Helium Core Mass Research Articles

1100 days in the life of the supernova 2018ibb

Stars with zero-age main sequence masses between 140 and 260 M⊙ are thought to explode as pair-instability supernovae (PISNe). During their thermonuclear runaway, PISNe can produce up to several tens of solar masses of radioactive nickel, resulting in luminous transients similar to some superluminous supernovae (SLSNe). Yet, no unambiguous PISN has been discovered so far. SN 2018ibb is a hydrogen-poor SLSN at z = 0.166 that evolves extremely slowly compared to the hundreds of known SLSNe. Between mid 2018 and early 2022, we monitored its photometric and spectroscopic evolution from the UV to the near-infrared (NIR) with 2–10 m class telescopes. SN 2018ibb radiated > 3 × 1051 erg during its evolution, and its bolometric light curve reached > 2 × 1044 erg s−1 at its peak. The long-lasting rise of > 93 rest-frame days implies a long diffusion time, which requires a very high total ejected mass. The PISN mechanism naturally provides both the energy source (56Ni) and the long diffusion time. Theoretical models of PISNe make clear predictions as to their photometric and spectroscopic properties. SN 2018ibb complies with most tests on the light curves, nebular spectra and host galaxy, and potentially all tests with the interpretation we propose. Both the light curve and the spectra require 25–44 M⊙ of freshly nucleosynthesised 56Ni, pointing to the explosion of a metal-poor star with a helium core mass of 120–130 M⊙ at the time of death. This interpretation is also supported by the tentative detection of [Co II] λ 1.025 μm, which has never been observed in any other PISN candidate or SLSN before. We observe a significant excess in the blue part of the optical spectrum during the nebular phase, which is in tension with predictions of existing PISN models. However, we have compelling observational evidence for an eruptive mass-loss episode of the progenitor of SN 2018ibb shortly before the explosion, and our dataset reveals that the interaction of the SN ejecta with this oxygen-rich circumstellar material contributed to the observed emission. That may explain this specific discrepancy with PISN models. Powering by a central engine, such as a magnetar or a black hole, can be excluded with high confidence. This makes SN 2018ibb by far the best candidate for being a PISN, to date.

Predicting the heaviest black holes below the pair instability gap

ABSTRACT Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M⊙, with stellar mass black holes (BHs) expected to ‘pile up’ towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an ‘impossibly’ heavy BH of 85 M⊙ as part of GW 190521 located inside the traditional PI gap, we realized that blue supergiant (BSG) progenitors with small cores but large hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed mesa stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semiconvection, and Z. We evolve low Z stars in the range 10−3 < Z/Z⊙ < ZSMC, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass-loss recipe to calculate it. We compute critical carbon–oxygen and helium core masses to determine our lower limit to PI physics, and we provide two equations for Mcore and Mfinal that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is MBH ≃ 93.3 M⊙. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the traditional boundary is not solely due to the shape of the initial mass function, but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.

Considering the Single and Binary Origins of the Type IIP SN 2017eaw

Current population synthesis modeling suggests that 30%–50% of Type II supernovae originate from binary progenitors; however, the identification of a binary progenitor is challenging. One indicator of a binary progenitor is that the surrounding stellar population is too old to contain a massive single star. Measurements of the progenitor mass of SN 2017eaw are starkly divided between observations made temporally close to core collapse, which show a progenitor mass of 13–15 M ⊙ (final helium-core mass MHe,core=4.4–6.0M⊙ —which is a more informative property than initial mass) and those from the stellar population surrounding the SN, which find M ≤ 10.8 M ⊙ ( MHe,core≤3.4M⊙ ). In this paper, we reanalyze the surrounding stellar population with improved astrometry and photometry, finding a median age of 16.8−1.0+3.2 Myr for all stars younger than 50 Myr ( MHe,core=4.7M⊙ ) and 85.9−6.5+3.2 Myr for stars younger than 150 Myr. 16.8 Myr is now consistent with the helium-core mass range derived from the temporally near-explosion observations for single stars. Applying the combined constraints to population synthesis models, we determine that the probability of the progenitor of SN 2017eaw being an initially single star is 65% compared to 35% for prior binary interaction. 85.9 Myr is inconsistent with any formation scenarios. We demonstrate that combining progenitor age constraints with helium-core mass estimates from red supergiant SED modeling, late-time spectra, and indirectly from light-curve modeling can help to differentiate single and binary progenitor scenarios and provide a framework for the application of this technique to future observations.

The white dwarf mass–orbital period relation under wind mass-loss

ABSTRACT Helium white dwarfs (HeWDs) are thought to form from low-mass red giant stars experiencing binary interaction. Because the helium core mass of a red giant star is closely related to the stellar radius, there exists well-known relation between the orbital period (Porb) and the mass (MWD) of the HeWDs, which is almost independent of the type of the companion star. Traditional derivation of the MWD–Porb relation generally neglected the effect of wind mass-loss from the red giants, while observations show that wind mass-loss from red giants in binary systems is systematically higher than that from isolated stars. In this work, we calculate binary evolution with tidally enhanced stellar wind (TEW) and find that it causes significantly scatter of the traditional MWD–Porb relation. The TEW can prevent the red giants from overflowing their Roche lobes and slow down the growth of the helium core, leaving a lower mass HeWD for given orbital period. This scenario may account for some of the HeWD binaries that deviate from the traditional MWD–Porb relation. However, we point out that observations of more HeWD binaries in wide orbits are needed to test the TEW model and to constrain the enhanced wind factor.

Asteroseismic Modeling of 1153 Kepler Red Giant Branch Stars: Improved Stellar Parameters with Gravity-mode Period Spacings and Luminosity Constraints

This paper reports the estimated stellar parameters of 1153 Kepler red giant branch stars determined with asteroseismic modeling. We use radial-mode oscillation frequencies, gravity-mode period spacings, Gaia luminosities, and spectroscopic data to characterize these stars. Compared with previous studies, we find that the two additional observed constraints, i.e., the gravity-mode period spacing and luminosity, significantly improve the precision of fundamental stellar parameters. The typical uncertainties are 2.9% for the mass, 11% for the age, 1.0% for the radius, 0.0039 dex for the surface gravity, and 0.5% for the helium core mass, making this the best-characterized large sample of red giant stars available to date. With better characterizations for these red giants, we recalibrate the seismic scaling relations and study the surface term on the red giant branch. We confirm that the surface term depends on the surface gravity and effective temperature, but there is no significant correlation with metallicity.

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.

Shell helium-burning hot subdwarf B stars as candidates for blue large-amplitude pulsators

Blue large-amplitude pulsators (BLAPs) are a newly discovered type of variable star. Their typical pulsation periods are on the order of a few tens of minutes, with relatively large amplitudes of 0.2–0.4 mag in optical bands, and their rates of period changes are on the order of 10−7 yr−1 (both positive and negative). They are extremely rare objects and attempts to explain their origins and internal structures have attracted a great deal of attention. Previous studies have proposed that BLAPs may be pre-white dwarfs, with masses around 0.3 M⊙, or core-helium-burning stars in the range of ∼0.7 − 1.1 M⊙. In this work, we use a number of MESA models to compute and explore whether BLAPs could be explained as shell helium-burning subdwarfs type B (SHeB sdBs). The models that best match existing observational constraints have helium core masses in the range of ∼0.45 − 0.5 M⊙. Our model predicts that the positive rate of period change may evolve to negative. The formation channels for SHeB sdBs involve binary evolution and although the vast majority of BLAPs do not appear to be binaries (with the exception of HD 133729), the observational constraints are still very poor. Motivated by these findings, we explored the Roche lobe overflow channel. Of the 304 binary evolution models we computed, about half of them are able to produce SHeB sdBs in long-period binaries that evade detection from the limited observations that are currently available.

Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3 M ⊙ whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer () beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10−2–1 M ⊙) and radii (∼1013–1016 cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr.

SN 1961V: A Pulsational Pair-instability Supernova

We explore a variety of models in which SN 1961V, one of the most enigmatic supernovae (SNe) ever observed, was a pulsational pair-instability supernova (PPISN). Successful models reproduce the bolometric light curve of the principal outburst and, in some cases, the emission 1 yr before and several years afterward. All models have helium-rich ejecta, bulk hydrogenic velocities near 2000 km s−1, and total kinetic energies of (4−8) × 1050 erg. Each eventually leaves behind a black hole remnant. Three subclasses of PPISN models are explored, each with two different choices of carbon abundance following helium burning. Carbon is an important parameter because shell carbon burning can weaken the explosion. The three subclasses correspond to situations where SN 1961V and its immediate afterglow were (a) a single event, (b) the first of two or more pulsational events separated by decades or centuries, or (c) the latter stages of a complex explosion that had already been going on for a year or more. For the low-carbon case, the main-sequence mass for SN 1961V’s progenitor would have been 100−115 M ⊙, its pre-SN helium core mass was 45−52 M ⊙, and the final black hole mass was 40−45 M ⊙. For the high-carbon case, these values are increased by roughly 20%−25%. In some PPISN models, a ∼1040 erg s−1 star-like object could still be shining at the site of SN 1961V, but it has more likely been replaced by a massive accreting black hole.

On the Diversity of Mixing and Helium Core Masses of B-type Dwarfs from Gravity-mode Asteroseismology

The chemical evolution of the galaxy is largely guided by the yields from massive stars. Their evolution is heavily influenced by their internal mixing, allowing the stars to live longer and yield a more massive helium core at the end of their main-sequence evolution. Asteroseismology is a powerful tool for studying stellar interiors by providing direct probes of the interior physics of the oscillating stars. This work revisits the recently derived internal mixing profiles of 26 slowly pulsating B stars observed by the Kepler space telescope, in order to investigate how well the mixing profiles can in fact be distinguished from one another as well as provide predictions for the expected helium core masses obtained at the end of the main-sequence evolution. We find that for five of these stars the mixing profile is derived unambiguously, while the remaining stars have at least one other mixing profile which explains the oscillations equally well. Convective penetration is preferred over exponential diffusive overshoot for ≈55% of the stars, while stratified mixing is preferred in the envelope (≈39%). We estimate the expected helium core masses obtained at the end of the main-sequence evolution and find them to be highly influenced by the estimated amount of mixing occurring in the envelopes of the stars.

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.

Comparing compact object distributions from mass- and presupernova core structure-based prescriptions

ABSTRACT Binary population synthesis (BPS) employs prescriptions to predict final fates, explosion or implosion, and remnant masses based on one or two stellar parameters at the evolutionary cutoff imposed by the code, usually at or near central carbon ignition. In doing this, BPS disregards the integral role late-stage evolution plays in determining the final fate, remnant type, and remnant mass within the neutrino-driven explosion paradigm. To highlight differences between a popular prescription, which relies only on the core and final stellar mass and emerging methods, which rely on a star’s presupernova core structure, we generate a series of compact object distributions using three different methods for a sample population of single and binary stars computed in BPASS. The first method estimates remnant mass based on a star’s carbon–oxygen (CO) core mass and final total mass. The second method uses the presupernova core structure based on recent bare CO-core models combined with a parameterized explosion criterion to first determine final fate and remnant type, then remnant mass. The third method associates presupernova helium-core masses with remnant masses determined from public explosion models which rely implicitly on core structure. We find that the core-/final mass-based prescription favours lower mass remnants, including a large population of mass gap black holes, and predicts neutron star masses which span a wide range, whereas the structure-based prescriptions favour slightly higher mass remnants, mass gap black holes only as low as 3.5 M⊙, and predict NS mass distributions which cluster in a narrow range.

The influence of initial orbital period on helium and carbon-oxygen core masses in massive case a binary systems with low accretion efficiency

The evolutionary models of 33 massive Case A binary systems in mass range from 30M? to 40M? with initial orbital periods of 3, 4 and 5 days, accretion efficiency of 10% and at the solar metallicity are presented. The models are obtained with the MESA (Modules for Experiments in Stellar Astrophysics) numerical code. The evolution is followed from a double O-type star, through Case A and Case mass transfer, to the formation of a carbon-oxygen (CO) core in the primary. The evolution of the secondary star in each binary system is further modeled with the same numerical code in an approximation of a single star, also to the formation of a carbon-oxygen core. The resulting helium core masses are in the range of 7.94M? - 13.19M? and 12.30M? and 19.12M? for primary and secondary stars, respectively. The carbon-oxygen core masses are between 5.26M? and 10M? for primaries and between 8.96M? and 15.32M? for secondaries. A clear influence of the initial orbital period on the resulting helium and CO core masses is demonstrated: primary stars in binary systems with initial orbital periods of 3, 4 and 5 days have on average about 15%, 8% and 2.5% smaller CO cores than single stars with the same initial masses. On the other hand, it was found that the correlation between the CO and helium core mass does not depend on the initial orbital period and can be approximated with the same linear fit for all binary systems. The CO/helium core mass ratio is found to be larger in binary systems than for single stars. It is also shown that the black hole formation limit for primary stars depends on the initial orbit and is between 33M? - 34M?, 32M? - 33M? and 30M? - 31M?, for the initial orbital periods of 3, 4 and 5 days, respectively. The resulting double compact objects are of two types: mixed neutron star - black hole systems (6 models) and double black holes (27 models). The resulting black hole masses are estimated to be in the range of 5M? to 17M?.

Evaluation of thermal- nuclear effects from pair-creation in the final fate very-massive stars

Thermonuclear conditions found in explosive massive-stars requirethe use of not only efficient, accurate but thermodynamically consistent stellar equation of state (EOS) routines.The use of tables to describe EoS involved in stellar models is very much needed in understanding the final fate of massive stars. Many massive-low metallicity stars end their life as pair creation supernova (PCSN) through the creation of electron-positron pairs.We used thermodynamically consistent EoS tables to numerically evaluate the thermonuclear effects of the electron electron-positron pair creation in rotating 150 and 200 Massive starsat SMC and rotating and non-rotating 500 M⊙at LMC.As expected, the effect of rotationofreducing the oxygen core masshad increasedthe thermal energy within the threshold of the pair-creation instability.Similarly, lower mass loss stars with SMC model produced higher thermal energies,which can cmpletely explode the stars as PCSNe without remnant.On the other hand, the non-rotating 500 M⊙ might have only reached the instability region due to its lower metallicity (compared to solar metallicity) that iscapable of suppressing the mass loss such that the thermonuclear energy maintains certain amount of elements into the pair creation region. At the final explosion of the stars, the helium core mass educed the thermal energies in trying to avoid the pair-creation region. Many implications of these results for the evolution and explosion of massive stars are discussed.

Dark matter capture and annihilation in stars: Impact on the red giant branch tip

Context.While stars have often been used as laboratories to study dark matter (DM), red giant branch (RGB) stars and all the rich phenomenology they encompass have frequently been overlooked by such endeavors.Aims.We study the capture, evaporation, and annihilation of weakly interacting massive particle (WIMP) DM in low-mass RGB stars (M = 0.8−1.4 M⊙).Methods.We used a modified stellar evolution code to study the effects of DM self-annihilation on the structure and evolution of low-mass RGB stars.Results.We find that the number of DM particles that accumulate inside low-mass RGB stars is not only constant during this phase of evolution, but also mostly independent of the stellar mass and to some extent stellar metallicity. Moreover, we find that the energy injected into the stellar core due to DM annihilation can promote the conditions necessary for helium burning and thus trigger an early end of the RGB phase. The premature end of the RGB, which is most pronounced for DM particles withmχ ≃ 100 GeV, is thus achieved at a lower helium core mass, which results in a lower luminosity at the tip of the red giant branch (TRGB). Although in the current WIMP paradigm, these effects are only relevant if the number of DM particles inside the star is extremely large, we find that for light WIMPs (mχ ≃ 4 GeV), relevant deviations from the standard TRGB luminosity (∼8%) can be achieved with conditions that can be realistic in the inner parsec of the Milky Way.

Predictions for the hydrogen-free ejecta of pulsational pair-instability supernovae

Present and upcoming time-domain astronomy efforts, in part driven by gravitational-wave follow-up campaigns, will unveil a variety of rare explosive transients in the sky. Here, we focus on pulsational pair-instability evolution, which can result in signatures that are observable with electromagnetic and gravitational waves. We simulated grids of bare helium stars to characterize the resulting black hole (BH) masses together with the ejecta composition, velocity, and thermal state. We find that the stars do not react “elastically” to the thermonuclear ignition in the core: there is not a one-to-one correspondence between pair-instability driven ignition and mass ejections, which causes ambiguity as to what is an observable pulse. In agreement with previous studies, we find that for initial helium core masses of 37.5 M⊙ ≲ MHe, init ≲ 41 M⊙, corresponding to carbon-oxygen core masses 27.5 M⊙ ≲ MCO ≲ 30.1 M⊙, the explosions are not strong enough to affect the surface. With increasing initial helium core mass, they become progressively stronger causing first large radial expansion (41 M⊙ ≲ MHe, init ≲ 42 M⊙, corresponding to 30.1 M⊙ ≲ MCO ≲ 30.8 M⊙) and, finally, also mass ejection episodes (for MHe, init ≳ 42 M⊙, or MCO ≳ 30.8 M⊙). The lowest mass helium core to be fully disrupted in a pair-instability supernova is MHe, init ≃ 80 M⊙, corresponding to MCO ≃ 55 M⊙. Models with MHe, init ≳ 200 M⊙ (MCO ≳ 114 M⊙) reach the photodisintegration regime, resulting in BHs with masses of MBH ≳ 125 M⊙. Although this is currently considered unlikely, if BHs from these models form via (weak) explosions, the previously-ejected material might be hit by the blast wave and convert kinetic energy into observable electromagnetic radiation. We characterize the hydrogen-free circumstellar material from the pulsational pair-instability of helium cores by simply assuming that the ejecta maintain a constant velocity after ejection. We find that our models produce helium-rich ejecta with mass of 10−3 M⊙ ≲ MCSM ≲ 40 M⊙, the larger values corresponding to the more massive progenitor stars. These ejecta are typically launched at a few thousand km s−1 and reach distances of ∼1012 − 1015 cm before the core-collapse of the star. The delays between mass ejection events and the final collapse span a wide and mass-dependent range (from subhour to 104 years), and the shells ejected can also collide with each other, powering supernova impostor events before the final core-collapse. The range of properties we find suggests a possible connection with (some) type Ibn supernovae.

Properties of OB star−black hole systems derived from detailed binary evolution models

Context. The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify the contribution of binary evolution to the formation of double black holes. Aims. A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples, such as the population in 30 Doradus in the Large Magellanic Cloud (LMC). Methods. With this purpose in mind, we analysed a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40 M⊙, and identified the model systems that potentially evolve into a binary consisting of a black hole and a massive main-sequence star. We then derived the observable properties of such systems, as well as peculiarities of the OB star component. Results. We find that ∼3% of the LMC late-O and early-B stars in binaries are expected to possess a black hole companion when stars with a final helium core mass above 6.6 M⊙ are assumed to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these black holes may be identified in spectroscopic binaries, either by large amplitude radial velocity variations (≳50 km s−1) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity (≳10 km s−1) and simultaneous rapid rotation of the OB star. The predicted mass ratios are such that main-sequence companions can be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be and X-ray binaries, and known massive black hole binaries supports our conclusion. Conclusions. We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general and for the formation of double black hole mergers in particular.

The mass discrepancy in intermediate- and high-mass eclipsing binaries: The need for higher convective core masses

Context. Eclipsing, spectroscopic double-lined binary star systems are excellent laboratories for calibrating theories of stellar interior structure and evolution. Their precise and accurate masses and radii measured from binary dynamics offer model-independent constraints and challenge current theories of stellar evolution. Aims. We aim to investigate the mass discrepancy in binary stars. This is the significant difference between stellar components’ masses measured from binary dynamics and those inferred from models of stellar evolution via positions of the components in the Teff − log g Kiel diagram. We study the effect of near-core mixing on the mass of the convective core of the stars and interpret the results in the context of the mass discrepancy. Methods. We fitted stellar isochrones computed from a grid of MESA stellar evolution models to a homogeneous sample of eleven high-mass binary systems. Two scenarios are considered where individual stellar components of a binary system are treated independent of each other and where they are forced to have the same age and initial chemical composition. We also study the effect of the microturbulent velocity and turbulent pressure on the atmosphere model structure and stellar spectral lines, and its link with the mass discrepancy. Results. We find that the mass discrepancy is present in our sample and that it is anti-correlated with the surface gravity of the star. No correlations are found with other fundamental and atmospheric parameters, including the stellar mass. The mass discrepancy can be partially accounted for by increasing the amount of near-core mixing in stellar evolution models. We also find that ignoring the microturbulent velocity and turbulent pressure in stellar atmosphere models of hot evolved stars results in the overestimation of their effective temperature by up to 8%. Together with enhanced near-core mixing, this can almost entirely account for the ∼30% mass discrepancy found for the evolved primary component of V380 Cyg. Conclusions. We find a strong link between the mass discrepancy and the convective core mass. The mass discrepancy can be solved by considering the combined effect of extra near-core boundary mixing and the consistent treatment in the spectrum analysis of hot evolved stars. Our binary modelling results in convective core masses between 17 and 35% of the stellar mass, which is in excellent agreement with the results from gravity-mode asteroseismology of single stars. This implies larger helium core masses near the end of the main sequence than have been anticipated so far.

Determining the size of the helium core of KIC 9970396 using asteroseismology: a red giant approaching the red giant bump

ABSTRACT Mixed modes are less affected by the surface effect and carry internal information of a star. Asteroseismic modelling of mixed modes can constrain the size of the helium core of a star. Previous work has shown that the evolution stage of KIC 9970396 may be near the red giant bump phase, which is one of the most interesting regions in the red giant evolution stage. It is known that there is a clear relation between the core and mass luminosity, following homological principles on the low-luminosity giant branch. Our aim in this paper is to determine the size of the helium core and constrain the internal structure of the star. Our approach is first to identify the most p-dominated frequencies in the observational mixed modes. The calculated frequencies are then fitted to the observational frequencies, which we choose not to correct for surface effect offsets. We have computed a grid of theoretical models to fit the l = 1 modes of KIC 9970396. The size of the helium core of KIC 9970396 is determined to be MHe = 0.229 ± 0.001 M⊙ and RHe = 0.03055 ± 0.00015 R⊙. The best-fitting model shows that KIC 9970396 is undergoing a stage that is between the first dredge-up event and the red giant bump. Compared with the results of our model, the previous core–mass luminosity relation overestimates the helium core mass by 0.009 M⊙ when the stellar luminosity of a red giant star is known.

The uncertain masses of progenitors of core-collapse supernovae and direct-collapse black holes

ABSTRACT We show that it is not possible to determine the final mass Mfin of a red supergiant (RSG) at the pre-supernova (SN) stage from its luminosity L and effective temperature Teff alone. Using a grid of stellar models, we demonstrate that for a given value of L and Teff, an RSG can have a range of Mfin as wide as 3 to 45 M⊙. While the probability distribution within these limits is not flat, any individual determination of Mfin for an RSG will be degenerate. This makes it difficult to determine its evolutionary history and to map Mfin to an initial mass. Single stars produce a narrower range that is difficult to accurately determine without making strong assumptions about mass-loss, convection, and rotation. Binaries would produce a wider range of RSG Mfin. However, the final Helium core mass $M_{\operatorname{He-core}}$ is well determined by the final luminosity and we find $\log (M_{\operatorname{He-core}}/\mathrm{M}_{\odot }) = 0.659 \log (L/\mathrm{L}_{\odot }) -2.630$. Using this relationship, we derive $M_{\operatorname{He-core}}$ for directly imaged SN progenitors and one failed SN candidate. The value of Mfin for stripped star progenitors of SNe IIb is better constrained by L and Teff due to the dependence of Teff on the envelope mass Menv for Menv ≲ 1 M⊙. Given the initial mass function, our results apply to the majority of progenitors of core-collapse SNe, failed SNe, and direct-collapse black holes.