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The massive semidetached binary with a long-term decreasing orbital period may involve a rapid mass-transfer phase in Case A, and thus, they are good astrophysical laboratories for investigating the evolution of massive binary stars. In this work, by using the long-term observational light curves from the Optical Gravitational Lensing Experiment project and other data in the low-metallicity Large Magellanic Cloud, four semidetached massive binaries with long-term decreases in the orbital periods are detected from 165 EB-type close binaries. It is found that the more massive component in S07798 is filling its Roche lobe, where the period decrease is caused by mass transfer from the primary to the secondary. However, the other three (S03065, S12631, S16873) are semidetached binaries with a lobe-filling secondary where the mass transfer between the components should cause the period to increase if the angular momentum is conservative. The long-term period decreases in these three systems may be caused by angular momentum loss. Additionally, the orbital periods of three systems (S03065, S07798, S16873) are detected to show cyclic variation with periods shorter than 11 yr, which can be plausibly explained by the presence of close-in third bodies in these massive binaries. Based on all of these results, it is suggested that the detected four semidetached binaries almost have multiplicity. The companion stars are crucial for the origin and evolution of these massive close binaries. [ABSTRACT FROM AUTHOR]

In this short review we present the recent progresses in modelling fast rotating stars in two dimensions. We thus give a brief description of the features of the public domain code ESTER that can compute self-consistently the structure and the large-scale flows (differential rotation and meridional circulation) of an axisymmetric stellar model of a (fast) rotating early-type main sequence star. We illustrate these modelling with the recent results obtained on Altair, a nearby extremely fast rotator. We then discuss the various way mixing takes place in the stably stratified radiative envelope of early-type stars, and especially in massive ones where the radiative winds add a new source of large-scale flows, which are shown to be strongly anisotropic and very difficult to represent in one dimension. [ABSTRACT FROM AUTHOR]

WR 137 (HD 192641) is a binary system consisting of a carbon-rich Wolf–Rayet (W-R) star and an Oe companion star in a 13 yr orbit. Near periastron, the winds of the two stars collide and form carbonaceous dust. We obtained three mid-infrared grism spectra of the system with SOFIA and FORCAST during the last year of SOFIA's operations in 2021 July, 2021 February, and 2022 May (Cycle 9). Within these spectra, we have identified several wind lines from He i, He ii, C iii, and C iv that are emitted from the W-R wind as well as a weak emission feature around 6.3–6.4 μ m that may have shifted its peak flux from 6.29 to 6.41 μ m through this time period. The weak feature grew as the continuum dust emission grew while the W-R emission appeared to decline due to lower contrast with the continuum. Furthermore, we observe that the peak of the feature shifts to redder wavelengths during the observations. We compare this feature to the unidentified infrared feature and other emission lines identified in dusty carbon-rich W-R (WC) binaries. For WR 137, we speculate that mixing of the winds in the system with the Oe star's disk is important for starting the dust formation and that it is less important as dust formation continues. Previous infrared photometry shows "minieruptions" of dust production, which could then be explained with variations of the Oe star disk. [ABSTRACT FROM AUTHOR]

Using a sample of 92 Galactic Be stars, we compare inclination angles (the angle between a star's rotation axis and the line of sight) determined from H α emission-line profile fitting to those determined by the spectroscopic signature of gravitational darkening. We find good agreement: 70% of the sample (64 out of 92 stars) is consistent with zero difference between the two methods using 1 σ errors, and there is a strong linear correlation coefficient between the two methods of r = +0.63 ± 0.05. There is some evidence that the H α profile fitting method overestimates the inclination angle for i ≲ 25°, perhaps due to the neglect of incoherent electron scattering on the H α line widths, while the gravitational darkening method underestimates the inclination angle for i ≳ 70°, perhaps due to the neglect of disk radiative transfer effects on the optical spectrum. Overall, it is demonstrated that a single H α spectrum of modest resolution and SNR can be used to extract a useful estimate for the inclination angle of an individual Be star. This allows equatorial rotation velocities for individual Be stars to be derived from v sin i measurements and will allow Be stars to be used to search for correlated spin axes in young, open clusters if unbiased (with respect to inclination) samples of Be stars are used. [ABSTRACT FROM AUTHOR]

We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (Δ I < 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parameters T eff = 42,000 K, log L / L ⊙ = 5.83 ± 0.15 , M / M ⊙ = 50 ± 9, and v sin i = 370 ± 40 km s−1. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric (e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493's mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors. [ABSTRACT FROM AUTHOR]

The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know that flaring occurs in the stars as hot as B-type ones. However, the structures of atmospheres of hot B-A stars crucially differ from the ones of late types, and thus the occurrence of flaring in B-A type stars requires some extension of our theoretical views of flare formation and therefore a detailed study of individual objects. Here we present the results of our spectral and photometric study of HD 36030, which is a B9 V star with flares detected by the TESS satellite. The spectra we acquired suggest that the star is in a binary system with a low-mass secondary component, but the light curve lacks any signs of periodic variability related to orbital motion or surface magnetic fields. Because of that, we argue that the flares originate due to magnetic interaction between the components of the system. [ABSTRACT FROM AUTHOR]

The massive semidetached binary with a long-term decreasing orbital period may involve a rapid mass-transfer phase in Case A, and thus, they are good astrophysical laboratories for investigating the evolution of massive binary stars. In this work, by using the long-term observational light curves from the Optical Gravitational Lensing Experiment project and other data in the low-metallicity Large Magellanic Cloud, four semidetached massive binaries with long-term decreases in the orbital periods are detected from 165 EB-type close binaries. It is found that the more massive component in S07798 is filling its Roche lobe, where the period decrease is caused by mass transfer from the primary to the secondary. However, the other three (S03065, S12631, S16873) are semidetached binaries with a lobe-filling secondary where the mass transfer between the components should cause the period to increase if the angular momentum is conservative. The long-term period decreases in these three systems may be caused by angular momentum loss. Additionally, the orbital periods of three systems (S03065, S07798, S16873) are detected to show cyclic variation with periods shorter than 11 yr, which can be plausibly explained by the presence of close-in third bodies in these massive binaries. Based on all of these results, it is suggested that the detected four semidetached binaries almost have multiplicity. The companion stars are crucial for the origin and evolution of these massive close binaries.

In this work, we update the catalog of OB stars based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope data release 7 and modify the OB stars’ selection criteria in spectral line indices’ space. The new catalog includes 37,778 spectra of 27,643 OB stars, of which 3827 OB stars are newly identified. The spectral subclasses of 27,643 OB stars are obtained using the automatic classification code MKCLASS. We find that the modified OB star selection criteria can better improve the completeness of late B-type stars by analyzing their spectral classification results given by MKCLASS. We also identify 3006 Be-type stars or candidates by examining the Balmer lines in their spectra and find that the frequency of our Be-type stars (10.9%) is consistent with previous results. The spatial distribution of OB stars indicates that they are mainly located in the Galactic disk. This new catalog of OB stars will provide valuable data for studying the structure and evolution of the Milky Way.

The spin–orbit alignment of binary stars traces their formation and accretion history. Previous studies of spin–orbit alignment have been limited to small samples, slowly rotating solar-type stars, and/or wide visual binaries that not surprisingly manifest random spin–orbit orientations. We analyze 917 Gaia astrometric binaries across periods P = 100–3000 days ( a = 0.5–5 au) that have B8-F1 IV/V primaries ( M _1 = 1.5–3 M _⊙ ) and measured projected rotational velocities v sin i . The primary stars in face-on orbits exhibit substantially smaller v sin i compared to those in edge-on orbits at the 6 σ level, demonstrating significant spin–orbit alignment. The primaries in our astrometric binaries are rotating more slowly than their single-star or wide-binary counterparts and therefore comprise the slow-rotator population in the observed bimodal rotational velocity distribution of early-type stars. We discuss formation models of close binaries where some of the disk angular momentum is transferred to the orbit and/or secondary spin, quenching angular momentum flow to the primary spin. The primaries in astrometric binaries with small mass ratios q = M _2 / M _1 < 0.3 possess even smaller v sin i , consistent with model predictions. Meanwhile, astrometric binaries with large eccentricities e > 0.4 do not display spin–orbit alignment or spin reduction. Using a Monte Carlo technique, we measure a spin–orbit alignment fraction of F _align = 75% ± 5% and an average spin reduction factor of 〈 S _align 〉 = 0.43 ± 0.04. We conclude that 75% of close A-type binaries likely experienced circumbinary disk accretion and probably formed via disk fragmentation and inward disk migration. The remaining 25%, mostly those with e > 0.4, likely formed via core fragmentation and orbital decay via dynamical friction.

Main-sequence radio pulse emitters (MRPs) are magnetic early-type stars that produce coherent radio emission observed in the form of periodic radio pulses. The emission mechanism behind this is the electron-cyclotron maser emission (ECME). Among all kinds of magnetospheric emission, ECME is unique due to its high directivity and intrinsically narrow bandwidth. The emission is also highly circularly polarized and the sign of polarization is opposite for the two magnetic hemispheres. This combination of properties makes ECME highly sensitive to the three-dimensional structures in the stellar magnetospheres. This is especially significant for late-B and A-type magnetic stars that do not emit other types of magnetospheric emission such as H α , the key probe used to trace magnetospheric densities. In this paper, we use an ultra-wideband observation (0.4–2 GHz) of a late B-type MRP HD 133880 to demonstrate how we can extract information on plasma distribution from ECME. We achieve this by examining the differences in pulse arrival times (“lags”) as a function of frequencies and qualitatively comparing those with lags obtained by simulating ECME ray paths in hot stars’ magnetospheres. This reveals that the stellar magnetosphere has a disk-like overdensity inclined to the magnetic equator with a centrally concentrated density that primarily affects the intermediate frequencies (400–800 MHz). This result, which is consistent with the recent density model proposed for hotter centrifugally supported magnetospheres, lends support to the idea of a unifying model for magnetospheric operations in early-type stars, and also provides further motivation to fully characterize the ECME phenomenon in large-scale stellar magnetospheres.

In order to explore how the ubiquitous stochastic low-frequency (SLF) variability of O-type stars is related to various stellar characteristics, we compiled a sample of 150 O-type stars observed via ground-based spectroscopic surveys, alongside photometric data obtained from the Transiting Exoplanet Survey Satellite (TESS). We analyzed 298 light curves obtained from TESS Sectors 1–65 for the stars in our sample. Leveraging the spectroscopic parameters, we used Bonnsai to determine masses, radii, fractional main-sequence ages, and mass-loss rates for stars of our sample. Subsequently, we identified possible correlations between the fitted parameters of SLF variability and stellar properties. Our analysis unveiled four significant correlations between the amplitude and stellar parameters, including mass, radius, fractional main-sequence ages, and mass-loss rate. For stars with ≳30 M _⊙ , we observed a decrease in characteristic frequency and steepness with increasing radius. Finally, we compared various physical processes that may account for the SLF variability with our results. The observed SLF variability may arise from the combined effects of the iron convection zone (FeCZ) and internal gravity waves (IGWs), with IGWs potentially more dominant in the early stages of stellar evolution, and the contribution of FeCZ becoming more significant as stars evolve. Meanwhile, our results indicate that the SLF variability of O-type stars bears certain signatures of the line-driven wind instability and granulation.

The detection of O- and B-type stars with extremely low-mass companions is very important for understanding the formation and evolution of binary stars. However, finding them remains a challenge because the low-mass components in such systems contribute such small flux to the total. During our search for pulsations among O- and B-type stars using the Transiting Exoplanet Survey Satellite (TESS) data, we found two short-period and B-type (B9) eclipsing binaries with orbital periods of 1.61613 and 2.37857 days. Photometric solutions of the two close binaries were derived by analyzing the TESS light curves with the Wilson–Devinney method. It is discovered that both of them are detached binaries with extremely low mass ratios of 0.067(2) for TIC 260342097 and 0.140(3) for TIC 209148631. The determined mass ratio indicates that TIC 260342097 is one of the lowest mass ratios among known B-type binary systems. We showed that the two systems have total eclipses with a broad and flat secondary minimum, suggesting that the photometric parameters could be derived reliably. The absolute parameters of the two binaries are estimated and it is found that the secondary components in the two systems are overluminous and oversize when compared with the normal low-mass and cool main-sequence (MS) stars. These findings may imply that the two systems are composed of a B-type MS primary and a cool pre-MS secondary with orbital periods shorter than 2.5 days. They are valuable targets to test theories of binary star formation and evolution.

UV wind-line variability in OB stars appears to be universal. In order to quantify this variation and to estimate its effect on a mass-loss rate determined from a single observation, we use the International Ultraviolet Explorer archive to identify nonpeculiar OB stars with well-developed but unsaturated Si iv λ 1400 doublets and at least 10 independent observations. This resulted in 1699 spectra of 25 stars. A simple model was used to translate the observed profile variations into optical depth variations and, hence, variations in measured mass-loss rates. These variations quantify the intrinsic error inherent in any single mass-loss rate derived from a single observation. The derived rates have an overall 1 σ variation of about 22%, but this appears to differ with T _eff , being as small at 8% for the hottest stars and up to 45% for the cooler ones. Furthermore, any single determination can differ from the mean by a factor of 2 or more. Our results also imply that mass-loss rates determined from nonsimultaneous observations (such as UV and ground-based data) need not agree. In addition, we use our results to examine the nature of the structures responsible for the variability. Our findings suggest that the optical depth variations result from optically very thick structures occulting more or less of the line of sight to the stellar disk. Further, the smaller optical depth variations in the hottest stars suggest that the structures responsible for the variations are disrupted in their more powerful winds.

The Tarantula Nebula (30 Doradus, 30 Dor) is the most important star-forming complex in the Local Group, offering a microscope on starburst astrophysics. At its heart lies the exceptionally rich young stellar cluster R136, containing the most massive stars known. Stellar winds and supernovae have carved 30 Dor into an amazing display of arcs, pillars, and bubbles. We present first results and advanced data-processing products from the 2 Ms Chandra X-ray Visionary Project, “The Tarantula—Revealed by X-rays” (T-ReX). The 3615 point sources in the T-ReX catalog include massive stars, compact objects, binaries, bright pre-main-sequence stars, and compact young stellar (sub)clusters in 30 Dor. After removing point sources and excluding the exceptionally bright supernova remnant N157B (30 Dor B), the global diffuse X-ray maps reveal hot plasma structures resolved at 1–10 pc scales, with an absorption-corrected total-band (0.5–7 keV) X-ray luminosity of 2.110 × 10 ^37 erg s ^−1 . Spatially resolved spectral modeling provides evidence for emission lines enhanced by charge-exchange processes at the interfaces. We identify a candidate for the oldest X-ray pulsar detected to date in 30 Dor, PSR J0538-6902, inside a newly resolved arcuate X-ray wind nebula, the Manta Ray. The long temporal baseline of T-ReX allowed monitoring of dozens of massive stars, several showing periodic variability tied to binary orbital periods, and captured strong flares from at least three low-mass Galactic foreground stars.

The measurement of atmospheric parameters is fundamental for scientific research using stellar spectra. The Chinese Space Station Telescope (CSST), scheduled to be launched in 2024, will provide researchers with hundreds of millions of slitless spectra for stars during a 10 yr survey. And machine learning has unparalleled efficiency in processing large amounts of data compared to manual processing. Here we studied the stellar parameters of early-type stars (effective temperature T _eff > 15,000 K) based on the design indicators of the CSST slitless spectrum and the machine learning algorithm, Stellar LAbel Machine. We used the Potsdam Wolf–Rayet (POWR) synthetic spectra library for cross validation. Then we tested the reliability of machine learning results by using the Next Generation Spectrum Library (NGSL) from Hubble Space Telescope observation data. In an ideal case for full-wavelength spectra without any noise, the average absolute value of the relative deviation is 2.7% (800 K) for T _eff , and 3.5% (0.11 c.g.s) for surface gravity $\mathrm{log}\ g$ . We use the spectra with the impact of interstellar extinction (AV = 0, 0.5, 1, 1.5, 2 mag) and radial velocity (RV = −50, −30, 0, 30, 50 km s ^−1 ) from the POWR library as the test set. When RV = 0 km s ^−1 and AV = 0 mag, the average value and standard deviation for 3 wavelength ranges (2550–4050 Å ( R = 287); 4050–6300 Å ( R = 232); 6300–10000 Å ( R = 207)) are –66 ± 3351 K, 550 ± 3536 K, and 356 ± 3616 K for T _eff , and 0.004 ± 0.224 c.g.s, –0.024 ± 0.246 c.g.s, and 0.01 ± 0.212 c.g.s for $\mathrm{log}\ g$ . When using the observed data from NGSL as the testing samples, the deviation of T _eff is less than 5% (1500 K), and the deviation of $\mathrm{log}\ g$ is less than 11% (0.33 c.g.s). In addition, we also test the influence of shifting of spectra on the parameters’ accuracy. The deviation of T _eff for the case with a shift of 5 Å and 10 Å are 3.6% (1100 K) and 4.3% (1300 K), respectively; the deviation of $\mathrm{log}\ g$ are 4.2% (0.13 c.g.s) and 5.1% (0.15 c.g.s). These results demonstrate that we can obtain relatively accurate stellar parameters of a population of early-type stars with the CSST slitless spectra and a machine-learning method.

Runaway stars are OB-type stars ejected from their birthplace with large peculiar velocities. The leading hypothesis addressed in their formation includes the supernova ejection mechanism and the dynamic ejection scenario. Identification of runaway populations is the first step to investigating their formation and evolution. Here we present our work of searching for Galactic runaway candidate stars from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) Medium Resolution Survey Data Release 8 (DR8) database. After studying the kinematic properties for a collection of 4432 early-type stars, predominantly B-type stars, using radial velocity measurements from LAMOST DR8 and astrometric solutions made by Gaia Data Release 3, we identified 229 runaway candidate stars. They span a wide distribution in projected rotational velocities. We investigated the Galactic spatial distribution of the runaway population and noticed that most of them likely reside within the Galactic thin disk. Based upon analyzing the Doppler shifts of the candidate stars, we found two binary runaway candidates displaying velocity variations with estimated orbital periods of 40 and 61 days.

We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period of P _A = 3.2254367 days that forms part of a higher-order multiple system in a configuration of (A+B)+C. We derived masses of the Aa+Ab binary of M _1 = 19.02 ± 0.12 and M _2 = 17.50 ± 0.13 M _ ⊙ , the radii of R _1 = 7.70 ± 0.05 and R _2 = 6.64 ± 0.06 R _⊙ , and temperatures of T _1 = 34,250 ± 500 K and T _2 = 33,750 ± 500 K. From the analysis of the radial velocities, we found a spectroscopic orbit of A in the outer A+B system with P _A+B = 195.8 days ( P _A+B / P _A ≈ 61). In the O − C analysis, we confirmed this orbit and found another component orbiting the A+B system with P _AB+C = 2550 days ( P _AB+C / P _A+B ≈ 13). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, M _B ≳ 10.7 M _⊙ . From the light travel time effect fit to the O − C data, we obtained the limit for the mass of the fourth component, M _C ≳ 7.3 M _⊙ . These extra components contribute about 20%–30% (increasing with wavelength) to the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 ± 0.06 mag, a reddening of 0.16 ± 0.02 mag, and an R _V of 3.2. This work is part of our ongoing project, which aims to calibrate the surface brightness–color relation for early-type stars.

The measurements of masses and luminosities of massive stars play an important role in understanding the formation and evolution of their host galaxies. In this work, we present the measurement of masses and luminosities of 2946 OB-type stars, including 78 O-type stars and 2868 B-type stars, based on their stellar parameters (effective temperature, surface gravity, and metallicity) and the Padova and Trieste Stellar Evolution Code isochrones model. Our results show that the median mass and luminosity of the 2946 OB-type stars are 5.4 M _⊙ and log( L / L _⊙ ) = 3.2 with median relative errors of 21.4% and 71.1%, respectively. A good agreement between our results estimated by using our method and those derived by using the orbital motions of binary stars from the literature is found for some B-type stars. In addition, we also fit the mass–luminosity relation of B-type stars by using our derived mass and the luminosity from Gaia Data Release 3.

We report metallicities for three ∼Gyr-old stars in the Milky Way nuclear star cluster (NSC) using high-resolution near-infrared spectroscopy. We derive effective temperatures from a calibration with Sc line strength, which yields results in good agreement with other methods, and metallicities from spectral fits to Fe i lines. Our derived metallicities range from −1.2 < [Fe/H] < + 0.5, a span of 1.7 dex. In addition we use isochrone projection to obtain masses of 1.6–4.3 M _⊙ , and ages assuming single-star evolution. The oldest of these stars is 1.5 Gyr while the youngest and most metal-rich is only 100 Myr. The wide range in metallicity poses interesting questions concerning the chemical evolution and enrichment of the NSC and adds to the evidence for the presence of a young, metal-rich population in the NSC. We suggest that the candidate intermediate-age, metal-poor ([Fe/H] = −1.2) star may be best explained as a blue straggler from an underlying old population.

We perform spectral fittings for O-type stars based on self-consistent wind solutions, providing Ṁ and ν(r directly derived from the initial stellar parameters. We introduce our two methods: m-CAK prescription and Lambert-procedure. The Lambert-procedure allows the calculation of consistent v (r) that reduce the number of free parameters when a spectral fitting using CMFGEN is performed, even without recalculation of the Ṁ. Spectra calculated from our Lambert-solutions show significant differences compared to the initial β -law CMFGEN models. For m-CAK prescription, self-consistent solutions provide values for theoretical Ṁ on the order of the most recent predictions from other studies. Later, we find a global fit with the RT code FASTWIND. This is an important step towards the determination of stellar and wind parameters without using β -law. Our m-CAK prescription is valid for the O-type stars with T eff ≥ 30 kK and log g ≥ 3.2. We expect that solutions introduced here to be extended to numerous studies about massive stars in future. [ABSTRACT FROM AUTHOR]

The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know that flaring occurs in the stars as hot as B-type ones. However, the structures of atmospheres of hot B-A stars crucially differ from the ones of late types, and thus the occurrence of flaring in B-A type stars requires some extension of our theoretical views of flare formation and therefore a detailed study of individual objects. Here we present the results of our spectral and photometric study of HD 36030, which is a B9 V star with flares detected by the TESS satellite. The spectra we acquired suggest that the star is in a binary system with a low-mass secondary component, but the light curve lacks any signs of periodic variability related to orbital motion or surface magnetic fields. Because of that, we argue that the flares originate due to magnetic interaction between the components of the system.

In this short review we present the recent progresses in modelling fast rotating stars in two dimensions. We thus give a brief description of the features of the public domain code ESTER that can compute self-consistently the structure and the large-scale flows (differential rotation and meridional circulation) of an axisymmetric stellar model of a (fast) rotating early-type main sequence star. We illustrate these modelling with the recent results obtained on Altair, a nearby extremely fast rotator. We then discuss the various way mixing takes place in the stably stratified radiative envelope of early-type stars, and especially in massive ones where the radiative winds add a new source of large-scale flows, which are shown to be strongly anisotropic and very difficult to represent in one dimension. [ABSTRACT FROM AUTHOR]

Massive stars (∼8–25 M _⊙ ) stripped of their hydrogen-rich envelopes via binary interaction are thought to be the main progenitors for merging neutron stars and stripped-envelope supernovae. We recently presented the discovery of the first set of such stripped stars in a companion paper. Here, we fit the spectra of 10 stars with new atmosphere models in order to constrain their stellar properties precisely. We find that the stellar properties align well with the theoretical expectations from binary evolution models for helium-core burning envelope-stripped stars. The fits confirm that the stars have high effective temperatures ( T _eff ∼ 50–100 kK), high surface gravities ( $\mathrm{log}g\sim $ 5), and hydrogen-poor/helium-rich surfaces ( X _H,surf ∼ 0–0.4) while showing for the first time a range of bolometric luminosities (10 ^3 –10 ^5 L _⊙ ), small radii (∼0.5–1 R _⊙ ), and low Eddington factors (Γ _e ∼ 0.006–0.4). Using these properties, we derive intermediate current masses (∼1–8 M _⊙ ), which suggest that their progenitors were massive stars (∼5–25 M _⊙ ) and that a subset will reach core-collapse, leaving behind neutron stars or black holes. Using the model fits, we also estimate the emission rates of ionizing photons for these stars, which agree well with previous model expectations. Further, by computing models for a range of mass-loss rates, we find that the stellar winds are weaker than predicted by any existing scheme ( ${\dot{M}}_{\mathrm{wind}}\lesssim {10}^{-9}$ M _⊙ yr ^−1 ). The properties of this first sample of intermediate-mass helium stars suggest they both contain progenitors of type Ib and IIb supernovae, and provide important benchmarks for binary evolution and population synthesis models.

Be X-ray binaries (Be XRBs) are high-mass X-ray binaries, with a neutron star or black hole orbiting and accreting material from a nonsupergiant B-star that is rotating at a near critical rate. These objects are prime targets to understand past binary interactions as the neutron star or black hole progenitor likely experienced Roche lobe overflow to spin up the Be star we observe now. The stellar variability can then allow us to explore the stellar structure of these objects. It was recently demonstrated that the high-mass X-ray binary CPD −29 2176 descended from an ultrastripped supernova and is a prime target to evolve into an eventual binary neutron star and kilonova. We present the photometric variability from both TESS and ASAS along with the spectral properties and disk variability of the system in this paper. All of the optical lines are contaminated with disk emission except for the He ii λ 4686 absorption line. The disk variability timescales are not the same as the orbital timescale, but could be related to the X-ray outbursts that have been recorded by Swift. We end our study with a discussion comparing CPD −29 2176 to classical Be stars and other Be X-ray binaries, finding the stellar rotation to be near a frequency of 1.5 cycles day ^−1 , and exhibiting incoherent variability in three frequency groups.

Although the majority of hot magnetic stars have extremely stable, ∼kilogauss strength surface magnetic fields with simple topologies, a subset undergoes small-scale explosions due to centrifugal breakout. The resulting small-scale flares are typically below the sensitivity of current magnetospheric diagnostics and do not generate detectable transient signatures. However, a recently reported radio flare from the hot magnetic star CU Vir suggests that some of the most energetic events do reach detectable levels. Motivated by this, we searched for transient radio sources in the first two epochs of the VLITE Commensal Sky Survey at the positions of 761 hot magnetic stars. We report three detections. A false-association analysis shows a less-than-1% probability that the sources are imaging artifacts. We then examine the stellar parameters of the three stars to understand if they are likely to produce flares. We conclude that while at this stage, we cannot make a definitive association of the detections with the stars, the current data are consistent with the hypothesis that the flares originate in the stellar magnetospheres.

We report ultra-wideband (0.4–4.0 GHz) observation of coherent radio emission via electron cyclotron maser emission (ECME) produced by the hot magnetic star HD 142990. With nearly perpendicular rotation and magnetic dipole axes, it represents an extreme case of oblique rotators. The large obliquity is predicted to cause a complex distribution of stellar wind plasma in the magnetosphere. It has been proposed that such a distribution will give rise to a nontrivial frequency dependence of ECME. Indeed we discovered strong frequency dependence of different pulse properties, such as the appearance of secondary pulses, different cutoff frequencies for pulses observed at different rotational phases, etc. But the unique feature that we observed is that while at sub-gigahertz frequencies, the star appears to produce ECME in the extraordinary mode, at gigahertz frequencies, the mode indicated by the pulse property is the ordinary mode. By considering the physical condition needed by such a scenario, we conclude that the required transition of the magnetoionic mode with frequency is unlikely to occur, and the most promising scenario is refraction caused by the complex plasma distribution surrounding the star. This suggests that the conventional way to deduce the magnetoionic mode based on ECME observed at a given frequency is not a reliable method for stars with large misalignment between their rotation and magnetic axes. We also find that ECME exhibits an upper cutoff at ≲3.3 GHz, which is much smaller than the frequency corresponding to the maximum stellar magnetic field strength.

WR 137 (HD 192641) is a binary system consisting of a carbon-rich Wolf–Rayet (W-R) star and an Oe companion star in a 13 yr orbit. Near periastron, the winds of the two stars collide and form carbonaceous dust. We obtained three mid-infrared grism spectra of the system with SOFIA and FORCAST during the last year of SOFIA’s operations in 2021 July, 2021 February, and 2022 May (Cycle 9). Within these spectra, we have identified several wind lines from He i , He ii , C iii , and C iv that are emitted from the W-R wind as well as a weak emission feature around 6.3–6.4 μ m that may have shifted its peak flux from 6.29 to 6.41 μ m through this time period. The weak feature grew as the continuum dust emission grew while the W-R emission appeared to decline due to lower contrast with the continuum. Furthermore, we observe that the peak of the feature shifts to redder wavelengths during the observations. We compare this feature to the unidentified infrared feature and other emission lines identified in dusty carbon-rich W-R (WC) binaries. For WR 137, we speculate that mixing of the winds in the system with the Oe star’s disk is important for starting the dust formation and that it is less important as dust formation continues. Previous infrared photometry shows “minieruptions” of dust production, which could then be explained with variations of the Oe star disk.

Massive contact binaries refer to the close binary systems in which the components have filled their respective Roche lobes and share a common envelope with early-type spectra. Twin binaries are a special type of binary system characterized by two components with nearly equal masses. The Magellanic Cloud, comprising the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud, is a fascinating galaxy that is distinct from the Milky Way. With its low metallicity, it serves as an ideal test bed for studying the formation and evolution of massive binaries and testing theoretical models. In this work, based on long-term observations with Optical Gravitational Lensing Experiment and MAssive Compact Halo Object in the LMC, we identified and performed comprehensive analyses of five massive twin contact binaries via the method of the light travel time effect and Wilson–Devinney code. The results show that all of these twin binaries are accompanied by low-mass third bodies. The third bodies have minimum masses ranging from 0.33 to 1.46 M _⊙ . Their orbital periods range from 4.34 to 12.03 yr. The maximum distances between the third bodies and the central binary systems range from 6.7 to 11.4 au. Remarkably, four out of the five massive twins have evolved into deep-contact binaries, which indicates that all of them may have originated from Case A mass transfer. These results strongly suggest the significant influence of the third body in the formation and evolution of massive contact binaries and may hold the key to unraveling the origins of massive binaries.

Magnetic hot stars refer to stars that have effective temperatures approximately in the range from 7000–50,000 K, and with large-scale globally organized magnetic fields. These magnetic fields exhibit strengths ranging from tens of Gauss to tens of kilo-Gauss. They are key in understanding the effects caused by magnetic fields in the stellar evolution. However, there are only three magnetic hot stars studied via a combination of spectropolarimetric and asteroseismic modeling. Combined with Transiting Exoplanet Survey Satellite sectors 1–56 data sets, we provided a photometric variability and stochastic low-frequency (SLF) variability study of 118 magnetic hot stars. Nine new rotating variable stars are identified. Using the Bayesian Markov Chain Monte Carlo framework, we fitted the morphologies of SLF variability for magnetic hot stars. Our analysis reveals that the magnetic hot stars in our sample have γ < 5.5 with the vast majority having 1 ≤ γ ≤ 3. The ν _char is primarily in the ranges of 0 day ^−1 < ν _char < 6.3 day ^−1 . The amplitude of SLF variability, log α _0 , shows a dominant distribution ranging from 0.8–3. No significant correlations are observed between the luminosity and fitting parameters, suggesting no clear dependence of SLF variability on stellar mass for our sample of magnetic hot stars with masses between approximately 1.5 M _⊙ < M < 20 M _⊙ . We found a significant negative correlation between the B _p and ν _char . This suppression effect of magnetic fields on ν _char may be a result of their inhibition of macroturbulence.

We present here the results of 8 yr of our near-simultaneous optical–near-infrared spectro-photometric monitoring of the bona fide FU Ori (FUor)-type candidate V2493 Cygnus (V2493 Cyg) starting from 2013 September to 2021 June. During our optical monitoring period (between 2015 October 16 and 2019 December 30), the V2493 Cyg is slowly dimming with an average dimming rate of ∼26.6 ± 5.6 mmag yr ^−1 in the V band. Our optical photometric colors show a significant reddening of the source post the second outburst pointing toward a gradual expansion of the emitting region post the second outburst. The mid-infrared colors, on the contrary, exhibit a blueing trend, which can be attributed to the brightening of the disk due to the outburst. Our spectroscopic monitoring shows a dramatic variation of the H α line as it transitioned from absorption feature to the emission feature and back. Such transition can possibly be explained by the variation in the wind structure in combination with accretion. Combining our time evolution spectra of the Ca ii infrared triplet lines with the previously published spectra of V2493 Cyg, we find that the accretion region has stabilized compared to the early days of the outburst. The evolution of the O i λ 7773 Å line also points toward the stabilization of the circumstellar disk post the second outburst.

Using a sample of 92 Galactic Be stars, we compare inclination angles (the angle between a star’s rotation axis and the line of sight) determined from H α emission-line profile fitting to those determined by the spectroscopic signature of gravitational darkening. We find good agreement: 70% of the sample (64 out of 92 stars) is consistent with zero difference between the two methods using 1 σ errors, and there is a strong linear correlation coefficient between the two methods of r = +0.63 ± 0.05. There is some evidence that the H α profile fitting method overestimates the inclination angle for i ≲ 25°, perhaps due to the neglect of incoherent electron scattering on the H α line widths, while the gravitational darkening method underestimates the inclination angle for i ≳ 70°, perhaps due to the neglect of disk radiative transfer effects on the optical spectrum. Overall, it is demonstrated that a single H α spectrum of modest resolution and SNR can be used to extract a useful estimate for the inclination angle of an individual Be star. This allows equatorial rotation velocities for individual Be stars to be derived from $v\sin i$ measurements and will allow Be stars to be used to search for correlated spin axes in young, open clusters if unbiased (with respect to inclination) samples of Be stars are used.

We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (Δ I < 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parameters T _eff = 42,000 K, $\mathrm{log}L/{L}_{\odot }=5.83\pm 0.15$ , M / M _⊙ = 50 ± 9, and v sin i = 370 ± 40 km s ^−1 . Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric ( e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493’s mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors.

Ongoing studies of different groups of stars result in improving our knowledge of their fundamental parameters and evolutionary status. Also, they result in finding new phases of stellar evolution, which require theoretical explanation. At the same time, availability of large telescopes and sensitivity improvement of detectors shift the focus of many observational programs toward fainter and more distant objects. However, there are still many problems in our understanding of details of stellar evolution which can now be solved with small telescopes and observations of bright stars. Approaching these problems implies conducting surveys of large groups of stars and long-term monitoring of individual objects. In this talk, we present the results of recent international programs of photometric and spectral monitoring of several groups of early-type stars. In particular, we discuss the role of binarity in creation of the Be phenomenon and show examples of recently discovered binary systems as well as the problem of refining fundamental parameters of B and A type supergiants. Special attention will be paid to collaboration with the amateur community and use of échelle spectrographs mounted on small telescopes.

Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 in M33. These regions serve as a training set for studies of more distant clusters, which can no longer be resolved into individual stars. Observations of recently formed star clusters and extended regions in star-forming galaxies in the nearby universe beyond the Local Group are presented. Their interpretation relies on spectral synthesis models. The successes and failures of such models are discussed, and future directions are highlighted. I present a case study of the extraordinary star cluster and giant H II region in the blue compact galaxy II Zw 40. The review concludes with a preview of two upcoming Hubble Space Telescope programs: ULLYSES, a survey of massive stars in nearby galaxies, and CLASSY, a study of massive star clusters in star-forming galaxies.

We perform spectral fittings for O-type stars based on self-consistent wind solutions, providing Ṁ and ν(r directly derived from the initial stellar parameters. We introduce our two methods: m-CAK prescription and Lambert-procedure. The Lambert-procedure allows the calculation of consistent v (r) that reduce the number of free parameters when a spectral fitting using CMFGEN is performed, even without recalculation of the Ṁ. Spectra calculated from our Lambert-solutions show significant differences compared to the initial β -law CMFGEN models. For m-CAK prescription, self-consistent solutions provide values for theoretical Ṁ on the order of the most recent predictions from other studies. Later, we find a global fit with the RT code FASTWIND. This is an important step towards the determination of stellar and wind parameters without using β -law. Our m-CAK prescription is valid for the O-type stars with T eff ≥ 30 kK and log g ≥ 3.2. We expect that solutions introduced here to be extended to numerous studies about massive stars in future. [ABSTRACT FROM AUTHOR]

We present a spectroscopic analysis of Hubble Space Telescope/Cosmic Origins Spectrograph observations of three massive stars in the low metallicity dwarf galaxies IC 1613 and WLM. These stars, were previously observed with Very Large Telescope (VLT)/X-shooter by Tramper et al., who claimed that their mass-loss rates are higher than expected from theoretical predictions for the underlying metallicity. A comparison of the far ultraviolet (FUV) spectra with those of stars of similar spectral types/luminosity classes in the Galaxy, and the Magellanic Clouds provides a direct, model-independent check of the mass-loss-metallicity relation. Then, a quantitative spectroscopic analysis is carried out using the non-LTE (NLTE) stellar atmosphere code CMFGEN. We derive the photospheric and wind characteristics, benefiting from a much better sensitivity of the FUV lines to wind properties than Ha. Iron and CNO abundances are measured, providing an independent check of the stellar metallicity. The spectroscopic analysis indicates that Z/Z⊙ = 1/5, similar to a Small Magellanic Cloud-type environment, and higher than usually quoted for IC 1613 and WLM. The mass-loss rates are smaller than the empirical ones by Tramper et al., and those predicted by the widely used theoretical recipe by Vink et al. On the other hand, we show that the empirical, FUV-based, mass-loss rates are in good agreement with those derived from mass fluxes computed by Lucy. We do not concur with Tramper et al. that there is a breakdown in the mass-loss-metallicity relation. [ABSTRACT FROM AUTHOR]