arXiv subjects

Lauren E. Henderson

Publications and source records attributed to Lauren E. Henderson.

5 recordsLinked to original sources

arXiv2026

Constraints on the r-process Delay Time from Dwarf Spheroidal Galaxies

The ancient stellar populations and simple structure of the local dwarf spheroidal galaxies (dSphs) make them excellent laboratories for studying the chemical evolution of the early universe. In this paper, we fit one-zone galactic chemical evolution models to C, Mg, Si, Ca, Fe, Sr, Y, Ba, and Eu abundances in the dSphs Sculptor and Draco. By including the s- and r-processes in our model, we are able to place constraints on the timescale of r-process enrichment in these dSphs. We find that neutron star mergers cannot be the only source of the r-process in Sculptor and Draco, and that a prompt r-process source is required to explain the observed stellar abundances. When neutron star mergers are the only source of the r-process in our models, their minimum delay time must be just 14-16 Myr. The neutron-capture abundances in Sculptor and Draco can also be fit by models with a longer neutron star merger minimum delay time combined with a prompt r-process source that operates on the same timescale as massive star evolution, such as magneto-rotational supernovae. Additionally, the Sr and Y abundances in the dSphs set limits on the Sr and Y yields per r-process event. We find that theoretical r-process yields overestimate Sr and Y production by a factor of up to 100.

astro-ph.GA↗

arXiv2025

Population-Dependent r-process Scatter in the Globular Cluster M15

Multiple populations, defined by correlations between light element abundances, are an almost ubiquitous property of globular clusters. On the other hand, dispersions among the heavy elements are limited to a few rare clusters. In this letter, we present Mg, Y, Ba, La, and Eu measurements for 89 stars in M15 with errors < 0.4 dex from Keck/DEIMOS medium-resolution spectra. We find higher Ba, La, and Eu dispersions in the first generation of stars than in the second generation at a significance of $\ge$2 $σ$. This is evidence for inhomogeneous mixing of gas during the formation of the first generation of stars, which subsequently became well-mixed prior to the formation of the second generation of stars. If the r-process event that caused the abundance dispersions was born with the first population of stars, it must be an r-process site with a short delay time.

astro-ph.SR↗

arXiv2025

Estimating Fe and Mg Abundances in the Milky Way Dwarf Galaxies Using Subaru/HSC and DEIMOS

We investigate the chemical abundance distributions of the Fornax, Sculptor, Ursa Minor, and Draco dwarf galaxies using Subaru/HSC photometric data. The HSC dataset, which includes broadband g and i filters and the narrowband NB515 filter, offers sensitivity to iron and magnesium abundances as well as surface gravity, enabling the identification of giant stars and foreground dwarfs. For analysis, we selected a total of 6713 giant candidates using a Random Forest regressor trained on medium-resolution (R ~ 6000) Keck/DEIMOS spectroscopic data. Our analysis reveals the extent of radial metallicity gradients in the galaxies. Such trends, not detectable in earlier studies, are now captured owing to the substantially enlarged sample size and areal coverage provided by the HSC data. These results are also consistent with chemical abundance patterns previously observed in the central regions through spectroscopic studies. Furthermore, we infer that Fornax underwent extended star formation, whereas Sculptor formed both metal-poor and metal-rich stars over a shorter time. Ursa Minor and Draco appear to have experienced brief, intense star formation episodes leading to nearly extinguished star formation. This study underscores the critical role of the expanded HSC dataset in revealing chemical gradients that were previously inaccessible. Future work incorporating additional spectra of metal-poor stars and age-sensitive isochrone modeling will enable more accurate maps of chemical abundance distributions.

astro-ph.GA↗

arXiv2025

Neutron-capture Element Abundances of 491 Stars in Milky Way Dwarf Satellite Galaxies from Medium-Resolution Spectra

The chemical compositions of evolved stars in Local Group dwarf spheroidal galaxies (dSphs) provide insight into the galaxy's past star formation and nucleosynthesis. Neutron-capture element abundances are especially interesting. In particular, s-process elements can provide a third chemical clock for resolving star formation histories in addition to core collapse and Type Ia supernovae. Likewise, the primary sites of the r-process are still areas of extensive research. Until now, the number of stars with neutron-capture element abundances in dSphs has been limited by the need for stars bright enough for high-resolution spectroscopy. We present abundance measurements of the neutron-capture elements Sr, Y, Ba, and Eu with errors < 0.4 dex - as well as new measurements of Mg - in 491 stars in Sculptor, Fornax, Draco, Sextans, and Ursa Minor. The large number of stars in our sample is possible because we used medium-resolution spectra from the DEIMOS spectrograph, assembling the largest homogeneous set of neutron-capture abundances in dwarf spheroidal galaxies to date. By utilizing the abundances of both s- and r-process elements, we find evidence of an s-process contribution at early times in Sculptor from our measurements of [Ba/Fe]. This is a potential signature of s-process nucleosynthesis in fast-rotating massive stars. By comparing our measurements of [Eu/Fe] with [Mg/Fe], we show the need for an r-process source that has a short delay time to enrich stars in the dSphs. Thus, neutron star mergers are likely not the sole source of r-process material in dSphs.

astro-ph.GA↗

arXiv2022

A Bayesian Analysis of Physical Parameters for 783 Kepler (Near-)Contact Binaries: Extreme-Mass-Ratio Systems and a New Mass Ratio versus Period Lower Limit

Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform understanding of binary evolutionary pathways and end products. We use light curves and new optical spectroscopy to conduct a pilot study of ten (near-)contact systems in the long-period ($P$>0.5 d) tail of close binaries in the Kepler field. We use PHOEBE light curve models to compute Bayesian probabilities on five principal system parameters. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the light curves. Pilot study systems have extreme mass ratios $q$<0.32. Most are triples. Analysis of the unbiased sample of 783 0.15 d<$P$<2 d (near-)contact binaries results in 178 probable contact systems, 114 probable detached systems, and 491 ambiguous systems for which we report best-fitting and 16th/50th/84th percentile parameters. Contact systems are rare at periods $P$>0.5 d, as are systems with $q$>0.8. There exists an empirical mass ratio lower limit $q_{min}$($P$)$\approx$0.05--0.15 below which contact systems are absent, supporting a new set of theoretical predictions obtained by modeling the evolution of contact systems under the constraints of mass and angular momentum conservation. Pre-merger systems should lie at long periods and near this mass ratio lower limit, which rises from $q$=0.044 for $P$=0.74 d to $q$=0.15 at $P$=2.0 d. These findings support a scenario whereby nuclear evolution of the primary (more massive) star drives mass transfer to the primary, thus moving systems toward extreme $q$ and larger $P$ until the onset of the Darwin instability at $q_{min}$ precipitates a merger.

astro-ph.SR↗