Core Helium Burning Phase Research Articles

The Impact of Breathing Pulses during Core Helium Burning on the Core Chemical Structure and Pulsations of Hydrogen-rich Atmosphere White Dwarfs

Breathing pulses are mixing episodes that could develop during the core helium-burning phase of low- and intermediate-mass stars. The occurrence of breathing pulses is expected to bear consequences on the formation and evolution of white dwarfs, particularly on the core chemical structure, which can be probed by asteroseismology. We aim to explore the consequences of breathing pulses on the chemical profiles and pulsational properties of variable white dwarf stars with hydrogen-rich envelopes, known as ZZ Ceti stars. We compute stellar models with masses of 1.0M ⊙ and 2.5M ⊙ in the zero-age main sequence and evolve them through the core helium-burning phase to the thermal pulses on the asymptotic giant branch, and finally to advanced stages of white dwarf cooling. We compare the chemical structure of the core of white dwarfs whose progenitors have experienced breathing pulses during the core helium-burning phase with the case in which breathing pulses have not occurred. We find that when breathing pulses occur, the white dwarf cores are larger and the central abundances of oxygen are higher than for the case in which the breathing pulses are suppressed, in line with previous studies. However, the occurrence of breathing pulses is not sufficient to explain the large cores and the excessive oxygen abundances that characterize recently derived asteroseismological models of pulsating white dwarfs. We find absolute differences of up to ∼30 s when we compare pulsation periods of white dwarfs coming from progenitors that have experienced breathing pulses with the case in which the progenitors have not suffered breathing pulses.

Evidence for Evolved Stellar Binary Mergers in Observed B-type Blue Supergiants

Blue supergiants are the brightest stars in their host galaxies, and yet their evolutionary status has been a long-standing problem in stellar astrophysics. In this pioneering work, we present a large sample of 59 early B-type supergiants in the Large Magellanic Cloud with newly derived stellar parameters and identify the signatures of stars born from binary mergers among them. We simulate novel 1D merger models of binaries consisting of post main-sequence giants with helium-rich cores (primaries) and main-sequence companions (secondaries), and consider the effects of interaction of the secondary with the core of the primary along with the mixing induced by the merger in the envelope. Thereafter, the evolution of the newborn 17–43 M⊙ stars is followed until core-carbon depletion, close to their final pre-explosion stage. Unlike stars born alone with comparable masses, stars born from mergers of evolved binaries are blue throughout their core helium-burning phase and replicate the surface gravities and Hertzsprung–Russell diagram positions of most of our sample, thus indicating that B-type supergiants structurally resemble stars born from such mergers. Moreover, the large nitrogen-to-carbon and nitrogen-to-oxygen number ratios, coupled with helium enhancements exhibited by at least half our data sample, is uniquely reproduced by our merger models. Collectively, these findings provide compelling evidence toward the important role of binary mergers in producing the currently observed population of blue supergiants in our Universe.

Effect of nuclear reactions rates and core boundary mixing on the seismology of red clump stars

Context. Modeling of the stars in the red clump (RC), that is, core helium-burning stars that have gone through a He flash, is challenging because of the uncertainties associated with the physical processes in their core and during the helium flash. By probing the internal stellar structure, asteroseismology allows us to constrain the core properties of RC stars and eventually, to improve our understanding of this evolutionary phase. Aims. We aim to quantify the impact on the seismic properties of the RC stars of the two main core modeling uncertainties: core boundary mixing, and helium-burning nuclear reaction rates. Methods. Using the MESA stellar evolution code, we computed models with different core boundary mixing as well as different 3α and 12C(α, γ)16O nuclear reaction rates. We investigated the impact of these parameters on the period spacing ΔΠ, which is a probe of the region around the core. Results. We find that different core boundary mixing schemes yield significantly different period spacings, with differences of 30 s between the maximum ΔΠ value computed with semiconvection and maximal overshoot. We show that an increased rate of 12C(α, γ)16O lengthens the core helium-burning phase, which extends the range of ΔΠ covered by the models during their evolution. This results in a difference of 10 s between the models computed with a nominal rate and a rate multiplied by 2, which exceeds the observational uncertainties. The effect of changing the 3α reaction rate is comparatively small. Conclusions. The core boundary mixing is the main source of uncertainty in the seismic modeling of RC stars. Moreover, the effect of the 12C(α, γ)16O is non-negligible, even though it is difficult to distinguish from the effect of the mixing. This degeneracy could be seen more frequently in the future in the new seismic data from the PLATO mission and through theoretical constraints from numerical simulations.

Magnetic field breakout from white dwarf crystallization dynamos

ABSTRACT A convective dynamo operating during the crystallization of white dwarfs is one of the promising channels to produce their observed strong magnetic fields. Although the magnitude of the fields generated by crystallization dynamos is uncertain, their timing may serve as an orthogonal test of this channel’s contribution. The carbon–oxygen cores of $M\approx 0.5-1.0\, {\rm M}_{\odot }$ white dwarfs begin to crystallize at an age tcryst ∝ M−5/3, but the magnetic field is initially trapped in the convection zone – deep inside the CO core. Only once a mass of mcryst has crystallized, the convection zone approaches the white dwarf’s helium layer, such that the magnetic diffusion time through the envelope shortens sufficiently for the field to break out to the surface, where it can be observed. This breakout time is longer than tcryst by a few Gyr, scaling as tbreak ∝ tcrystf−1/2, where f ≡ 1 − mcryst/M depends on the white dwarf’s initial C/O profile before crystallization. The first appearance of strong magnetic fields B ≳ 1 MG in volume-limited samples approximately coincides with our numerically computed tbreak(M) – potentially signalling crystallization dynamos as a dominant magnetization channel. However, some observed magnetic white dwarfs are slightly younger, challenging this scenario. The dependence of the breakout process on the white dwarf’s C/O profile implies that magnetism may probe the CO phase diagram, as well as uncertainties during the core helium burning phase in the white dwarf’s progenitor, such as the 12C(α, γ)16O nuclear reaction.

An insight into Capella (α Aurigae): From the extent of core overshoot to its evolutionary history

Context. The binary star α Aurigae (otherwise known as Capella) is extremely important to understanding the core hydrogen and helium burning phases of stars, as its primary star is likely evolving through the core helium burning phase. Moreover, the masses of the star’s two components are ∼2.5 M⊙ and ∼2.6 M⊙, meaning they fall into a mass range in which the extension of the core overshoot during the main sequence phase is uncertain. Aims. We aim to derive the extent of the core overshoot experienced during the core burning phases and to test the efficiency of the convective transport of energy in the external envelope of Capella by comparing results from stellar evolution modelling with the results from observations. Methods. We considered evolutionary tracks calculated expressly for the present work, that is, for the primary and secondary stars of Capella. We determined the extent of the extra mixing from the core during the main sequence evolution and the age of the system by requiring that the effective temperatures and surface gravities of the model stars reproduce those derived from the observations at the same epoch. We further checked the consistency between the observed and predicted surface chemistry of the stars. Results. The consistency between results from stellar evolution modelling and the observations of Capella is found when extra mixing from the core is assumed, the extent of the extra-mixed zone being of the order of 0.25 HP. The age of the system is estimated to be 710 Myr. These results allow the observed surface chemistry to be well reproduced, particularly the recent determination of the 12C/13C ratio based on Large Binocular Telescope and Vatican Advanced Technology Telescope observations.

Sparse Logistic Regression for RR Lyrae versus Binaries Classification

RR Lyrae (RRL) stars are old, low-mass, radially pulsating variable stars in their core helium burning phase. They are popular stellar tracers and primary distance indicators since they obey well-defined period–luminosity relations in the near-infrared regime. Their photometric identification is not trivial; indeed, RRL star samples can be contaminated by eclipsing binaries, especially in large data sets produced by fully automatic pipelines. Interpretable machine-learning approaches for separating eclipsing binaries from RRL stars are thus needed. Ideally, they should be able to achieve high precision in identifying RRL stars while generalizing new data from different instruments. In this paper, we train a simple logistic regression classifier on Catalina Sky Survey (CSS) light curves. It achieves a precision of 87% at 78% recall for the RRL star class on unseen CSS light curves. It generalizes on out-of-sample data (ASAS/ASAS-SN light curves) with a precision of 85% at 96% recall. We also considered a L1-regularized version of our classifier, which reaches 90% sparsity in the light-curve features with a limited trade-off in accuracy on our CSS validation set and—remarkably—also on the ASAS/ASAS-SN light-curve test set. Logistic regression is natively interpretable, and regularization allows us to point out the parts of the light curves that matter the most in classification. We thus achieved both good generalization and full interpretability.

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Context. Asteroseismology provides constraints on the core rotation rate for hundreds of low- and intermediate-mass stars in evolved phases. Current physical processes tested in stellar evolution models cannot reproduce the evolution of these core rotation rates. Aims. We investigate the efficiency of the internal angular momentum redistribution in red giants during the hydrogen-shell and core-helium burning phases based on the asteroseismic determinations of their core rotation rates. Methods. We computed stellar evolution models with rotation and model the transport of angular momentum by the action of a sole dominant diffusive process parameterised by an additional viscosity in the equation of angular momentum transport. We constrained the values of this viscosity to match the mean core rotation rates of red giants and their behaviour with mass and evolution using asteroseismic indicators along the red giant branch and in the red clump. Results. For red giants in the hydrogen-shell burning phase, the transport of angular momentum must be more efficient in more massive stars. The additional viscosity is found to vary by approximately two orders of magnitude in the mass range M ∼ 1–2.5 M⊙. As stars evolve along the red giant branch, the efficiency of the internal transport of angular momentum must increase for low-mass stars (M ≲ 2 M⊙) and remain approximately constant for slightly higher masses (2.0 M⊙ ≲ M ≲ 2.5 M⊙). In red clump stars, the additional viscosities must be an order of magnitude higher than in younger red giants of similar mass during the hydrogen-shell burning phase. Conclusions. In combination with previous efforts, we obtain a clear picture of how the physical processes acting in stellar interiors should redistribute angular momentum from the end of the main sequence until the core-helium burning phase for low- and intermediate-mass stars to satisfy the asteroseismic constraints.

Seismic Cartography of White-Dwarf Interiors From the Toulouse-Montréal Optimal-Design Approach

Probing internal properties of white-dwarf stars has been amongst the earliest objectives of asteroseismology, following the first discovery in the late 1960s of non-radial pulsations in these evolved compact stars. It was swiftly recognized that white-dwarf pulsators could offer new opportunities to unravel their inner structure and dynamics from the observed low-degree, low-order gravity (g-)modes. From these early days on, many approaches have been attempted to fully exploit this potential, with various levels of success. Here, we review the most recent efforts from our group to perform a complete seismic cartography of white-dwarf interiors. Our approach involves new models incorporating flexible internal profiles for the main chemical constituents (H, He, C, and O) that are optimized, along with other fundamental parameters (Teff and log g), to determine the stellar structure that best reproduces the observed period spectrum of a given star. The method is meant to reduce as much as possible solution dependency relative to stellar evolution uncertainties. The outcome is a full seismic model of the pulsating white-dwarf star under consideration, including its internal core and envelope chemical stratification. Searching for seismic solutions that do not depend on stellar evolution calculations is a key requirement of this strategy. Late stages of evolution that ultimately shape the inner structure of white dwarf stars are known to rely on still uncertain processes. One of our hopes is to be able to test these processes, therefore requiring that seismic models do not incorporate strong preconceived expectations from evolutionary models. We present and discuss results obtained so far from the application of this method to a handful of DB and DA pulsators. In all cases, significant qualitative improvements of the seismic solutions is obtained, providing as an outcome strong quantitative constraints on the core chemical structure of these stars. In particular, we consistently find that the homogeneous C/O mixed core, inherited from the core helium-burning phase, is ∼40% larger (in mass) and ∼15% richer in oxygen (in mass fraction) than expected from standard evolution calculations. Such results constitute precious guidelines for modeling late stages of stellar evolution and better understanding their constitutive physics. As an illustration of this, we show that the central oxygen mass fraction measured by seismology can indeed be reproduced when helium-burning cores experience the so-called breathing pulses. The latter are usually suppressed in standard evolution calculations, as the result of an old debate whether such events are real or numerical artefacts. In other words, our seismic determination of the central amount of oxygen in white dwarfs provides evidence that breathing pulses occurred in the core of their progenitors and should not be dismissed in models after all.

<sup>22</sup>Ne(α, n)<sup>25</sup>Mg: The key neutron source in massive stars

<p indent="0mm">The origin of heavy elements from iron to uranium in the universe is an important frontier direction for nuclear astrophysics. It is considered to be one of the 11 greatest unanswered questions of physics in this century. The neutron capture reaction can overcome the Coulomb barrier and is the key way to generate heavy elements in the universe. The average life of free neutrons is only about <sc>15 min.</sc> Therefore, the neutron source is a hot topic for understanding the origin of heavy elements. There are two main neutron sources in the s-process nucleosynthesis, one is the ¹³C(α, n)¹⁶O reaction, the other is the ²²Ne(α, n)²⁵Mg reaction. The ¹³C(α, n)¹⁶O reaction mainly provides neutrons for the s-process nucleosynthesis of small or medium mass stars, and affects the nucleosynthesis of nuclides with the mass numbers from 90 to 209; while the ²²Ne(α, n)²⁵Mg reaction mainly provides neutrons for the s-process nucleosynthesis of massive stars and affects the nucleosynthesis of nuclides with the mass number less than 90. The ²²Ne(α, n)²⁵Mg reaction takes place in the core helium burning phase and the subsequent shell carbon burning phase of massive stars. Due to the difficulty of experiment measurement, the data of the ²²Ne(α, n)²⁵Mg reaction rate is still lacking, which leads to the problem of understanding effective neutron supply in the nucleosynthesis of the heavy elements in massive stars. Limited by cosmic ray backgrounds, a large number of key reactions in hydrostatic nuclear burning phases cannot be accurately measured. Therefore, direct measurement of these reactions has gradually moved from the earth-surface to underground laboratories. The Laboratory for Underground Nuclear Astrophysics (LUNA) experimental platform in Italy is a mature underground nuclear astrophysics laboratory in the world. Since the 1990s, direct measurement of many nuclear reactions has been carried out. Jinping Underground Nuclear Astrophysics Laboratory (JUNA) is a new experimental platform. JUNA is located in the world’s deepest underground laboratory—China JinPing Underground Laboratory (CJPL). JUNA team has successfully developed a <sc>10 mA</sc> low-energy accelerator and experimental terminal, which aims to conduct experimental research on a number of key nuclear reactions in hydrostatic hydrogen and helium burnings of stars in an underground environment with very low background. Since its operation in the underground laboratory at the end of 2020, JUNA has completed the first phase of experimental tasks and successfully studied several key nuclear reactions including the neutron source reaction¹³C(α, n)¹⁶O. In this paper, we review the motivation of the key neutron source ²²Ne(α, n)²⁵Mg in the origin of heavy elements and in the massive stars, the latest research progress of this neutron source and future research plan, especially the underground direct measurement experimental plan. As the key neutron source reaction of the s-process, the ²²Ne(α, n)²⁵Mg reaction is of great significance for understanding the origin of heavy elements in the universe. As the first experiment in the second phase of JUNA, the ²²Ne(α, n)²⁵Mg reaction is expected to achieve world-class research results and provides the crucial nuclear input for understanding the nucleosynthesis in weak s-process.

Bonn Optimized Stellar Tracks (BoOST)

Massive and very massive stars can play important roles in stellar populations by ejecting strong stellar winds and exploding in energetic phenomena. It is therefore imperative that their behavior be properly accounted for in synthetic model populations. We present nine grids of stellar evolutionary model sequences, together with finely resolved interpolated sequences and synthetic populations, of stars with 9–500M⊙and with metallicities ranging from Galactic metallicity down to 1/250Z⊙. The stellar models were computed with the Bonn evolutionary code with consistent physical ingredients, and covering core hydrogen- and core helium-burning phases. The interpolation and population synthesis were performed with our newly developed routineSYNSTARS. Eight of the grids represent slowly rotating massive stars with a normal or classical evolutionary path, while one grid represents fast-rotating, chemically homogeneously evolving models. The grids contain data on stellar wind properties such as estimated wind velocity and kinetic energy of the wind, as well as common stellar parameters such as mass, radius, surface temperature, luminosity, mass-loss rate, and surface abundances of 34 isotopes. We also provide estimates of the helium and carbon-oxygen core mass for calculating the mass of stellar remnants. The Bonn Optimized Stellar Tracks (BoOST) project is published as simple tables that include stellar models, interpolated tracks, and synthetic populations. Covering the broadest mass and metallicity range of any published massive star evolutionary model sets to date, BoOST is ideal for further scientific applications such as star formation studies in both low- and high-redshift galaxies.

Formation of GW190521 from stellar evolution: the impact of the hydrogen-rich envelope, dredge-up, and 12C(α, γ)16O rate on the pair-instability black hole mass gap

ABSTRACT Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between ≈40–65 and ≈120 M⊙. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of $85^{+21}_{-14}$ M⊙. Here, we investigate the main uncertainties on the PI mass gap: the 12C(α, γ)16O reaction rate and the H-rich envelope collapse. With the standard 12C(α, γ)16O rate, the lower edge of the mass gap can be 70 M⊙ if we allow for the collapse of the residual H-rich envelope at metallicity Z ≤ 0.0003. Adopting the uncertainties given by the starlib database, for models computed with the 12C(α, γ)16O rate $-1\, \sigma$, we find that the PI mass gap ranges between ≈80 and ≈150 M⊙. Stars with MZAMS &amp;gt; 110 M⊙ may experience a deep dredge-up episode during the core helium-burning phase, that extracts matter from the core enriching the envelope. As a consequence of the He-core mass reduction, a star with MZAMS = 160 M⊙ may avoid the PI and produce a BH of 150 M⊙. In the $-2\, {}\sigma {}$ case, the PI mass gap ranges from 92 to 110 M⊙. Finally, in models computed with 12C(α, γ)16O $-3\, {}\sigma {}$, the mass gap is completely removed by the dredge-up effect. The onset of this dredge-up is particularly sensitive to the assumed model for convection and mixing. The combined effect of H-rich envelope collapse and low 12C(α, γ)16O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.

Separation between RR Lyrae and type II Cepheids and their importance for a distance determination: the case of omega Cen

The separation between RR Lyrae (RRLs) and type II Cepheid (T2Cs) variables based on their period is debated. Both types of variable stars are distance indicators, and we aim to promote the use of T2Cs as distance indicators in synergy with RRLs. We adopted new and existing optical and near-infrared (NIR) photometry ofωCen to investigate several diagnostics (color-magnitude diagram, Bailey diagram, Fourier decomposition of the light curve, and amplitude ratios) for their empirical separation. We found that the classical period threshold at one day is not universal and does not dictate the evolutionary stage: V92 has a period of 1.3 days but is likely to be still in its core helium-burning phase, which is typical of RRLs. We also derived NIR period-luminosity relations and found a distance modulus of 13.65 ± 0.07 (err.) ± 0.01 (σ) mag, in agreement with the recent literature. We also found that RRLs and T2Cs obey the same period-luminosity relations in the NIR. This equivalence provides the opportunity of adopting RRLs+T2Cs as an alternative to classical Cepheids to calibrate the extragalactic distance scale.

Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA

Abstract We conduct a rigorous examination of the nearby red supergiant Betelgeuse by drawing on the synthesis of new observational data and three different modeling techniques. Our observational results include the release of new, processed photometric measurements collected with the space-based Solar Mass Ejection Imager instrument prior to Betelgeuse’s recent, unprecedented dimming event. We detect the first radial overtone in the photometric data and report a period of 185 ± 13.5 days. Our theoretical predictions include self-consistent results from multi-timescale evolutionary, oscillatory, and hydrodynamic simulations conducted with the Modules for Experiments in Stellar Astrophysics software suite. Significant outcomes of our modeling efforts include a precise prediction for the star’s radius: . In concert with additional constraints, this allows us to derive a new, independent distance estimate of pc and a parallax of mas, in good agreement with Hipparcos but less so with recent radio measurements. Seismic results from both perturbed hydrostatic and evolving hydrodynamic simulations constrain the period and driving mechanisms of Betelgeuse’s dominant periodicities in new ways. Our analyses converge to the conclusion that Betelgeuse’s ≈400 day period is the result of pulsation in the fundamental mode, driven by the κ-mechanism. Grid-based hydrodynamic modeling reveals that the behavior of the oscillating envelope is mass-dependent, and likewise suggests that the nonlinear pulsation excitation time could serve as a mass constraint. Our results place α Orionis definitively in the early core helium-burning phase of the red supergiant branch. We report a present-day mass of 16.5–19 M ⊙—slightly lower than typical literature values.

Fitting formulae for evolution tracks of massive stars under extreme metal-poor environments for population synthesis calculations and star cluster simulations

ABSTRACT We have devised fitting formulae for evolution tracks of massive stars with 8 ≲ M/M⊙ ≲ 160 under extreme metal-poor (EMP) environments for log (Z/Z⊙) = −2, −4, −5, −6, and −8, where M⊙ and Z⊙ are the solar mass and metallicity, respectively. Our fitting formulae are based on reference stellar models which we have newly obtained by simulating the time evolutions of EMP stars. Our fitting formulae take into account stars ending with blue supergiant (BSG) stars, and stars skipping Hertzsprung gap phases and blue loops, which are characteristics of massive EMP stars. In our fitting formulae, stars may remain BSG stars when they finish their core Helium burning phases. Our fitting formulae are in good agreement with our stellar evolution models. We can use these fitting formulae on the sse, bse, nbody4, and nbody6 codes, which are widely used for population synthesis calculations and star cluster simulations. These fitting formulae should be useful to make theoretical templates of binary black holes formed under EMP environments.

Theoretical investigation of the Humphreys–Davidson limit at high and low metallicity

Context. Current massive star evolution grids are not able to simultaneously reproduce the empirical upper luminosity limit of red supergiants, the Humphrey–Davidson (HD) limit, nor the blue-to-red (B/R) supergiant ratio at high and low metallicity. Although previous studies have shown that the treatment of convection and semi-convection plays a role in the post-main-sequence (MS) evolution to blue or red supergiants (RSGs), a unified treatment for all metallicities has not been achieved so far. Aims. We focus on developing a better understanding of what drives massive star evolution to blue and red supergiant phases, with the ultimate aim of reproducing the HD limit at varied metallicities. We discuss the consequences of classifying B and R in the B/R ratio and clarify what is required to quantify a relatable theoretical B/R ratio for comparison with observations. Methods. For solar, Large Magellanic Cloud (50% solar), and Small Magellanic Cloud (20% solar) metallicities, we develop eight grids of MESA models for the mass range 20–60 M⊙ to probe the effect of semi-convection and overshooting on the core helium-burning phase. We compare rotating and non-rotating models with efficient (αsemi = 100) and inefficient semi-convection (αsemi = 0.1), with high and low amounts of core overshooting (αov of 0.1 or 0.5). The red and blue supergiant evolutionary phases are investigated by comparing the fraction of core He-burning lifetimes spent in each phase for a range of masses and metallicities. Results. We find that the extension of the convective core by overshooting αov = 0.5 has an effect on the post-MS evolution that can disable semi-convection, leading to more RSGs, but a lack of BSGs. We therefore implement αov = 0.1, which switches on semi-convective mixing, but for standard αsemi = 1 would result in an HD limit that is higher than observed at low Z (Large and Small Magellanic Clouds). Therefore, we need to implement very efficient semi-convection of αsemi = 100, which reproduces the HD limit at log L/L⊙ ∼ 5.5 for the Magellanic Clouds while simultaneously reproducing the Galactic HD limit of log L/L⊙ ∼ 5.8 naturally. The effect of semi-convection is not active at high metallicities because the envelope structure is depleted by strong mass loss such that semi-convective regions could not form. Conclusions. Metallicity-dependent mass loss plays an indirect, yet decisive role in setting the HD limit as a function of Z. For a combination of efficient semi-convection and low overshooting with standard Ṁ(Z), we find a natural HD limit at all metallicities.

Core–Envelope Coupling in Intermediate-mass Core-helium Burning Stars

Abstract Stars between two and three solar masses rotate rapidly on the main sequence, and the detection of slow core and surface rotation in the core-helium burning phase for these stars places strong constraints on their angular momentum transport and loss. From a detailed asteroseismic study of the mixed-dipole mode pattern in a carefully selected, representative sample of stars, we find that slow core rotation rates in the range reported by prior studies are a general phenomenon and not a selection effect. We show that the core rotation rates of these stars decline strongly with decreasing surface gravity during the core He-burning phase. We argue that this is a model-independent indication of significant rapid angular momentum transport between the cores and envelopes of these stars. We see a significant range in core rotation rates at all surface gravities, with little evidence for a convergence toward a uniform value. We demonstrate using evolutionary models that measured surface rotation periods are a biased tracer of the true surface rotation distribution, and we argue for using stellar models for interpreting the contrast between core and surface rotation rates. The core rotation rates we measure do not have a strong mass or metallicity dependence. We argue that the emerging data strongly favor a model where angular momentum transport is much more efficient during the core He-burning phase than in the shell-burning phases that precede and follow it.

Type IIP Supernova Progenitors and Their Explodability. I. Convective Overshoot, Blue Loops, and Surface Composition

Abstract We present the evolution of massive star progenitors of supernovae of type IIP. We take the example of the nearby and well-studied SN 2013ej. We explore how convective overshoot affects the stellar structure, surface abundances, and effective temperature of massive stars, using the Modules for Experiments in Stellar Astrophysics. In particular, models with moderate overshoot (f = 0.02–0.031) show the presence of blue loops in the Hertzsprung–Russell diagram with a red to blue excursion (log10[T eff/K] from &lt;3.6 to &gt;4.0) and transition back to red, during the core helium-burning phase. Models with overshoot outside this range of f values kept the star in the red supergiant state throughout the post-helium-ignition phases. The surface CNO abundance shows enrichment post-main-sequence and again around the time when helium is exhausted in the core. These evolutionary changes in surface CNO abundance are indistinguishable in the currently available observations due to large observational uncertainties. However, these observations may distinguish between the ratio of surface nitrogen to oxygen at different evolutionary stages of the star. We also compare the effects of convective overshoot on various parameters related to likelihood of explosion of a star as opposed to collapse to a black hole. These parameters are the compactness parameter, M 4, and μ 4. The combination μ 4 × M 4, and μ 4 have similar variations with f and both peak at f = 0.032. We find that all of our 13 models are likely to explode.

Grids of stellar models with rotation

The effects of rotation on stellar evolution are particularly important at low metallicity, when mass loss by stellar winds diminishes and the surface enrichment due to rotational mixing becomes relatively more pronounced than at high metallicities. Here we investigate the impact of rotation and metallicity on stellar evolution. Using similar physics as in our previous large grids of models at Z = 0.002 and Z = 0.014, we compute stellar evolution models with the Geneva code for rotating and nonrotating stars with initial masses (Mini) between 1.7 and 120 M⊙ and Z = 0.0004 (1/35 solar). This is comparable to the metallicities of the most metal poor galaxies observed so far, such as I Zw 18. Concerning massive stars, both rotating and nonrotating models spend most of their core-helium burning phase with an effective temperature higher than 8000 K. Stars become red supergiants only at the end of their lifetimes, and few red supergiants are expected. Our models predict very few to no classical Wolf–Rayet stars as a results of weak stellar winds at low metallicity. The most massive stars end their lifetimes as luminous blue supergiants or luminous blue variables, a feature that is not predicted by models with higher initial metallicities. Interestingly, due to the behavior of the intermediate convective zone, the mass domain of stars producing pair-instability supernovae is smaller at Z = 0.0004 than at Z = 0.002. We find that during the main sequence (MS) phase, the ratio between nitrogen and carbon abundances (N/C) remains unchanged for nonrotating models. However, N/C increases by factors of 10–20 in rotating models at the end of the MS. Cepheids coming from stars with Mini &gt; 4 − 6 M⊙ are beyond the core helium burning phase and spend little time in the instability strip. Since they would evolve towards cooler effective temperatures, these Cepheids should show an increase of the pulsation period as a function of age.

Low-metallicity massive single stars with rotation

Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z⊙) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20−130 M⊙) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf–Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.

Constraining transport of angular momentum in stars

Context. Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. Aims. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M⊙ stars. Methods. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M⊙. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. Results. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity νadd around 107 cm2 s−1 provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the νadd during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.