Ionise hard: interstellar PO${}^{+}$ detection

Abstract

We report the first detection of the phosphorus monoxide ion (PO${}^{+}$) in the interstellar medium. Our unbiased and very sensitive spectral survey towards the G+0.693$-$0.027 molecular cloud covers four different rotational transitions of this molecule, two of which ($J$=1$-$0 and $J$=2$-$1) appear free of contamination from other species. The fit performed, assuming Local Thermodynamic Equilibrium conditions, yields a column density of $N$=(6.0$\pm$0.7)$\times$10${}^{11}$ cm${}^{-2}$. The resulting molecular abundance with respect to molecular hydrogen is 4.5$\times$10${}^{-12}$. The column density of PO${}^{+}$ normalised by the cosmic abundance of P is larger than those of NO${}^{+}$ and SO${}^{+}$, normalised by N and S, by factors of 3.6 and 2.3, respectively. The $N$(PO${}^{+}$)/$N$(PO) ratio is 0.12$\pm$0.03, more than one order of magnitude higher than those of $N$(SO${}^{+}$)/$N$(SO) and $N$(NO${}^{+}$)/$N$(NO). These results indicate that P is more efficiently ionised in the ISM than N and S. We have performed new chemical models that confirm that the PO${}^{+}$ abundance is strongly enhanced in shocked regions with high values of cosmic-ray ionisation rates (10${}^{-15}-$10${}^{-14}$ s${}^{-1}$), as occurs in the G+0.693$-$0.027 molecular cloud. The shocks sputter the interstellar icy grain mantles, releasing into the gas phase most of their P content, mainly in the form of PH${}_3$, which is converted into atomic P, and then ionised efficiently by cosmic rays, forming P${}^{+}$. Further reactions with O${}_2$ and OH produce PO${}^{+}$. The cosmic-ray ionisation of PO might also contribute significantly, which would explain the high $N$(PO${}^{+}$)/$N$(PO) observed. The relatively high gas-phase abundance of PO${}^{+}$ with respect to other P-bearing species stresses the relevance of this species in the interstellar chemistry of P.

Accepted in Frontiers in Astronomy and Space Sciences