  To investigate how the abundance of N2H+ 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 H2 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 20 K to 30 K.
This makes them good candidates to study chemical evolution of molecules (such
as N2H+) 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 N2H+. We found that when the
dust temperature is above 27 K, the abundance of N2H+ begins to decrease or
reaches a plateau. We regard this is because that in the photodissociation
regions (PDRs) around classical HII regions, N2H+ is destroyed by free
electrons heavily. However, when the dust temperature is below 27 K, the
abundance of N2H+ 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.
