To investigate how the abundance of N _ { 2 } H ^ { + } varies as massive clumps evolve , here we present a multi-wavelength study toward six molecular clouds .
All of these clouds contain several massive clumps in different evolutionary stages of star formation .
Using archival data of Herschel InfraRed Galactic Plane Survey ( Hi-GAL ) , we made H _ { 2 } column density and dust temperature maps of these regions by the spectral energy distribution ( SED ) method .
We found all of the six clouds show distinct dust temperature gradients , ranging from \sim 20 K to \sim 30 K. This makes them good candidates to study chemical evolution of molecules ( such as N _ { 2 } H ^ { + } ) in different evolutionary stages of star formation .
Our molecular line data come from Millimeter Astronomy Legacy Team Survey at 90 GHz ( MALT90 ) .
We made column density and then abundance maps of N _ { 2 } H ^ { + } .
We found that when the dust temperature is above 27 K , the abundance of N _ { 2 } H ^ { + } begins to decrease or reaches a plateau .
We regard this is because that in the photodissociation regions ( PDRs ) around classical HII regions , N _ { 2 } H ^ { + } is destroyed by free electrons heavily .
However , when the dust temperature is below 27 K , the abundance of N _ { 2 } H ^ { + } increases with dust temperature .
This seems to be inconsistent with previous chemical models made in low-mass star-forming regions .
In order to check out whether this inconsistency is caused by a different chemistry in high-mass star-forming clumps , higher angular resolution observations are necessary .