arXiv subjects

Chase L. Smith

Publications and source records attributed to Chase L. Smith.

3 recordsLinked to original sources

arXiv2026

Using GALEX UV Excess to Search for Metal-poor Halo Stars

Metal-poor solar-type stars display a significant reduction in metal-line blanketing at short wavelengths, leading to an excess of near-ultraviolet (NUV) flux compared to their metal-rich counterparts. We utilize GALEX NUV and $\it{Gaia}$ DR3 photometry along with ground-based spectroscopy to establish a correlation between NUV excess and [Fe/H]. We construct a sample of 492 solar-type (F5-G9) halo stars with NUV excess and measured metallicitices. We perform our own observations with the KOSMOS spectrograph at Apache Point Observatory's 3.5m telescope to measure the abundances of 13 halo stars, 11 of which did not have previous metallicity measurements. Our targeted 13 halo stars span $-$2.92 $<$ [Fe/H] $<$ $-$1.97 and are all $\alpha$ enhanced with [$\alpha$/Fe] = 0.05-0.73. For our full sample of 492 objects, we find an anti-correlation between NUV excess and [Fe/H] that is statistically significant at the 8$\sigma$ level. GALEX NUV excess can be used to distinguish very metal-poor (VMP) stars ([Fe/H] $<$ $-$2) from their metal-rich counterparts. However, there is significant dispersion in the relation due to NUV chromospheric variability caused by rotational effects and magnetic cycle activity. The NUV chromospheric variability inhibits our ability to reliably distinguish extremely metal-poor (EMP) stars ([Fe/H] $<$ $-$3) from VMP stars based on photometry alone. UV spectra of EMP halo stars are needed to better calibrate their atmospheric properties and variability.

astro-ph.SR↗

arXiv2025

PAH Marks the Spot: Digging for Buried Clusters in Nearby Star-forming Galaxies

The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST datasets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschen$α$ and H$α$ recombination lines, and the 3.3 $μ$m polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction $\left< A_V \right> = 6.0$ mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of $10^3$ $M_{\odot}$. We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.

astro-ph.GA↗

arXiv2024

Spin-Orbit Alignment of Early-type Astrometric Binaries and the Origin of Slow Rotators

The spin-orbit alignment of binary stars traces their formation and accretion history. Previous studies of spin-orbit alignment have been limited to small samples, slowly rotating solar-type stars, and/or wide visual binaries that not surprisingly manifest random spin-orbit orientations. We analyze 917 Gaia astrometric binaries across periods $P$ = 100-3,000 days ($a$ = 0.5-5 au) that have B8-F1 IV/V primaries ($M_1$ = 1.5-3 M$_{\odot}$) and measured projected rotational velocities $v$sin$i$. The primary stars in face-on orbits exhibit substantially smaller $v$sin$i$ compared to those in edge-on orbits at the 6$σ$ level, demonstrating significant spin-orbit alignment. The primaries in our astrometric binaries are rotating more slowly than their single-star or wide-binary counterparts and therefore comprise the slow-rotator population in the observed bimodal rotational velocity distribution of early-type stars. We discuss formation models of close binaries where some of the disk angular momentum is transferred to the orbit and/or secondary spin, quenching angular momentum flow to the primary spin. The primaries in astrometric binaries with small mass ratios $q$ = $M_2$/$M_1<$0.3 possess even smaller $v$sin$i$, consistent with model predictions. Meanwhile, astrometric binaries with large eccentricities $e>$0.4 do not display spin-orbit alignment or spin reduction. Using a Monte Carlo technique, we measure a spin-orbit alignment fraction of $F_{\rm align}$ = 75% $\pm$ 5% and an average spin reduction factor of $\langle S_{\rm align} \rangle$ = 0.43 $\pm$ 0.04. We conclude that 75% of close A-type binaries likely experienced circumbinary disk accretion and probably formed via disk fragmentation and inward disk migration. The remaining 25%, mostly those with $e>$0.4, likely formed via core fragmentation and orbital decay via dynamical friction.

astro-ph.SR↗