Aaron Dotter

Context. Convection and rotation are both key processes in stellar evolution modelling. While standard mixing-length theory (MLT) provides a widely used modelling of convection, it neglects the effects of rotation on convective transport. Aims. We investigate how rotating mixing-length theory (R-MLT), which accounts for the influence of rotation on convection, affects the internal structure, convective mixing, and angular momentum transport in stellar models in comparison to the standard non-rotating MLT. Methods. Using the MESA stellar structure and evolution software, we model the main-sequence evolution of a 5 M _⊙ star, for three cases: non-rotating, rotating with standard MLT for modelling convection, and rotating with R-MLT in convection zones, with the initial rotation rate set to 20 percent of the critical (Keplerian) value at the surface for the rotating models. Results. We find that R-MLT reduces both the convective velocity and mixing length in the stellar core, leading to a smaller convective diffusion coefficient and a ∼20 percent reduction in the extent of the convective overshooting region. While the overall size of the convective core remains nearly unchanged, R-MLT changes the resulting chemical gradient at the core–envelope boundary, shifting the peak of the Brunt–Väisälä frequency and modifying the angular momentum transport in that region. Conclusions. Including the effects of rotation in the treatment of convection through R-MLT introduces measurable structural and transport differences, underscoring the importance of incorporating rotation–convection coupling in models of stars.

We update and expand the MESA Isochrones and Stellar Tracks (MIST) database to include variations in the α -capture elements, specifically [ α /Fe] = −0.2, 0, +0.2, +0.4, and +0.6 for the full range of −3 ≤ [Fe/H] ≤ +0.5 already covered by MIST. Variations in [ α /Fe] are included in a self-consistent manner from the stellar interior models to the synthetic spectra used to translate these models in the observational plane. We describe a number of updates to the physics utilized in these models as well as new information provided by the models. We validate the models with comparisons to other stellar evolution models including the previous generation of MIST and other models from the literature. MIST data products including stellar evolution tracks, isochrones, and bolometric correction tables can be obtained from the MIST project website, https://mist.science . All necessary files to reproduce MIST models are available from Zenodo.

We present a substantial update to the MESA Isochrones and Stellar Tracks (MIST) library, extending the MIST model grids and isochrones down the white dwarf (WD) cooling sequence with realistic physics for WD cooling timescales. This work provides a large grid of MESA models for carbon–oxygen core WDs with hydrogen atmospheres (spectral type DA/DC), descended from full prior stellar evolution calculations. The model tracks, isochrones, and WD cooling timescale contours are available on the MIST project website and at doi: 10.5281/zenodo.15242047 . Our WD models provide a very large, publicly available grid with detailed physics for WD cooling timescales: realistic interior and envelope compositions, with element diffusion and heavy-element sedimentation, nuclear burning at the base of the WD hydrogen envelope, core crystallization, and C/O phase separation. As a large grid of open-source stellar evolution models, these WD models provide both out-of-the-box model tracks for comparison with observations and a framework for building further WD models to investigate variations in WD physics.

We present new stellar population models, α -MC, self-consistently taking into account nonsolar [ α /Fe] abundances for both isochrones and stellar spectra. The α -MC models are based on α -enhanced MIST isochrones and C3K spectral libraries, which are publicly available in FSPS. Our new models cover a wide range of ages ( log ( age / yr ) = 5.0 - 10.3 ), metallicities ([Fe/H] = [−2.5, +0.5] in steps of 0.25, [ α /Fe] = −0.2, +0.0, +0.2, +0.4, +0.6), and wavelengths (0.1–2.5 μ m). We investigate the separate and combined effects of α -enhanced isochrones and stellar spectral libraries on simple stellar populations, including their broadband colors, spectral indices, and full spectra. We find that the primary effect of α -enhancement in isochrones is to lower the overall continuum levels and redden the continuum shapes, while α -enhancement in stellar spectra mainly affects individual spectral lines. At constant [Fe/H], α -enhancement has significant impacts on the broadband colors by ∼0.1–0.4 mag across all ages (0.01–10 Gyr). The effects of α -enhancement on colors at fixed [Z/H] are smaller, by ∼0.1–0.2 mag. The spectral indices involving α -elements, Ca4227 and Mg b , increase with [ α /Fe] (both at fixed [Fe/H] and fixed [Z/H]) due to enhanced α -abundances. At constant [Fe/H], α -enhancement weakens most Fe-sensitive and hydrogen Balmer lines. Our new self-consistent α -enhanced models will be essential in deriving accurate physical properties of high-redshift galaxies, where α -enhancement is expected to be common.

Whether considering rare astrophysical events on cosmological scales or unresolved stellar populations, accurate models must account for the integrated contribution from the entire history of star formation upon which that population is built. Here, we describe the second version of POSYDON, an open-source binary population synthesis code based on extensive grids of detailed binary evolution models computed using the MESA code, which follows both stars’ structures as a binary system evolves through its complete evolution from the zero-age main sequence, through multiple phases of mass transfer and supernovae, to their death as compact objects. To generate synthetic binary populations, POSYDON uses advanced methods to interpolate between our large, densely spaced grids of simulated binaries. In our updated version of POSYDON, we account for the evolution of stellar binaries across a cosmological range of metallicities, extending from 10⁻⁴ to 2 Z _⊙, including grids specifically focused on the Small and Large Magellanic Clouds (0.2 and 0.45 Z _⊙). In addition to describing our model grids and detailing our methodology, we outline several improvements to POSYDON. These include the incorporation of single stars in stellar populations, a treatment for stellar mergers, and a careful modeling of “reverse-mass transferring” binaries in which a once-accreting star later becomes a donor star. Our simulations are focused on binaries with at least one high-mass component, such as those that host neutron stars and black holes, and we provide postprocessing methods to account for the cosmological evolution of metallicity and star formation as well as rate calculations for transient events.

From >1000 orbits of HST imaging, we present deep homogeneous resolved star color–magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies (−6 ≥ M _V ≥ −17) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs, we find: (i) The median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within 1.8 Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. (ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. (iii) ~50% of our M31 satellites show prominent ancient star formation (>12 Gyr ago) followed by delayed quenching (8–10 Gyr ago), which is not commonly observed among the MW satellites. (iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts shows that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.

We present age estimates for over 8100 subgiant branch (SGB) stars in Omega Centauri (ω Cen) to study its star formation history. Our large data set, which combines multi-wavelength Hubble Space Telescope photometry with MUSE metallicities, provides an unprecedented opportunity to measure individual stellar ages. We do this by fitting each star’s photometry and metallicity with theoretical isochrones that are embedded with an empirical [C + N + O]–[Fe/H] relation specific to ω Cen. The bulk of the stars have ages between 13 and 10 Gyr, with the mean stellar age being 12.08 ± 0.01 Gyr and the median age uncertainty being 0.68 Gyr. From these ages we construct the most complete age–metallicity relation for ω Cen to date. We find that the mean age of stars decreases with increasing metallicity and find two distinct streams in the age–metallicity plane, hinting at different star formation pathways. We derive an intrinsic spread in the ages of 0.75 ± 0.01 Gyr for the whole cluster, with the age spread showing a clear increase with metallicity. We verify the robustness of our age estimations by varying isochrone parameters and constraining our systematics. We find the C + N + O relation to be the most critical consideration for constraining the age–metallicity relation. We also present an SGB chromosome map with age information. In the future these stellar ages could be combined with chemical abundances to study age differences in subpopulations and uncover the chemical evolution history of this massive nuclear star cluster.

We present an analysis of high-resolution optical spectra recorded for 30 stars of the split extended main-sequence turnoff of the young (∼40 Myr) Small Magellanic Cloud globular cluster NGC 330. Spectra were obtained with the Michigan/Magellan Fiber System and Magellan Inamori Kyocera Echelle spectrographs located on the Magellan-Clay 6.5 m telescope. These spectra revealed the presence of Be stars, occupying primarily the cool side of the split main sequence. Rotational velocity ( v sin i ) measurements for most of the targets are consistent with the presence of two populations of stars in the cluster: one made up of rapidly rotating Be stars ( 〈 v sin i 〉 ≈ 200 km s⁻¹) and the other consisting of warmer stars with slower rotation ( 〈 v sin i 〉 ≈ 50 km s⁻¹). Core emission in the Hδ photospheric lines was observed for most of the Hα emitters. The shell parameter computed for the targets in our sample indicates that most of the observed stars should have inclinations below 75°. These results confirm the detection of Be stars obtained through photometry but also reveal the presence of narrow Hα and Hδ features for some targets that cannot be detected with low-resolution spectroscopy or photometry. Asymmetry variability of Hα line profiles on the timescales of a few years is also observed and could provide information on the geometry of the decretion disks. Observations revealed the presence of nebular Hα emission, strong enough in faint targets to compromise the extraction of spectra and to impact narrow-band photometry used to assess the presence of Hα emission.

We present Augustus, a catalog of distance, extinction, and stellar parameter estimates for 170 million stars from 14 mag < r < 20 mag and with ∣b∣ > 10° drawing on a combination of optical to near-infrared photometry from Pan-STARRS, 2MASS, UKIDSS, and unWISE along with parallax measurements from Gaia DR2 and 3D dust extinction maps. After applying quality cuts, we find 125 million objects have “high-quality” posteriors with statistical distance uncertainties of ≲10% for objects with well-constrained stellar types. This is a substantial improvement over the distance estimates derived from Gaia parallaxes alone and in line with the recent results from Anders et al. We find the fits are able to reproduce the dereddened Gaia color–magnitude diagram accurately, which serves as a useful consistency check of our results. We show that we are able to detect large, kinematically coherent substructures in our data clearly relative to the input priors, including the Monoceros Ring and the Sagittarius Stream, attesting to the quality of the catalog. Our results are publicly available at doi:10.7910/DVN/WYMSXV. An accompanying interactive visualization can be found at http://allsky.s3-website.us-east-2.amazonaws.com.