arXiv subjects

Benjamin D. C. Lowe

Publications and source records attributed to Benjamin D. C. Lowe.

2 recordsLinked to original sources

arXiv2026

Detailed Abundance Determination of Metal-Poor Stars with X-Shooter II. - Chemically Disentangling the Halo, Disk and GSE

We present a detailed chemical analysis of seven extremely metal-poor (EMP) star candidates observed with X-Shooter, combining them with 16 EMP candidates from Paper I. We measured abundances for 16 elements, showing excellent agreement with previously published results. The sample was further extended using high-resolution literature data for 315 metal-poor stars. The full sample was then kinematically separated into prograde disk, retrograde disk, Gaia-Sausage Enceladus (GSE) and halo classifications. Combining dynamics with chemistry, we demonstrate that the prograde disk exhibits a distinct negative linear trend in [Sc/Mg] with increasing metallicity, with a slope of -0.6 dex per dex. This contrasts with the halo trend at -0.04 dex per dex, a difference significant at the 3.95$σ$ level. Within the metallicity range -4.2 $\leq$ [Fe/H] $\leq$ -1.9, the prograde disk trend is driven by low [Mg/Fe] at lower metallicities, along with low [Sc/Fe] at higher metallicities. This could be due to reduced early Mg enrichment in the progenitor prograde disk, followed by subsequent Mg enrichment, possibly associated with a later gas accretion event. However, the physical origin of the higher-metallicity Sc depletion remains unexplained by current nucleosynthesis models. The result remains significant at the >3$σ$ level across kinematic classifications derived from a different Galactic potential. Additionally, we also identified an r-I star with enhanced Ti, moderately-enhanced Sc, and depleted in C (unrelated to its evolutionary state). These abundances suggest a massive jet-induced hypernova progenitor, though a measurement of Zn is needed to verify this.

astro-ph.GA↗

arXiv2026

Detailed Abundance Determination of Metal-Poor Stars with X-Shooter I. Unusual Chemistry in Halo Stars

We present a detailed chemical analysis study of 16 candidate metal-poor stars, previously identified with 2dF + AAOmega, using X-Shooter spectra and the Korg 1D local thermodynamic equilibrium spectral synthesis code. We confirm the earlier metallicity estimates and reveal six extremely metal-poor ([Fe/H] $< -3$) stars in the current sample. Two of these stars, including the most metal poor at [Fe/H] = $-3.89 \pm 0.07$, are kinematically associated with the Gaia-Sausage-Enceladus (GSE) accretion event, increasing the number of known GSE stars with [Fe/H] $< -3.5$ to eight. From the X-Shooter spectra we determine abundances for 16 elements, with the element-to-iron abundance ratios generally consistent with high-resolution studies of Galactic halo stars. Within the sample, we identify three peculiar stars: the first is a GSE nitrogen enhanced metal-poor ([N/Fe] $= 1.60 \pm 0.10$ and [C/Fe] $= 0.23 \pm 0.08$) star with unusually high Na ([Na/Fe] $= 2.26 \pm 0.07$) and Li (A(Li)$_{\rm 3DNLTE} = 1.90 \pm 0.08$) abundances, but lacking enhancements in [Al/Fe] or [Mg/Fe]. The second is a halo r-II star significantly enhanced in Sr ([Sr/Ba] $= 0.39 \pm 0.08$), suggesting mixture of r-process and s-process enrichment, uncommon for r-II stars. Whilst the third is a halo star very depleted in N ([N/Fe] $< -1.11$), with low C ([C/Fe] $= -0.33 \pm 0.08$) and otherwise 'normal' [X/Fe] abundances, suggesting enrichment with Type II supernova that proceeds enrichment from massive asymptotic giant branch stars. This study reveals the substantial degree of chemical diversity in the stellar populations which assembled the early Milky Way.

astro-ph.GA↗