Experiments In Stellar Astrophysics Research Articles

Bridging theory and observations in stellar pulsations: the impact of convection and metallicity on the instability strips of classical and type-II cepheids

ABSTRACT The effect of metallicity on the theoretical and empirical period–luminosity relations of Cepheid variables is not well understood and remains a highly debated issue. Here, we examine empirical colour–magnitude diagrams (CMDs) of Classical and Type-II Cepheids in the Magellanic Clouds and compare those with the theoretically predicted instability strip (IS) edges. We explore the effects of incorporating turbulent flux, turbulent pressure, and radiative cooling into the convection theory on the predicted IS at various metallicities using Modules for Experiments in Stellar Astrophysics – Radial Stellar Pulsations. We find that the edges become redder with the increasing complexity of convection physics incorporated in the fiducial convection sets, and are similarly shifted to the red with increasing metallicity. The inclusion of turbulent flux and pressure improves the agreement of the red edge of the IS, while their exclusion leads to better agreement with observations of the blue edge. About 90 per cent of observed stars are found to fall within the predicted bluest and reddest edges across the considered variations of turbulent convection parameters. Furthermore, we identify and discuss discrepancies between theoretical and observed CMDs in the low-effective temperature and high-luminosity regions for stars with periods greater than ∼20 d. These findings highlight the potential for calibrating the turbulent convection parameters in stellar pulsation models or the prediction of a new class of rare, long-period, ‘red Cepheids’, thereby improving our understanding of Cepheids and their role in cosmological studies.

Asteroseismological Analysis of the Non-Blazhko RRab Star EPIC 248846335 in the LAMOST–Kepler/K2 Project

We conduct an asteroseismological analysis on the non-Blazhko ab-type RR Lyrae star EPIC 248846335, employing the Radial Stellar Pulsations module of the Modules for Experiments in Stellar Astrophysics based on the set of stellar parameters. The atmospheric parameters T eff = 6933 ± 70 K, log g = 3.35 ± 0.50, and [Fe/H] = −1.18 ± 0.14 are estimated from low-resolution spectra of LAMOST DR9. The luminosity L = 49.70−1.80+2.99 L ⊙ and mass M = 0.56 ± 0.07 M ⊙ are calculated, respectively, using the distance provided by Gaia and the metallicity estimated from the low-resolution spectra. The Fourier parameters of the light curves observed by K2 and radial velocity (RV) curves determined from the medium-resolution spectra of LAMOST DR10 are also calculated in this work. The period of the fundamental mode of the star and the residuals r of the Fourier parameters between the models and observations serve to select an optimal model, whose stellar parameters are T eff = 6700 ± 220 K, log g = 2.70, [Fe/H] = −1.20 ± 0.2, M = 0.59 ± 0.05 M ⊙, and L = 56.0 ± 4.2 L ⊙. The projection factors are constrained as 1.20 ± 0.02 and 1.59 ± 0.13 by the blue- and red-arm observed velocities with their corresponding RV curves derived from the best-fit model, respectively. The precise determination of stellar parameters in ab-type RR Lyrae stars is crucial for understanding the physical processes that occur during pulsation and for providing a deeper understanding of their period–luminosity relationship.

No X-Rays or Radio from the Nearest Black Holes and Implications for Future Searches

Astrometry from the Gaia mission was recently used to discover the two nearest known stellar-mass black holes (BHs), Gaia BH1 and Gaia BH2. These objects are among the first stellar-mass BHs not discovered via X-rays or gravitational waves. Both systems contain ∼1 M ⊙ stars in wide orbits (a ≈ 1.4 au, 4.96 au) around ∼9 M ⊙ BHs, with both stars (solar-type main sequence star, red giant) well within their Roche lobes in Gaia BH1 and BH2, respectively. However, the BHs are still expected to accrete stellar winds, leading to potentially detectable X-ray or radio emission. Here, we report observations of both systems with the Chandra X-ray Observatory, the Very Large Array (for Gaia BH1) and MeerKAT (for Gaia BH2). We did not detect either system, leading to X-ray upper limits of L X < 9.4 × 1028 and L X < 4.0 × 1029 erg s−1 and radio upper limits of L r < 1.6 × 1025 and L r < 1.0 × 1026 erg s−1 for Gaia BH1 and BH2, respectively. For Gaia BH2, the non-detection implies that the accretion rate near the horizon is much lower than the Bondi rate, consistent with recent models for hot accretion flows. We discuss implications of these non-detections for broader BH searches, concluding that it is unlikely that isolated BHs will be detected via interstellar medium accretion in the near future. We also calculate evolutionary models for the binaries’ future evolution using Modules for Experiments in Stellar Astrophysics, and find that Gaia BH1 will be visible as a symbiotic BH X-ray binary for 5–50 Myr. Since no symbiotic BH X-ray binaries are known, this implies either that fewer than ∼104 Gaia BH1-like binaries exist in the Milky Way, or that they are common but have evaded detection.

OGLE-BLAP-009 – a case study for the properties and evolution of blue large-amplitude pulsators

ABSTRACT Blue large-amplitude pulsators (BLAPs) make up a rare class of hot pulsating stars with effective temperatures of ≈30 000 K and surface gravities of 4.0–5.0 dex (cgs). The evolutionary origin and current status of BLAPs is not well understood, largely based on a lack of spectroscopic observations and no available mass constraints. However, several theoretical models have been proposed that reproduce their observed properties, including studies that identify them as pulsating helium-core pre-white dwarfs (He-core pre-WDs). We present here follow-up high-speed photometry and phase-resolved spectroscopy of one of the original 14 BLAPs, OGLE-BLAP-009, discovered during the Optical Gravitational Lensing Experiment. We aim to explore its pulsation characteristics and determine stellar properties such as mass and radius in order to test the consistency of these results with He-core pre-WD models. Using the mean atmospheric parameters found using spectroscopy, we fit a spectral energy distribution to obtain a preliminary estimate of the radius, luminosity, and mass by making use of the Gaia parallax. We then compare the consistency of these results to He-core pre-WD models generated using Modules for Experiments in Stellar Astrophysics, with predicted pulsation periods implemented using gyre. We find that our mass constraints are in agreement with a low-mass He-core pre-WD of ≈0.30 M⊙.

Testing Modules for Experiments in Stellar Astrophysics (MESA)

Regular, automated testing is a foundational principle of modern software development. Numerous widely used continuous integration systems exist, but they are often not suitable for the unique needs of scientific simulation software. Here we describe the testing infrastructure developed for and used by the Modules for Experiments in Stellar Astrophysics (MESA) project. This system allows the computationally demanding MESA test suite to be regularly run on a heterogeneous set of computers and aggregates and displays the testing results in a form that allows for the rapid identification and diagnosis of regressions. Regularly collecting comprehensive testing data also enables longitudinal studies of the performance of the software and the properties of the models it generates.

MESA-Web: A cloud resource for stellar evolution in astronomy curriculum

We present MESA-Web,a cloud resource with an online interface to the Modules for Experiments in Stellar Astrophysics (MESA) software instrument. MESA-Web allows learners to evolve stellar models without the need to download and install MESA. Since being released in 2015, MESA-Web has delivered over 17,000 calculations to over 2,200 unique learners and currently performs about 11 jobs per day. MESA-Web can be used as an educational tool for stars in the classroom or for scientific investigations.We report on new capabilities of MESA-Web introduced since its 2015 release including learner-supplied nuclear reaction rates, custom stopping conditions, and an expanded selection of input parameters. To foster collaboration we have created a Zenodo MESA-Web community hub, where instructors can openly share examples of using MESA-Web in the classroom. We discuss two examples in the current community hub. The first example is a lesson module on Red Giant Branch stars that includes a suite of exercises designed to fit a range of learners and a Jupyter workbook for additional analysis. The second example is lesson materials for an upper-level Astronomy majors course in Stars and Radiation that includes an assignment verifying some of the expected trends that are presented in a popular stellar physics textbook.

Analysing mixed modes of the four solar-like oscillating subgiant stars

ABSTRACT The observation of an unprecedented number of solar-like oscillating subgiant (SG) stars by the Kepler and TESS missions is crucial for the asteroseismic characterization of these stars, stellar population studies, and the study of stellar evolution theories. Owing to these missions, the fundamental parameters of the solar-like oscillating stars are precisely calculated from the evolution codes using the observed oscillation frequencies. Herein, we considered four solar-like oscillating SG stars. We obtained the fundamental parameters of the SG stars by constructing interior models using asteroseismic and non-asteroseismic observed parameters. The interior models of the four SG stars are constructed using the Modules for Experiments in Stellar Astrophysics code to effectively determine the fundamental properties. Using this method, the four solar-like oscillating SG stars are found to have masses and radii within the ranges of 1.16–1.75 M⊙ and 2.26–3.17 R⊙, respectively. The estimation accuracy of the typical asteroseismic radius, mass, and age is increased by fitting the observed and model reference frequencies. The typical uncertainties of the mass and radius are 3–4 ${{\ \rm per\ cent}}$ and 1–2 ${{\ \rm per\ cent}}$, respectively. Furthermore, the observed l = 1 frequencies, which showed a mixed mode for the first time, were also fitted to the models. Information regarding the gravity and density of the helium core was obtained by examining the mixed modes. Moreover, new asteroseismic methods for determining the age of SG stars are developed for the first time in this study.

Overview and Validation of the Asteroseismic Modeling Portal v2.0

The launch of NASA’s Kepler space telescope in 2009 revolutionized the quality and quantity of observational data available for asteroseismic analysis. While Kepler was able to detect solar-like oscillations in hundreds of main-sequence and subgiant stars, the Transiting Exoplanet Survey Satellite is now making similar observations for thousands of the brightest stars in the sky. The Asteroseismic Modeling Portal (AMP) is an automated and objective stellar model-fitting pipeline for asteroseismic data, which was originally developed to use models from the Aarhus Stellar Evolution Code. We briefly summarize an updated version of the AMP pipeline that uses Modules for Experiments in Stellar Astrophysics, and we present initial modeling results for the Sun and several solar analogs to validate the precision and accuracy of the inferred stellar properties.

Modeling fast radio burst heating a main-sequence companion star in a close binary using MESA

The exact origin of fast radio bursts (FRBs) remains a mystery. The repeating fast radio burst source, FRB20200120E, was discovered in a globular cluster containing old stellar populations. Yang (2021) suggested that this FRB might be in close binaries with low-mass main-sequence (MS) stars. They analytically investigated the observational consequences caused by the heating of FRB radio radiation onto the low-mass MS companion star in a close binary, suggesting that the radio radiation emitted by FRB could make the MS companion star more luminous and detectable in future multi-wavelength follow-up observations for a Galactic FRB. We revisited the study of Yang (2021) by numerically modeling the detailed process of FRB heating onto an MS companion with 1D stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA). Our results are consistent with the trends derived from the analytical model of Yang (2021), except that the typical re-emission luminosities of our main sequence (MS) models, caused by the heating from FRBs, are generally dimmer by about two orders of magnitude compared to his findings, and our models have a longer re-emission timescale. This may indicate that the searches of the optical transients caused by the radio radiation heating companion star are more likely to be successful within a distance of 0.3 Mpc.

The Study of Helium Variations in Star Clusters Using China Space Station Telescope

The China Space Station Telescope (CSST) is a 2 m space-based optical-UV telescope. Its primary goal is to carry out a high-resolution photometric imaging survey of a 17,500 square degree sky area using the on board Survey Camera. With its wide field of view (1.1 square degrees) and a mosaic imager containing 640 million pixels, studying the different populations of stars within star clusters is highly feasible. The aim of this study is to assess the CSST’s ability to distinguish between stellar populations with varying helium abundance levels, with the help of Modules for Experiments in Stellar Astrophysics. The results of the CSST’s photometry for these different populations are presented by transferring the models into the CSST Survey Camera photometric system. The findings confirm that helium-enriched stellar populations will have unique patterns in the color–magnitude diagrams under the CSST photometric system, compared to normal stellar populations. The CSST, with its filters and wide field of view of the Survey Camera, provides a new avenue for the study of multiple populations in star clusters.

Multi-campaign asteroseismic analysis of eight solar-like pulsating stars observed by the K2 mission

The NASA K2 mission that succeeded the nominal Kepler mission observed several hundred thousand stars during its operations. While most of the stars were observed in single campaigns of ∼80 days, some of them were targeted for more than one campaign. We perform an asteroseismic study of a sample of eight solar-like stars observed during K2 Campaigns 6 and 17, allowing us access to up to 160 days of data. With these two observing campaigns, we determine not only the stellar parameters but also study the rotation and magnetic activity of these stars. We first extract the light curves for the two campaigns using two different pipelines, EVEREST and Lightkurve. The seismic analysis is done on the combined light curve of C6 and C17, where the gap between them was removed and the two campaigns were ‘stitched’ together. We determine the global seismic parameters of the solar-like oscillations using two different methods: one using the A2Z pipeline and the other the Bayesian apollinaire code. With the latter, we also perform the peak-bagging of the modes to characterize their individual frequencies. By combining the frequencies with the Gaia DR2 effective temperature and luminosity, and metallicity for five of the targets, we determine the fundamental parameters of the targets using the IACgrids based on the MESA (Modules for Experiments in Stellar Astrophysics) code. We find that four of the stars are on the main sequence, two stars are about to leave it, and two stars are more evolved (a subgiant and an early red giant). While the masses and radii of our targets probe a similar parameter space compared to the Kepler solar-like stars, with detailed modeling, we find that for a given mass our more evolved stars seem to be older than previous seismic stellar ensembles. We calculate the stellar parameters using two different grids of models, one incorporating and one excluding the treatment of diffusion, and find that the results agree generally within the uncertainties, except for the ages. The ages obtained using the models that exclude diffusion are older, with differences of greater than 10% for most stars. The seismic radii and the Gaia DR2 radii present an average difference of 4% with a dispersion of 5%. Although the agreement is relatively good, the seismic radii are slightly underestimated compared to Gaia DR2 for our stars, the disagreement being greater for the more evolved ones. Our rotation analysis provides two candidates for potential rotation periods but longer observations are required to confirm them.

Residual eccentricity of an Earth-like planet orbiting a red giant Sun

Context. The late phases of the orbital evolution of an Earth-like planet around a Sun-like star are revisited in order to consider the effect of density fluctuations associated with convective motions inside the star. Aims. Such fluctuations produce a random perturbation of the stellar outer gravitational field that excites a small residual eccentricity in the orbit of the planet. This counteracts the effects of tides, which tend to circularize the orbit. Methods. We computed the power spectrum of the outer gravitational field fluctuations of the star in the quadrupole approximation and studied their effects on the orbit of the planet using a perturbative approach. The residual eccentricity is found to be a stochastic variable showing a Gaussian distribution. Results. Adopting a model of the stellar evolution of our Sun computed with Modules for Experiments in Stellar Astrophysics (MESA), we find that the Earth will be engulfed by the Sun when it is close to the tip of the red giant branch phase of evolution. We find a maximum mean value of the residual eccentricity of ~0.026 immediately before engulfment. Considering an Earth-mass planet with an initial orbital semimajor axis sufficiently large to escape engulfment, we find that the mean value of the residual eccentricity is greater than 0.01 for an initial separation of up to ~l.4 au. Conclusions. The engulfment of the Earth by the red giant Sun is found to be a stochastic process instead of being deterministic as assumed in previous studies. If an Earth-like planet escapes engulfment, its orbit around its remnant white dwarf (WD) star will be moderately eccentric. Such a residual eccentricity of on the order of a few hundredths can play a relevant role in sustaining the pollution of the WD atmosphere by asteroids and comets, as observed in several objects.

Carbon–Oxygen Phase Separation in Modules for Experiments in Stellar Astrophysics (MESA) White Dwarf Models

We enhance the treatment of crystallization for models of white dwarfs (WDs) in the stellar evolution software Modules for Experiments in Stellar Astrophysics (MESA) by implementing carbon–oxygen (C/O) phase separation. The phase separation process during crystallization leads to transport of oxygen toward the centers of WDs, resulting in a more compact structure that liberates gravitational energy as additional heating that modestly slows WD cooling timescales. We quantify this cooling delay in MESA C/O WD models over the mass range 0.5–1.0 M ⊙, finding delays of 0.5–0.8 Gyr for typical C/O interior profiles. MESA WD cooling timescales including this effect are generally comparable to other WD evolution models that make similar assumptions about input physics. When considering phase separation alongside 22Ne sedimentation, however, we find that both MESA and BaSTI WD cooling models predict a more modest sedimentation delay than the latest LPCODE models, and this may therefore require a reevaluation of previously proposed solutions to some WD cooling anomalies that were based on LPCODE models of 22Ne sedimentation. Our implementation of C/O phase separation in the open-source stellar evolution software MESA provides an important tool for building realistic grids of WD cooling models, as well as a framework for expanding on our implementation to explore additional physical processes related to phase transitions and associated fluid motions in WD interiors.

Giant Planet Engulfment by Evolved Giant Stars: Light Curves, Asteroseismology, and Survivability

About ten percent of Sun-like (1–2 M ⊙) stars will engulf a 1–10 M J planet as they expand during the red giant branch (RGB) or asymptotic giant branch (AGB) phase of their evolution. Once engulfed, these planets experience a strong drag force in the star’s convective envelope and spiral inward, depositing energy and angular momentum. For these mass ratios, the inspiral takes ∼10–102 yr (∼102–103 orbits); the planet undergoes tidal disruption at a radius of ∼1 R ⊙. We use the Modules for Experiments in Stellar Astrophysics (MESA) software instrument to track the stellar response to the energy deposition while simultaneously evolving the planetary orbit. For RGB stars, as well as AGB stars with M p ≲ 5 M J planets, the star responds quasi-statically but still brightens measurably on a timescale of years. In addition, asteroseismic indicators, such as the frequency spacing or rotational splitting, differ before and after engulfment. For AGB stars, engulfment of an M p ≳ 5 M J planet drives supersonic expansion of the envelope, causing a bright, red, dusty eruption similar to a “luminous red nova.” Based on the peak luminosity, color, duration, and expected rate of these events, we suggest that engulfment events on the AGB could be a significant fraction of low-luminosity red novae in the Galaxy. We do not find conditions where the envelope is ejected prior to the planet’s tidal disruption, complicating the interpretation of short-period giant planets orbiting white dwarfs as survivors of common envelope evolution.

White dwarf mass growth in cataclysmic variables: roles of dwarf novae

ABSTRACT The disc instability mechanism (DIM) is widely accepted to account for the transient behaviour of dwarf novae (DNe), which experience short outbursts separated by long quiescence. The duty cycle (the ratio between the outburst duration and the recurrence time) determines the amount of accreted mass by the white dwarf (WDs) during outbursts, thus playing an important role in the long-term binary evolution. Employing the code of Modules for Experiments in Stellar Astrophysics, we systemically investigate the influence of the duty cycles on the evolution of DNe and the mass growth of accreting carbon-oxygen (CO) WDs. Our calculations show that, while the DIM can considerably influence the accretion process, efficient WD-mass growth requires a particular range of the duty cycle. For WDs with the initial masses of 0.6, 0.7, and 1.1 M⊙, these duty cycles are 0.006$\, \le$d$\, \le$0.007, d = 0.005, and, d = 0.003 and the accumulated mass of the WDs can reach 0.1, 0.13, and 0.21 M⊙, respectively. In all of our simulations, no CO WDs can grow their masses to the explosion mass of Type Ia supernovae of about 1.38 M⊙. Because of a much short time-scale of the outburst state, the final donor-star masses and orbital periods are insensitive to the duty cycles. Therefore, we propose that the DIM in DNe could alleviate the WD mass problem to some extent.

Accreting white dwarfs: effect of WD composition on helium ignition during slow accretion

Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon–oxygen (CO) white dwarf (WD) from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on various parameters during the accretion of helium-rich matter at a slow rate. We have used the computer simulation code Modules for Experiments in Stellar Astrophysics (MESA) to understand the variations in the properties, such as specific heat ( $$C_P$$ ) and degeneracy parameter ( $$\eta $$ ). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the difference in carbon abundance in a CO-WD. We found that WDs of the lower abundance of carbon, accrete slightly longer before the onset of helium ignition.

Theoretical investigation of the occurrence of tidally excited oscillations in massive eccentric binary systems

Context. Massive and intermediate-mass stars reside in binary systems at a much higher rate than low-mass stars. At the same time, binaries containing massive main-sequence (MS) component(s) are often characterised by eccentric orbits, and can therefore be observed as eccentric ellipsoidal variables (EEVs). The orbital phase-dependent tidal potential acting on the components of EEVs can induce tidally excited oscillations (TEOs), which can affect the evolution of the binary system. Aims. We investigate how the history of resonances between the eigenmode spectra of the EEV components and the tidal forcing frequencies depends on the initial parameters of the system, limiting our study to the MS. Each resonance is a potential source of TEO. We are particularly interested in the total number of resonances, their average rate of occurrence, and their distribution in time. Methods. We synthesised 20 000 evolutionary models of the EEVs across the MS using Modules for Experiments in Stellar Astrophysics (MESA) software for stellar structure and evolution. We considered a range of masses of the primary component from 5 to 30 M⊙. Later, using the GYRE stellar non-adiabatic oscillations code, we calculated the eigenfrequencies for each model recorded by MESA. We focused only on the l = 2, m = 0, +2 modes, which are suspected of being dominant TEOs. Knowing the temporal changes in the orbital parameters of simulated EEVs and the changes in the eigenfrequency spectra for both components, we were able to determine so-called resonance curves, which describe the overall chance of a resonance occurring and therefore of a TEO occurring. We analysed the resonance curves by constructing basic statistics for them and analysing their morphology using machine learning methods, including the Uniform Manifold Approximation and Projection (UMAP) tool. Results. The EEV resonance curves from our sample are characterised by a striking diversity, including the occurrence of exceptionally long resonances or the absence of resonances for long evolutionary times. We find that the total number of resonances encountered by components in the MS phase ranges from ∼102 to ∼103, mostly depending on the initial eccentricity. We also noticed that the average rate of resonances is about an order of magnitude higher (∼102 Myr−1) for the most massive components in the assumed range than for EEVs with intermediate-mass stars (∼101 Myr−1). The distribution of resonances over time is strongly inhomogeneous, and its shape depends mainly on whether the system is able to circularise its orbit before the primary component reaches the terminal-age main sequence (TAMS). Both components may be subject to increased resonance rates as they approach the TAMS. Thanks to the low-dimensional UMAP embeddings performed for the resonance curves, we argue that their morphology changes smoothly across the resulting manifold for different initial EEV conditions. The structure of the embeddings allowed us to explore the whole space of resonance curves in terms of their morphology and to isolate some extreme cases. Conclusions. Resonances between tidal forcing frequencies and stellar eigenfrequencies cannot be considered rare events for EEVs with massive and intermediate-mass MS stars. On average, we should observe TEOs more frequently in EEVs that contain massive components than those that contain intermediate-mass ones. The TEOs will be particularly well pronounced for EEVs whose component(s) are close to the TAMS, which calls for observational verification. Given the total number of resonances and their rates, TEOs may play an important role in the transport of angular momentum within massive and intermediate-mass stars (mainly near the TAMS).

Modules for Experiments in Stellar Astrophysics (MESA): Time-dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite-difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron-degenerate ignition events. We strengthen MESA’s implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA, we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator-split nuclear burning mode. We close by discussing major updates to MESA’s software infrastructure that enhance source code development and community engagement.

WOCS 4540: Detailed Analysis of a very Long Orbital Period Blue Straggler

WOCS 4540 is the longest orbital period (P orb = 3030 days) blue straggler star (BSS)—white dwarf (WD) pair in the old open cluster NGC 188. It also contains one of the most luminous BSS in the cluster. Prior Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy measured a WD mass of 0.53 M ⊙, indicative of a carbon–oxygen WD and suggesting previous mass transfer from an asymptotic giant branch (AGB) star. Detailed modeling of the system evolution, including red giant branch phase wind mass transfer, AGB wind Roche-lobe overflow, and regular Roche-lobe overflow, is done with Modules for Experiments in Stellar Astrophysics. The best-fit model produces excellent agreement with a wide array of observational constraints on the BSS, the WD, and the binary system. To produce the observed luminosity and effective temperature of the BSS, all three donor mass-transfer mechanisms contribute similarly to build a 1.5 M ⊙ BSS. The overall mass-transfer efficiency is 55%. Regular Roche-lobe overflow occurs only during the largest AGB thermal pulse, but yields a very high accretion rate at 75% efficiency and briefly (less than 1 Myr) a very high luminosity boost from the accretor.

The influence of metallicity on helium and CO core masses in massive stars

We present the results of 58 detailed evolutionary models of massive single stars and close binary systems with the Solar and Small Magellanic Cloud (SMC) metallicity computed with the MESA (Modules for Experiments in Stellar Astrophysics) numerical code. Helium core masses of single stars (30 M? - 75 M?) with metallicities of 0.02 and 0.0021 are in the range of 9.26 M? - 29.56 M? and 11.62 M? - 33.96 M?, respectively. Their carbon-oxygen (CO) core masses are between 5.44 M? and 25.04 M? vs. 8.23 M? and 28.38 M? for the Solar vs. SMC metallicity, accounting for an average difference of 25%. To investigate the influence of metallicity on helium and carbon-oxygen core masses in massive close Case A binary systems, detailed evolutionary models of binary systems in the mass range of 30 M? to 40 M? are calculated. The initial orbital period is set to 3 days and the accretion efficiency to 10%. The helium core mass range for primary stars with lower metallicity is 10.61 - 16.21 M? vs. 7.94 - 11.69 M? for z = 0.02. The resulting CO core masses of primary stars for the SMC metallicity are on average about 50% larger than for the Solar metallicity, so the effect is more prominent than in the case of single stars. The black hole formation limit for primary stars with the SMC metallicity is under 30 M?. While the least massive primary stars with Solar metallicity end up as neutron stars, all primary stars with the SMC metallicity and all secondary stars complete their evolution as black holes. The double compact objects resulting from the presented models are of two types: mixed neutron star-black hole systems (4 models) and double black holes (18 models). We also derive the relation between the final helium core mass and the carbon-oxygen core mass and show that it does not depend on metallicity. We confirm the CO/helium core mass ratio to be larger in binary systems than for single stars.