The Galactic Distribution of Phosphorus: A Survey of 163 Disk and Halo Stars

TL;DR

This study measures phosphorus abundances in 163 stars to understand its cosmic evolution, revealing that P is mainly produced by massive stars and showing differences between galactic components, challenging existing models.

Contribution

First large-scale survey of phosphorus in stars across the Milky Way, providing new insights into P production and galactic chemical evolution.

Findings

P is under-predicted by models across observed metallicities.

[P/Fe] differs by ~0.1 dex between thin and thick disk stars.

P most closely resembles alpha-element evolution in the disk.

Abstract

Phosphorus (P) is a critical element for life on Earth yet the cosmic production sites of P are relatively uncertain. To understand how P has evolved in the solar neighborhood, we measured abundances for 163 FGK stars over a range of -1.09 $<$ [Fe/H] $<$ 0.47 using observations from the Habitable-zone Planet Finder (HPF) instrument on the Hobby-Eberly Telescope (HET). Atmospheric parameters were calculated by fitting a combination of astrometry, photometry, and Fe I line equivalent widths. Phosphorus abundances were measured by matching synthetic spectra to a P I feature at 10529.52 angstroms. Our [P/Fe] ratios show that chemical evolution models generally under-predict P over the observed metallicity range. Additionally, we find that the [P/Fe] differs by $\sim$ 0.1 dex between thin disk and thick disk stars that were identified with kinematics. The P abundances were compared with $\alpha$-elements, iron-peak, odd-Z, and s-process elements and we found that P in the disk most strongly resembles the evolution of the $\alpha$-elements. We also find molar P/C and N/C ratios for our sample match the scatter seen from other abundance studies. Finally, we measure a [P/Fe] = 0.09 $\pm$ 0.1 ratio in one low-$\alpha$ halo star and probable Gaia-Sausage-Enceladus (GSE) member, an abundance ratio $\sim$ 0.3 - 0.5 dex lower than the other Milky Way disk and halo stars at similar metallicities. Overall, we find that P is likely most significantly produced by massive stars in core collapse supernovae (CCSNe) based on the largest P abundance survey to-date.