Jamie Tayar

There are only three white dwarfs known to possess metal-enriched polar caps due to magnetically guided accretion. These rare systems offer a unique opportunity to study planetesimal accretion and how magnetic fields influence the spreading of material across the atmosphere. To better study these processes however, more targets are needed. Here we report our findings from an observing campaign on the magnetic white dwarf LHS 2534. Time-resolved spectra at the MMT show clear variability in the strength of the Na i and Cr i features. We find that the strengths of these features vary over a period of 19.46 and 19.36 hr respectively, closely matching the 20.22 hr period determined from K2 photometry. Our findings demonstrate the ability to detect and study these variable objects with spectroscopy, and time-series observations of similar targets in the future will provide more insight into how these atmospheres form.

In the Milky Way disk, there is a strong trend linking stellar age to surface element abundances. Here, we explore this relationship with a data set of 8803 red-giant and red-clump stars with both asteroseismic data from NASA Kepler Mission and surface abundances from the Sloan Digital Sky Survey (SDSS)-V Milky Way Mapper (MWM) survey. We find, with a k -nearest-neighbors approach, that the [Mg/H] and [Fe/Mg] abundance ratios predict asteroseismic ages to an accuracy of about 2 Gyr for the majority of stars. That said, there are substantial outlier stars whose surface abundances do not match their asteroseismic ages. Because asteroseismic ages for these stars are fundamentally based on density or mass, these outliers are mass-transfer candidates. Stars whose surface abundances predict an older age (lower mass) than what’s seen in the asteroseismology are mass accretor candidates (MACs); stars whose abundances predict a younger age (higher mass) than the asteroseismology age are mass donor candidates (MDCs). We create precise control samples, matched according to (1) surface abundances and (2) asteroseismic ages, for both the MAC and MDC stars; we use these to find slight differences in rotational velocity, [C/N], and [Na/Mg] between the mass-transfer candidates and their abundance neighbors. We find no drastic differences in kinematics, orbital invariants, UV excess, or other stellar abundances between outliers and their abundance neighbors. We deliver 377 mass-transfer candidates for follow-up observations. This project implicitly suggests a fundamental limit on the reliability of asteroseismic ages, and supports existing evidence that age-abundance outliers are products of binary mass transfer.

A growing understanding of stellar processes that alter surface chemical abundances over time has opened new avenues for using these changes as probes of stellar properties. On the main sequence and near the turnoff, stellar surface abundances are affected by gravitational settling and radiative acceleration, collectively known as atomic diffusion. In this work, we use Sloan Digital Sky Survey/APOGEE DR17/DR19 data to investigate atomic diffusion in the open clusters NGC 752 and Ruprecht 147, thereby constraining how these signatures vary with age. From the analysis of Fe, C, N, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Co, and Ni, we find significant abundance differences between stars near the turnoff and the cooler main sequence, where warmer stars are depleted relative to the cooler main-sequence stars at the ≥1 σ level for all elements available in the analysis. These abundance differences are consistent with the signatures expected from atomic diffusion and are further supported by comparisons with stellar models that include diffusion. By fitting the observed T _eff –[Fe/H] patterns with MIST isochrones, we obtain best-fit ages of 1.0 Gyr for NGC 752 and 2.5–3.2 Gyr for Ruprecht 147. Carbon is a key diagnostic, as it shows both atomic diffusion and extra-mixing signatures associated with first dredge-up. For NGC 752, the coolest stars in our sample, which provide the best proxy for the initial cluster composition, yield [Fe/H] _CS = 0.01 ± 0.02(±0.05) dex. For Ruprecht 147, we obtain [Fe/H] _CS = 0.17 ± 0.00(±0.05) dex. Our findings further constrain atomic diffusion models, suggesting that atomic diffusion affects age estimates of stars near the main-sequence turnoff.

Open clusters are fundamental laboratories for investigating stellar and Galactic evolution, and serve as important benchmarks for asteroseismic analyses. Using a “boutique” method to analyze TESS photometry, we study red giants in two old supersolar-metallicity open clusters: NGC 188 and NGC 6791. By comparing Kepler Space Telescope (Kepler) and TESS observations for NGC 6791, similar oscillation mode frequencies are recovered, however we find a systematic offset of 2.2% with a scatter of 9% in the ν _max measurements. We attribute this discrepancy to the lower signal-to-noise ratio of the TESS data for these relatively faint stars. For the brighter cluster NGC 188, we present new seismic measurements in 17 red giants. We estimate average seismic masses for the red giant branch (RGB) of M _{RGB,NGC 188} = 1.13 ± 0.04(rand.) − 0.19 + 0.12 (sys.) M _⊙ and red clump of M _{RC,NGC 188} = 1.11 ± 0.01(rand.) − 0.19 + 0.11 (sys.) M _⊙ , consistent with independent mass estimates for this cluster and with similar precision to previous Kepler studies. From the difference between the average evolutionary phase masses, we estimate an integrated RGB mass loss of Δ M = 0.02 ± 0.04(rand.) ± 0.01(sys.) M _⊙ , supporting the evidence for lower mass loss at higher metallicities. Using asteroseismology and chemical abundances, we identify three binary interaction candidates: two undermassive stars and one overmassive star potentially exhibiting dipole-mode suppression. Finally, we derive an average seismic cluster age of 7.0 ± 0.9 Gyr, in good agreement with previous literature ages. Our analysis demonstrates the strong potential of TESS asteroseismology for open clusters, and motivates extending this investigation to other TESS clusters that span a wider range of ages and metallicities.

In this paper we present the public release of the Yale Rotating Evolution Code (YREC). YREC is a stellar evolution code that covers brown dwarfs and stars across a wide range of masses, and evolutionary states from the pre-main sequence through helium burning. We summarize the key ingredients of the code, document the code performance, and discuss its strengths and limitations. We present libraries of input files, documentation, sample use cases, and scripts. In addition to usage as a research tool, we highlight the utility of the code for educational purposes.

We constrain the formation history of the Milky Way bulge using a two-infall galactic chemical evolution (GCE) algorithm implemented in the N’OMEGA+ code. We recover a best-fit scenario in which the bulge forms through an early, rapid starburst ( t ₁ ∼ 0.1 Gyr, τ ₁ ∼ 0.09 Gyr, and star formation efficiency (SFE) ∼ 3 Gyr ⁻¹ ), followed by a delayed, lower-mass second infall ( t ₂ ∼ 5.1 Gyr, τ ₂ ∼ 1.7 Gyr, and σ ₂ ∼ 0.69). Our model adopts mass- and metallicity-dependent nucleosynthetic yields from modern stellar grids and explores a wide GCE parameter space in infall timing, SFE, mass partitioning, initial mass function upper mass, and type Ia supernova normalization, optimized via a hybrid genetic algorithm with Markov Chain Monte Carlo refinement. The later infall features a reduced SFE (ΔSFE ∼ 0.72), reproducing the metal-rich peak of the bulge metallicity distribution function (MDF) and the decline in [ α /Fe] at high [Fe/H]. Our model naturally favors the M. Joyce et al. age–metallicity relation over the ages in T. Bensby et al. Degeneracy and principal component analyses show that the infall history, SFE, and mass partitioning are strongly covariant—the bulge’s observed MDF, abundance trends, and age distribution constrain only their combinations, not each parameter independently. The results support a composite bulge origin—an early, rapid collapse builds the majority of the mass, while a younger component is required to match the late-stage enrichment.

Mapping the local and distant Universe is key to our understanding of it. For decades, the Sloan Digital Sky Survey (SDSS) has made a concerted effort to map millions of celestial objects to constrain the physical processes that govern our Universe. The most recent and fifth generation of SDSS (SDSS-V) is organized into three scientific “mappers”: the Milky Way Mapper, which aims to chart the various components of the Milky Way and constrain its formation and assembly; the Black Hole Mapper, which focuses on understanding supermassive black holes in distant galaxies across the Universe; and the Local Volume Mapper, which uses integral field spectroscopy to map the ionized interstellar medium in the Local Group. This paper describes the scope and content for the nineteenth data release (DR19) of SDSS, which is the most substantial to date in SDSS-V. DR19 is the first to contain data from all three mappers. Additionally, we also describe nine value-added catalogs that enhance the science that can be conducted with the SDSS-V data. Finally, we discuss how to access SDSS DR19 and provide illustrative examples and tutorials.

The fifth-generation Sloan Digital Sky Survey (SDSS-V) is conducting the first all-sky low-resolution spectroscopic survey of the Milky Way’s (MW) stellar halo. We describe the stellar parameter pipeline for the SDSS-V halo survey, which simultaneously models spectra, broadband photometry, and parallaxes to derive stellar parameters, metallicities, alpha abundances, and distances. The resulting Baryon Oscillation Spectroscopic Survey (BOSS)- MINESweeper catalog is validated across a wide range of stellar parameters and metallicities using star clusters and a comparison to high-resolution spectroscopic surveys. We demonstrate several scientific capabilities of this dataset: identifying the most chemically peculiar stars in our Galaxy, discovering and mapping distant halo substructures, and measuring the all-sky dynamics of the MW on the largest scales. The BOSS- MINESweeper catalog for SDSS DR19 is publicly available and will be updated for future data releases.

NASA’s Transiting Exoplanet Survey Satellite mission has identified at least 158,000 oscillating red giants, increasing the known sample by roughly an order of magnitude. After validating that these measurements are reliable to 5% for up to 90% of red giants, we make custom stellar evolution models using Modules for Experiments in Stellar Astrophysics in order to estimate ages for ∼132,794 of these stars to an average uncertainty of 23%. We show that these ages follow similar distributions to those observed in other samples such as Kepler, with small differences likely resulting in the Galactic volume probed. We provide these ages to the community to enable future Galactic archeology analyses.