The Binarity at LOw Metallicity (BLOeM) survey is an ESO large programme designed to obtain multi-epoch spectroscopy for 929 massive stars in the Small Magellanic Cloud (SMC). It will provide binary fractions and orbital configurations of binary systems and search for dormant black hole binary candidates (OB+BH). We present projected rotational velocities (v sin i) of all sources, and, using the multiplicity properties presented in previous papers, we derive the v sin i distributions of apparent single stars, single-lined spectroscopic (SB1) binaries, and SB2 systems. We identify a locus in the Hertzsprung-Russell diagram where rotational velocities decrease significantly; we interpret this feature as broadly corresponding to the terminal-age main sequence. The main-sequence cohort is distinguished by a broad range of v sin i values, but with a strong peak in the distribution in the range 30-60 km s(-1), which is close to the resolution limit of similar to 30 km s(-1), indicating the presence of many upper limits. Sources in this low v sin i peak are distributed throughout the main sequence and are also present in the SB1 sample, though less prominent than in the single-star distribution. A preliminary analysis of the lowest v sin i cohort, which includes SB1 systems, implies that roughly one-third may be nitrogen rich, and we speculate that this cohort is a mix of pristine single stars, long-period binaries, and merger products. The SB2 systems appear to be mostly short-period binaries in synchronous rotation, and their v sin i estimates are distributed around a mean value of similar to 140 km s(-1). Higher v sin i sources are also present in the single and SB1 systems, all of which have a tail to higher v sin i values. This is consistent with tidal and mass-transfer effects. The supergiants, with a few exceptions, have low v sin i, and the bulk of these systems is essentially unresolved at the current spectral resolution (similar to 30 km s(-1)).

Binary evolution plays a central role in producing rapidly rotating stars. Previous studies have shown that mass gainers in binaries can reach critical rotation after accreting only modest amounts of material, particularly during thermal-timescale Case B mass transfer, in which tidal spin-down is ineffective due to wide orbits. However, such rapid accretion often drives the mass gainer out of thermal equilibrium, and its subsequent spin evolution during thermal relaxation has not been analyzed in depth. In this study, we construct a suite of accreting detailed single-star models with different accretion prescriptions, which inflate and spin up to critical rotation during the accretion. After the accretion has ended, the models relax thermally and deflate. We find that the ratio of surface to critical angular velocity decreases to subcritical values during thermal contraction, with the magnitude of this decrease correlating with the degree of thermal disequilibrium at the end of accretion. This reduction in fractional critical rotation is even stronger when internal angular momentum transport is inefficient. Detailed binary models show the same trend, indicating that the results from our toy single-star models also apply to real binary evolution. Our results highlight that binary mass transfer does not always produce critically rotating stars, but instead may yield a wide range of spin rates depending on the mass transfer and accretion history. Our findings offer new insights into the rotational properties of mass gainers in binaries, stellar merger products, and newly formed massive stars following accretion.

A significant fraction of stars experience close interactions, including collisions resulting from gravitational encounters and mergers within close binary systems. These processes can produce more massive stars that may give rise to relatively rare objects such as blue stragglers. Distinguishing the outcomes of collisions and mergers is challenging yet essential for interpreting observations. This study utilizes the magnetohydrodynamics code AREPO to simulate collisions and mergers of 5–10 M _⊙ main-sequence stars, systematically comparing the properties of the resulting products. Both collisions and mergers yield more massive, strongly magnetized, rapidly and differentially rotating stars with cores enriched in hydrogen, but notable quantitative differences emerge. Merger products exhibit core hydrogen fractions up to 10% higher than those of collision products. In both scenarios, turbulent mixing amplifies magnetic field energies by 9–12 orders of magnitude. However, magnetic fields in small-impact-parameter collision products display small-scale reversals that may dissipate over time, whereas merger products and large-impact-parameter collision products develop large-scale ordered, potentially long-lived magnetic fields. Additionally, only merger products display magnetically driven, bipolar outflows with radial velocities up to 300 km s ^−1 . These distinctions may result in different long-term evolutionary outcomes, which warrant further investigation in future studies.

Understanding the transfer of mass and angular momentum in binary interactions is crucial for modeling the evolution of any interacting binary after the first mass-transfer phase. Mass-transfer physics assumptions shape the predictions for later stages of binary evolution, such as the immediate progenitors of stripped-envelope supernovae and gravitational-wave mergers. We constrain the efficiency and stability of thermal-timescale mass transfer in massive binary evolution using the observed population of 62 massive interacting binaries on the main sequence ("Algols") in the Milky Way and the Large and Small Magellanic Clouds. We find that purely conservative or nonconservative mass transfer cannot explain the current mass ratio and orbital period of all massive Algols. Angular momentum conservation rules out conservative mass transfer in similar to 28% of massive Algols in the SMC. About three-quarters of all massive Algols are consistent with having undergone inefficient mass transfer (less than or similar to 50%), while the remaining systems, mostly residing in the LMC and the Milky Way, require mass transfer to have been more efficient than 25%. For our fiducial assumption on the extent of envelope stripping, the current sample of massive Algols does not require mass transfer to be efficient at the shortest orbital periods (similar to 2 days) at any metallicity. We find evidence that mass transfer on the main sequence needs to be stable for initial accretor-to-donor mass ratios as unequal as similar to 0.6. Unless biased by observational selection effects, the massive Algols in the SMC seem to have undergone less efficient mass transfer than those in the LMC and the Milky Way.

Context. The majority of massive stars are born with a close binary companion. How this affects their evolution and fate is still largely uncertain, especially at low metallicity. Aims. We derive synthetic populations of massive post-interaction binary products and compare them with corresponding observed populations in the Small Magellanic Cloud (SMC). Methods. We analyse 53298 detailed binary evolutionary models computed with MESA. Our models include the physics of rotation, mass and angular momentum transfer, magnetic internal angular momentum transport, and tidal spin-orbit coupling. They cover initial primary masses of 5-100 M-circle dot, initial mass ratios of 0.3-0.95, and all initial periods for which interaction is expected, 1-3162 d. They are evolved through the first mass transfer and the donor star death, and a a possible ensuing Be X-ray binary phase, and they end when the mass gainer leaves the main sequence. Results. In our fiducial synthetic population, 8% of the OB stars in the SMC are post-mass-transfer systems, and 7% are merger products. In many of our models, the mass gainers are spun up and expected to form Oe/Be stars. While our model underpredicts the number of Be X-ray binaries in the SMC, it reproduces the main features of their orbital period distribution and the observed number of SMC binary WR stars. We further expect similar to 50 OB+BH binaries below and similar to 170 above the 20 d orbital period. The long-period OB+BH binaries might produce merging double black holes. However, their progenitors, the predicted long-period WR+OB binaries, are not observed. Conlcusions. While the comparison with the observed SMC stars supports many physics assumptions in our high-mass binary models, a better match for the large number of observed OBe stars and Be X-ray binaries likely requires a lower merger rate and/or a higher mass transfer efficiency during the first mass transfer. The fate of the initially wide O star binaries remains particularly uncertain.

Red supergiants (RSGs) represent a late evolutionary stage of massive stars. Recent observations reveal that the observed luminosity range of RSGs in young open clusters is wider than expected from single-star evolution models. Binary evolution effects have been suggested as a possible explanation. Here, we analyze 3670 detailed binary-evolution models, as well as corresponding single-star models, to probe the contribution of binary mass transfer and binary mergers to the luminosity distribution of RSGs in star clusters with ages up to 100 Myr. We confirm that the expected luminosity range of RSGs in a coeval population can span a factor of 10, as a consequence of mergers between two main-sequence stars, which reproduces the observed RSG luminosity ranges in rich clusters well. While the luminosity increase as consequence of mass transfer is more limited, it may help to increase the number of overluminous RSGs. However, our results also demonstrate that binary effects alone are insufficient to account for the number of RSGs found with luminosities of up to 3 times those predicted by current single-star models. We discuss observational accuracy, rotational mixing, age spread, and intrinsic RSG variability as possible explanations. Further observations of RSGs in young open clusters, in particular studies of their intrinsic brightness variability, appear crucial for disentangling these effects.

Context. Massive star evolution plays a crucial role in astrophysics; however, its study is subject to large uncertainties. This problem becomes more severe by the majority of massive stars being born in close binary systems, whose evolution is affected by interactions among their components. Aims. We want to constrain major uncertainties in massive binary star evolution, particularly with respect to the efficiency and the stability of the first mass-transfer phase .Methods. We used the rapid population synthesis code COMBINE to generate synthetic populations of post-interaction binaries, assuming constant mass-transfer efficiency. We employed a new merger criterion that adjusts self-consistently to any prescribed mass-transfer efficiency. We tailored our synthetic populations to be comparable to the expected binary populations in the Small Magellanic Cloud (SMC). Results. We find that the observed populations of evolved massive binaries cannot be reproduced with a single mass-transfer efficiency. Instead, a rather high efficiency (greater than or similar to 50%) is needed to reproduce the number of Be stars and Be/X-ray (BeXB) binaries in the SMC, while a low efficiency (similar to 10%) leads to a better agreement with the observed number of Wolf-Rayet (WR) stars. We constructed a corresponding mass-dependent mass-transfer efficiency recipe to produce our fiducial synthetic SMC post-interaction binary population. It reproduces the observed number and properties of the BeXBs and WR binaries rather well; furthermore, it is not in stark disagreement with the observed OBe star population. It predicts around 170 massive stars with neutron star companion, of which 140 are Be stars, and about 170 systems disrupted by the supernova, of which 150 are Be stars. Overall, 20% of all post-interaction systems contain a helium star. It also predicts two large, as-yet-unobserved populations of OB + BH binaries: about 100 OB + BH systems with rather small orbital periods (less than or similar to 20 d) and around 40 longer period OBe + BH systems. Conclusions. Continued searches for massive binary systems will strongly advance our understanding of their evolution.

Rapidly rotating classical OBe stars have been proposed as the products of binary interactions, and the fraction of Be stars with compact companions implies that at least some are. However, to constrain the interaction physics spinning up the OBe stars, a large sample of homogeneously analysed OBe stars with well-determined binary characteristics and orbital parameters are required. We investigate the multiplicity properties of a sample of 18 Oe, 62 Be, and two Of?p stars observed within the BLOeM survey in the Small Magellanic Cloud. We analyse the first nine epochs of spectroscopic observations obtained over approximately three months in 2023. Radial velocities (RVs) of all stars are measured. Applying commonly-used binarity criteria we classify objects as binaries, binary candidates, and apparently single (RV stable) objects. We further inspect the spectra for double-lined spectroscopic binaries and cross-match with catalogues of X-ray sources and photometric binaries. We classify 14 OBe stars as binaries, and an additional 11 as binary candidates. The two Of?p stars are apparently single. Two more objects are most likely currently interacting binaries. Without those, the observed binary fraction for the OBe sample (78 stars) is f_OBe_obs=0.18+/-0.04 (f_obs_cand=0.32+/-0.05 including candidates). This fraction is less than half of that measured for OB stars in BLOeM. Combined with the lower fraction of SB2s, this suggests that OBe stars have indeed fundamentally different binary properties than OB stars. We find no evidence for OBe binaries with massive compact companions, in contrast to expectations from binary population synthesis. Our results support the binary scenario as an important formation channel for OBe stars, as post-interaction binaries may have been disrupted or the stripped companions of OBe stars are harder to detect.

At high metallicity, a majority of massive stars have at least one close stellar companion. The evolution of such binaries is subject to strong interaction processes, which heavily impact the characteristics of their life-ending supernova and compact remnants. For the low-metallicity environments of high-redshift galaxies, constraints on the multiplicity properties of massive stars over the separation range leading to binary interaction are crucially missing. Here we show that the presence of massive stars in close binaries is ubiquitous, even at low metallicity. Using the Very Large Telescope, we obtained multi-epoch radial velocity measurements of a representative sample of 139 massive O-type stars across the Small Magellanic Cloud, which has a metal content of about one-fifth of the solar value. We find that 45% of them show radial velocity variations that demonstrate that they are members of close binary systems, and predominantly have orbital periods shorter than 1 year. Correcting for observational biases indicates that at least 70-6+11%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$7{0}_{-6}<^>{+11}\%$$\end{document} of the O stars in our sample are in close binaries, and that at least 68-8+7%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$6{8}_{-8}<^>{+7}\%$$\end{document} of all O stars interact with a companion star during their lifetime. We found no evidence supporting a statistically significant trend of the multiplicity properties with metallicity. Our results indicate that multiplicity and binary interactions govern the evolution of massive stars and determine their cosmic feedback and explosive fates.

Given the uncertain evolutionary status of blue supergiant stars, their multiplicity properties hold vital clues to better understand their origin and evolution. As part of The Binarity at LOw Metallicity (BLOeM) campaign in the Small Magellanic Cloud, we present a multi-epoch spectroscopic survey of 128 supergiant stars of spectral type B5-F5, which roughly correspond to initial masses in the 6-30 M-circle dot range. The observed binary fraction for the B5-9 supergiants is 25 +/- 6% (10 +/- 4%) and 5 +/- 2% (0%) for the A-F stars, which were found using a radial-velocity (RV) variability threshold of 5 km s(-1) (10 km s(-1)) as a criterion for binarity. Accounting for observational biases, we find an intrinsic multiplicity fraction of less than 18% for the B5-9 stars and 8( -7)(+9) % for the AF stars, for the orbital periods up to 10(3.5) days and mass ratios (q) in the 0.1 < q < 1 range. The large stellar radii of these supergiant stars prevent short orbital periods, but we demonstrate that this effect alone cannot explain our results. We assessed the spectra and RV time series of the detected binary systems and find that only a small fraction display convincing solutions. We conclude that the multiplicity fractions are compromised by intrinsic stellar variability, such that the true multiplicity fraction may be significantly smaller. Our main conclusions from comparing the multiplicity properties of the B5-9- and AF-type supergiants to that of their less evolved counterparts is that such stars cannot be explained by a direct evolution from the main sequence. Furthermore, by comparing their multiplicity properties to red supergiant stars, we conclude that the AF supergiant stars are neither progenitors nor descendants of red supergiants.

The growing database of gravitational-wave (GW) detections with the binary black holes (BHs) merging in the distant Universe contains subtle insights into their formation scenarios. One of the puzzling properties of detected GW sources is the possible (anti)correlation between mass ratio q of BH-BH binaries and their effective spin. We use rapid binary evolution models to demonstrate that the isolated binary evolution followed by efficient tidal spin-up of stripped helium core produces a similar pattern in Xeff vs q distributions of BH-BH mergers. In our models, the progenitors of unequal BH-BH systems in the stable mass transfer formation scenario are more likely to efficiently shrink their orbits during the second Roche-lobe overflow than the binaries that evolve into nearly equal-mass component systems. This makes it easier for unequal-mass progenitors to enter the tidal spin-up regime and later merge due to GW emission. Our results are, however, sensitive to some input assumptions, especially, the stability of mass transfer and the angular momentum loss during non-conservative mass transfer. We note that mass transfer prescriptions widely adopted in rapid codes favor the formation of BH-BH merger progenitors with unequal masses and moderate separations. We compare our results with detailed stellar model grids and find reasonable agreement after appropriate calibration of the physics models. We anticipate that future detections of unequal-mass BH-BH mergers could provide valuable constraints on the role of the stable mass transfer formation channel. A significant fraction of BH-BH detections with mass ratio q in range (0.4 - 0.7) would be consistent with the mass ratio reversal scenario during the first, relatively conservative mass transfer, and a non-enhanced angular momentum loss during the second, highly non-conservative mass transfer phase.

Massive binary evolution models are needed to predict massive star populations in star-forming galaxies, the supernova diversity, and the number and properties of gravitational wave sources. Such models are often computed using so-called rapid binary evolution codes, which approximate the evolution of the binary components based on detailed single star models. However, about one-third of the interacting massive binary stars undergo mass transfer during core hydrogen-burning (Case A mass transfer), whose outcome is difficult to derive from single star models. For this work, we used a large grid of detailed binary evolution models for primaries in the initial mass range 10-40 M-circle dot with a Large and Small Magellanic Cloud composition, to derive analytic fits for the key quantities needed in rapid binary evolution codes, that is, the duration of core hydrogen-burning, and the resulting donor star mass. We find that systems with shorter orbital periods produce up to 50% lighter stripped donors and have a lifetime up to 30% larger than wider systems. Both quantities strongly depend on the initial binary orbital period, but the initial mass ratio and the mass-transfer efficiency of the binary have little impact on the outcome. Our results are easily parameterisable and can be used to capture the effects of Case A mass transfer more accurately in rapid binary evolution codes.

Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z circle dot). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P less than or similar to 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960-4570 & Aring; resolving power R = 6200; typical signal-to-noise ratio(S/N) approximate to 70-100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 less than or similar to Teff/kK less than or similar to 45 and 3.7 < log L/L-circle dot < 6.1 and initial masses of 8 less than or similar to Mini less than or similar to 80 M-circle dot. The sample comprises 159 O-type stars, 331 early B-type (B0-3) dwarfs and giants (luminosity classes V-III), 303 early B-type supergiants (II-I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries.

Massive stars mainly form in close binaries, where their mutual interactions can profoundly alter their evolutionary paths. Evolved binaries consisting of a massive OB-type main-sequence star with a stripped helium star or a compact companion represent a crucial stage in the evolution towards double compact objects, whose mergers are (potentially) detectable via gravitational waves. The recent detection of X-ray quiet OB+black hole binaries and OB+stripped helium star binaries has set the stage for discovering more of these systems in the near future. In this work, based on 3670 detailed binary-evolution models and using empirical distributions of initial binary parameters, we compute the expected population of such evolved massive binaries in coeval stellar populations, including stars in star clusters and in galaxies with starburst activities, for ages up to 100 Myr. Our results are vividly illustrated in an animation that shows the evolution of these binaries in the color-magnitude diagram over time. We find that the number of OB+black hole binaries peaks around 10 Myr, and OB+neutron star binaries are most abundant at approximately 20 Myr. Both black holes and neutron stars can potentially be found in populations with ages up to 90 Myr. Additionally, we analyze the properties of such binaries at specific ages. We find that OB+helium stars and OB+black hole binaries are likely to be identifiable as single-lined spectroscopic binaries. Our research serves as a guide for future observational efforts to discover such binaries in young star clusters and starburst environments.

Blue straggler stars are unique main-sequence stars that appear more luminous, hotter, and therefore younger, than their coeval counterparts. In star clusters, these stars are located above the cluster turn-off in the Hertzsprung-Russell diagram or color-magnitude diagram. First identified in the 1950s, these stars are found across diverse environments, from sparse galactic fields to dense star clusters. They are crucial for understanding stellar and binary evolution and star cluster dynamics. Despite extensive research, many challenges concerning their properties and origin mechanisms remain unresolved. This chapter delves into the properties and origins of blue stragglers, examining how theoretical tools are employed to study them and the implications of each proposed formation mechanism. We assess how contemporary observational data either support or challenge these theoretical predictions. Continued theoretical and observational efforts are essential for advancing our understanding of these enigmatic stars.

The detection of split main sequences (MSs) associated with young clusters ($\lesssim$600 Myr) has caught lots of attention. A prevailing scenario is that a bimodality of stellar rotation distribution drives the MS bifurcation. Nevertheless, the origin of the stellar rotation dichotomy remains unclear. Hypotheses involving tidally-locked binaries or blue straggler stars (BSSs) are proposed to explain the observed split MSs. This work examines if the long-term dynamical evolution of star clusters can produce the observed split MSs, through high-performance $N$-body simulation. As a prototype example, the young massive cluster NGC 1856 exhibits an apparent MS bifurcation. Our simulation reports that at the age of NGC 1856, tidally-locked binaries are fully mixed with single stars. This is consistent with the observation that there is no significant spatial difference between blue MS and red MS stars. However, we find that only high mass-ratio binaries can evolve to the tidally-locked phase at the age of the NGC 1856. These tidally-locked binaries will populate a much redder sequence than the MS of single stars rather than a blue MS, which is inconsistent with the hypothesis. The number of tidally-locked binaries cannot account for the observation. Our simulation shows that BSSs produced by binary interactions do populate the blue periphery in the color-magnitude diagram, and their spatial distribution shows a similar pattern of single stars. However, the number of BSSs does not fit the observation.

The origin of initial rotation rates of stars, and how a star's surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the 35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). We develop a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multi-band Hubble Space Telescope photometry and Multi Unit Spectroscopic Explorer spectroscopy. This allows us to homogeneously constrain the physical parameters of over 250 B and Be stars, brighter than mF814W = 18.8 mag. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B stars. The rotational velocities vary as a function of the star's position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. The rotational velocities of the 35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the bulk of the B stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different vsini-characteristics of NGC 330 compared to younger populations can be understood in this framework.

Spectroscopic observations of stars in young open clusters have revealed evidence for a dichotomous distribution of stellar rotational velocities, with 10-30% of stars rotating slowly and the remaining 70-90% rotating fairly rapidly. At the same time, high-precision multiband photometry of young star clusters shows a split main sequence band, which is again interpreted as due to a spin dichotomy. Recent papers suggest that extreme rotation is required to retrieve the photometric split. Our new grids of MESA models and the prevalent SYCLIST models show, however, that initial slow (0-35% of the linear Keplerian rotation velocities) and intermediate (50-65% of the Keplerian rotation velocities) rotation are adequate to explain the photometric split. These values are consistent with the recent spectroscopic measurements of cluster and field stars, and are likely to reflect the birth spin distributions of upper main-sequence stars. A fraction of the initially faster-rotating stars may be able to reach near-critical rotation at the end of their main-sequence evolution and produce Be stars in the turn-off region of young star clusters. However, we find that the presence of Be stars up to two magnitudes below the cluster turnoff advocates for a crucial role of binary interaction in creating Be stars. We argue that surface chemical composition measurements may help distinguish these two Be star formation channels. While only the most rapidly rotating, and therefore nitrogen-enriched, single stars can evolve into Be stars, slow pre-mass-transfer rotation and inefficient accretion allows for mild or no enrichment even in critically rotating accretion-induced Be stars. Our results shed new light on the origin of the spin distribution of young and evolved B-type main sequence stars.

In the past years, we have undertaken an extensive investigation of LMC and SMC star clusters based on HST data. We present photometry and astrometry of stars in 101 fields observed with the WFC/ACS, UVIS/WFC3 and NIR/WFC3 cameras. These fields comprise 113 star clusters. We provide differential-reddening maps and illustrate various scientific outcomes that arise from the early inspection of the photometric catalogs. In particular, we provide new insights on the extended main-sequence turn-off (eMSTO) phenomenon: i) We detected eMSTOs in two clusters, KMHK361 and NGC265, which had no previous evidence of multiple populations. This finding corroborates the conclusion that the eMSTO is a widespread phenomenon among clusters younger than 2 Gyr. ii) The homogeneous color-magnitude diagrams (CMDs) of 19 LMC clusters reveal that the distribution of stars along the eMSTO depends on cluster age. iii) We discovered a new feature along the eMSTO of NGC1783, which consists of a distinct group of stars going on the red side of the eMSTO in CMDs composed of ultraviolet filters. Furthermore, we derived the proper motions of stars in the fields of view of clusters with multi-epoch images. Proper motions allowed us to separate the bulk of bright field stars from cluster members and investigate the internal kinematics of stellar populations in various LMC and SMC fields. As an example, we analyze the field around NGC346 to disentangle the motions of its stellar populations, including NGC364 and BS90, young and pre-MS stars in the star-forming region associated with NGC346, and young and old field stellar populations of the SMC. Based on these results and the fields around five additional clusters, we find that young SMC stars exhibit elongated proper-motion distributions that point toward the LMC, thus bringing new evidence for a kinematic connection between the LMC and SMC.

Massive star feedback affects the evolution of galaxies, where the most massive stars may have the largest impact. The majority of massive stars are born as members of close binary systems. In this work, we investigated detailed evolutionary models of very massive binaries (30−90 M⊙) with Large Magellanic Cloud (LMC) metallicity. We identify four effects defying the conventional knowledge of binary evolution, which are all related to the proximity of the models to the Eddington limit. We find that the majority of systems undergo mass transfer during core hydrogen burning. During the ensuing nuclear timescale evolution, many mass donors remain more massive than their companions (‘reverse Algols’), and nuclear timescale mass transfer may be interrupted or absent altogether. Furthermore, due to the elevated luminosity-to-mass ratio, many of the core-hydrogen-burning donors may develop Wolf-Rayet-type winds at luminosities where single stars would not. We identify observational counterparts of very massive reverse Algol binaries in the LMC and discuss their contribution to the observed hydrogen-rich Wolf-Rayet stars. We argue that understanding very massive Algol systems is key to predicting the advanced evolution of very massive binaries, including their ability to evolve into observable gravitational wave sources.