Evolution Of Massive Stars Research Articles

A Metallicity Dependence on the Occurrence of Core-collapse Supernovae

Core-collapse supernovae (CCSNe) are widely accepted to be caused by the explosive death of massive stars with initial masses ≳8 M ⊙. There is, however, a comparatively poor understanding of how properties of the progenitors—mass, metallicity, multiplicity, rotation, etc.—manifest in the resultant CCSN population. Here, we present a minimally biased sample of nearby CCSNe from the All-Sky Automated Survey for Supernovae survey whose host galaxies were observed with integral-field spectroscopy using MUSE at the Very Large Telescope. This data set allows us to analyze the explosion sites of CCSNe within the context of global star formation properties across the host galaxies. We show that the CCSN explosion site oxygen abundance distribution is offset to lower values than the overall H ii region abundance distribution within the host galaxies. We further split the sample at dex and show that within the subsample of low-metallicity host galaxies, the CCSNe unbiasedly trace the star formation with respect to oxygen abundance, while for the subsample of higher-metallicity host galaxies, they preferentially occur in lower-abundance star-forming regions. We estimate the occurrence of CCSNe as a function of oxygen abundance per unit star formation and show that there is a strong decrease as abundance increases. Such a strong and quantified metallicity dependence on CCSN production has not been shown before. Finally, we discuss possible explanations for our result and show that each of these has strong implications not only for our understanding of CCSNe and massive star evolution but also for star formation and galaxy evolution.

Modeling of the High-velocity Jet Powered by the Massive Star MWC 349A

MWC 349A is a massive star with a well-known circumstellar disk rotating following a Keplerian law, and an ionized wind launched from the disk surface. Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) carried out toward this system, however, have revealed an additional high-velocity component in the strong, maser emission of hydrogen radio recombination lines (RRLs), suggesting the presence of a high-velocity ionized jet. In this work, we present 3D non-LTE radiative transfer modeling of the emission of the H30α and H26α maser lines, and of their associated radio continuum emission, toward MWC 349A. By using the MORELI code, we reproduce the spatial distribution and kinematics of the high-velocity emission of the H30α and H26α maser lines with a high-velocity ionized jet expanding at a velocity of ∼250 km s−1, surrounded by MWC 349A’s wide-angle ionized wind. The bipolar jet, which is launched from MWC 349A’s disk, is poorly collimated and slightly misaligned with respect to the disk rotation axis. Thanks to the unprecedented sensitivity and spatial accuracy provided by ALMA, we also find that the already known, wide-angle ionized wind decelerates as it expands radially from the ionized disk. We briefly discuss the implications of our findings for understanding the formation and evolution of massive stars. Our results show the huge potential of RRL masers as powerful probes of the innermost ionized regions around massive stars and of their high-velocity jets.

Carbon burning at stellar energies

Fusion reactions with light nuclei play an essential role in understanding the energy production, the nucleosynthesis of chemical elements and the evolution of massive stars. The measurement of key fusion reactions at stellar energies is thus of interest, but highly challenging since the associated cross sections are extremely small, of the sub-nanobarn range. Among these reactions, the fusion of carbon nuclei, which drives the stellar carbon burning phase, is deeply connected with essential microscopic features such as the impact of symmetries, the access to quantum states, emerging of resonances or Pauli repulsion effects. These may manifest themselves in exceptional behaviour of the S-factor of this particular system and the precision of extrapolations to deep sub-barrier energies is limited. The present contribution discusses recent experimental results of the nuclear astrophysics community on the measurement of the carbon + carbon fusion reaction down to the astrophysics region. The interplay between nuclear structure, nucleosynthesis and stellar evolution is addressed.

The B-type Binaries Characterisation Programme – II. VFTS 291: a stripped star from a recent mass transfer phase

ABSTRACT Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity (K1 = 93.7 ± 0.2 km s−1). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ∼1.5–2.5 $\, \mathrm{M}_\odot$ dominating the spectrum, and an early B-type main-sequence companion of 13.2 ± 1.5 $\, \mathrm{M}_\odot$. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 $\, \mathrm{M}_\odot$ primary with an 8 $\, \mathrm{M}_\odot$ companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.

Clumping and X-rays in cooler B supergiant stars

Context. B supergiants (BSGs) are evolved stars with effective temperatures between ~10 to ~30 kK. Knowing the properties of these objects is important to understand massive star evolution. Located on the cool end of the line-driven wind regime, the study of their atmospheres can help us to understand the physics of their winds and phenomena such as the bi-stability jump. Aims. Despite being well-studied stars, key UV features of their spectra have so far not been reproduced by atmosphere models for spectral types later than B1. In this study, we aim to remedy this situation by performing quantitative spectral analyzes that account for the effects of X-rays and clumping in the wind. In addition, we also briefly investigate the evolutionary status of our sample stars based on the stellar parameters we obtained. Methods. We determined photospheric and wind parameters via quantitative spectroscopy using atmosphere models computed with CMFGEN and PoWR. These models were compared to high-resolution UV and optical spectra of four BSGs: HD206165, HD198478, HD53138, and HD164353. We further employed GENEC and MESA tracks to study the evolutionary status of our sample. Results. When including both clumping and X-rays, we obtained a good agreement between synthetic and observed spectra for our sample stars. For the first time, we reproduced key wind lines in the UV, where previous studies were unsuccessful. To model the UV spectra, we require a moderately clumped wind (fV∞ ≳ 0.5). We also infer a relative X-ray luminosity of about 10−7.5 to 10−8, which is lower than the typical ratio of 10−7. Moreover, we find a possible mismatch between evolutionary mass predictions and the derived spectroscopic masses, which deserves deeper investigation as this might relate to the mass-discrepancy problem present in other types of OB stars. Conclusions. Our results provide direct spectroscopic evidence that both X-rays and clumping need to be taken into account to describe the winds of cool BSGs. However, their winds seem to be much less structured than in earlier OB-type stars. Our findings are in line with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The evolutionary status of BSGs seems to be diverse with some objects potentially being post-red supergiants or merger products. The obtained wind parameters provide evidence for a moderate increase of the mass-loss rate around the bi-stability jump.

Fundamental parameters of two O9-type giant stars: the (former) spectral classification standard HD 93249 A and ALS 12502 A

ABSTRACT The evolution of massive stars is not completely understood. Several phenomena affect their birth, life, and death, multiplicity being one of them. In this context, the OWN and MONOS projects are systematically observing O- and WN-type stars whose multiplicity status is unknown. Their major goal considers the necessity of determining absolute parameters of massive stars. We have collected spectra of HD 93249 A and ALS 12502 A aiming at characterizing their binary nature. For both stars, we analysed high-resolution spectra and combined them with Transiting Exoplanet Survey Satellite (TESS) observations to be compared with binary models constructed by means of the phoebe code. We discovered that the radial velocity of HD 93249 A varies with a period of 2.97968 ± 0.00001 d and that the system presents ellipsoidal light variations. We disentangled the composite spectra and classified its components as O9 III and B1.5 III, respectively. Confirmed as a spectroscopic binary, HD 93249 A can no longer be used as spectral classification standard. ALS 12502 A turned out to be a detached eclipsing binary in the TESS and Gaia data. These results enable us to determine absolute parameters for each component in the system.

Energy rates due to Fe isotopes during presupernova evolution of massive stars

This work presents the microscopic calculation of energy rates (γ-ray heating and (anti)neutrino cooling rates) due to weak decay of selected Fe Isotopes. The isotopes have astrophysical significance during the presupernova evolution of massive stars. The energy rates are calculated using the pn-QRPA model and compared with the independent particle model (IPM), large scale shell model (LSSM) and recent shell model calculation (GXPF1J). The reported (anti)neutrino cooling rates are smaller by up to two orders of magnitude at low core temperature values than the IPM rates. The two calculations compare well at T = 30 GK. The comparison of cooling rates with the LSSM is interesting. The pn-QRPA cooling rates due to even-even Fe isotopes are smaller (up to 2 orders of magnitude). For the odd-A isotopes, the reported rates are bigger up to an order of magnitude. The pn-QRPA computed cooling rates are, up to 2 orders of magnitude, bigger when compared with the GXPF1J calculation. The γ-ray heating rates due to electron capture rates rise with the temperature and density values of the stellar core. On the other hand, the γ-ray heating due to β-decay increases with the core temperature values but decreases by orders of magnitude when the stellar core stiffens. The pn-QRPA computed γ heating rates are bigger (up to 3 orders of magnitude) at high temperatures and densities (for the case of 55,56Fe) when compared with the recent shell model results. Owing to the importance of energy rates, this study may contribute to a realistic simulation of presupernova evolution of massive stars.

Modelling stellar evolution in mass-transferring binaries and gravitational-wave progenitors with metisse

ABSTRACT Massive binaries are vital sources of various transient processes, including gravitational-wave mergers. However, large uncertainties in the evolution of massive stars, both physical and numerical, present a major challenge to the understanding of their binary evolution. In this paper, we upgrade our interpolation-based stellar evolution code metisse to include the effects of mass changes, such as binary mass transfer or wind-driven mass loss, not already included within the input stellar tracks. metisse’s implementation of mass loss (applied to tracks without mass loss) shows excellent agreement with the sse fitting formulae and with detailed mesa tracks, except in cases where the mass transfer is too rapid for the star to maintain equilibrium. We use this updated version of metisse within the binary population synthesis code bse to demonstrate the impact of varying stellar evolution parameters, particularly core overshooting, on the evolution of a massive (25 and 15 M⊙) binary system with an orbital period of 1800 d. Depending on the input tracks, we find that the binary system can form a binary black hole or a black hole–neutron star system, with primary (secondary) remnant masses ranging between 4.47 (1.36) and 12.30 (10.89) M⊙, and orbital periods ranging from 6 d to the binary becoming unbound. Extending this analysis to a population of isolated binaries uniformly distributed in mass and orbital period, we show that the input stellar models play an important role in determining which regions of the binary parameter space can produce compact binary mergers, paving the way for predictions for current and future gravitational-wave observatories.

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

Context. Massive stars lose a large fraction of their mass to radiation-driven winds throughout their entire life. These outflows impact both the life and death of these stars and their surroundings. Aims. Theoretical mass-loss rates of hot, massive stars are derived to be used in applications such as stellar evolution. The behaviour of these rates in the OB-star regime is analysed, and their effects on massive-star evolution predictions studied. Methods. Dynamically consistent models are computed by solving the spherically symmetric, steady-state equation-of-motion for a large grid of hot, massive stars with different metallicities. The radiative acceleration is derived from non-local thermodynamic equilibrium radiative transfer in the co-moving frame, and all models cover a large spatial range from deep subsonic atmospheric layers into the radiation-driven and highly supersonic wind outflow. The resulting mass-loss rates are used to derive a simple scaling recipe with stellar parameters (luminosity, mass, effective temperature, and metallicity), and the new recipe is used to evaluate some first impacts upon massive-star evolution tracks. Results. We provide a new prescription for steady-state, radiation-driven mass-loss from hot, massive stars depending on their fundamental parameters. In accordance with our previous work, the rates for O stars are lower a factor of ~3 than the rates typically used in previous stellar-evolution calculations, where differences generally decrease with increasing luminosity and temperature. For cooler B giants/supergiants we find larger discrepancies, of up to one or even two orders of magnitude. This arises because we do not find any systematic increase in mass-loss rates below the so-called bi-stability region; indeed, our results do not show any sign of a significant bi-stability jump within the parameter range covered by the grid (Teff ≥ 15 kK). Due to the lower mass-loss rates we find that massive-star envelopes are not easily stripped by means of standard steady-state winds, making it difficult to create classical Wolf-Rayet stars via this channel. Moreover, since the stars retain more mass right before they die as supernovae, our new rates make it possible to create black holes of higher masses than in previous models, even at Galactic metallicity. However, a remaining key uncertainty regarding these predictions concerns unsteady mass-loss for very high-luminosity stars close to the Eddington limit as well as the impact of non-line-driven winds.

Event Rate of Strongly Lensed Gravitational Waves of Stellar Binary Black Hole Mergers Produced by Dynamical Interactions

Gravitational waves emitted from stellar binary black hole (sBBH) mergers can be gravitationally lensed by intervening galaxies and detected by future ground-based detectors. A great amount of effort has been put into the estimation of the detection rate of lensed sBBH originating from the evolution of massive binary stars (EMBS channel). However, sBBHs produced by the dynamical interaction in dense clusters (dynamical channel) may also be dominant in our universe and their intrinsic distribution of physical properties can be significantly different from those produced by massive stars, especially mass and redshift distribution. In this paper, we investigate the event rate of lensed sBBHs produced via the dynamical channel by Monte Carlo simulations and the number is yr−1 for the Einstein telescope and yr−1 for Cosmic Explorer, of which the median is about ∼2 times the rate of sBBHs originating from the EMBS channel (calibrated by the local merger rate density estimated for the dynamical and the EMBS channel, i.e., and , respectively). Therefore, one may constrain the fraction of both the EMBS and dynamical channels through the comparison of the predicted and observed number of lensed sBBH events statistically.

Validity of Brink-Axel Hypothesis for calculations of allowed stellar weak rates of heavy nuclei

The knowledge of beta-decay transitional probabilities and Gamow-Teller (GT) strength functions from highly excited states of nuclides is of particular importance for applications to astrophysical network calculations of nucleosynthesis in explosive stellar events. These quantities are challenging to achieve from measurements or computations using various nuclear models. Due to unavailability of feasible alternatives, many theoretical studies often rely on the Brink-Axel (BA) hypothesis, that is, the response of strength functions depends merely on the transition energy of the parent nuclear ground state and is independent of the underlying details of the parent state, for the calculation of stellar rates. BA hypothesis has been used in many applications from nuclear structure determination to nucleosynthesis yield in the astrophysical matter. We explore here the the validity of BA hypothesis in the calculation of stellar beta-decay (BD) and electron capture (EC) weak rates of fp- and fpg-shell nuclides for GT transitions. Strength functions have been computed employing the fully microscopic proton-neutron QRPA (quasi-particle random-phase approximation) within a broad density, ρY e = (10-1011) [g cm−3], and temperature, T = (1−30) [GK], grid relevant to the pre-collapse astrophysical environment. Our work provides evidence that the use of the approximation based on the BA hypothesis does not lead to reliable calculations of excited states strength functions under extreme temperature-density conditions characteristic of presupernova and supernova evolution of massive stars. Weak rates obtained by incorporating the BA hypothesis in the calculation of strength functions substantially deviate from the rates based on the state-by-state microscopically calculated strength functions. Deviation in the two calculations becomes significant as early as neon burning phases of massive stars. The deviation in the calculation of BD rates is even more pronounced, reaching up to three orders of magnitude.

A rare outburst from the stealthy BeXRB system Swift J0549.7−6812

ABSTRACT Swift J0549.7−6812 is a Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting an ∼6 s pulse period. Like many such systems, the variable X-ray emission is believed to be driven by the underlying behaviour of the mass donor Be star. In this paper, we report on X-ray observations of the brightest known outburst from this system, which reached a luminosity of ∼8 × 1037 erg s−1. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from Optical Gravitational Lens Explorer (OGLE) and Gaia. The latter strongly suggest a binary period of 46.1 d. All the observational data indicate that Swift J0549.7−6812 is a system that spends the vast majority of its time in X-ray quiescence, or even switched off completely. This suggests that occasional observations may easily miss it, and many similar systems, and thereby underestimate the massive star evolution numbers for the LMC.

Connecting the young pulsars in Milky Way globular clusters with white dwarf mergers and the M81 fast radio burst

ABSTRACT The detections of four apparently young radio pulsars in the Milky Way globular clusters are difficult to reconcile with standard neutron star formation scenarios associated with massive star evolution. Here, we discuss formation of these young pulsars through white dwarf mergers in dynamically old clusters that have undergone core collapse. Based on observed properties of magnetic white dwarfs, we argue neutron stars formed via white dwarf merger are born with spin periods of roughly $10{\!-\!}100\,$ ms and magnetic fields of roughly $10^{11}{\!-\!}10^{13}\,$ G. As these neutron stars spin down via magnetic dipole radiation, they naturally reproduce the four observed young pulsars in the Milky Way clusters. Rates inferred from N-body cluster simulations as well as the binarity, host cluster properties, and cluster offsets observed for these young pulsars hint further at a white dwarf merger origin. These young pulsars may be descendants of neutron stars capable of powering fast radio bursts analogous to the bursts observed recently in a globular cluster in M81.

Discovering New B[e] Supergiants and Candidate Luminous Blue Variables in Nearby Galaxies

Mass loss is one of the key parameters that determine stellar evolution. Despite the progress we have achieved over the last decades we still cannot match the observational derived values with theoretical predictions. Even worse, there are certain phases, such as the B[e] supergiants (B[e]SGs) and the Luminous Blue Variables (LBVs), where significant mass is lost through episodic or outburst activity. This leads to various structures forming around them that permit dust formation, making these objects bright IR sources. The ASSESS project aims to determine the role of episodic mass in the evolution of massive stars, by examining large numbers of cool and hot objects (such as B[e]SGs/LBVs). For this purpose, we initiated a large observation campaign to obtain spectroscopic data for ∼1000 IR-selected sources in 27 nearby galaxies. Within this project we successfully identified seven B[e] supergiants (one candidate) and four Luminous Blue Variables of which six and two, respectively, are new discoveries. We used spectroscopic, photometric, and light curve information to better constrain the nature of the reported objects. We particularly noted the presence of B[e]SGs at metallicity environments as low as 0.14 Z⊙.

Compact object mergers: exploring uncertainties from stellar and binary evolution with sevn

ABSTRACT Population-synthesis codes are an unique tool to explore the parameter space of massive binary star evolution and binary compact object (BCO) formation. Most population-synthesis codes are based on the same stellar evolution model, limiting our ability to explore the main uncertainties. Here, we present the new version of the code sevn, which overcomes this issue by interpolating the main stellar properties from a set of pre-computed evolutionary tracks. We describe the new interpolation and adaptive time-step algorithms of sevn, and the main upgrades on single and binary evolution. With sevn, we evolved 1.2 × 109 binaries in the metallicity range 0.0001 ≤ Z ≤ 0.03, exploring a number of models for electron-capture, core-collapse and pair-instability supernovae, different assumptions for common envelope, stability of mass transfer, quasi-homogeneous evolution, and stellar tides. We find that stellar evolution has a dramatic impact on the formation of single and BCOs. Just by slightly changing the overshooting parameter (λov = 0.4, 0.5) and the pair-instability model, the maximum mass of a black hole can vary from ≈60 to ≈100 M⊙. Furthermore, the formation channels of BCOs and the merger efficiency we obtain with sevn show significant differences with respect to the results of other population-synthesis codes, even when the same binary-evolution parameters are used. For example, the main traditional formation channel of BCOs is strongly suppressed in our models: at high metallicity (Z ≳ 0.01) only <20 per cent of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions >70 per cent.

The IACOB project

Context. Blue supergiants (BSGs) are key objects for studying the intermediate phases of massive star evolution because they are very useful to constrain evolutionary models. However, the lack of a holistic study of a statistically significant and unbiased sample of these objects has lead to several long-standing questions about their physical properties and evolutionary nature to remain unsolved. Aims. This paper and other upcoming papers of the IACOB series are focused on studying from a pure empirical point of view a sample of about 500 Galactic O9–B9 stars with luminosity classes I and II (plus 250 late O- and early B-type stars with luminosity classes III, IV, and V) that cover distances up to ≈4 kpc from the Sun. Methods. We compiled an initial set of ≈11 000 high-resolution spectra from ≈1600 Galactic late O- and B-type stars. We used a novel spectroscopic strategy based on a simple fitting of the Hβ line to select stars in a specific region of the spectroscopic Hertzsprung–Russel diagram. We evaluated the completeness of our sample using the Alma Luminous Star catalog (ALS III) and Gaia-DR3 data. Results. We show the benefits of the proposed strategy for identifying BSGs that are descended in the context of single star evolution from stellar objects that are born as O-type stars. The resulting sample reaches a high level of completeness with respect to the ALS III catalog, gathering ≈80% of all-sky targets brighter than Bmag < 9 located within 2 kpc. However, we identify the need for new observations in specific regions of the southern hemisphere. Conclusions. We have explored a very fast and robust method for selecting BSGs. This provides a valuable tool for large spectroscopic surveys such as WEAVE-SCIP or 4MIDABLE-LR, and it highlights the risk of using spectral classifications from the literature. Upcoming studies will make use of this large and homogeneous spectroscopic sample to study the specific properties of these stars in detail. We initially provide first results for their rotational properties (in terms of projected rotational velocities, v sin i).

Evolution of rotating massive stars with new hydrodynamic wind models

Context. Mass loss due to radiatively line-driven winds is central to our understanding of the evolution of massive stars in both single and multiple systems. This mass loss plays a key role in modulating the stellar evolution at different metallicities, particularly in the case of massive stars with M* ≥ 25 M⊙. Aims. We extend the evolution models introduced in Paper I, where the mass-loss recipe is based on the simultaneous calculation of the wind hydrodynamics and the line acceleration, by incorporating the effects of stellar rotation. Methods. As in Paper I, we introduce a grid of self-consistent line-force parameters (k, α, δ) for a set of standard evolutionary tracks using GENEC. Based on this grid, we analysed the effects of stellar rotation, CNO abundances, and He/H ratio on the wind solutions to derive additional terms for the recipe with which we predict the self-consistent mass-loss rate, Ṁsc. With this, we generated a new set of evolutionary tracks with rotation for MZAMS = 25, 40, 70, and 120 M⊙, and for metallicities Z = 0.014 (Galactic) and 0.006 (Large Magellanic Cloud). Results. In addition to the expected correction factor due to rotation, the mass-loss rate decreases when the surface becomes more helium rich, especially in the later moments of the main-sequence phase. The self-consistent approach gives lower mass-loss rates than the standard values adopted in previous GENEC evolution models. This decrease strongly affects the tracks of the most massive models. Weaker winds allow the star to retain more mass, but also more angular momentum. As a consequence, weaker wind models rotate faster and show a less efficient mixing in their inner stellar structure at a given age. Conclusions. The self-consistent tracks predict an evolution of the rotational velocities through the main sequence that closely agrees with the range of v sin i values found by recent surveys of Galactic O-type stars. As subsequent implications, the weaker winds from self-consistent models also suggest a reduction of the contribution of the isotope 26Al to the interstellar medium due to stellar winds of massive stars during the MS phase. Moreover, the higher luminosities found for the self-consistent evolutionary models suggest that some populations of massive stars might be less massive than previously thought, as in the case of Ofpe stars at the Galactic centre. Therefore, this study opens a wide range of consequences for further research based on the evolution of massive stars.

Nucleosynthesis of Binary-stripped Stars

The cosmic origin of the elements, the fundamental chemical building blocks of the universe, is still uncertain. Binary interactions play a key role in the evolution of many massive stars, yet their impact on chemical yields is poorly understood. Using the MESA stellar evolution code, we predict the chemical yields ejected in wind mass loss and the supernovae of single and binary-stripped stars. We do this with a large 162-isotope nuclear network at solar metallicity. We find that binary-stripped stars are more effective producers of the elements than single stars, due to their increased mass loss and an increased chance to eject their envelopes during a supernova. This increased production by binaries varies across the periodic table, with F and K being more significantly produced by binary-stripped stars than single stars. We find that the 12C/13C could be used as an indicator of the conservativeness of mass transfer, as 13C is preferentially ejected during mass transfer while 12C is preferentially ejected during wind mass loss. We identify a number of gamma-ray-emitting radioactive isotopes that may be used to help constrain progenitor and explosion models of core-collapse supernovae with next-generation gamma-ray detectors. For single stars we find that 44V and 52Mn are strong probes of the explosion model, while for binary-stripped stars it is 48Cr. Our findings highlight that binary-stripped stars are not equivalent to two single stars and that detailed stellar modeling is needed to predict their final nucleosynthetic yields.

Constraints on the Binarity of the WN3/O3 Class of Wolf–Rayet Stars* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

The WN3/O3 Wolf–Rayet (WR) stars were discovered as part of our survey for WRs in the Magellanic Clouds. The WN3/O3s show the emission lines of a high-excitation WN star and the absorption lines of a hot O-type star, but our prior work has shown that the absorption spectrum is intrinsic to the WR star. Their place in the evolution of massive stars remains unclear. Here we investigate the possibility that they are the products of binary evolution. Although these are not WN3+O3 V binaries, they could still harbor unseen companions. To address this possibility, we have conducted a multiyear radial velocity study of six of the nine known WN3/O3s. Our study finds no evidence of statistically significant radial velocity variations, and allows us to set stringent upper limits on the mass of any hypothetical companion star: for probable orbital inclinations, any companion with a period less than 100 days must have a mass <2M ⊙. For periods less than 10 days, any companion would have to have a mass <1M ⊙. We argue that scenarios where any such companion is a compact object are unlikely. The absorption lines indicate a normal projected rotational velocity, making it unlikely that these stars evolved with the aid of a companion star that has since merged. The modest rotation also suggests that these stars are not the result of homogenous evolution. Thus it is likely that these stars are a normal but short-lived stage in the evolution of massive stars.

Observational Properties of 155 O- and B-type Massive Pulsating Stars

O- and B-type (OB-type) pulsating stars are important objects for studying the structure and evolution of massive stars through asteroseismology. A large amount of data from various sky surveys provides an unprecedented opportunity to search for and study this kind of variable star. We identify 155 OB-type pulsating stars or candidates, including 38 Oe/Be stars or candidates, from the data observed by TESS, LAMOST, and Gaia, which are almost new. Among the 155 objects, 87 samples are identified as slowly pulsating B (SPB) stars including 37 objects with pure low-frequency and 50 objects with both low- and high-frequency pulsation, and 14 samples are identified as β Cephei pulsating variable (BCEP) stars with both low- and high-frequency pulsation. An H-R diagram shows that these SPB and BCEP stars are mainly located in their instability regions and in the evolutionary stage of the main sequence with mass ranges of 2.5–20 M ⊙ and 7–20 M ⊙, respectively. Two special objects show Fourier spectra similar to BCEP stars but with different positions in H-R, period–temperature (P-T), and period–luminosity (P-L) diagrams. Meanwhile, 52 other targets are identified as candidates of OB-type pulsating stars. We also derive the preliminary results of the P-L relation for SPB and BCEP stars, respectively. This work also indicates that in addition to the H-R diagram, the P-T and P-L diagrams are also very useful for the classification of SPB and BCEP stars. Further detailed analysis of these objects can dramatically increase our understanding of the theories of evolution and structure for massive OB-type pulsating stars.