On the origin of phosphorus nitride in star-forming regions

TL;DR

This study investigates the formation of phosphorus nitride (PN) in star-forming regions, revealing that PN can originate from shocks due to dust grain sputtering or from colder, quiescent gas through other pathways, depending on the environment.

Contribution

It provides the first measurement of PN excitation temperatures and compares chemical tracers to identify multiple formation mechanisms in star-forming cores.

Findings

PN excitation temperatures range from 5-30 K.

PN line profiles correlate with SiO and SO in most targets.

PN can form via shocks or in cold, quiescent gas.

Abstract

We present multi-transition observations of PN towards a sample of nine massive dense cores in different evolutionary stages. Using transitions with different excitation conditions, we have found for the first time that the excitation temperatures of PN are in the range 5-30 K. To investigate the main chemical route for the PN formation (surface-chemistry vs. gas-phase chemistry), and the dominant desorption mechanism (thermal vs. shock), we have compared our results with those obtained from molecules tracing different chemical and physical conditions (SiO, SO, CH3OH, and N2H+). We have found that the PN line profiles are very well correlated with those of SiO and SO in six out of the nine targets, which indicates that PN may be released by sputtering of dust grains due to shocks. This finding is corroborated by a faint but statistically significant positive trend between the PN abundance and those of SiO and SO. However, in three objects the PN lines have no hints of high velocity wings, which indicates an alternative origin of PN. Overall, our results indicate that the origin of PN is not unique, as it can be formed in protostellar shocks, but also in colder and more quiescent gas through alternative pathways.