ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets

Abstract: To understand how Phosphorus-bearing molecules are formed in star-forming regions, we have analysed ALMA observations of PN and PO towards the massive star-forming region AFGL 5142, combined with a new analysis of the data of the comet 67P/Churyumov-Gerasimenko taken with the ROSINA instrument onboard Rosetta. The ALMA maps show that the emission of PN and PO arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. PO is more abundant than PN in most of the spots, with the PO/PN ratio increasing as a function of the distance to the protostar. Our data favor a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by UV photons from the protostar allows efficient formation of the two species in the cavity walls. Our analysis of the ROSINA data has revealed that PO is the main carrier of P in the comet, with PO/PN>10. Since comets may have delivered a significant amount of prebiotic material to the early Earth, this finding suggests that PO could contribute significantly to the phosphorus reservoir during the dawn of our planet. There is evidence that PO was already in the cometary ices prior to the birth of the Sun, so the chemical budget of the comet might be inherited from the natal environment of the Solar System, which is thought to be a stellar cluster including also massive stars.

ALMA and ROSINA Detections of Phosphorus-bearing Molecules

The detection and analysis of phosphorus-bearing molecules in star-forming regions and comets present vital insights into the origins of life's essential elements. This paper delves into the interstellar chemistry of phosphorus, using ALMA observations and ROSINA data to elucidate the path from interstellar clouds to cometary bodies.

Summary of Findings

The study focuses on phosphorus-bearing molecules in the massive star-forming region AFGL 5142, integrating observations from ALMA and data from the ROSINA instrument aboard Rosetta. The presence of PN and PO in the cavity walls of a bipolar outflow marks a notable spatial distribution, with PO consistently showing a greater abundance than PN. ROSINA data from comet 67P/Churyumov-Gerasimenko reveal PO as the predominant phosphorus carrier, with a minimal presence of other phosphorus-bearing species like PN, PH₃, and CP.

Implications and Claims

1. Phosphorus Chemistry: The paper posits that phosphorus chemistry in star-forming regions involves multiple phases: hydrogenation of atomic phosphorus on dust grains, sputtering due to shocks, and subsequent conversion to PO and PN via gas-phase photochemistry driven by UV photons. This photochemical process is highlighted by the increasing PO/PN ratio observed with distance from the protostar.

2. Interstellar to Cometary Transfer: The cometary analysis suggests that phosphorus-bearing molecules like PO may have originated from the interstellar medium, supporting a pathway from star-forming regions to cometary bodies. The similar PO/PN ratios in both AFGL 5142 and 67P propose a direct chemical lineage.

3. Contribution to Prebiotic Chemistry: The paper discusses the potential of comets to contribute phosphorus to early Earth, highlighting PO as a key source due to its higher abundance. This supports PO's role in prebiotic chemistry, forming phosphates essential for biomolecules.

Future Prospects

The research underscores the need for more detailed theoretical models to confirm the proposed photochemical pathways and the mechanisms of phosphorus transfer from star-forming regions to planetary systems. Spectroscopic studies with higher resolution will further refine our understanding of the elemental makeup of cometary ices. Moreover, this study suggests further exploration of cometary impacts as a source of phosphorus on primordial Earth, contributing to the understanding of life's chemical precursors.

Conclusion

This comprehensive study sheds light on the interstellar pathways of phosphorus from star-forming regions to comets, with implications for prebiotic chemistry and the development of life. It opens avenues for future research into molecular cloud chemistry and the astrophysical processes facilitating phosphorus deposition on planetary surfaces.