2016ApJ...833..248Y

Red Midcourse Space Experiment (MSX) Sources (RMSs) are regarded as excellent candidates of massive star-forming regions. In order to characterize the chemical properties of massive star formation, we made a systematic study of 87 RMSs in the southern sky, using archival data taken from the Atacama Pathfinder Experiment Telescope Large Area Survey of the Galaxy (ATLASGAL), the Australia Telescope Compact Array, and the Millimetre Astronomy Legacy Team Survey at 90 GHz (MALT90). According to previous multiwavelength observations, our sample could be divided into two groups: massive young stellar objects and H ii regions. Combined with the MALT90 data, we calculated the column densities of N₂H⁺, C₂H, HC₃N, and HNC and found that they are not much different from previous studies made in other massive star-forming regions. However, their abundances are relatively low compared to infrared dark clouds (IRDCs). The abundances of N₂H⁺ and HNC in our sample are at least 1 mag lower than those found in IRDCs, indicating chemical depletions in the relatively hot gas. Besides, the fractional abundances of N₂H⁺, C₂H, and HC₃N seem to decrease as a function of their Lyman continuum fluxes (N_L), indicating that these molecules could be destroyed by UV photons when H ii regions have formed inside. We also find that the C₂H abundance decreases faster than HC₃N with respect to N_L. The abundance of HNC has a tight correlation with that of N₂H⁺, indicating that it may be also preferentially formed in cold gas. We regard our RMSs as being in a relatively late evolutionary stage of massive star formation.