  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.
