDCs using multiple molecular lines, highlighting their evolutionary diversity and molecular abundance ratios.

Findings

IRDCs are at various evolutionary stages from starless to UCHII regions.

Integrated intensity ratios differ from previous measurements, indicating environmental effects.

Strong correlations among HNC, HCO+, and HCN intensities suggest linked chemical evolution.

Abstract

We have studied 14 southern infrared dark clouds (IRDCs) using the data taken from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey and the GLIMPSE and MIPSGAL mid-infrared survey of the inner Galaxy. The physical and chemical characteristics of the 14 IRDCs are investigated using N2H+(1-0), HNC(1-0), HCO+(1-0), and HCN(1-0) molecular lines. We find that the 14 IRDCs are in different evolutionary stages from the "starless" to the sources with an UCHII region. Three IRDCs are detected to have the star forming activities. The integrated intensity ratios I(HCO+)/I(HCN), I(N2H+)/I(HCN), and I(HNC)/I(HCN) are all about 1.5, which is different from the previous measurements, suggesting that the integrated intensity ratios may be affected by the cloud environments. The integrated intensities of HNC, HCO+ and HCN show a tight correlation for the 14 IRDCs, implying a close link to the chemical evolution of these three species in the IRDCs. The derived excitation temperature for each IRDC is less than 25 K. We estimate the abundances of the four molecules from 10^(-11) to 10^(-9), and the average abundance ratios N(HNC)/N(HCN) = 1.4+ -0.50, N(HNC)/N(HCO+) = 1.74 + - 0.22, N(HCN)/N(HCO+)= 1.21 + - 0.41.