We present our measurements of the amplitude of photometric and spectroscopic variability due to clumping in the wind of Wolf-Rayet (WR) stars. Photometric variability was assessed using TESS light-curves, while spectroscopic variations were obtained from almost 20 years of monitoring of nearly 100 classical (presumably single) stars. Our results show an apparent dependence of the variability amplitude with the stars’ surface temperature and/or terminal velocity. Our interpretation is that it supports the idea that the dominating driver of the clumps in WR winds is a sub-surface convection region.

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.

We present the analysis of the dust properties of the Wolf-Rayet nebulae M 1-67 and RCW 58 around the WN8h stars WR 124 and WR 40, respectively. Modeling with the photoionization code Cloudy shows that in both nebulae the IR spectral energy distributions and ionized gas properties can be reproduced by a dust shell consisting of two populations of dust grains. Furthermore, taking into account the initial mass, the morphology and the kinematics of the nebulae we propose M 1-67 and RCW 58, together with their progenitor stars, as the first observational evidences of post-common envelope evolution in nebulae around massive stars.

We present X-ray and spectropolarimetric observations of the WN+O binaries WR71 and WR97, which are analogs of the well-studied V444 Cygni. The combined results have the potential to constrain the locations and properties of wind interaction regions in these binaries, give clues to their subsequent evolution, and address the commonalities among WR+O systems.

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.

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.

Expanding nebulae are produced by mass loss from stars, especially during late stages of evolution. We describe the algorithms and methods implemented in the radiation-magnetohydrodynamics (MHD) code PION for highly scalable simulations using static mesh-refinement. We present results from 3D MHD simulations of bow shocks around runaway massive stars, and of the expansion of a fast wind from a Wolf-Rayet star into the slow wind from a previous red supergiant phase of evolution. PION is free software that can be downloaded from https://www.pion.ie/

We discovered a nebula in the low-metallicity (Z=0.1 ) nearby ( Mpc) dwarf galaxy NGC 4068, which reveals broad Hα line profile and unusual emission line fluxes in its spectrum. The object also shows significant nitrogen overabundance, not typical for metal-poor environment. We assumed that the nebula could be ionized by an evolved massive star with , Wolf-Rayet or Blue Supergiant, and built models of the nebula using Cloudy and CMFGEN codes. Our models successfully reproduce the optical emission spectrum of the object, including the peculiar [SII]/[NII] ratio and the presence of the HeII λ4686 line.

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.

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.

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.

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 .

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.

This contribution presents recent advances in identifying the stellar upper mass limit using simulations of UV radiative feedback during the star formation process. Generally, due to computational costs and a focus on au to parsec scales, simulations of massive star formation do not trace the flow of material to distances closer than a few au from the forming star. However, UV line-acceleration acts directly on accreting material in the sub-au circumstellar region, thereby efficiently ablating the surface layers off the protostellar disk. For stars on the order of a few hundred solar masses, this disk destruction rate exceeds the accretion rate, destroying the disk faster than it is replenished, and setting a maximum stellar mass as a function of metallicity that can be attained by single star formation channels.

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.

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).

The recent generation of dedicated wide-field, high-cadence sky-surveys have overwhelmed discovery statistics for all manner of extra-galactic transients, and uncovered new phenomena seemingly linked to the demise of massive stars. For the more established classes of transients, such as core-collapse supernovae, surges in discoveries are allowing true population studies to provide quantitative constraints not only on the explosion properties, but also on the progenitor populations. Crucially, such population insights are benefiting from creation of samples of transients constructed with largely unbiased methods for discovery and characterisation. Surrounding these discoveries are increasing samples of extreme transients that do not fit the standard core-collapse paradigm - requiring the invocation of exotic progenitor stars and placing demands on the stellar evolution of such systems. Here I will provide a high-level observationally-driven overview of recent results related to massive stellar transients.

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.

Supernova properties in radio strongly depend on their circumstellar environment and they are an important probe to investigate the mass loss of supernova progenitors. Recently, core-collapse supernova observations in radio have been assembled and the rise time and peak luminosity distribution of core-collapse supernovae in radio has been obtained. In this talk, we will discuss the constraints on the mass-loss prescriptions of red supergiants obtained from the assembled radio properties of Type II supernovae. We take a couple of mass-loss prescriptions for red supergiants, calculate the rise time and peak luminosity distribution based on them, and compare the results with the observed distribution. We found that the widely spread radio rise time and peak luminosity distribution of Type II supernovae can only be explained by mass-loss prescriptions having strong dependence on the luminosity. Red supergiant mass-loss prescriptions should have steep luminosity dependence in the supernova progenitor range.

Long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe) are expected to result from massive star deaths. However to date, there has been no direct observational measurement of their cloud collapse timescales nor progenitor lifetimes to help constrain their mass. Our analyses of z 2 LGRB afterglow spectra and Hubble Space Telescope images find a higher fraction of host galaxies that are interacting, have a close companion, and/or may have experienced a recent galaxy ‘fly by’ as compared to the general z 2 galaxy population. A smaller set of z 2 SLSNe suggests a similar result. Under the hypothesis that galaxy interactions induce cloud collapse and star formation near their closest approach, we explore measurements of the host and companion galaxy velocities and separations at the time of the LRGB/SLSN event as a direct physical means to measure the timescale of cloud collapse plus progenitor star lifetime.