Supermassive stars represent a promising avenue for seeding the (super-)massive black holes observed in the centres of massive galaxies. In these proceedings I review the motivation on the need for supermassive stars as a progenitor pathway for seeding massive black holes. I discuss the currently understood limitations of seeds produced by less massive stars (i.e. remnants from the first generation of stars) and advocate that more massive stars - with masses up to M∗ ∼ 105Mȯ - formed under the conditions of hierarchical structure formation, in rare haloes, are the favoured pathway. Finally, I discuss some recent high resolution simulations demonstrating the formation of supermassive stars in early galaxies.

We used interferometric observations made with the CHARA Array of 25 B-type stars and 6 O-type stars to obtain precise measurements of angular size, radius, and effective temperature to test stellar atmospheric models for massive stars. Our measured angular diameters range from 1.09 milli-arcseconds (mas) for β Tau down to 0.11 mas for 10 Lac, the smallest star yet resolved with the CHARA Array. The rotational oblateness of the rapidly rotating star ζ Oph is directly measured for the first time. We collected ultraviolet to infrared spectrophotometry for all sample stars and derived temperatures, angular diameters, and reddening estimates that best fit the spectra. There is generally good agreement between the observed and spectral fit angular diameters for the O and B stars, indicating that the fluxes predicted from model atmospheres are reliable. The derived and model temperatures for the O stars are also in fair agreement, however the sample size is small and several of the O stars results we consider to be preliminary. On the other hand, the temperatures derived from angular diameters and fluxes tend to be larger (by ≈ 4%) for the B stars than those from published results based on analysis of the line spectrum (Gordon et al. 2018, 2019).

In this contribution we present the results from a1 homogeneous quantitative spectroscopic analysis of ∼400 Galactic O-type stars targeted by the IACOB and OWN surveys. The ultimate objective is to perform a modern reassessment of one of the long-standing problems in the field of massive stars: the elusive detection of mid O-type stars close to the “canonical” theoretical ZAMS. We first provide statistically significant evidence of the existence of a clear lack of stars in our sample populating the region of the spectroscopic HR diagram approximately delimited by the theoretical ZAMS, the ∼ 40 and ∼ 70 M⊙ single evolutionary tracks and the 2 Myr isochrone. We then evaluate if this empirical result could be a result of possible limitations of our analysis strategy and/or the existence of potential observational biases affecting the compiled sample. Once both explanations are investigated, we evaluate the possibility that a modification of the efficiency of mass accretion during the star formation process could lead to a new (corrected) theoretical ZAMS in better accordance with our empirical results.

The role of mass loss from massive stars, especially episodic mass loss, is one of the outstanding open questions facing stellar evolution theory. Multiple lines of evidence are pointing to violent, episodic mass-loss events being responsible for removing a large part of the massive stellar envelope, especially in low-metallicity galaxies. The ERC ASSESS project aims to determine whether episodic mass loss is a dominant process in the evolution of the most massive stars by conducting the first extensive, multi-wavelength survey of evolved massive stars in the nearby Universe. The project hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to investigate the properties of evolved targets in nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models. In this work we present some of our first observational results from the galaxies NGC 6822 and IC 10 obtained with OSIRIS (GTC).

Wolf-Rayet (WR) stars comprise a class of stars whose spectra are dominated by strong, broad emission lines that are associated with copious mass loss. In the massive-star regime, roughly 90% of the known WR stars are thought to have evolved off the main sequence. Dubbed classical WR (cWR) stars, these hydrogen-depleted objects represent a crucial evolutionary phase preceding core collapse into black holes, and offer a unique window into hot-star wind physics. Their formation is thought to be rooted in either intrinsic mass-loss or binary interactions. Results obtained from analyses using contemporary model atmospheres still fail to reconcile the derived properties of WR stars with predictions from stellar evolution. Importantly, stellar evolution models cannot reproduce the the bulk of cWR stars, a problem that becomes especially severe at subsolar metallicity. Next-generation model atmospheres and upcoming observational campaigns to hunt for undetected companions promise a venue for progress.

We study the apsidal motion in close eccentric massive binaries. Measuring the rate of apsidal motion in such a system gives insight into the internal structure and evolutionary state of the stars. We focus on CPD-41° 7742, for which independent studies in the past showed large discrepancies in the longitude of periastron of the orbit, hinting at the presence of apsidal motion. We perform a consistent analysis of all observational data to solve this apparent discrepancy and report the first determination of apsidal motion in this system. This study confirms the need for enhanced mixing in the stellar evolution models of the primary star to reproduce the observational properties. This points towards larger convective cores than usually considered.

The formation of multiples has seen some significant progress over the past years mainly due to the advent and the expansion of high-angular resolution facilities. Star-forming regions are the laboratories where massive stars can be caught right after their formation phase. Still, the observational constraints and the properties of young multiple systems are poorly documented. These proceedings contain recent results about the multiplicity properties of six young O-type stars in the M17 star-forming region, observed by the means of near-IR interferometric observations, which have provided insight into the origin of massive close binaries in a cluster environment.

Mergers of neutron stars and black holes are nowadays observed routinely thanks to gravitational-wave astronomy. In the isolated, binary-evolution channel, a common-envelope phase of a red supergiant and a compact object is crucial to sufficiently shrink the orbit and thereby enable a merger via gravitational-wave emission. Here, we use the outcome of three-dimensional hydrodynamic common-envelope simulations of a 9.4 solar mass red supergiant and a 5 solar mass black-hole to explore the further evolution and final fate of the remnant binary. The binary system undergoes another phase of mass transfer during which it is visible as an X-ray binary. We find that the donor star does not explode as an ultra-stripped supernova because of the large remaining envelope mass, but as a Type Ib/c supernova. Supernova kicks are actually required to sufficiently perturb the orbit and thus facilitate a merger within a Hubble time via gravitational-wave emission.

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.

UV wind line variability in OB stars appears to be universal. We review the evidence that the variability is due to large, dense, optically thick structures rooted in or near the photosphere. Using repeated observations and a simple model we translate observed profile variations into optical depth variations and, consequently, variations in measured mass loss rates. Although global rates may be stable, measured rates vary. Consequently, profile variations infer how mass loss rates determined from UV wind lines vary. These variations quantify the intrinsic error inherent in any mass loss rate derived from a single observation. These derived rates can differ by factors of 3 or more. Our results also imply that rates from non-simultaneous observations (such as UV and ground based data) need not agree. Finally, we use our results to examine the nature of the structures responsible for the variability.

While we have growing numbers of massive star observations, it remains unclear how efficient the key physical processes such as mass loss, convection and rotation are, or indeed how they impact each other. We reconcile this with detailed stellar evolution models, yet these models have their own drawbacks with necessary assumptions for 3-dimensional processes like rotation which need to be adapted into 1-dimensional models. The implementation of empirical mass-loss prescriptions in stellar evolution codes can lead to the extrapolation of base rates to unconstrained evolutionary stages leading to a range of uncertain fates. In short, there remain many free parameters and physical processes which need to be calibrated in order to align our theory better with upcoming observations. We have tested various processes such as mass loss and internal mixing, including rotational mixing and convective overshooting, against multiple observational constraints such as using eclipsing binaries, the Humphreys-Davidson limit, and the final masses of Wolf-Rayet stars, across a range of metallicities. In fact, we developed a method of disentangling the effects of mixing and mass loss in the ‘Mass-Luminosity Plane’ allowing direct calibration of these processes. In all cases, it is important to note that a combined appreciation for both stellar winds and internal mixing are important to reproduce observations.

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.

We present Color Magnitude Diagrams (CMDs) created by using our database of 43,340 synthetic CMFGEN spectra. For each calculated spectra we measure the absolute flux by Johnson and Gaia photometry filters and build synthetic CMDs.

The wealth of information obtained about massive stars from observations at ultraviolet (UV) wavelengths has been fundamental to understand their structure and the mechanisms regulating their evolution. There are however, important aspects that have not yet been addressed due to the lack of data and the relevant instrumentation such as the role of binarity and magnetic fields or the impact of low metallicity in the evolution of massive stars. There are plans to develop UV spectropolarimeters, UV monitors and very efficient telescopes with high collecting surfaces that will revolutionize the field. In this contribution, a short update on the current and foreseen UV instrumentation is provided.

Hot, massive stars are known to host unstable, radiation-driven outflowing winds, giving rise to dense clumps of material which severely affect the diagnostic techniques used to derive wind properties of massive stars. Most of the current diagnostic models account for wind inhomogeneities by assuming a one-component medium consisting of optically thin clumps, and maintaining a smooth velocity-field. However, this neglects important light-leakage effects through porous channels in-between the clumps. These light-leakage effects have recently been incorporated in the stellar atmosphere modelling code FASTWIND, and here we will present quantitative mass-loss results from a combined Ultraviolet-Optical wind analysis of O-supergiants in the Galaxy. Using a genetic-algorithm fitting-approach, we systematically investigate the impact the wind physics has on derived stellar and wind parameters, and how this depends on metallicity and spectral type. We compare our findings with earlier results (which do not take into account such light-leakage effects), to standard mass-loss rates usually included in evolution model studies of massive stars, and with theoretical predictions of clumping properties. We will also present the first systematic empirical constraints on the new wind parameters, associated with light-leakage, and compare these with theoretical predictions.

I describe (i) our recent updates on first star formation, with particular emphasis on their binaries, (ii) formation of low-metallicity stars and the transition of their initial mass functions with metal enrichment, and finally (iii) formation of supermassive stars from slightly metal-enriched gas by the newly found super-competitive accretion channel.

Despite the important role mass loss in the red supergiant phase plays in controlling stellar evolution and massive stars’ final supernova fates, a theoretical explanation of the mechanism driving this mass loss has been elusive. In this contribution we present a recent breakthrough [Kee et∼al., 2021] showing that turbulent pressure alone is sufficient to markedly extend the atmospheres of red supergiants and allow a wind to be launched. The resulting theory provides a fully analytic prescription for red supergiant mass-loss rates. Moreover, the theoretical mass-loss rates computed from observationally inferred turbulent velocities are in overall good agreement with observationally inferred red supergiant mass loss. A particularly interesting aspect of this theory is that it is not sensitive to metallicity, providing important implications for stellar evolution and the so-called “red-supergiant problem” for supernova progenitors in various environments.

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.

Rotation plays an important role in the structure and evolution of massive stars. It leads to deviation from spherical symmetry for very fast rotating stars, mixing in otherwise unmixed radiative regions and generally increased mass loss. In addition, magnetic fields interact with rotation and lead to significant transport of angular momentum. In this article, we review the various rotational and magnetic instabilities present in massive stars and their implementation in one-dimensional stellar evolution codes. We then focus on their impact on the evolution of single rotating stars. Finally, we compare rotating models to observations and discuss ways to disentangle between various uncertainties.