Physical and chemical properties of Red MSX Sources in the southern sky:
  HII regions
  We have studied the physical and chemical properties of 18 southern Red
Midcourse Space Experiment Sources (RMSs), using archival data taken from the
Atacama Pathfinder Experiment (APEX) Telescope Large Area Survey of the Galaxy,
the Australia Telescope Compact Array, and the Millimeter Astronomy Legacy Team
Survey at 90 GHz. Most of our sources have simple cometary/unresolved radio
emissions at 4.8 and/or 8.6GHz. The large number of Lyman continuum fluxes (NL)
indicates they are probably massive O- or early B-type star formation regions.
Archival IRAS infrared data are used to estimate the dust temperature, which is
about 30 K of our sources. Then, the H2 column densities and the
volume-averaged H2 number densities are estimated using the 0.87 mm dust
emissions. Large-scale infall and ionized accretions may be occurring in
G345.4881+00.3148. We also attempt to characterize the chemical properties of
these RMSs through molecular line (N2H+ (1-0) and HCO+ (1-0)) observations.
Most of the detected N2H+ and HCO+ emissions match well with the dust emission,
implying a close link to their chemical evolution in the RMSs. We found that
the abundance of N2H+ is one order of magnitude lower than that in other
surveys of infrared dark clouds, and a positive correlation between the
abundances of N2H+ and HCO+. The fractional abundance of N2H+ with respect to
H2 seems to decrease as a function of NL. These observed trends could be
interpreted as an indication of enhanced destruction of N2H+, either by CO or
through dissociative recombination with electrons produced by central UV
photons.
