2024A&A...682A..74F

Context. The astrochemistry of the important biogenic element phosphorus (P) is still poorly understood, but observational evidence indicates that P-bearing molecules are likely associated with shocks.
Aims. We study P-bearing molecules and some shock tracers towards one of the chemically richest hot molecular cores, G31.41+0.31, in the framework of the project “G31.41+0.31 Unbiased ALMA sPectral Observational Survey” (GUAPOS), which is being carried out with the Atacama Large Millimeter Array (ALMA).
Methods. We observed the molecules PN, PO, SO, SO₂, SiO, and SiS through their rotational lines in the spectral range 84.05- 115.91 GHz covered by the GUAPOS project.
Results. PN is clearly detected, while PO is tentatively detected. The PN emission arises from two regions southwest of the hot core peak, named regions 1 and 2 here, and is undetected or tentatively detected towards the hot core peak. The PN and SiO lines are very similar both in spatial emission morphology and spectral shape. Region 1 is partly overlapping with the hot core and is warmer than region 2, which is well separated from the hot core and located along the outflows identified in previous studies. The SO, SO₂, and SiS emissions are also detected towards the PN-emitting regions 1 and 2, but arise mostly from the hot core. Moreover, the column density ratio SiO/PN remains constant in regions 1 and 2, while SO/PN, SiS/PN, and SO2/PN decrease by about an order of magnitude from region 1 to region 2, indicating that SiO and PN have a common origin even in regions with different physical conditions. The PO/PN ratio in region 2, where PO is tentatively detected, is ∼0.6-0.9, which is in line with the predictions of pure shock models.
Conclusions. Our study provides robust confirmation of previous observational evidence that PN emission is tightly associated with SiO and is likely a product of shock chemistry, as the lack of a clear detection of PN towards the hot core allows us to rule out relevant formation pathways in hot gas. We propose the PN-emitting region 2 as a new astrophysical laboratory for shock-chemistry studies.