Mass loss is a key property to understand stellar evolution and in particular for low-metallicity environments. Our knowledge has improved dramatically over the last decades both for single and binary evolutionary models. However, episodic mass loss although definitely present observationally, is not included in the models, while its role is currently undetermined. A major hindrance is the lack of large enough samples of classified stars. We attempted to address this by applying an ensemble machine-learning approach using color indices (from IR/Spitzer and optical/Pan-STARRS photometry) as features and combining the probabilities from three different algorithms. We trained on M31 and M33 sources with known spectral classification, which we grouped into Blue/Yellow/Red/B[e] Supergiants, Luminous Blue Variables, classical Wolf-Rayet and background galaxies/AGNs. We then applied the classifier to about one million Spitzer point sources from 25 nearby galaxies, spanning a range of metallicites (). Equipped with spectral classifications we investigated the occurrence of these populations with metallicity.

Wolf-Rayet stars are regarded as candidates for progenitors of core-collapse supernovae, and they are expected to be progenitors of long gamma-ray bursts. These types of stars are considered to be fast rotators. Their high rotation speed breaks the sphericity of the star and leads to an axisymmetric wind density structure. In such a case, the electron scattering takes place in a nonspherical environment, and as a result, we might expect an intrinsic polarization. We present a 2.5D radiation hydrodynamic stellar wind model of these stars. The model simulations account for the deformation of the stellar surface due to rotation, gravity darkening, and nonradial forces. We computed the polarization from the density variable of the hydrodynamic model, derived the upper limit of rotational velocities, and found no conflict with the previous studies of Wolf-Rayet stars.

Most massive stars (up to 100%) are thought to be in binary systems. The multiplicity of massive stars seems to be intrinsically linked to their formation and evolution, and so Massive Young Stellar Objects are key in observing this early stage of star formation. We have surveyed hundreds of MYSOs across the Galaxy from the RMS catalogue, using UKIDSS and VVV point source data. Preliminary results show binary fractions of 44±3% for the UKIDSS sample and 32±3% for the VVV sample. In addition we use the K-band magnitudes as a proxy for the companion mass, and find a significant fraction of the detected companions have estimated mass ratios greater than 0.5, which suggests a deviation from the capture formation scenario.

Gravitational-wave (GW) observations are revealing the population of compact objects from a new angle. Yet their stellar progenitors remain uncertain because few observational clues on their progenitors exist. Theoretical models typically assume that the progenitor evolution can be approximated with single-star models. We explore how binary evolution affects the pre-supernova (SN) structure of stars, and the resulting distribution of compact object remnants. We focus on the differences in the core properties of single stars and of donor stars that transfer their outer layers in binary systems and become binary-stripped. We show that the final structures of binary-stripped stars that lose their outer layers before the end of core helium burning are systematically different compared to single stars. As a result, we find that binary-stripped stars tend to explode more easily than single stars and preferentially produce neutron stars and fewer black holes, with consequences for GW progenitors.

The galactic binary star LB-1 contains a recently stripped B-type star. Comparing its properties to detailed binary star models shows that tidal braking and magnetic torques lead to low surface rotational velocities in the stripped donors after Roche-lobe overffiow. Models without magnetic torques cannot reproduce the observed low surface rotation.

The jittering jets explosion mechanism of core collapse supernovae proposes that stochastic convective motion, amplified by instabilities, results in intermittent accretion disk that launches jittering jets that explode the star. We conduct one-dimensional simulations of a wide range of stellar masses and show that the convective motion in the pre-collapse stellar core, when scaled to corresponding three-dimensional simulations, supply sufficient angular momentum fluctuation to form the intermittent accretion disk the launches the jittering jets. The resulting neutron star masses are consistent with observations.

Recent observations of Type II supernovae have revealed that their red-supergiant progenitors lose a significant amount of mass during the last years of their evolution. However, because it is difficult to discover supernovae within days of explosion, the diversity of mass loss in red supergiants has not yet been fully mapped. This talk presented the case of SN 2021yja, which was serendipitously imaged within hours of explosion and observed with a sub-day cadence during its rise to peak. From the exceptionally long plateau period and the high nickel mass, we infer a relatively massive red-supergiant progenitor star. However, archival imaging from the Hubble Space Telescope places a stringent upper limit of on its progenitor mass. We discuss these conflicting constraints in the context of the larger sample of exploding red supergiants. Our analysis helps illuminate the poorly understood mechanism(s) behind red-supergiant mass loss.

We use the RIOTS4 sample of SMC field OB stars to determine the origin of massive runaways in this low-metallicity galaxy using Gaia proper motions, together with stellar masses obtained from RIOTS4 data. These data allow us to estimate the relative contributions of stars accelerated by the dynamical ejection vs binary supernova mechanisms, since dynamical ejection favors faster, more massive runaways, while SN ejection favors the opposite trend. In addition, we use the frequencies of classical OBe stars, high-mass X-ray binaries, and non-compact binaries to discriminate between the mechanisms. Our results show that the dynamical mechanism dominates by a factor of 2 – 3. This also implies a significant contribution from two-step acceleration that occurs when dynamically ejected binaries are followed by SN kicks. We update our published quantitative results from Gaia DR2 proper motions with new data from DR3.

Mass-loss is a key parameter throughout the evolution of massive stars. In this work we probe the radial clumping stratification of OB stars in the intermediate and outer wind regions (r ≳ 2R*; r, radial distance to photosphere), derive upper limits for mass-loss rates, Ṁmax, and compare them to current theoretical mass-loss recipes implemented in evolutionary models. A key conclusion of our analysis regards the derived upper-limit mass-loss rates of B supergiants, independently of clumping, which calls for an urgent revision of the role recombination of iron-like elements plays in determining the mass-loss rates of objects that cross the bi-stability region, and a careful analysis of corresponding effects for stellar evolution models.

We present the results of our analysis of 122 light-curves from 50 Wolf-Rayet (WR) stars using a red+white noise analysis, where we compare the fitted red noise features with stellar parameters to assess the presence of correlations with stellar parameters. A significant correlation between the amplitude of variability α0 and v∞ was found for the whole sample, along with several other correlations satisfying the Spearman-Rank p<0.001 criterion for both He-burning and WNh stars. Our results are compatible with several plausible processes that can have an influence on the level of variability in the winds of these stars, including a subsurface convection zone and core-generated internal gravity waves.

Massive stars play crucial roles in astrophysical settings across cosmic history, and thus it is a fundamental problem to understand whether their formation processes are universal or diverse in various galactic environments. In particular, metallicity is the essential characteristic of cosmic evolution. Our theoretical studies have suggested some degrees of metallicity dependence of massive star formation. In the extremely metal-poor case of , protostellar disks are significantly unstable, and the photoionization feedback is more efficient. We also execute an ALMA survey targeting massive protostars in the Large Magellanic Clouds (LMC) with . We found that the outflow properties of LMC protostars (mass, momentum, energy) are consistent with those of Galactic protostars, suggesting the universality of massive star formation at least in the range of .

We report on a study of 9 nearby primitive galaxies observed by Hubble’s COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. By “primitive galaxies,” we mean galaxies having a low stellar mass, and low gas metallicity, , whether they are local or at high redshift. We find that far-UV spectra of these galaxies show evidence of hard radiation, including X-rays. Following Thuan et al. (2004), we identify these galaxies as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of extremely strong X-radiation. Following Heger et al. (2003), we suggest that the accreting black hold is produced by direct collapse of stars having initial masses greater than 50 . The X-radiation produced by black hole disk directly affects the surrounding interstellar medium, and many of these effects are observable in far-UV spectra.

In this paper, we give a brief description of the Polstar instrument, an ultraviolet (UV) spectropolarimetric, Midex-class mission proposed to NASA to study the winds of massive stars as well as interstellar medium and protoplanetary discs topics.

Our knowledge of massive star evolution is limited by uncertainties linked with multi-dimensional processes taking place in stellar interiors. Important examples are convective boundary mixing (CBM) and entrainment, which are implemented in 1D stellar evolution models assuming simplified prescriptions. 3D hydrodynamics models can improve these prescriptions by studying realistic multi-D processes for a short timerange (minutes or hours). In these proceedings, we present results coming from a new set of high-resolution hydrodynamics simulations of a neon-burning shell in a massive star, and discuss how the entrainment law can be calibrated from 3D models and then used to improve 1D stellar evolution prescriptions.

Line-driven stellar winds are ubiquitous among hot massive stars. In some cases they can become so strong, that the whole star is cloaked by an optically thick wind. The strong outflow gives rise to large emission lines, defining the class of so-called Wolf-Rayet (WR) stars. While being major players in the evolution of massive stars, the formation of heavy black holes, and the distribution of elements, the occurrence and nature of WR winds is still quite enigmatic.

A promising instrument towards a better theoretical understanding are stellar atmospheres allowing for a consistent inclusion of the hydrodynamics. By coupling stellar and wind parameters and the inclusion of a detailed non-LTE radiative transfer, they allow us to go beneath the observable layers and study the onset of WR-type winds. Establishing larger sets of models, we were able to make ground-breaking progress by identifying trends with mass and metallicity that deviate significantly from present empirical descriptions. Our modelling efforts reveal a complex picture for WR-type winds with strong, non-linear dependencies. Besides covering metallicity and mass, we further identify surface hydrogen as an important ingredient to retain WR-type mass loss at lower metallicity. Here, we present a summary of recent insights on the nature and onset of WR-type winds in massive stars including the consequences for stellar evolution, remaining open questions, and current efforts to overcome them.

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.

The emission line spectra of WR stars are often formed completely in the optically thick stellar wind. Hence, any assumption on the wind velocity law in a spectral analysis has a profound impact on the determination of the stellar parameters. By comparing Potsdam Wolf-Rayet (PoWR) model spectra calculated with different β laws, we show that the velocity field heavily influences the spectra: by using the appropriate β laws, the entire range of late and early types can be covered with the same stellar model.

Very metal-poor massive stars hold the key to interpret high-redshift star-forming galaxies and the early reionization epoch, but also contemporary events such as gravitational waves. To study these objects in resolved environments, we need to resort to dwarf irregular galaxies far from the potential wells of M31 and the Milky Way, and therefore distant. While the archives, recently boosted by the ULLYSES and XSHOOTU programs, store a healthy dataset of massive stars in the Milky Way and the Magellanic Clouds, the number of observed targets with poorer metal content than the SMC (1/5 Zȯ) is dramatically small. This paper reviews the state of observations of very metal-poor massive stars, assessing what can be realistically learned about their physics and evolution with current instrumentation, and arguing whether or not near-future facilities can remedy the gaps in the knowledge that remain.

. 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.

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.