On the origin of phosphorus nitride in star-forming regions

Abstract

We present multitransition observations of phosphorus nitride (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 versus gas-phase chemistry), and the dominant desorption mechanism (thermal versus 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 indicate 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 not only in protostellar shocks, but also in colder and more quiescent gas through alternative pathways.