arXiv subjects

Alexander Yelland

Publications and source records attributed to Alexander Yelland.

3 recordsLinked to original sources

arXiv2026

Sr and Ba yields of the First Generation(s) of stars: Constraints from metal-poor stars

We present our chemical abundance analysis of ten new extremely metal-poor stars with $-4.05\leq\mbox{[Fe/H]}\leq-2.33$, based on high-resolution (R $\sim28,000$) Magellan/MIKE spectra. Eight of our stars have low heavy-element abundances of $\mbox{[Sr/H]}<-4.5$ and $\mbox{[Ba/H]}<-4.0$, making them Small Accreted Stellar System (SASS) stars. Four are hyper neutron-capture-element poor with $\mbox{[Sr/H]}<-5.0$, including Gaia DR3 5729400267359655680, which sets a new record for the lowest detected Sr abundance of $\mbox{[Sr/H]} =-6.4$. We identify four distinct [Sr/Ba] groups within the wider SASS star population which span a large range from $\mbox{[Sr/Ba]} =-2.0$ to +1.6, pointing to multiple types of progenitor events and different nucleosynthesis processes/sites. To explore the origins of this large [Sr/Ba] range, we adopt site-agnostic Sr yields of $\mbox{[Sr/H]}=-6$, $-5.75$, $-5.42$, and $-4.93$ for the four groups. Applying those yields suggests that the majority of SASS stars formed from gas enriched by $\sim$1-10 progenitor events, consistent with expectations from their extremely metal-poor nature. We thus attribute the [Sr/H] abundance scatter to intrinsic variations in the Sr yield per nucleosynthesis site/event. Our proposed Sr yields for each [Sr/Ba] group and associated nucleosynthesis origin are a reasonable and representative approximation, good to within a factor of a few, and can constrain future theoretical heavy element nucleosynthesis calculations in early core-collapse supernovae.

astro-ph.SR↗

arXiv2026

Carbon Abundances in Metal-Poor Stars Reveal Distinct Galaxy and Star Formation Pathways in the Early Universe

Carbon-enhanced metal-poor (CEMP; with $\rm{[Fe/H]} \le -2.0$ and $\rm{[C/Fe]} \ge 0.7$) stars preserve information about early chemical enrichment, low-mass star formation, and the hierarchical assembly of galaxies. In this study, we have compiled an extensive literature sample of 1032 stellar carbon abundances spanning the metal-poor Milky Way halo (437 stars), 21 ultra-faint dwarf galaxies (UFDs; 102 stars), seven classical dwarf spheroidal galaxies (254 stars), three accreted dwarf galaxies (90 stars), the Small Accreted Stellar Systems (SASS; 77 stars), and eleven stellar streams (72 stars). We establish the fractions of CEMP stars for each of these systems and categories. Generally, the low-mass UFDs possess the high fractions at low metallicities, whereas the more massive classical dwarf galaxies have relatively few CEMP stars. This behavior reveals a new low-metallicity Magnitude ($M_{\rm V}$)--CEMP Fraction relation across the dwarf satellite galaxy population. The high CEMP fractions in surviving UFDs suggest their enrichment was dominated by faint supernovae, as higher energy input would likely have quenched star production. The low CEMP fractions in classical dwarfs imply predominantly in situ formation rather than assembly from smaller systems. Using $\rm{[C/H]}$ abundances, we also probe early low-mass star formation. Eight stars lie within or near the theoretical ''forbidden zone'', indicating that dust-induced cooling, alongside fine-structure line cooling, contributed to early star formation. These rare dust-cooled stars may have formed in UFD-like systems that did not survive. Overall, the metal-poor Milky Way halo appears to have assembled from many different dwarf galaxies, with CEMP halo stars being contributed by early UFD-like systems and non-CEMP halo stars by intermediate-sized halos that later formed classical dwarfs.

astro-ph.GA↗

arXiv2025

Early r-process Enrichment and Hierarchical Assembly Across the Sagittarius Dwarf Galaxy

Dwarf galaxies like Sagittarius (Sgr) provide a unique window into the early stages of galactic chemical evolution, particularly through their metal-poor stars. By studying the chemical abundances of stars in the Sgr core and tidal streams, we can gain insights into the assembly history of this galaxy and its early heavy element nucleosynthesis processes. We efficiently selected extremely metal-poor candidates in the core and streams for high-resolution spectroscopic analysis using metallicity-sensitive photometry from SkyMapper DR2, and Gaia DR3 XP spectra and proper motions. This allowed us to obtain a high-purity selection of Sgr members based on stellar kinematics while reducing the chances of potential contamination from the Milky Way halo. We present a sample of 37 Sgr stars with detailed chemical abundances, of which we identify 10 extremely metal-poor (EMP; $\rm{[Fe/H]} \le -3.0$) stars, 25 very metal-poor (VMP; $\rm{[Fe/H]} \le -2.0$) stars, and 2 metal-poor (MP; $\rm{[Fe/H]} \le -1.0$) stars. This sample increases the number of extremely metal-poor Sgr stars analyzed with high-resolution spectroscopy by a factor of five. Of these stars, 15 are identified as members of the Sgr tidal stream, while the remaining 22 are associated with the core. We derive abundances for up to 20 elements and identify no statistically significant differences between the element abundance patterns across the core and stream samples. Intriguingly, we identify stars that may have formed in ultra-faint dwarf galaxies that accreted onto Sgr, in addition to patterns of C and r-process elements distinct from the Milky Way halo. Over half of the sample shows a neutron-capture element abundance pattern consistent with the scaled solar pure r-process pattern, indicating early r-process enrichment in the Sgr progenitor.

astro-ph.GA↗