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Description
Phosphorus is one of the crucial elements for life. It plays a central role in the structure of essential biotic molecules, such as nucleic acids (DNA and RNA), phospholipids (the skin of all cellular membranes) and the adenosine triphosphate (ATP), from which all forms of life assume energy (Pasek & Lauretta 2005).
Despite its importance, the chemistry of Phosphorus in the interstellar medium (ISM) is still poorly known.
The molecule of PN is one of the two P-bearing molecules detected in star-forming regions (PN & PO) (e.g. Turner & Bally 1987, Fontani et al. 2016, Rivilla et al. 2016), but it is still not clear under which conditions it is formed.
PN is a crucial species to understand the chemistry of interstellar P, because it has been proposed as precursor of other P-bearing species like PO, HNNP, HNPN, and HPNN (Rivilla et al. 2016, Bhasi et al. 2016). Moreover, PN-based derivatives have been proposed as very plausible prebiotic agents in the early Earth (Karki et al. 2017).
The few detections ($\sim$6) of PN reported in the literature before 2016 are associated with warm and turbulent sources, or even shocked material. However, Fontani et al. (2016) found some high-mass star-forming cores with PN(2-1) line widths smaller than 5 km/s. This indicates that PN can also arise from relatively quiescent and cold gas. This information challenges theoretical models that invoke either high desorption temperatures or grain sputtering from shocks to release phosphorus into the gas phase (Turner & Bally 1987, Charnley & Millar 1994).
In order to investigate the main chemical route that leads to the formation of PN and the main mechanism of desorption, we present multi-line observations of PN towards 9 massive dense cores in different evolutionary stages. We compared the results for the molecule of PN with other molecules, tracing different chemical and physical conditions: $\mathrm{SiO, SO, CH_{3}OH, N_{2}H^{+}}$.
The main result of our analysis is that in six out of nine sources the most important release mechanism of PN seems to be sputtering of dust grains in shocked regions, in good agreement with recent results in Galactic Center clouds (Rivilla et al. 2018). In fact, in these six sources the line profiles of PN are very well correlated with those of the two shock tracers SiO and SO. Moreover, the abundances of PN and those of SiO and SO show a faint but not negligible positive trend.
Nevertheless, this can not be the only mechanism since the line profiles of the three remaining sources do not show high-velocity wings (associated with shocked material), but narrow line widths. This confirms the previous results of Fontani et al. (2016), and reinforces the conclusion that the origin of PN is not to be considered unique, since it could form in both shocked and quiescent gas.
