Chemical evolution of Red MSX Sources in the southern sky
  Red Midcourse Space Experiment (MSX) Sources (RMSs) are regarded as excellent
candidates of massive starforming 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 (NL), 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 NL. 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.
