The phosphorus availability in star-forming regions is fundamental for the formation of several prebiotic molecules. Phosphorous is quite abundant on Earth (P/H ~ 10-3) compared to its cosmic abundance (P/H ~ 2.57 x 10-7). However, only the P-bearing molecules PO and PN have been detected in the gas-phase of star forming regions, with an abundance relative to hydrogen of about 10-9 -10-10. This phosphorus depletion in the gas phase suggests that the majority of P may be carried by semi-refractory species and/or locked in the condensed phase. Recently, phosphorus, carried by PO fragments, was detected on comet 67P/Churyumov–Gerasimenko during the Rosetta mission, hinting to the existence of semi-refractory P species containing the POx moiety. We propose PO2, PO3, HPO, H2PO, HPO2, and HPO3 as potential carriers of condensed phase P in the interstellar medium (ISM). First, we used DFT calculation at the M06-2X/aug-cc-pVDZ level of theory to calculate the desorption energy (DE) of such species, and determined that these molecules are more refractory than water (DE&gt;8000K). This implies that such POx and HPOx species, if formed on grains, are expected to stay in the solid phase in most astrochemical environments. Next, we tested the formation and the behavior of these species during the cloud collapse and stellar warm-up phase using the astrochemical code UCLCHEM. We used a phosphorus chemical reaction network available in the literature to which we added a sequence of hydrogen and oxygen atomic addition reactions. These reactions allow for the formation on grains of PO2, PO3, HPO, H2PO, HPO2, and HPO3 starting from PO. We include both the Langmuir-Hinshelwood (LH) and the Eley-Rideal (ER) reaction mechanisms for each of the hydrogenation and oxygenation reactions. Starting from a diffuse cloud scenario we modeled the evolution of the abundance of P-bearing species during the cold collapse and during the following warm-up phase. Preliminary results show that by the end of the collapse phase there is a significant build-up of the (H)POx (x=2,3) species on grains. The formation of such species appears to be more efficient via ER mechanisms. The evaluation of the molecular abundances of P-carries in relation to the temperature and to the initial phosphorus abundance is under study. We expect for these (H)POx molecules to not be easily desorbed in the gas-phase during the warm-up and therefore constitute a potential reservoir of condensed-phase phosphorus in the ISM.

Computational study of the formation of (H)POx, x=2,3 molecules on interstellar grains during cloud evolution

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