Local Group (LG) very metal-poor massive stars are the best proxy for the First Stars of the Universe, fundamental to model the early evolution of the first galaxies, and key to unravelling new evolutionary pathways restricted to low metallicities, such as chemically homogeneous evolution. Yet, due to the great leap in distance required to reach metal-poor galaxies of the LG and vicinity, no comprehensive spectroscopic studies have been carried out at sub-$$$$ metallicities so far.

We focus on the massive star population of the 1/10Ȯ galaxy Sextans A. After five observing campaigns at the 10.4-m Gran Telescopio Canarias (GTC), we have assembled a spectroscopic catalogue of more than 150 OB stars. This catalogue will be fundamental to test stellar evolution at very low metallicity, to detect the first binary systems at 1/10Ȯ and to unveil the most recent star formation history of this galaxy.

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/

A detailed X-ray study of a massive binary called HD 93205 has been made using long-term XMM-Newton observations. The X-ray spectrum of HD 93205 displays negligible counts above 5 keV. The two thermal plasma emission models with average temperature values as ∼ 0.20 and ∼ 0.60 keV are required to explain the X-ray spectra. The X-ray flux variations in the binary are noticed to be in qualitative agreement with the wind-wind collision model but with a few deviations from the expected 1/D trend (D is the binary separation).

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.

This contribution presents new results on two members of the class of post-Red Supergiants, IRAS 17163-3907, the central star of the Fried Egg nebula and IRC +10420. New optical spectra in the blue spectral range confirm their spectral type to be of A-supergiant class. Our VLTI/GRAVITY K-band interferometry reveals that the neutral Na i 2.2 μm line emitting region is smaller than that of the hydrogen Brγ emission. This can be explained with the hydrogen emission the result of collisional excitation populating the higher levels in a neutral region instead them being populated through recombination in an ionised environment as mostly inferred in stellar winds. Finally, the central star of the Fried Egg nebula, has undergone 3 distinct mass loss episodes over the last hundreds of years. As it is likely that at least the last mass loss event occurred when the star was already a Yellow Hypergiant and not a Red Supergiant, we put forward the bi-stability mechanism as explanation for the mass loss.

The first stars in the Universe have inherited their composition from primordial nucleosynthesis, so they have no metal. These stars, which are also named population III (pop III) stars, began the process of reionization in the Universe and contributed to the metal enrichment with heavy elements. Previous studies showed that they should have been rotating fast due to small or no angular momentum loss, reaching easily the critical velocity since they are massive and have very low stellar winds, thus their mass loss is very low or zero. Our aim is to study how the production of primary nitrogen is affected due to high rotation in the pop III stars. So, we compared grids of pop III stars with zero, average, and high rotation. All these models have been computed using Geneva code (GENEC) in the mass range of 9M⊙ ≤ Mini ≤ 120M⊙. Due to the rotational mixing, the carbon produced in the He-burning core is diffused towards the H-burning shell, triggering the CNO cycle and producing primary nitrogen. In some models the transition of the shell from a pp-chain H-burning to a CNO H-burning induces a strong energy release and a complete change of the stellar structure and the nucleosynthesis. The production of nitrogen is boosted for the high rotation models.

Spectroscopy can decode the radiation from stars in an appropriate way and derive many properties of different stellar objects. In this work we seek to derive simultaneously stellar and wind parameters of massive stars. To model the data we use the radiative transport code Fastwind with the hydrodynamic solutions derived using our stationary code Hydwind as input, instead of the β-law. Then, ISOSCELES, our grid of stellar atmosphere and hydrodynamic models of massive stars, is used to derive the physical properties of the observed spectra through spectral line fittings. This quantitative spectroscopic analysis provide an estimation about the line–force parameters, whose theoretical calculations are complex. In addition, we expect to confirm that the hydrodynamic δ-slow solutions, describe quite reliable the radiation line-driven winds of A and late B supergiant stars and, at the same time, explain disagreements between observational data and theoretical models for the Wind–Momentum Luminosity Relationship (WLR).

Episodic mass loss is not understood theoretically, neither accounted for in state-of-the-art models of stellar evolution, which has far-reaching consequences for many areas of astronomy. We introduce the ERC-funded ASSESS project (2018-2024), which 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. It hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to derive physical parameters of ∼1000 dusty, evolved massive stars in ∼25 nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models, and quantify the duration and frequency of episodic mass loss as a function of metallicity. The approach involves applying machine-learning algorithms to select dusty, luminous targets from existing multi-band photometry of nearby galaxies. We present the first results of the project, including the machine-learning methodology for target selection and results from our spectroscopic observations so far. The emerging trend for the ubiquity of episodic mass loss, if confirmed, will be key to understanding the explosive early Universe and will have profound consequences for low-metallicity stars, reionization, and the chemical evolution of 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.

Super star cluster (SSC) A1 in starburst galaxy NGC 3125 has the strongest broad He II λ1640 emission line ever observed in the nearby Universe and constitutes an important template for interpreting observations of galaxies that are located out to a redshift of z∼3. We use observations of SSC A1 obtained with the Cosmic Origins Spectrograph (COS) on board of the Hubble Space Telescope (HST) in order to check if there is a contribution of nebular emission to the He II line. In addition, we compare the COS G130M + G160M observations of A1 (1150 – 1750∘A) to the latest Charlot & Bruzual population synthesis models, which account for Very Massive Stars (VMS) of up to 300 Mȯ. A model with Z = 0.008 and age = 2.4 Myr provides a very reasonable fit to the C III λ1175, N V λ1240, C IV λ1550, He II λ1640, and N IV λ1718 stellar-wind features, although the O V λ1371 line is not well reproduced. Overall, our results show the great improvement of stellar evolution and population synthesis models over the past decade, and in particular, the improved formulation of stellar mass loss rates.

The core of the cluster R136 in the Large Magellanic Cloud hosts the most massive stars known. The high mass-loss rates of these stars strongly impact their surroundings, as well as the evolution of the stars themselves. To quantify this impact accurate mass-loss rates are needed, however, uncertainty about the degree of inhomogeneity of the winds (‘wind clumping’), makes mass-loss measurements uncertain. We combine optical and ultraviolet HST/STIS spectroscopy of 56 stars in the core of R136 in order to put constraints on the wind structure, improving the accuracy of the mass-loss rate measurements. We find that the winds are highly clumped, and use our measured mass-loss rates to test theoretical predictions. Furthermore we find, for the first time, tentative trends in the wind-structure parameters as a function of mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped than those with lower mass-loss rates.

In this poster, using the POSYDON code, we present results on binary progenitors of stripped-envelope SNe and their companions. We find that most progenitors are expected to explode, according to typical SN prescriptions (in contrast to single star progenitors). We also show the expected masses and position in the HR diagram of the companions of these SNe at the moment of explosion, allowing us to do a first statistical comparison with the compiled sample of observational detections (or upper limits) on these companions.

Luminous blue variables (LBVs) and B[e] supergiants (B[e]SGs) are some of the most massive stars that display extreme and puzzling behavior. Their rarity indicates that they belong to short evolutionary phases or short-lived phenomena in the post-main sequence evolution of massive stars. However, their strong mass loss and episodic mass eruptions may be crucially impacting massive star evolution. LBVs are a group of evolved massive stars that exhibit irregular variability and eruptive mass loss. Various subtypes, including S Doradus variables, giant eruptions, and pre-supernova outbursts, exist. The physical cause of the LBV phenomenon remains heavily debated. B[e]SGs have strong forbidden line emission and infrared excess from dust that are thought to arise in a circumstellar disk or torus. The formation mechanism of their disk-like structures is yet to be settled. The evolutionary phases of LBVs and B[e]SGs and their connection to other evolved massive stars are important unanswered questions in massive star evolution.

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 a detailed spectroscopic analysis of the only known eclipsing high mass X-ray binary with a black hole companion, M33 X-7. We obtained the first UV spectra of the system accompanied by X-ray observations, taken at three key orbital phases. We performed a detailed analysis of X-Ray, UV, and archival optical spectra using stellar atmosphere models which shed light on the interaction of the stellar wind with the black hole. Our new analysis suggests a large reduction in component masses compared to previous results. Our one-dimensional calculations confirm that the photoionization by the X-ray radiation can significantly change the ionization structure and diminish the wind accelerations. For this system standard wind-fed accretion scenario alone cannot explain the observed X-ray luminosity, indicating an additional mass overflow towards the black hole. Our evolutionary models suggest that the system is transitioning towards a common envelope stage in which both components merge.

The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a mystery till today. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields. The massive star τ Sco is a well-known member of the group of magnetic stars and is a blue straggler given its apparently young age compared to that of other members of the Upper Scorpius association. Here, we present 3D magnetohydrodynamic simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain τ Sco’s magnetic field, apparent youth and other observed characteristics. We argue that field amplification in stellar mergers is a general mechanism to form strongly-magnetised massive stars. Such stars are promising progenitors of magnetars, which may give rise to some of the enigmatic fast radio bursts, and their supernova explosions may be affected by the strong magnetic fields.

The VLT/FLAMES Tarantula Survey (Evans et al. 2011) identified a group of slowly-rotating nitrogen-rich O-type stars that cannot be explained by current evolutionary models. Here we present high-quality VLT/UVES observations of four of these stars that allow a detailed quantitative spectroscopic analysis. We present the analysis of the spectra with a genetic algorithm, and discuss the future steps to be taken to further investigate the cause of the nitrogen enrichment.

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.

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.

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.