Phosphorus chemistry in star forming regions:  

understanding the formation of the molecule of PN

Together with Carbon, Oxygen, Hydrogen and Nitrogen, Phosphorus is one of the key element for Life as we know it on Earth. The tight connection of Life and Phosphorus is shown by the presence of this element in several biologically relevant molecules: it plays a central role in 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. [1]



All the Phosphorus in the Universe is created through nuclear reactions in high mass stars (stars with M>8 M⦿ ) and it is injected to the interstellar medium (ISM) through supernovae explosions [2], but the chemistry of this element during the process of star formation is still poorly known.
The only two P-bearing molecules detected in dense star forming clouds are the two simple molecules of PN and PO. Until recent years the few number of detections were not enough to understand which are the physical conditions that favour the formation of P-bearing molecules. In particular, PN is a crucial species to understand the chemistry of interstellar P, as it has been proposed as precursor of other P-bearing species like PO, HNNP, HNPN, and HPNN [3,4]. Moreover, PN-based derivatives have been proposed as very plausible prebiotic agents in the early Earth [5].
For these reasons, the molecule of PN has been the central point of the study led at the Arcetri Observatory by a group of researchers, including Chiara Mininni (PhD Student), Francesco Fontani, Victor Manuel Rivilla and Maite Beltrán. In this study the molecule of PN has been observed towards nine high-mass star forming regions in different evolutionary stages, in its rotational transitions at 1 mm and 2.1 mm. The data were obtained using the IRAM-30m, located at Pico Veleta in Spain, and were integrated with the observation of PN line at 3.2 mm presented in [6]. PN was detected, at least in one transition, in all the nine sources, regardless of the different evolutionary stages.
The detection of more than one spectral line of PN has allowed the researchers to calculate the abundances of this molecule in the sources, using the method of Rotational Diagrams. These abundances has been compared to those of other well known molecules, used as tracers of different physical condition and chemical pathways:
  • SiO and SO: they are present in the nuclei of dust grains and their abundances in gas phase are enhanced in regions of shocks.
  • CH3OH: it forms on the surface of dust grains and it is mainly released in gas phase due to thermal heating.
  • N2H+: it forms via gas-phase chemical reactions.
The comparisons of the abundances and of the line widths of the lines seem to exclude any correlation between PN and CH3OH, while there is a faint but statistically significant positive trend between the abundances of PN and those of N2H+, SiO and SO (see Figure 1)
Figure 1: In the panel on the left is shown the plot of the abundances of PN against the abundances of the molecule of SO, while in the right panel the PN abundances is plotted against those of CH3OH. The red line is the best linear fit; the angular coefficient of it is reported in the upper left corner of each panel. Different color refers to evolutionary stages (High Mass Starless Core, High Mass Protostellar Object and Ultra Compact HII region)
The main result of the analysis is that in six out of nine sources line profiles of PN are very well correlated with those of the two shock tracers SiO and SO (see Figure 2).
This, together with the positive trend shown by the abundances, seems to point out that in 2/3 of the sample the most important release mechanism of PN is sputtering of dust grains in shocked regions, in good agreement with recent results in Galactic Center clouds [7].
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 (these sources has been labeled as Narrow (N), while the previous as Broad (B) in Figure 1 and 2).
Figure 2: For each of the nine sources we present the overplot of the PN (3-2) line (in red) and the shock tracer SiO (2-1) line (in black). The lines of the molecule of PN are multiplied for an appropriate factor (reported in red in the upper left corner of each panel), in order to be more visible. The Broad sources (B) shows good agreement in the lines profile, while this is not true for Narrow sources (N)
This confirms the results of Fontani et al. (2016), who found line widths for the line at 3.2 mm lower than 5 km/s in some sources, 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.
Paper: “On the origin of phosphorus nitride in star-forming regions”, accepted for publication in Monthly Notices of the Royal Astronomical Society Letters
Authors: C. Mininni, F. Fontani, V. M. Rivilla, M. T. Beltrán, P. Caselli & A. Vasyunin
[1] Pasek and Lauretta, Astrobiology 5, 515-535 (2005)
[2] Koo et al., Science 342, pp 1346-1348 (2013)
[3] Rivilla et al., ApJ 826, 161 (2016)
[4] Bhasi et al., JTCC 16, 1750075 (2017)
[5] Karki et al., Life 7, 32 (2017)
[6] Fontani et al., ApJ 822, L30 (2016)
[7] Rivilla et al., MNRAS 475, L30-L34 (2018)