Chemical Evolution of N 2 H + in Six Massive Star-forming Regions

Abstract

To investigate how the abundance of N 2 H + varies as massive clumps evolve, here we present a multiwavelength study toward six molecular clouds. All of these clouds contain several massive clumps in different evolutionary stages of star formation. Using archival data of the 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 method. We found that all of the six clouds show distinct dust temperature gradients, ranging from ∼20 to ∼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 comes from the 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 that this is because in the photodissociation regions around classical H ii regions, N 2 H + is heavily destroyed by free electrons. However, when the dust temperature is below 27 K, the abundance of N 2 H + increases with the dust temperature. This seems to be inconsistent with previous chemical models made in low-mass star-forming regions. In order to investigate whether this inconsistency is caused by a different chemistry in high-mass star-forming clumps, higher angular resolution observations are necessary.