The \emph{R}-process Alliance: A Bright, Strongly \emph{R}-process-enhanced Extremely Metal-poor Star Observed with GHOST

TL;DR

This paper analyzes four extremely metal-poor stars, focusing on a strongly r-process-enhanced star, using high-resolution spectra to understand their chemical compositions and origins, including potential links to neutron star mergers and Galactic accretion events.

Contribution

It provides detailed chemical and kinematic analyses of EMP stars, identifying a strongly r-process-enhanced star and comparing its heavy-element pattern with models and other stellar patterns.

Findings

G256353 shows strong r-process enhancement with [Eu/Fe] = +1.20.

The best-fit supernova model suggests progenitors of 20-30 solar masses.

G256353's heavy-element pattern aligns with neutron star merger models and the Solar r-process pattern.

Abstract

We present a detailed chemical-abundance and kinematic analysis of four extremely metal-poor (EMP; [Fe/H] $\leq -3.0$) stars identified from \textit{Gaia} BP/RP data in our ongoing search for the most primitive stars. This includes a primary target, \textit{Gaia}~DR3~2563539603865382656 (hereafter G256353), a strongly $r$-process-enhanced star with [Eu/Fe]~$= +1.20$ and [Ba/Eu]~$= -0.64$. Our results are based on high-resolution, high-signal-to-noise GHOST spectra from Gemini-South. For the full sample, we statistically match the light-element abundances with those predicted from Population\,III supernova models. The ``best-fit'' model suggests massive progenitors with stellar masses of M$_{\star}\sim$ 20-30\,M$_\odot$. In addition, we determine orbital histories for all of the stars. We find that Gaia~DR3~2887334237669844480 appears to be kinematically associated with Atari, an accreted structure in the Galactic disk. This star has low abundance ratios of strontium ([Sr/Fe] = $-$1.09) and barium ([Ba/Fe] = $-$0.37), which supports an accretion origin. For G256353, we determine chemical abundances for 15 neutron-capture elements. We compare the observed heavy-element pattern for G256353 with that of the Sun, HD~222925, and two neutron star merger models. The $r$-process elements in G256353 align reasonably well with HD~222925, the scaled-Solar pattern (except for the first peak), and a recent predicted pattern associated with neutron star mergers. This consistency reinforces the universality of the main $r$-process across diverse astrophysical environments.