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.

The Intensity Interferometry technique consists of measuring the spatial coherence (visibility) of an object via its intensity fluctuations over a sufficient range of telescope separations (baselines). This allows us to study the size, shape and morphology of stars with an unprecedented resolution. Cherenkov telescopes have a set of characteristics that coincidentally allow for Intensity Interferometry observations: very large reflective surfaces, sensitivity to individual photons, temporal resolution of nanoseconds and the fact that they come in groups of several telescopes. In the recent years, the MAGIC Collaboration has developed a deadtime-free Intensity Interferometry setup for its two 17 m diameter Cherenkov telescopes that includes a 4-channel GPU-based real-time correlator, 410–430 nm filters and new ways of splitting its primary mirrors into submirrors using Active Mirror Control (AMC). With this setup, MAGIC can operate as a long-baseline optical interferometer in the baseline range 40–90 m, which translates into angular resolutions of 0.5-1 mas. Additionally, thanks to its AMC, it can simultaneously measure the zero-baseline correlation or, by splitting into submirrors, access shorter baselines under 17 m in multiple u-v plane orientations. The best candidates to observe with this technique are relatively small and bright stars, in other words, massive stars (O, B and A types). We will present the science cases that are currently being proposed for this setup, as well as the prospects for the future of the system and technique, like the possibility of large-scale implementation with CTA.

Massive stars emit X-rays. Despite modest X-ray luminosities of single hot massive stars, the ongoing large observing campaigns combining X-ray and UV spectroscopy provide a tomographic view of stellar winds. It is now established that X-ray radiation is modulated with stellar rotation and shows the same period as discrete absorption components (DACs) in the UV resonance lines. The latter are associated with corotating interaction regions (CIRs) in stellar winds, therefore the mechanisms responsible for generation of X-rays and CIRs appear to be physically linked. Binary massive stars with accreting compact companions – high-mass X-ray binaries (HMXBs) – are routinely observed by modern X-ray observatories at Mpc distances. Joint observations in X-ray and UV allow to determine realistic properties of these systems. The brightest sources among HMXBs are ultraluminous X-ray sources (ULXs). Their powerful radiation is an important source of stellar feedback. HMXBs are the products of massive binary evolution and are typically found in the vicinity of young massive star clusters. The superstar clusters blow hot superbubbles which fill large areas in star-forming dwarf galaxies. Recent models show that X-ray emission from superbubbles is likely the dominant source of He ii ionization in metal-poor star-forming dwarf galaxies. To conclude, X-ray observations provide an important window for studying massive stars and their feedback near and far.

We have studied the validity of the historical Cygnus OB associations and have found that many do not show the kinematic coherence expected for true OB associations. We have revisited these groups by photometrically identifying thousands of OB stars across the region with an SED fitting process which combines photometry, astrometry, spectral and evolutionary models. We applied a flexible clustering method and identified seven kinematically-coherent new OB associations. We observe a distinct correlation between position and velocity for two sets of these associations that suggests an expansion pattern. Tracing the motion of the stars back in the past we find that the sets were at their closest around 7.9 and 8.5 Myr ago. We discuss whether this expansion is a natural by-product of the commonly observed size - velocity dispersion relation of molecular clouds, or requires feedback to initiate the dispersal.

The Alicante Survey of MAssive Stars in Hii Regions (A-SMASHeR) is aimed at finding the ratio of massive stars that are born in isolation. We present LIRIS/WHT images and EMIR/GTC spectra of the massive stellar content in A-SMASHeR regions. Our preliminary analysis yields ∼20% of regions hosting relatively (or truly) isolated massive stars.

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.

At the time of this meeting, the latest Gaia data release is EDR3, published on 3 December 2020, but the next one, DR3, will appear soon, on 13 June 2022. This contribution describes, on the one hand, Gaia EDR3 results on massive stars and young stellar clusters, placing special emphasis on how a correct treatment of the astrometric and photometric calibration yields results that are simultaneously precise and accurate. On the other hand, it gives a brief description of the exciting results we can expect from Gaia DR3.

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.

A fundamental question for theories of massive star formation is whether OB stars can form in isolation. We assess the contribution of any in-situ OB star formation by using 210 field OB stars in the Small Magellanic Cloud (SMC) from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4). We search for tiny, sparse clusters around our target OB stars using cluster-finding algorithms. Employing statistical tests, we compare these observations with random-field data sets. We find that ∼5% of our target fields do show evidence of higher central stellar densities, implying the presence of small clusters. This frequency of small clusters is low and within errors, it is also consistent with the field OB population being composed entirely of runaway and walkaway stars. Assuming this small cluster fraction is real, it implies that some OB stars may form in highly isolated conditions. The low frequency could be caused by these clusters evaporating on a short timescale. However, another interpretation is that the low fraction of small clusters is observed because these form rarely, or not at all, implying a higher cluster lower-mass limit and generally consistent with a relationship between maximum stellar mass (mmax) and the cluster mass (Mcl).

We study the He II λ4686 emission south of multiple-star system HD 5980, which hosts two WN-type stars. We use optical VLT FORS1 long-slit spectra. The observations are close to the eclipse phase. Broad He II emission with a SNR >5 is observed as far as 7.6 pc from HD 5980. The He II emission that is closest to HD 5980 is 1.2 pc directly south and has a redshifted component with FWHM 1450 km s-1 originating in the eclipsing WN5-6 star, and a blueshifted component with FWHM 600 km s-1. The second component cannot be explained by either the nearly- eclipsed WN star or the nearby supernova remnant, SNR B0057-724. We suggest that the additional He II emission comes from the colliding winds of the two WN 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.

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.

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.

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.

Over the last two decades there have been considerable advances in modelling the spectra of massive stars and supernovae (SNe). Despite this progress, there are still numerous uncertainties that affect the accuracy of models. For massive stars, convection, instabilities, clumping, and our inability to model stellar winds self-consistently likely introduce systematic errors into our analyses. For SNe, and particularly for core-collapse SNe, departures from spherical symmetry strongly affect observed spectra and need to be taken into account. There are also issues with clumping, and mixing processes (both in the progenitor and the SN explosion) that need to be resolved. For both massive stars and SNe, the accuracy and availability of atomic data continues to be an ongoing issue influencing analyses.

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.

Massive stars are the objects that condition the evolution of the interstellar medium by the amount of energy released during their lives and especially by their death as a supernova explosion. All the data provided by the previous and ongoing missions of ground and space telescopes have saturated us with the amount of information, which is no longer easy to process daily by human routines. To this end, we present the development of a massive star spectroscopic interactive database designed for scientific research.

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.

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.

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.