We substantially update the capabilities of the open source software package Modules for Experiments in Stellar Astrophysics (MESA), and its one-dimensional stellar evolution module, MESA star. Improvements in MESA star's ability to model the evolution of giant planets now extends its applicability down to masses as low as one-tenth that of Jupiter. The dramatic improvement in asteroseismology enabled by the space-based Kepler and CoRoT missions motivates our full coupling of the ADIPLS adiabatic pulsation code with MESA star. This also motivates a numerical recasting of the Ledoux criterion that is more easily implemented when many nuclei are present at non-negligible abundances. This impacts the way in which MESA star calculates semi-convective and thermohaline mixing. We exhibit the evolution of 3-8 M ? stars through the end of core He burning, the onset of He thermal pulses, and arrival on the white dwarf cooling sequence. We implement diffusion of angular momentum and chemical abundances that enable calculations of rotating-star models, which we compare thoroughly with earlier work. We introduce a new treatment of radiation-dominated envelopes that allows the uninterrupted evolution of massive stars to core collapse. This enables the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. We substantially modify the way in which MESA star solves the fully coupled stellar structure and composition equations, and we show how this has improved the scaling of MESA's calculational speed on multi-core processors. Updates to the modules for equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions are also provided. We describe the MESA Software Development Kit that packages all the required components needed to form a unified, maintained, and well-validated build environment for MESA. We also highlight a few tools developed by the community for rapid visualization of MESA star results.

/pdf/modules-for-experiments-in-stellar-astrophysics-mesa-planets-29j324a9bq.pdf

Modules for Experiments in Stellar Astrophysics (MESA): Planets, Oscillations, Rotation, and Massive Stars

Evolutionary population synthesis models for a wide range of metallicities, ages, star formation histories, initial mass functions and horizontal branch morphologies, including blue morphologies at high metallicity, are computed. The model output comprises spectral energy distributions, colours, stellar M/L ratios, bolometric corrections and near-infrared (IR) spectral line indices. The energetics of the post main sequence evolutionary phases are evaluated with the fuel consumption theorem. The impact on the models of the stellar evolutionary tracks (in particular with and without overshooting) is assessed. We find modest differences in synthetic broad-band colours as induced by the use of different tracks in our code [e.g. �( V − K ) ∼ 0.08 mag, �( B − V ) ∼ 0.03 mag]. Noticeably, these differences are substantially smaller than the scatter among other models in the literature, even when the latter adopt the same evolutionary tracks. The models are calibrated with globular cluster data from the Milky Way for old ages, and the Magellanic clouds plus the merger remnant galaxy NGC 7252, both for young ages of ∼0.1‐2 Gyr, in a large wavelength range from the U band to the K band. Particular emphasis is put on the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) phase. We show that this evolutionary phase is crucial for the modelling of young stellar populations by direct comparison with observed spectral energy distributions of Magellanic cloud clusters, which are characterized by relatively high fluxes, both blueward and redward of the V band. We find that the combination of the near-IR spectral indices C2 and H2O can be used to determine the metallicity of ∼1 Gyr stellar populations. As an illustrative application, we re-analyse the spectral energy distributions of some of the high-z galaxies (2.4 z 2.9) observed with the Spitzer Space Telescope by Yan et al. Their high rest-frame near-IR fluxes is reproduced very well with the models including TP-AGB stars for ages in the range ∼0.6‐1.5 Gyr, suggesting formation redshifts for these objects around z ∼ 3‐6. Ke yw ords: stars: AGB and post-AGB ‐ stars: evolution ‐ galaxies: evolution ‐ galaxies: stellar content ‐ cosmology: early Universe.

/pdf/evolutionary-population-synthesis-models-analysis-of-the-2nvt8b3ulb.pdf

Evolutionary population synthesis: models, analysis of the ingredients and application to high‐z galaxies

The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs) to translate the evolution of a multimetallicity, multi-age set of stars into a prediction for the time-evolution of the integrated light from that set of stars. Each of these necessary inputs carries significant uncertainties that have until now received little systematic attention. The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte Carlo Markov Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following. (1) Including uncertainties in stellar evolution, stellar masses at z ~ 0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z ~ 2, the masses of bright red galaxies are uncertain at the ~0.6 dex level. (2) Either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally pulsating AGB phase. (3) Conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time. Any possible evolution in the IMF, as suggested by several independent lines of evidence, will only exacerbate this problem. (4) Assuming a distribution of stellar metallicities within a galaxy, rather than a fixed value as is usually assumed, can yield important differences when considering bands blueward of V, but is not a concern for redder bands. Spectroscopic information may alleviate some of these concerns, though uncertainties in the stellar spectral libraries and the importance of nonsolar abundance ratios have not yet been systematically investigated in the SPS context.

The propagation of uncertainties in stellar population synthesis modeling. i. the relevance of uncertain aspects of stellar evolution and the initial mass function to the derived physical properties of galaxies

We present the results of a new series of nongray calculations of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K This theory encompasses most of the mass/age parameter space occupied by substellar objects and is the first spectral study down to 100 K These calculations are in aid of the multitude of searches being conducted or planned around the world for giant planets and brown dwarfs and reveal the exotic nature of the class Generically, absorption by H2 at longer wavelengths and H2O opacity windows at shorter wavelengths conspire to redistribute flux blueward Below 1200 K, methane is the dominant carbon bearing molecule and is a universal diagnostic feature of EGP and brown dwarf spectra We find that the primary bands in which to search are Z (~105 ?m), J (~12 ?m), H (~16 ?m), K (~22 ?m), M (~5 ?m), and N (~10 ?m), that enhancements of the emergent flux over blackbody values, in particular in the near infrared, can be by many orders of magnitude, and that the infrared colors of EGPs and brown dwarfs are much bluer than previously believed In particular, relative to J and H, the K band flux is reduced by CH4 and H2 absorption Furthermore, we conclude that for Teff's below 1200 K most or all true metals may be sequestered below the photosphere, that an interior radiative zone is a generic feature of substellar objects, and that clouds of H2O and NH3 are formed for Teff's below ~400 and ~200 K, respectively This study is done for solar-metallicity objects in isolation and does not include the effects of stellar insulation Nevertheless, it is a comprehensive attempt to bridge the gap between the planetary and stellar realms and to develop a nongray theory of objects from 03MJ (Saturn) to 70MJ (~007 M?) We find that the detection ranges for brown dwarf/EGP discovery of both ground- and space-based telescopes are larger than previously estimated

https://iopscience.iop.org/article/10.1086/305002/pdf

A Nongray Theory of Extrasolar Giant Planets and Brown Dwarfs

The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs). The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte-Carlo Markov-Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following: 1) including uncertainties in stellar evolution, stellar masses at z~0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z~2, the masses of bright red galaxies are uncertain at the ~0.6 dex level; 2) either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally-pulsating asymptotic giant branch phase; 3) conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K-band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time; 4) The more plausible assumption of a distribution of stellar metallicities, rather than a fixed value as is usually assumed, can have significant effects on the interpretation of colors blueward of the V-band. (ABRIDGED)

The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies

This series of papers comprises a systematic exploration of the hypothesis that the far-ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short-wavelength (900-3000 A) observations

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. We use the term `Extreme Horizontal Branch' (EHB) to refer to HB sequences of constant mass that do not reach the thermally-pulsing stage on the AGB. These models evolve after core helium exhaustion

https://arxiv.org/pdf/astro-ph/9311022.pdf

Ultraviolet Radiation from Evolved Stellar Populations -- I. Models

Strong mass loss on the red giant branch (RGB) can result in the formation of extreme horizontal branch (EHB) stars. The EHB stars spend most of their He core and shell burning phase at high temperatures and produce copious ultraviolet flux. They have very small hydrogen envelopes and occupy a small range in mass. We have computed evolutionary RGB models with mass loss for stars with a range of metallicities at initial masses < 1.1 Msun corresponding to populations ages between 12.5 and 14.5 Gyr. We used the Reimers formula to characterize mass loss, but investigated a larger range of the mass loss efficiency parameter, eta, than is common. To understand how the number of EHB stars varies with metallicity in a stellar population we considered how the zero-age horizontal branch (ZAHB) is populated. The range in eta producing EHB stars is comparable to that producing `mid-HB' stars. Somewhat surprisingly, neither the range nor magnitude of eta producing EHB stars varies much metallicity. In contrast, the range of eta producing mid-HB stars decreases with increasing metallicity. Hence the HB of populations with solar metallicity and higher, such as expected in elliptical galaxies and spiral bulges, will be bimodal if the distribution covers a sufficiently large range in eta.

The Origin of Extreme Horizontal Branch Stars

We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441 All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems This may have important implications for the interpretation of the integrated spectra of elliptical galaxies The cause of the phenomenon remains uncertain We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue against dynamical effects playing a role

/pdf/discovery-of-extended-blue-horizontal-branches-in-two-metal-ri5p33g6y5.pdf

Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters*

https://arxiv.org/pdf/astro-ph/9511017.pdf

Ben Dorman

Papers

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Ultraviolet Radiation from Evolved Stellar Populations -- I. Models

The Origin of Extreme Horizontal Branch Stars

Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters*

The Origin of Extreme Horizontal Branch Stars