Low metallicity (Z) massive stars are among the main feedback agents in the early Universe and in present-day blue dwarf galaxies. The nearby star-forming SMC galaxy offers conditions which resemble those at redshift z∼2 i.e. where modern galaxies formed and star formation peaked. Here we present the recent results about the nature of the eclipsing O-type binary in the SMC, AzV 476, to gain insights on the properties of massive stars and binaries at earlier cosmic epochs. We find that the primary has surprisingly low mass while being much brighter and hotter than the secondary. To place the measured stellar properties in the evolutionary context we modeled the system and confirm that AzV 476 is a post-interaction binary with the primary already being core helium (He) burning, while still having a hydrogen-rich (H-rich) envelope. These results constrain massive binary evolutionary scenarios and guide the searches of stripped stars in low-Z environments.

Luminous Blue Variable stars (LBVs) are rare and enigmatic. Often cited as evolutionary stages in the single-star evolution, the idea that binary evolution produces the LBV state was already considered, 30 years ago.

It is now commonly accepted that a significant part of massive stars are born in multiple systems. One aspect that also emerged is that massive stars have on average at least two companions, i.e. they are triples. This immediately implies that a number of LBVs should have evolved as part of multiple systems.

While some LBVs are confirmed as binaries, different methods were used to derive their multiplicity, with different results. We report on a systematic search for multiplicity using spectroscopy, interferometry in a sample of 20 LBVs. Spectroscopy provides us with a bias-corrected binary fraction of $\[62_{ - 24}^{ + 38}\]$%, and a percentage of 50–70% is found from interferometry. This has a high impact on the way that these objects might be formed.

The temperature independent part of the Humphreys-Davidson (HD) limit sets the boundary for evolutionary channels of massive stars that either end their lives as red supergiants (RSGs) or as the hotter blue supergiants (BSGs) and Wolf-Rayet stars. Recent downward revision of most luminous RSGs the Galaxy below log(L / L⊙) ≈ 5.5, more in line with the Magellanic Clouds, might hint towards a metallicity (Z)-independent HD limit. We present MESA single star models in the 15-40 M⊙ range and study the different Z-dependent processes that could potentially affect the location of the upper luminosity limit of RSGs.

Since massive stars form preferentially as members of close binary systems, we use dense grids of detailed binary evolution models to explore how binary evolution shapes the main-sequence morphology of young star clusters. We propose that binary mergers might be the origin of the blue main sequence stars in young star clusters. Our results imply that stars may either form by accretion, or through a binary merger, and that both paths lead to distinctly different spins, magnetic fields, and stellar mass distributions.

In this contribution, we explore the question on the formation of multiple massive stellar systems via disk fragmentation with the help of the highest-resolution simulations to date of a fragmenting disk in the context of massive star formation. The simulations start from a collapsing cloud of 200 solar masses, followed by the formation of an accretion disk that develops spiral arms and fragments. Due to the high resolution of our grid, we are able to self-consistently form the fragments without the need for a subgrid module such as sink particles. We track the formed fragments into the first stages of companion formation, which allows us to give an estimate of the multiplicity of the final system due to disk fragmentation. We find in total around ∼6 fragments, some at orbits of ∼ 1000 au, and some close (possibly spectroscopic) companions.

MWC 656 has been reported as classical Be star with a black hole companion. Revisited spectral variability properties render this unlikely, with a hot subdwarf more probable.

Exploring the low-mass end of the companion mass function around massive stars is of crucial importance to constrain massive star formation theories. We present a high-contrast imaging study of 20 O- and early B-type stars in the Scorpius OB1 association. From the analysis of VLT/SPHERE data, we identify a total of 789 sources. The data probe the brown dwarf regime around massive stars, resulting in the discovery of large-separation multiple systems with mass-ratios as low as 0.001 (comparable to Jupiter-Sun mass-ratio).

One significant difficulty in reliable quantification of the rates of mass-loss from hot, massive stars lies in uncertainties associated with quantifying temporal and spatial variability within stellar winds. The consequences of low-metallicity conditions for wind structure also merit continued investigation. We present initial results from ULLYSES data with the aim of identifying structure within the stellar winds of early B type supergiants with sub-solar metallicities in the Large and Small Magellanic Clouds. We demonstrate how single-epoch ULLYSES data can be used to investigate significant wind structure for these stars.

We investigate the physical properties of dust in the environment of three core-collapse supernovae (CCSNe) through mid-infrared (mid-IR) spectral energy distribution (SED) modeling (both analytical and numerical methods) and interpret our results within a Bayesian framework. We provide evidence that the observed late-time mid-IR excess of the SNe can be described by dust models. We conclude that in case of various types of SNe, numerical dust models with a shell-like geometry can be reconciled with analytical models, regarding the essential properties of dust grains.

Mass loss through stellar winds plays a dominant role in the evolution of massive stars. Very massive stars (VMSs, > 100Mȯ) display Wolf-Rayet spectral morphologies (WNh) whilst on the main-sequence. Bestenlehner (2020) extended the elegant and widely used stellar wind theory by Castor, Abbott & Klein (1975) from the optically thin (O star) to the optically thick main-sequence (WNh) wind regime. The new mass-loss description is able to explain the empirical mass-loss dependence on the Eddington parameter and is suitable for incorporation into stellar evolution models for massive and very massive stars. The prescription can be calibrated with the transition mass-loss rate defined in Vink & Gräfener (2012). Based on the stellar sample presented in Bestenlehner et al. (2014) we derive a mass-loss recipe for the Large Magellanic Cloud using the new theoretical mass-loss prescription of Bestenlehner (2020).

Adequate stellar atmosphere models are prerequisite to derive robust stellar parameters from spectroscopic analyses. I will briefly review recent results obtained with the Potdam Wolf-Rayet (PoWR) model atmosphere code, which is applicable to all types of hot stars. Using multi-wavelength observations including the UV, we analyzed large samples of massive stars at various metallicities, gaining important insights on their cosmic role and the feedback to their environment.

A recent extension of PoWR allows to compose the model atmosphere from two zones. A rapidly rotating star, e.g., might possess a cooler equatorial region with a slow wind, and two polar cones with higher photospheric temperature and fast wind. For two examples of rapidly rotating O-type stars, we demonstrate that such model can reproduce wind-line profiles which otherwise would stay inconsistent. Fast rotation, which prevails in particular at low metallicities, thus might bias empirically derived parameters, having implications for feedback as well as for angular-momentum losses of SN and GRB progenitors.

Upcoming large-scale spectroscopic surveys such as WEAVE and 4MOST will provide thousands of spectra of massive stars, which need to be analysed in an efficient and homogeneous way. Studies on massive stars are usually based on samples of a few hundred objects which pushes current spectroscopic analysis tools to their limits because visual inspection is necessary to verify the spectroscopic fit.

The novel spectroscopic analysis pipeline takes advantage of the statistics that large samples provide, and determines the model error to account for imperfections in stellar atmosphere codes due to simplified, wrong or missing physics. Considering observational plus model uncertainties improve spectroscopic fits. The pipeline utilises the entire spectrum rather than selected diagnostic lines allowing a wider range of temperature from B to early O stars to be analysed. A small fraction of stars like peculiar, contaminated or spectroscopic binaries require visual inspection, which are identified through their larger uncertainties.

B-type supergiants show enormous potential as resourceful tools to address a wide range of astrophysical questions concerning stellar atmospheres, stellar and galactic evolution and the cosmic distance scale. For the purposes of a comprehensive analysis of these objects we test a hybrid non-LTE approach – line-blanketed model atmospheres computed under the assumptions of local thermodynamic equilibrium (LTE) in combination with non-LTE line-formation calculations. An observational sample of 14 Galactic B-type supergiants with masses below about 30 Mȯ is investigated on the basis of high-resolution Echelle spectra. The results of this analysis – atmospheric and fundamental stellar parameters, the characterisation of the interstellar sightlines to the objects, as well as derived spectroscopic distances and multi-species abundances – are subjected to multiple tests of consistency.

There is evidence that some red supergiants (RSGs) experience phases of episodic mass-loss. These episodes yield more extreme mass-loss rates, further stripping the envelope of the RSG, significantly affecting the further evolution towards the final collapse of the star. Mass lost through RSG outbursts/superwinds will flow outwards and form dust further out from the surface and this dust may be detected and modelled. Here, we aim to derive the surface properties and estimate the global properties of Mid-IR bright RSGs in the Magellanic Clouds. These properties will then be compared to evolutionary predictions and used for future spectral energy distribution fitting studies to measure the mass-loss rates from present circumstellar dust.

In this paper, we present a glimpse of our observations of two Wolf-Rayet (WR) nebulae, NGC2359 and NGC6888 obtained with the SITELLE imaging Fourier transform spectrograph. The data are of unprecedented spatial coverage and cover a broad wavelength range.

We present the results of a magnitude-limited spectroscopic survey of Galactic Wolf-Rayet stars with the HERMES spectrograph mounted on the Mercator telescope. Using cross-correlation to measure radial velocities, we measured the observed binary fractions of the Galactic carbon- (WC) and nitrogen-rich (WN) Wolf-Rayet stars to be and . We used Monte-Carlo simulations with a Bayesian framework to derive the intrinsic multiplicity properties and found and . We find that the majority of WN binaries reside in short-period systems, similar to O stars. However, the orbital period distribution of the Galactic WC population peaks at 5000 d, a discrepancy that challenges our current understanding of binary evolution in Wolf-Rayet stars.

B supergiants (BSGs) lie on the cool end of line-driven wind regime, such that the study of their atmospheres can help us to understand the physics of line-driven winds. So far key features of their spectra, especially in the UV region, could not be reproduced consistently with atmosphere models. This represents a significant gap in our knowledge of their physical properties and behavior, which is particularly striking for BSGs on the cool side of the Bi-Stability Jump (cooler than B1). To address this problem, we analysed a sample of Galactic cool BSGs, with sufficient UV and optical coverage. None of our targets are detected in X-rays with only upper limits existing for some of them.

. We present UVIT/Astrosat UV photometry of the RSG population of the Small Cloud galaxy (SMC). As RSGs are extremely faint in the far-UV, these observations directly probe potential companion stars. From a sample of 861 SMC RSGs, we find 88 have detections at far-UV wavelengths: a clear signature of binarity. Stellar parameters are determined for both components, which allows us to study - for the first time - the mass-ratio (q) distribution of RSG binary systems. We find a flat mass-ratio distribution best describes the observations up to MRSG ∼15M⊙. We account for our main observing bias (i.e. the limiting magnitude of the UVIT survey) to determine the intrinsic RSG binary fraction of 18.8 ± 1.5 %, for mass-ratios in the range 0.3.<q<1.0 and orbital periods approximately in the range 3<log P[days]<8.

The evolutionary link between Red Supergiants and Luminous Blue variables is interesting, but still poorly understood. We present the results of a study of the Galactic candidate luminous blue variable Wray 15-906, revealed via the detection of its infrared circumstellar shell (of ≍2 pc in diameter) with the Wide-field Infrared Survey Explorer (WISE) and the Herschel Space Observatory. Using the stellar atmosphere code CMFGEN and the Gaia parallax, we found that Wray 15-906 is a relatively low-luminosity, log(L/Lȯ) ≍ 5.4, star with a temperature of 25±2 kK. In the framework of single star evolution, the obtained results suggest that Wray 15-906 is a post-red supergiant star with an initial mass of ≍ 25Mȯ and that before exploding as a supernova it could transform for a short time into a WN11h star. The presence of a shell with a mass 2.9±0.5Me indicates that Wray 15-906 has suffered substantial mass loss in the recent past.

The young open cluster NGC 6231 hosts a rich population of O-type binary stars. We study several of these eccentric short-period massive eclipsing binaries and assess their fundamental parameters. The properties of these systems make them interesting targets to study tidally induced apsidal motion. The analysis of apsidal motion offers a powerful means to obtain information about the internal structure of the stars. Indeed, since the rate of apsidal motion in a binary system is proportional to the internal structure constants of the stars composing it, its value gives direct insight into the internal structure and evolutionary state of these stars. Stellar evolution models are constructed based on the observationally-determined fundamental parameters and a theoretical rate of apsidal motion is inferred. The results are striking: Adopting standard stellar evolution models yields a theoretical rate of apsidal motion much larger than the observational value. This discrepancy results from the standard models predicting too low an efficiency of internal mixing and thus too homogenous stars in terms of density. By enforcing the theoretical rates of apsidal motion to match the observational values, enhanced mixing is required, through a large overshooting parameter and/or additional turbulent/rotational mixing. Our analysis leads to the conclusion that the chemically mixed cores in those massive stars must be more extended than anticipated from standard models.