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 0.1Â pc , separated by a projected distance of only 0.12Â 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 , NGCÂ 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 .