Confirmation of interstellar phosphine towards asymptotic giant branch star IRC+10216

Abstract

Phosphorus (P) is an important element for the chemical evolution of galaxies and many kinds of biochemical reactions. Phosphorus is one of the crucial chemical compounds in the formation of life on our planet. In an interstellar medium, phosphine (PH${}_3$) is a crucial biomolecule that plays a major role in understanding the chemistry of phosphorus-bearing molecules, particularly phosphorus nitride (PN) and phosphorus monoxide (PO), in the gas phase or interstellar grains. We present the first confirmed detection of phosphine (PH${}_3$) in the asymptotic giant branch (AGB) carbon-rich star IRC+10216 using the Atacama Large Millimeter/Submillimeter Array (ALMA) band 6. We detect the $J$ = 1${}_0$$-$0${}_0$ rotational transition line of PH${}_3$ with a signal-to-noise ratio (SNR) of $\ge$3.5$\sigma$. This is the first confirmed detection of phosphine (PH${}_3$) in the ISM. Based on LTE spectral modelling, the column density of PH${}_3$ is (3.15$\pm$0.20)$\times$10${}^{15}$ cm${}^{-2}$ at an excitation temperature of 52$\pm$5 K. The fractional abundance of PH${}_3$ with respect to H${}_2$ is (8.29$\pm$1.37)$\times$10${}^{-8}$. We also discuss the possible formation pathways of PH${}_3$ and we claim that PH${}_3$ may be created via the hydrogenation of PH${}_2$ on the grain surface of IRC+10216.

Accepted for publication in Journal of Astrophysics and Astronomy