Phosphorus-rich stars

In the long term, identifying the progenitor of the P-rich stars may contribute to the search for the source of P in our Galaxy. We performed a detailed chemical abundance analysis based on the high resolution near-infrared (H-band) spectra from the latest data release (DR17) of the APOGEE-2 survey. Employing the BACCHUS code, we measured the abundances of 13 elements in the inspected sample, which is mainly composed of a recent collection of Si-enhanced giants.

We also analyzed the orbital motions and compared the abundance results to possible nucleosynthetic formation scenarios, as well as to detailed GCE models. These models were produced with the OMEGA+ chemical evolution code, using four different massive star yield sets to investigate different scenarios for massive star evolution. We enlarged the sample of confirmed P-rich stars from 16 to a group of 78 giants, representing the largest sample of P-rich stars to date, including the first detection of a P-rich star in a Galactic globular cluster.

Significant enhancements in O, Al, Si and Ce, as well as systematic correlations among the studied elements, unveil the unique chemical fingerprint of the P-rich stars. In contrast, the high [Mg/Fe] and [(C+N)/Fe] found in some of the P-rich stars with respect to P-normal stars is, due to the present uncertainties, not confirmed over the full sample. Strikingly, the strong over-abundance in the alpha-element Si is accompanied by normal Ca and S abundances, at odds with present stellar nucleosynthesis models of massive stars.

Our analysis of the orbital motion showed that the P-rich stars do not belong to a locally specific population in the Galaxy. In addition, we confirm that the majority of the sample stars are not part of binary systems.