Interferometric Observations of High-Mass Star-Forming Clumps with
  Unusual N2H+/HCO+ Line Ratios
  The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey has detected
high-mass star-forming clumps with anomalous N$_2$H$^+$/HCO$^+$(1-0) integrated
intensity ratios that are either unusually high ("N$_2$H$^+$ rich") or
unusually low ("N$_2$H$^+$ poor"). With 3 mm observations from the Australia
Telescope Compact Array (ATCA), we imaged two N$_2$H$^+$ rich clumps,
G333.234-00.061 and G345.144-00.216, and two N$_2$H$^+$ poor clumps,
G351.409+00.567 and G353.229+00.672. In these clumps, the N$_2$H$^+$ rich
anomalies arise from extreme self-absorption of the HCO$^+$ line.
G333.234-00.061 contains two of the most massive protostellar cores known with
diameters of less than 0.1 pc, separated by a projected distance of only 0.12
pc. Unexpectedly, the higher mass core appears to be at an earlier evolutionary
stage than the lower mass core, which may suggest that two different epochs of
high-mass star formation can occur in close proximity. Through careful analysis
of the ATCA observations and MALT90 clumps (including the G333, NGC 6334, and
NGC 6357 star formation regions), we find that N$_2$H$^+$ poor anomalies arise
at clump-scales and are caused by lower relative abundances of N$_2$H$^+$ due
to the distinct chemistry of H II regions or photodissociation regions.
