Programme

This program uses the California/USA time zone

Monday 15

Sesión 1 | Session Chair: Nicole St-Louis

Large surveys and key observations

Massive stars are the engines of the Cosmos, shaping their environments and driving galaxy evolution across cosmic time. Yet, this general textbook picture faces many challenges when trying to turn abstract insights into quantitative predictions. Recent discoveries, such the surprisingly high metallicity and early nitrogen enrichment in high-redshift galaxies discovered by JWST, are challenging current descriptions of massive star evolution add new pieces to a puzzle that is yet everything but complete.

The oncoming era of large surveys will help us to extend our knowledge. Advances in computational modeling create the potential to reach breakthroughs in our theoretical understanding and add further missing puzzle pieces. Yet, to resolve current problems and conflicting conclusions, we will also need to reconsider what we think we know. Are the objects we observe what we think they are? Are the models we use describing what is actually going on? And what can we learn from previous misconceptions?

In this opening talk, I will present and discuss major open questions regarding massive stars, reaching from individual stars and stellar systems back to the first galaxies, recent discoveries as well as unsolved theoretical issues.
Over the past few decades, we have been experiencing a golden era in massive star research, particularly from an observational perspective. An ever-growing volume of high-quality, multi-wavelength data is becoming available for large samples of stars—not only within our Galaxy, but also in nearby galaxies spanning a wide range of metallicities.

In this talk, I will provide a general overview of the main characteristics of several large-scale (primarily spectroscopic) surveys specifically designed to advance our understanding of the physical properties and evolution of massive stars. I will also summarize key findings derived from the analysis of these datasets, including the realization that interpreting certain observables in the context of massive star evolution is significantly more complex than previously anticipated.
Gaia has been the mission that has contributed the most to Galactic astronomy in the current century. In this talk I will discuss what we have learned/are learning/will learn about massive stars with Gaia both from the instrumental and scientific points of view.

From the instrumental point of view, I will discuss astrometry, photometry, spectrophotometry, high-resolution spectroscopy, and variability in previous Gaia data releases and in the incoming DR4. From the scientific point of view, I will discuss distances, the sample of known (and unknown) massive stars, cluster membership and IMF, runaways, Galactic rotation, detection of emission line objects, extinction.
Ullyses is a large Director's Discretionary program for the Hubble Space Telescope devoting ~1000 orbits to establish a spectroscopic ultraviolet spectral library of young high- and low-mass stars. About half of these orbits are dedicated to massive stars in the Large and Small Magellanic Clouds as well as Local Group dwarf galaxies.

The XShootU collaboration aims to complement the UV data for massive stars by VLT/XSHOOTER spectra covering the full optical to near-infrared wavelength range. In this talk I will discuss the publicly available high-level data products produced by the XShootU consortium, which include corrections for slit losses and telluric absorption, absolute flux calibration, and rectification to the continuum. I will also give an overview of the key scientific results produced by the consortium so far.
We present results from a new pipeline applied to two large spectroscopic samples of Magellanic Cloud OB stars, 174 LMC+SMC stars from the XShootU survey (Vink et al. 2022) plus 778 SMC stars from the BLOeM survey (Shenar et al. 2024). The pipeline (Bestenlehner et al. 2024) utilises large grids of FASTWIND models and is designed to automatically analyse large spectroscopic samples.

Results from the XShootU study, reveal median O (B) evolutionary masses of 46 Msun (27 Msun) and 32 Msun (20 Msun) for the LMC and SMC, respectively (Bestenlehner et al. 2025a). Pipeline results support higher wind momenta in LMC stars with respect to the SMC, incorporating empirical wind velocities from ULLYSES.

The SMC BLOeM study comprises 137 O-type & 641 B-type stars (Bestenlehner et al. 2025b), with median masses of 20 Msun and 12 Msun, respectively, and has the advantage of multi-epoch spectroscopy for the identification of short period binaries. Rotational velocities are strikingly different for single stars and binaries, with evidence of a bimodality for O stars. Satisfactory agreement is achieved with respect to resource-intensive literature studies, offering the possibility of efficient analysis of upcoming surveys (4MOST, WEAVE).
This talk presents an integrated overview of our recent results from the BLOeM campaign in the Small Magellanic Cloud (Z = 0.2 Z⊙). Drawing on the initial nine epochs of VLT/FLAMES spectroscopy, we investigated the multiplicity properties of 929 massive stars across a wide range of masses and temperatures.

Our findings reveal contrasting binary properties across evolutionary stages: O-type stars show an intrinsic close binary fraction of at least 70%, while early B-type dwarfs/giants exhibit an even higher fraction (80%), contrasting with values reported for higher-metallicity samples. The evolved B0-B3 supergiants display a significantly lower binary fraction (40%). This fraction further decreases to 8% for the cooler supergiants (A-F), although intrinsic variability is suspected to contribute, suggesting that the multiplicity fraction could be even lower.

OBe stars exhibit distinct binary properties compared to other OB stars, supporting a post-interaction origin. These results have profound implications for understanding massive star evolution at low metallicity, including the production of exotic transients, gravitational-wave progenitors, and ionising radiation in the early Universe.
In the era of large spectroscopic surveys, a vast amount of spectra of massive stars will be gathered and supplemented by the wealth of astrometric and photometric data provided by the Gaia satellite. Released data will mean a major step forward in the study of massive stars, giving us the chance to create statistically significant samples to explore the role of almost any parameter.

In this contribution, I will introduce to the community the Multi-wavelength Exploration of massIve star-forminG regions and ASsociations project (MEIGAS), its current status and long-term plans for conducting comprehensive studies in the major galactic and extragalactic star-forming regions and OB associations.

Benefiting from current and forthcoming data from large scale spectroscopic surveys such as WEAVE, 4MOST, GES, or XShootU, as well as complementary observations at different wavelength ranges, the project aims to achieve crucial and complementary information to adequately characterize these regions and their stellar content, something imperative to improve our understanding of star formation and poorly known evolutionary pathways of massive stars.
Massive stars are pivotal drivers of galactic chemo-dynamical evolution. Despite their relevance, several important aspects of their nature and evolution remain poorly constrained. A main limitation in trying to narrow down the existing uncertainties is the use of small or incomplete samples.

Fortunately, large-scale spectroscopic surveys together with Gaia kinematics offers the opportunity to analyze the physical, chemical, kinematical, and binary characteristics of thousands of massive stars in the Galaxy. However, the study of mixed populations born under different initial conditions can lead to the wrong interpretation of the results. In this context, young open clusters and associations become ideal laboratories given the similar age and initial composition of their stellar populations.

In this presentation, I will focus on Per OB1, one of the known young associations with more massive stars in the Galaxy and home to the h and χ Persei double cluster. I will present recent findings on the physical, chemical, kinematical, and binary properties of Per OB1's complete population of luminous OB stars, and I will compare these findings with those obtained from the analysis of a thousand Galactic OB stars within the IACOB project.
Stripped stars—massive stars that have lost their hydrogen envelopes due to interaction with a binary companion—are predicted to be hot, compact, and strong emitters in the extreme UV. Consequently, a subset of these stars can be identified by a UV excess, appearing bluewards of the MS. However, exploiting this signature requires UV data for many stars—a challenge due to uncertain distances and reddening in the Galaxy coupled with the high degree of crowding in nearby Galaxies.

Here, I present a new UV catalog containing over 700K sources in the Magellanic Clouds, where low extinction, known distances, and testable metallicity effects provide an ideal environment. I identify hundreds of candidates in each galaxy, with brightnesses and colors that agree with expectations for stripped stars of ~2–8 M☉, bridging the gap between subdwarfs and WR stars.

I show how these candidates are being confirmed by showcasing their optical spectra, all of which match predictions for stripped star binaries. Finally, I show how these systems are being characterized by discussing an interesting case study of a stripped star in a short period (4.2 h) orbit around a compact object companion, a possible progenitor of a compact object merger.
We present a multi-epoch, radial velocity (RV) survey of 55 field OB and OBe stars of the SMC Wing from the RIOTS4 survey obtained with the Magellan IMACS and M2FS multi-object spectrographs. We obtain a binary fraction of at least 36% (60% with candidate binaries), divided equally between OB and OBe binaries.

The RV data reveal that OB binaries appear to favor more circular, tighter orbits, while OBe binaries suggest higher eccentricities. Our RV results are also used to set constraints on companion masses, and for some cases, other orbital parameters. We identify 4 candidate black hole binaries.

We compare these results to BPASS models of OB and OBe stars that assume OBe stars are binary mass gainers ejected by the companion supernovae. We predict the frequencies of black-hole, neutron-star, and stripped-star companions, and the distributions of binary orbital parameters and compare these to our observations. Our results indicate that the models are broadly consistent with the binary origin scenario for OBe stars, and they predict an even larger number of post-supernova OB binaries.
We offer a review of ongoing spectroscopic surveys of massive stars and their local environment as part of the Sloan Digital Sky Survey. The Milky Way Mapper (MWM) OB program is expected to obtain ~10^5 multi-epoch optical (BOSS, 3600–10400 Å, R~2000) and near-infrared (APOGEE, 1.5–1.7 μm, R~22500) spectra for OBA targets in the Milky Way and the Magellanic Clouds. This program will offer an unprecedented panorama of their properties across the Galactic disk ecosystem, allowing us to detail kinematic distributions, their impact on the ISM, and their relation to the evolution of young star clusters.

As part of the SDSS V survey, we are also exploring the interstellar medium in the Milky Way, the Magellanic Clouds, and galaxies of the Local Group with the Local Volume Mapper (LVM). The LVM is mapping the warm ionized ISM through 3D spectroscopy (3600–9800 Å, R~4000) using a state-of-the-art wide field IFU (0.165 sq deg, 1800 f), aiming to explore hundreds of H II regions, from the solar neighborhood to nearby galaxies. The MWM-LVM synergy sets one magnificent stage for OB star science in the next few years.

Sesión 2 | Session Chair: Aida Wofford

High-z stars: observed and synthetic.

The formation of the first stars marks the end of the cosmic dark ages, thus initiating the fundamental transition from the simple initial conditions imprinted in the early Universe to a state of increasing complexity. Based on multi-scale cosmological simulations, the field has developed a “standard model” of first star formation, with the key prediction of a top-heavy initial mass function, and possibly hints at the important role of rapid rotation.

Furthermore, there are theoretical reasons to expect a boosted star formation efficiency, proceeding in a bursty, stochastic fashion. With the JWST, we can now confront this theoretical framework with a network of empirical constraints, driving a period of exciting progress in our understanding. In this talk, I will summarize this dynamic development, and suggest pathways towards further advances.
JWST has opened up a new era of gas-phase chemical abundance studies for galaxies in the early Universe. Massive stars play many key roles in galaxy chemical evolution: as a primary enrichment channel for metals, as drivers of feedback and outflows from their deaths as supernovae, and as sources of ionizing photons that power the nebular line emission used to derive chemical abundances.

The sensitivity of JWST's spectrographs has opened up many new avenues of exploration, including measuring metallicities of faint and very high redshift sources, calculating metallicities using the temperature-based direct method, and constraining detailed multi-element gas-phase abundance patterns. I will summarize what we have already learned about chemical enrichment and evolution at early times in the JWST era and some of the questions we have yet to solve.
I will describe the huge observational progresses made since 2016: review LyC detections from z~0 to z<4, and summarise what we learned.

I will give an overview of radiation hydrodynamical simulations of Cosmic Reionization, describing assumptions concerning the input LyC spectrum, and focussing on galaxy scale properties: which results seem robust/universal.
Very massive stars, with initial masses exceeding 100 solar masses, play a crucial role in shaping the chemical evolution of the early universe. Their explosive deaths as pair-instability supernovae could significantly enhance Fe/O and S/O ratios in very low-metallicity environments, providing a distinct chemical signature of their contribution.

Sulfur, with its strong emission lines in the galaxy spectra and its minimal depletion by dust, could serve as a robust and alternative tracer of early enrichment. Population III (Pop III) stars, the first generation of stars formed from primordial gas, remain elusive but can be detected indirectly through the HeII1640 emission line produced by their intense ionizing radiation.

By post-processing galaxies from cosmological simulations and modelling the emission line spectrum, we can compute the expected HeII1640 line strengths, providing crucial insights into detecting Pop III candidates with next-generation telescopes. These advancements offer critical insights into early nucleosynthesis, galaxy formation, and the role of very massive stars in the chemical enrichment history of the universe.
I will describe recent advances in understanding massive stars in high redshift galaxies from the James Webb Space Telescope. The JWST Advanced Deep Extragalactic Survey (JADES) has discovered the most distant galaxies so far, spectroscopically confirmed with NIRSpec, including a very UV-luminous galaxy at z=14 where bursty star formation or a top-heavy initial mass function may be brightening the UV more than seen at lower redshift.

In another galaxy within the epoch of reionization we have evidence of a Balmer jump due to nebular continuum, and hot enough stars to produce enough ionizing photons such that the two-photon emission becomes visible, again hinting at a top-heavy IMF. We have also discovered a nitrogen-rich galaxy, GN-z11, at z=10.6 where the N/O abundance ratio is significantly above solar, suggesting runaway stellar collisions in a dense cluster and potential Super Massive Star formation.

The halo of GN-z11 has a patch of HeII1640 emission which may be consistent with Population III stars in the outer regions.
Massive stars have a profound impact on their host galaxies: they dominate galaxy spectra at most wavelengths, drive early interstellar enrichment, and provide energetic feedback to regulate future star formation. Despite their key role, many aspects of their appearance and evolution at early cosmic times remain poorly understood.

Our team established the ubiquity of α-enhanced ([O/Fe]~0.4), metal-poor ([Fe/H]~-0.8) massive star populations at high redshift. I will share the latest results from CECILIA, our JWST/NIRSpec program that obtained ultra-deep (30 hr) rest-optical spectra of individual z>2 galaxies.

These observations contain myriad spectral features sensitive to the properties of ionized gas and massive stars, and can be combined with rest-UV spectra of the same galaxies to constrain the shape of massive stars' ionizing spectra and facilitate direct comparisons with stellar population synthesis models. I will conclude by highlighting areas where we need improved stellar models to interpret observations of high-z galaxies and opportunities to use current and future galaxy studies to characterize these chemically distinct massive stars.
Detections of high-ionization line emission in z>6 galaxies continue to puzzle the community in the JWST era, and cast some doubt on our ability to confidently interpret the earliest phases of galaxy assembly.

Disentangling the uncertain contributions of Pop III star clusters, early accreting black holes, and other potential sources will remain a key challenge for this new era of extragalactic astrophysics.

In this talk I will focus on work closer to home in calibrating the uncertain contribution of extremely metal-poor massive star populations, where puzzling detections of lines like He II are still debated. I will highlight results from an ongoing He II narrowband survey (HelgI) at Magellan, designed to identify highly-ionized nebulae alongside wind-driving hot stars, and discuss how combining these observations with deep spectroscopy of unresolved dwarfs beyond the Local Group is key to completing our picture of high-z emission sources.
Understanding the processes in place during the epoch of reionization (EoR) is a frontier goal of observational cosmology, centered on uncovering the sources of reionization.

Local reionization-era analogs are commonly used as proxies for EoR galaxies; however, these analogs are often too massive. The low-mass dwarf starburst galaxy Pox 186 (≈10^5 M☉) offers physical conditions representative of EoR systems.

We discuss comprehensive panchromatic data from the far-UV to the far-IR, emphasizing HST and JWST observations. We model the stellar population, ISM, and dust using Starburst99 and BPASS, and address challenges in these models to inform the next generation of stellar atmosphere and evolution models.
We present the properties of six Type II supernovae (0.675 < z < 3.61) discovered by the JWST Advanced Deep Extragalactic Survey (JADES) transient survey through light-curve modeling. Two SNe exhibit high explosion energies (~3×10^51 erg), while four have typical energies (0.5–2×10^51 erg).

The fraction of high-energy Type II SNe may be higher at high redshifts due to lower metallicity, but small sample size and observational biases limit firm conclusions.

We found that introducing confined, dense circumstellar matter improves light-curve fits, suggesting its frequent presence at high z. More high-redshift Type II SNe are required to explore differences between nearby and distant events, but this work offers the first glimpse into the high-redshift population.
Over the last decade, dozens of transients comprising individual, luminous stars at background lensed galaxies have been discovered by HST and JWST through strong lensing by galaxy clusters and microlensing by intracluster stars.

Since only massive stars can be detected as such transients, the detection rate directly correlates with the abundance of massive stars and thus probes the high-mass end of the IMF in lensed galaxies.

As proof-of-concept, we model the transient detection rate in the “Spock” galaxy (z ≈ 1) and reproduce observed blue transient rates sensitive to the IMF. Bayesian analysis favors a Salpeter-like IMF over a top-heavy one.

With upcoming JWST surveys, more lensed transient events will be found at various redshifts, offering a novel probe of the high-mass IMF in the early universe.

Tuesday 16

Sesión 3 | Session Chair: Jorick Vink

Stellar evolution and atmosphere models: theory

A very distinguishing characteristic of massive stars is their proximity to the classical Eddington limit. This leads to radiation-dominated, highly turbulent envelopes and atmospheres and to powerful radiation-driven outflows, where interactions with a multitude of spectral lines are key. In this talk we will review the current state-of-the-art of such line-driven winds for hot, massive stars, discussing analytic and numerical models as well as their confrontation with various observational constraints. In the spirit of the main theme of this conference, we will pay particular attention to how stellar wind properties are expected to change with metallicity, and how outflows from (very) massive stars in the early Universe might differ from their present-day counterparts.
Binary interactions shape the evolution of the most massive stars, leading to significant deviations from the evolutionary pathways possible in single star evolution. These processes impact the universe at large scales and result in high energy events such as peculiar supernovae and gravitational wave sources. To understand these outcomes it is important to assess binary evolution in early stages, ranging from pre-interaction, roche-lobe overflow and post-interaction phases. I will discuss the current progress in our understanding of mass-transferring binaries, covering the impact of this process on the donor star (with the possible production of a stripped star), as well as the response of its companion. Of particular importance in recent years is the identification of bloated stripped stars caught immediately after interaction which provides a snapshot of the end-states of mass transfer, and I will discuss how their properties constrain orbital evolution and the efficiency of mass transfer. I will also emphasize that many of the uncertain processes in massive binary star evolution can also be assessed through the study of intermediate mass systems, for which the physics in early evolutionary phases does not differ significantly.
Massive stars play a key role in the Universe via their radiatiave, kinetic and chemical feedback. Understanding their evolution and fate is thus critical. For this purpose, we can use stellar evolution models to predict how the structure and composition of massive stars change with time. These models require many important physical ingredients, e.g. nuclear reaction, mass loss, rotation, magnetic fields, binarity and convection. In this talk I will focus on mass loss, rotation and convection and present large grids of one-dimensional (1D) models as well as detailed 3D hydrodynamic simulations and the synergies between the two. I will also review the key current uncertainties and their impact on theoretical predictions.
Stellar atmosphere models have long been computed in the 1D spherically symmetric, steady state assumption. Whilst this is has been necessary in the past to keep the modeling efforts feasible, the last few years have seen developments which have recently culminated in the first generation of multi-D time dependent models.

These new multi-D models show us that the typical assumptions made in previous 1D models (clumping descriptions, ...) are perhaps not the best description of what is actually happening in stellar atmospheres. Whilst the computationally heavy 3D models are too expensive to use in modern grid-fitting techniques, they can be used to better inform 1D models.

In this talk, I will talk about the recent advancements in multi-D modeling of hot massive star atmospheres, and I will talk about what is to come in the next generation of models. In addition, I will discuss how these multi-D models inform us to rethink 1D models with an updated description which is better at representing the structure formation inside these models as compared to the current 2-component medium. A key result here is for example the influence of turbulent pressure on the overall atmospheric structure.
Born with masses greater than 100 Msun, very massive stars (VMS) are extreme sources of radiative, mechanical, and chemical feedback. Due to their high luminosity, individual VMS can dominate the integrated light of stellar populations, emphasizing the need for accurate models of such stars, particularly in the context of JWST observations of unresolved galaxies in the early universe. In this talk, I will present our current work supplementing evolutionary models computed in GENEC with detailed atmosphere models computed in PoWR to obtain a more comprehensive picture of VMS. As part of this effort, we have computed, for the first time, a series of deep atmospheric models of VMS that cover the entire region of the hot iron bump. Our results show that this approach avoids the opacity-induced inflation predicted by the evolutionary code during the main sequence, which significantly impacts the predicted effective temperatures of VMS and reveals new uncertainties in spectroscopic diagnostics. To address these inconsistencies, I will conclude by discussing our ongoing work to improve the physical treatment of the iron bump region in our 1D PoWR code, particularly by including a more realistic treatment of radiatively-driven turbulence.
Unveiling massive stars’ internal structure and the physical origin and efficiency of the internal mixing processes? It is now possible using the apsidal motion rate (AMR) in close eccentric binaries! The AMR depends on the tidal interactions occurring between the stars and is proportional to k2, a measure of the star’s inner density profile. The AMR is commonly derived from the eclipses’ times of minima, made possible thanks to the high precision TESS/Kepler observations. I propose an innovative approach: derive the AMR from radial velocities obtained over a VERY long timescale combined with lightcurves to get high-accuracy consistent physical and orbital parameters for the binaries. I highlight recent results concerning the two most massive binaries studied this way.

Confronting standard stellar models with observations reveals the famous k2-discrepancy: models predict too high a k2 for the stars, i.e. stars with too low a density contrast. GENEC models including tidally-enhanced rotational mixing finally lift this discrepancy! Tightened to the observed binaries, the models reveal the instabilities allowing to reproduce the stellar density profiles. It paves the way for the next generation of rotating single/binary stars models.
Stars stripped of their hydrogen-rich envelopes are long known to be products of interacting binary systems. Binary stellar mergers are accepted as most common way to form low mass hot subdwarfs, and are also invoked to explain high mass single Wolf Rayet stars at low metallicity. The new sample of intermediate-mass helium-rich stars (Drout et al. 2023) bridges the mass gap between subdwarfs and Wolf Rayets, offering a unique opportunity to study whether stellar mergers contribute to the full mass range of stripped stars. We propose that the merger of two helium stars can form long-lived, intermediate to high-mass stripped products. The channel sees a stable mass transfer episode stripping the primary into its helium-rich core; rejuvenation and expansion to Roche lobe of the secondary follows; common envelope evolution drives the merger of the helium cores. We produce detailed binary evolution models to investigate initial conditions and population properties of such merger products. Our results reveal that stable mass transfer between a Main Sequence primary and a similar mass secondary at low metallicity leads most favorably to intermediate, long lived helium star products, substantiating the properties of the observed sample.
Massive stars drive the chemical and mechanical evolution of their host galaxies and thus understanding their current properties and evolution is essential for our understanding of the universe as a whole. However, the detailed modeling of massive star atmospheres is complex and computationally intensive due to the fact that the LTE assumption is not valid in this regime, often requiring significant resources and time to obtain the surface parameters of a given system. With the advent of large spectroscopic surveys and next generation multi-object spectrographs on the horizon, the need for efficient and accurate modeling tools has become increasingly important. To this end, we have developed an emulator for the NLTE radiative transfer code FASTWIND using a series of deep neural networks each trained to emulate individual spectral lines. In this talk, I will discuss the architecture of the neural networks, the training process, and the results of our initial tests. I will demonstrate that the emulator can reproduce the results of FASTWIND with high accuracy and I will discuss how this emulator can can act as a drop-in replacement of FASTWIND in certain cases. Furthermore, I will present a neural network based modelling suite that we have developed that allows the user to apply several different fitting techniques to model observed spectra, and I will discuss how these compare with other currently available methods in terms of accuracy, speed and computational cost. Finally, I will discuss the potential applications of this emulator in the context of large spectroscopic surveys and how this proof of concept work can be expanded to include more complex emulators in the future.
Realistic atmosphere models have progressively been included through various methods in state-of-the-art low- and intermediate-mass stars evolution calculations. At the same time atmosphere models for massive stars have become increasingly more realistic over the last 50 years, with the progressive inclusion of non-LTE and non-gray effects, wind extension, and line-blanketing. However all these improvements have never been included in massive stars evolution calculation, which in their vast majority still use the Eddington gray approximation. In this talk I will present preliminary results of the inclusion of realistic atmosphere models in the evolution calculations of massive stars.

Sesión 4 | Session Chair: Takashi Moriya

Stellar evolution and atmospheres: observations

On the route towards merging neutron stars, binary population synthesis predicts a large number of post-interaction systems with massive stars that have been stripped off their outer layers. Yet, observations of such stars in the intermediate-mass regime are rare.

Furthermore, recent observations of a growing population of 'bloated' or partially stripped stars, a phase predicted to be significantly shorter-lived than compact, hot stripped stars, challenge current binary evolution models. In this talk, I will present recent advancements in the study of these partially stripped stars in Be binaries.

I will focus on their identification using multiwavelength and multi-epoch spectral data, along with the spectral analysis of both components in the binary using stellar atmosphere models. Using this, we uncover their fundamental parameters and surface abundances.

Additionally, using UV data, we probe their stellar winds and compare their mass-loss rates and wind velocities to commonly used recipes. I will discuss the evolutionary nature and the implications of these discovered systems, particularly addressing the challenge posed by the observed 'bloated' stars and the need for refinements in current evolutionary models.
Model atmosphere analyses provide observational constraints that can be used to test models of OB stars to answer key questions of massive star research. In the focus are elemental abundances and atmospheric parameters, which in particular in the era of precise and accurate parallax measurements by the Gaia mission can be used to derive fundamental stellar parameters. An overview is given of the state-of-the-art of quantitative spectroscopy of OB stars, concentrating on one hand on the question how well information can be extracted from observations, which is of particular interest on the eve of the upcoming large-scale spectroscopic surveys. On the other hand, existing observational results are confronted with model predictions, illuminating successes but also pointing out the open questions.
Massive X-ray binaries are fascinating astrophysical systems that offer valuable insights into some of the most extreme physical processes in the universe. These binary systems provide a unique opportunity to understand the end products of stellar evolution, specifically neutron stars and black holes.

As precursors to gravitational waves and short gamma-ray bursts, massive X-ray binaries play a crucial role in studying a key stage of binary stellar evolution and help us comprehend the pathways leading to some of the most exotic and extreme astrophysical objects.

In my talk, I will provide an overview of multiwavelength studies of massive X-ray binaries, highlighting some of the significant recent findings. The focus will primarily be on the largest subclass of massive X-ray binaries, known as Be X-ray binaries.

The complex interactions between the Be disc and the neutron star in these systems remain a puzzling area of research. Additionally, I will offer an overview of a related class of binary systems called gamma-ray binaries, where many of the mysteries center around the unknown nature of the compact objects involved.
The B[e] phenomenon discovered nearly 50 years ago features the presence of forbidden emission lines due to extended and dense circumstellar gas and large IR excesses due to the radiation from circumstellar dust in a wide variety of objects from pre-main-sequence stars to Planetary Nebulae. It also shows up in a small group of supergiants that includes Luminous Blue Variables, such as Eta Carinae.

Over the years, some of them were proven to be binary systems, but the presence of a secondary component in other is still elusive. At the same time, there is growing evidence that the B[e] phenomenon can be due to binary mergers or interactions in triple systems.

Our studies of mostly Galactic objects with the B[e] phenomenon over the last 20 years resulted in finding many new such objects, determination of their intrinsic properties, and revealing evolutionary history of discovered binary systems. We will present our results on the most luminous objects, compare their properties with other groups of evolved stars, and discuss the current understanding of processes that lead to the creation of the B[e] phenomenon in intermediate-mass and massive stellar systems.
Mass-loss is a key mechanism affecting the evolution of massive stars. However, the real mass-loss rates at the low-metallicity, low-luminosity regime have yet to be constrained and only have upper limits.

In this work, we present the panchromatic analysis of a sample of SMC O-stars from the weak to the strong-wind regimes, including the Br-alpha line as a sensitive mass-loss diagnostic for weak-winded stars. This new diagnostic line is only made possible with JWST-NIRSPEC, and is crucial to compensate for the insensitivity of classical mass-loss diagnostics such as H-alpha and UV P-Cygni profiles in the weak-winded regime.

We use CMFGEN atmosphere models to analyse the observed spectra and derive the best-fitting stellar parameters. These models account for wind inhomogeneities and X-ray emission from wind shocks.

This work will deepen our understanding of the radiation-driven wind theory and its caveats, and its repercussions on our current knowledge of massive star evolution.
Metal-poor massive stars have wide-ranging impacts on their low-mass host galaxies, both locally and at high redshift. Theory predicts that their radiation-driven winds are weak compared to their higher-metallicity counterparts, with important implications for stellar evolution, feedback, and ionizing photon production.

However, few FUV and optical spectra of individual metal-poor OB stars exist to validate models of stellar mass loss due to the large distance of even the closest galaxies more metal-poor than the SMC (20% Solar). I will present new HST/COS and Keck/KCWI spectra of three O stars in very low-metallicity (3-14% Solar) galaxies.

PoWR atmosphere models fit to these observations constrain the stellar mass-loss rates and terminal wind speeds, favoring weaker winds than predicted by widely adopted theoretical wind recipes at these low metallicities.

Finally, I will describe the Treasury of Extremely Metal-Poor O Stars (TEMPOS), a new Large HST program assembling a FUV spectral atlas to scale this analysis to a larger sample. TEMPOS will provide a much-needed empirical benchmark for the models necessary to understand stellar evolution and feedback in metal-poor galaxies from the local universe to the epoch of reionization.
AT 2016blu is a supernova impostor that has experienced 21 eruptions, recurring every 113 ± 2 days. These eruptions are likely driven by periastron encounters in an eccentric binary system, in which the primary star is an LBV.

Spectroscopic analysis shows that AT 2016blu’s spectrum remains consistent over time, resembling that of hot LBVs. At some epochs, the Hα profile exhibits multi-component P Cygni absorptions with velocities significantly exceeding the FWHM of Hα, which are most likely associated with a companion.

No correlation is observed between the spectral features and either the phase or the magnitude, suggesting an interaction between a companion and a variable or inhomogeneous primary wind in an orbit with only mild eccentricity.

The eruptive behavior and spectral characteristics of AT 2016blu closely resemble those observed in the precursor of SN 2009ip before its supernova explosion, as well as the periastron encounters in Eta Carinae before its giant eruption. This suggests that AT 2016blu may be approaching a major evolutionary milestone, making it a target of great interest.
Red Supergiants (RSGs) represent the final evolutionary stage of massive stars before core-collapse supernova explosions. However, the scarcity of luminous RSGs as supernova progenitors, along with observational evidence of warm, post-RSG objects, suggests a blueward evolution before the explosion.

Since the 1980s, WOH G64 has been regarded as the most extreme RSG in the Large Magellanic Cloud due to its exceptional size, high mass-loss rate, and proximity to the Humphreys-Davidson limit. Long-term photometry revealed an abrupt change in 2014, despite the absence of an outburst.

Subsequent spectroscopy revealed an unprecedented transition, with WOH G64 now exhibiting B[e] spectral features. We discovered that WOH G64 has transitioned to a Yellow Hypergiant and is part of a massive symbiotic system with a B-star companion.

Our ongoing spectroscopic monitoring will clarify whether a silent eruption, binary interactions, or a pre-supernova wind phase drove the transition. As one of the most extreme massive stars, WOH G64 offers a unique opportunity to witness stellar evolution in real time, providing crucial clues for the final phases of massive stars and their resulting supernovae.
Massive stars hold key to the evolution of galaxies due to their feedback into the surrounding medium in the form of energy, momentum and chemically enriched material. Most of this feedback happens during their post main sequence evolution, whose exact details are still uncertain.

Large samples of massive stars in different phases of evolution and chemical conditions are required to comprehensively address this issue. Such samples are historically hard to assemble beyond the Local group because of the difficulty of identifying individual stars beyond the Local Group due to the faintness and crowding.

The arrival of JWST promises to improve the situation. We have started a program to carry out analysis of all point sources catalogued by DOPHOT in the JWST and HST images of nearby galaxies to identify massive stars in different evolutionary phases using the technique of Resolved Stellar Photometry.

The study allows us to obtain the number of O-stars and BSGs and RSGs. A comparison of the ratio of these numbers with that expected from different stellar evolutionary codes would help us to improve the current understanding of massive star evolution. Results for M51, M83 and NGC628 would be presented in this work.

Wednesday 17

Sesión 5 | Session Chair: Miriam García

Asteroseismology, spectropolarimetry, interferometry

Asteroseismology is the study and interpretation of stellar pulsations. Such pulsations are observed as variations in the surface brightness of the star and travel deep into the stellar interiors, thereby providing direct probes of the structure and the physical processes that dominate the lives of stars.

With this review talk, I will briefly explain how we can use stellar pulsations to study interior mixing, rotation, and magnetic fields. I will then discuss recent results focusing on O- and B-type stars in single or binary systems, open clusters, and OB associations.
Magnetic fields play an important role in the formation and evolution of stars. In massive stars observational studies of their magnetic fields have demonstrated how they can strongly impact the stellar evolution.

In this talk I will describe how high-resolution spectropolarimetry allows to characterize the properties of stellar magnetic fields. I will then review our current knowledge on massive stars magnetism, and its impact on stellar evolution.

Finally I will discuss the different theories on the origin of stellar magnetic fields, and the future avenues to go further in that domain.
Despite their rarity, massive stars play a fundamental role in the evolution of their host galaxies. Through their powerful ionizing radiation, these cosmic engines are responsible for a large amount of the global energy budget of their host galaxies and for shaping their interstellar medium.

Their strong winds not only deposit large amounts of momentum and energy, but also make them the main sources of CNO and heavy elements to the interstellar medium. However, even considering their importance, there are still many open questions, especially about their formation, evolution, mass-loss and multiplicity.

Therefore, for this talk, I will present the results obtained by our group using speckle interferometry on small telescopes and optical interferometry at the VLTI to obtain high-angular resolution observations, derive the multiplicity ratio and describe the circumstellar medium of Be, B[e] and LBV stars. In addition, I will present the new project OCEANS (Overcoming Challenges in the Evolution And Nature of massive Stars) involving an international multidisciplinary network of researchers to study massive stars.
After decades of efforts, optical long-baseline interferometry has become a mainstream observational technique in terms of operation robustness and user friendliness. Its use has however remained limited in our massive star community despite the unique high-angular resolution window it offers into fundamental properties of massive stars.

In this presentation, I propose to provide an overview of the contribution of interferometry to massive star science in the last decade, covering topics such as OB and WR binaries, companion mass function, initial parameter distributions and stability of triple systems, WR stellar winds and other cornerstone observations on massive star formation.

I will also briefly outline the outlook promised by ongoing upgrades and future instrument projects.
Most stars have angular diameters of less than one milliarcsecond (mas), making optical interferometry essential for direct size and shape measurements. Historically, optical amplitude interferometers have dominated this field.

However, OB main-sequence stars are not the ideal targets for these instruments: they are brighter in short wavelengths where amplitude interferometry becomes very technically challenging and generally have angular diameters beyond their typical resolution. A new generation of optical intensity interferometers, capable of observing in the Blue band, are emerging by taking advantage of the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes.

Among those, the MAGIC + LST1 Stellar Intensity Interferometer (SII) is the most sensitive. Once fully commissioned, it will be capable of resolving stars with angular diameters between 0.3 to 1 mas up to magnitude 6 in B in just a few hours. During its first year of operations the MAGIC + LST1 SII has been able to resolve the shape of the Be star gam Cas. In addition to this new measurement for gam Cas, prospects for similar measurements on other Be stars as well as for massive close binaries will be shown in this presentation.
Massive stars start their lives surrounded by accretion disks and launching powerful protostellar jets that carry mass and angular momentum away. For a long time, the exact mechanism that drives early protostellar jets was unknown.

Using very long-baseline interferometry in the star-forming region IRAS 21078+5211, recent detailed observations of water masers have revealed for the first time the kinematics of individual streamlines of the protostellar jet emerging from the accretion disk (at ~20 au away from the forming massive star). At the same time, high-resolution simulations of protostellar jets have enabled us to study in detail the dynamical processes driving the jet.

In this contribution, I present these simulations (which include self-gravity, resistivity and radiation transport) and perform a detailed comparison between the streamlines predicted by the simulations and the observed water maser patterns. As a result, we conclude that protostellar jets are launched magneto-centrifugally.

Finally, I discuss how protostellar jets can remove angular momentum from the protostar, helping us uncover the origin of rotation on massive stars upon their arrival into the main sequence.
Macro physics refers to the large-scale physical processes and structures governing stellar evolution, including convection, rotation and internal gravity waves (IGWs). Through several projects, we've investigated 3D macro physics using computationally intensive stellar hydrodynamics simulations with the PPMstar code developed by Paul Woodward (University of Minnesota) for non-rotating and rotating stars.

I'll present new findings from main-sequence stars on core-convection boundary structure, convection penetration extent, and IGW excitation and propagation—including whether IGWs cause mixing in stably stratified regions.

Building on earlier work, our simulations now encompass almost the entire main-sequence 25 M☉ star, including a thin near-surface convection zone. These comprehensive simulations provide new insights into how both core and envelope convection contribute to the asteroseismically observed stochastic low-frequency excess.

I'll also discuss radiation-pressure dominated core-convection in supermassive main-sequence stars, double-shell He-H convection in a 45 M☉ Pop III star, and how IGWs observed in these simulations suggest the first massive stars evolved very differently from contemporary counterparts.
The majority of massive stars do not live alone, and their various interactions with their companion(s) can drastically change their further evolution. In the most extreme case, interactions such as mass transfer can result in a stellar merger. At least ~10% of mass-transferring binary systems are expected to merge.

Hence, we expect the resulting merger products to be common in populations of single stars. Neglecting a star’s possible merger origin could lead to errors in its inferred parameters.

Several properties, such as slow rotation, peculiar surface abundances, and strong large-scale magnetic fields, have been attributed to stars being merger products. However, these surface diagnostics alone are usually insufficient to unambiguously identify merger products.

We explore if and how the predicted differences in the interior structures of merger products and stars born as single stars manifest in their stellar pulsations. More specifically, we demonstrate the potential of asteroseismology to distinguish merger products from “genuine single” massive main-sequence and blue-supergiant stars and discuss the steps that still need to be taken before this can be put into practice.
The winds of WR stars exhibit two main forms of structure: stochastic small-scale clumping and large-scale organized patterns, the latter often linked to Corotating Interaction Regions (CIRs) driven by rotational modulation. WR6 is a prime example of such structured winds, showing strong variability with a 3.76-day period.

Variation patterns are found to be stable over several weeks and then evolve in nature, which is often interpreted as the signature of CIRs shaping the wind dynamics. Spectropolarimetry is a powerful tool to investigate wind structures.

I will present CFHT/Espadons observations obtained in 2009, 2010 and a new dataset over 20 days in 2024 that reveal periodic variability that support the CIR interpretation. Our data show significant linear polarization variations across several He II lines and lines of other ions such as He I, C IV, N IV and N V.

These changes can be partly attributed to the dilution of continuum polarization by unpolarized line flux, but this mechanism alone cannot fully explain the observations. Additional effects such as intrinsic line polarization and occultation effects are required to account for the complex variations seen predominantly in the blue-shifted portion of the lines.

Thursday 18

Sesión 6 | Session Chair: Hugues Sana

Final products of massive stars

In this talk I will give an overview, focusing on literature from the last year in the field, about what we have learned about massive star formation, lives, and deaths from gravitational-wave observations.
The gravitational collapse of massive stars at the end of their life marks the beginning of core-collapse supernovae (CCSN), which play a central role in the formation of compact objects, the dynamics and chemical evolution of their host galaxies, and the emission of multi-messenger signals in the form of electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays. Numerical models of the exploding massive stars have reached a high degree of complexity, as state-of-the-art 3D magnetohydrodynamic simulations of CCSN routinely include now multi-dimensional neutrino transport schemes, nuclear equations of state, and the effects of general relativity. However, the role of rotation and magnetic fields, along with the complex dynamics that couples them, is still a challenging subject that requires more in-depth studies.

In this talk I will review the recent developments in the 3D modeling of magnetized CCSN, with some emphasis on the dynamo processes that can amplify magnetic fields during the explosion. I will show the dynamical impact of magnetic fields on the onset of the explosion, the formation of the central proto-neutron star, the associated multi-messenger emission, and the nucleosynthesis of new heavy elements.
At the end of their lives the most massive stars are expected to collapse into black holes (BHs). The detection of an 85 Solar-mass BH from GW 190521 appeared to present a fundamental problem as to how such heavy BHs would exist.

Using systematic explorations with the MESA stellar evolution code and new mass-loss physics we (Vink, Winch+) show that for stellar models with non-extreme assumptions, 100 solar-mass stars at reduced metallicity (below 10% solar) can produce blue supergiant progenitors with core masses sufficiently small to remain below the fundamental pair-instability limit, yet at the same time lose an amount of mass via stellar winds that is small enough to end up inside the second mass gap. The key physical points are (i) the proper consideration of internal mixing, and (ii) stellar wind physics. Our modelling provides a robust scenario that not only doubles the maximum BH mass set by pair instability, but also allows us to probe the maximum stellar BH mass as a function of metallicity in a physically sound framework.
A variety of evidence suggests the majority of stars are born in stellar clusters or associations. For the most massive young clusters (that may be similar to the progenitors of the old globular clusters observed in the Milky Way), dynamical interactions play a crucial role in shaping the properties of stars and, ultimately, the compact objects formed.

In this talk I will discuss the dynamics of massive stars and black holes in young stellar clusters (ages less than 100 Myr). Informed by results from realistic N-body simulations, I will address a number of physical processes including the formation of very massive stars through runaway stellar collisions, the collapse of very massive stars into intermediate-mass black holes, as well as black hole growth through accretion of ambient gas and/or through successive mergers with other black holes.

Additionally, I will describe the key (uncertain) properties of young clusters at birth that play an essential role in the features and event rates of these various processes. Finally, I will discuss the role of young star clusters (and old star clusters) in the dynamical formation of binary black hole mergers that may be similar to those observed as gravitational wave sources by LIGO/Virgo.
Recent studies on the evolution of massive stars have shown that the core compactness at the moment of core-collapse is a bimodal function of the final Carbon-Oxygen core mass. The compactness parameter, a proxy for core density, may be strongly correlated to the outcome of the core-collapse, namely the mass of the remnant and whether it is a black hole or neutron star. Together, these results indicate that the black hole mass spectrum should also be bimodal, with a narrow peak around 8-10 solar masses, a broader bump rising again at around 18 solar masses, and a prominent gap in between. We implement this bimodal black hole mass model into population synthesis to investigate its impact on the chirp masses of merging binary black holes. We find that an apparent dip in the observed LVK chirp masses around 13 solar masses can be readily reproduced with this model but not with other, traditional core-collapse models. The upcoming LVK O4 data release will be critical in testing the validity of the 13 solar mass dip, and therefore the bimodal black hole mass model. .
A great deal of discussion has surrounded the origin of the ~35 M☉ “bump” in the primary mass distribution of LVK binary black hole mergers. The presence of a “pileup” at a few solar masses had already been expected as a consequence of pulsational pair-instability supernovae (PPISNe). Recent work, however, has supported the occurrence of the PPISNe pileup at higher masses leaving the 35 M☉ bump unexplained. In this talk, I will explore the feasibility of producing the 35 M☉ bump through chemically homogeneously evolving (CHE) stars.

CHE stars are stars with rapid rotation that induces efficient mixing between core and envelope early in their lives. Mixing becomes more efficient for higher masses and shorter spin periods, favoring massive stars tidally locked in close orbits. CHE stars are intrinsically less likely to lead to stellar mergers, as they lose a distinct core-envelope structure and remain compact throughout their lives. These stars are thus likely progenitors of close pairs of BHs above the dominant ~10 M☉ peak.

I will explore the main uncertainties related to CHE stars, how their interplay might lead to a peaked BH mass distribution, and the conditions necessary for this peak to match the 35 M☉ bump.
The interaction of a supernova shock and ejecta with circumstellar material (CSM) creates a reverse movie of the mass-loss history of the progenitor star. Recent optical observations have shown some stars experience much more extreme mass loss than initially thought. However, while the optical is only sensitive to the most extreme mass loss, ultraviolet (UV) wavelengths probe the full range of observed mass loss.

I will discuss mass loss constraints from the growing sample of Type II supernovae with UV time-series spectroscopy, including late time observations. Combining these observations with early optical spectra traces the mass-loss history of the red supergiant progenitors in some cases showing an evolution from quiescent mass loss to much more extreme mass loss (e.g. eruptive or super wind) and in other cases showing no evidence of extreme mass loss. Understanding CSM interaction in the UV is critical to interpreting the light curves from the Rubin Observatory and the Roman Space Telescope, neither of which include UV wavelengths or spectroscopy, and which will push our sample of type II supernovae to significantly higher redshift, illuminating mass loss in massive stars in the early Universe.
One of the main challenges in modeling massive stars to the onset of core-collapse is the computational bottleneck of nucleosynthesis during late burning stages. The large number of isotopes formed makes the simulations computationally intensive and prone to numerical instability.

To overcome this barrier, we design a nuclear neural network (NNN) framework to replace the nuclear reaction network solver in stellar evolution codes following oxygen core depletion. The NNN takes the temperature, density and composition of a burning region as input and predicts the resulting isotopic abundances and nuclear energy generation rates.

We find that the NNN successfully emulates the results obtained with large nuclear networks, which are crucial for multidimensional simulations, at a computational cost comparable to that of the small commonly used networks. While further work is needed to integrate NNN trained models into stellar evolution codes, this approach is promising for facilitating large-scale generation of core-collapse supernova progenitors with higher physical fidelity, thus advancing our understanding of the explosion mechanism as well as neutron star and black hole formation, supernova kicks and gravitational wave progenitors.
Gravitational wave (GW) palaeontology requires accurate modelling of stellar binaries. In modelling binary interactions, uncertainties in the efficiency and stability of mass transfer, tidal strengths and internal mixing percolate through our understanding of progenitors of stripped-envelope supernovae and GW mergers. Without empirical constraints, degeneracies in the binary parameter space may not be resolved simply by an exponential increase in the observed GW mergers or supernovae in the upcoming decades.

Currently undergoing mass transfer on the main sequence, the so-called 'Algol' binaries are a unique testbed for in-situ constraints on mass transfer efficiency and stability, tidal strength and internal mixing physics. We revisit every observed massive Algol binary in the Milky Way, Large and Small Magellanic Cloud, using analytical arguments to derive empirical constraints on the mass transfer efficiency and stability. Angular momentum conservation arguments rule out conservative mass transfer in 31 of 61 massive Algols and favour an inefficient mass transfer (< 50%), in agreement with detailed binary evolution models. A few massive Algols require mass transfer to be stable down to at least an initial mass ratio of 0.5.

Sesión 7 | Session Chair:Claus Leitherer

Population synthesis models and feedback

I will review the role of interacting massive binary stars on the spectrophotometric properties of stellar populations as a function of age and metallicity predicted by synthesis models.

The most relevant differences with respect to single star models occur in the ultraviolet, including the number of H and He ionizing photons.

This spectral range is now observable with JWST in distant galaxies and the use of appropriate models is fundamental for the correct interpretation of these observations.
Massive stars eject strong winds that affect their evolution. When in a binary system, their winds collide and emit radiation across the spectrum, providing an opportunity to study the stars and the interaction between them. There are many physical effects involved in the colliding wind problem, and its complexity requires 3D numerical simulations. I will present simulations of colliding winds in massive binary systems that include a detailed treatment of wind ejection, orbital motion, clumpiness, and other effects. I will discuss the results of systematic simulations that were used to determine the general conditions that may lead to accretion onto the star with the weaker wind, and demonstrate new relationships between the mass accretion rate and the ratio of the stellar wind momentum and the Bondi–Hoyle–Lyttleton accretion rate. Additionally, I will present simulations of mass ejection during giant eruptions in LBVs, including interactions and accretion onto the companion star, and discuss the implications for the formation scenario of LBVs. I will also present new stellar evolution simulations revealing the effects of mass ejection and mass accretion at high rates on massive stars.
During the Big Bang, only light elements such as hydrogen and helium were produced. Carbon and heavier elements are created inside stars and are ejected when they die. Iron-peak and neutron-capture elements are further produced by binaries—Type Ia supernovae and neutron star mergers, respectively.

Detailed elemental abundances of individual stars in the Milky Way have been extremely useful for constraining stellar nucleosynthesis and the origin of elements. Not only metallicities (Z, [Fe/H], or log O/H) but also elemental abundance ratios evolve in galaxies, which can be used to constrain galaxy evolution and stellar astrophysics.

With the James Webb Space Telescope, unexpected elemental-abundance ratios (namely high N/O) are detected for very high redshift star-forming galaxies, which can be explained with Wolf-Rayet stars under intermittent star formation or may indicate the existence of very or super massive stars linking to the origin of super-massive black holes. Using more elements (CNO, Ne, and Ar), it will be possible to constrain these scenarios.

I will show predictions with cosmological hydrodynamical simulations and summarize the possible role of massive stars in cosmic chemical enrichment.
Hot, massive stars are essential components of stellar populations, contributing significantly to the ionising spectrum and setting key mechanical, chemical, and energetic feedback properties. They become increasingly extreme at low metallicity, which is especially relevant for the early universe, but our view of sub-solar massive stars is limited.

Population models are missing coverage below 0.2 Z⊙ and above 120 M⊙, leaving us unable to reproduce new observations of massive star populations from the local universe to high redshift. We have updated the Starburst99 code with a new comprehensive set of evolutionary tracks (GENEC) and atmosphere models (FASTWIND/PoWR), increasing the upper mass limit to 500 M⊙, including the effects of rotation, and extending our capability to metallicities representative of the Milky Way, LMC, SMC, IZw18, and the early universe.

We see significant increases in ionising fluxes, key stellar and nebular emission line strengths, as well as improved estimates of wind luminosities, spectral features, colours, and chemical yields. In this talk I will quantify our new predictions and discuss them in the context of the latest results from large surveys with HST/JWST (e.g. ULLYSES; JADES/CEERS).
In general, open-access spectral synthesis models are not optimized to analyse extremely low metallicity environments in which Very Massive Stars (VMS, >200 M⊙) might be present. The stellar population synthesis code Starburst99 aims to keep up with these advances with the release of their next update: Starburst25 (Hawcroft et al., 2025).

Amongst the improvements in this update are the inclusion of Population III stars, the possibility to calculate models with VMS, the use of independent stellar evolutionary tracks for each metallicity, and stellar spectra consistent with the track metallicity.

We use the photoionization code Cloudy to calculate an open-access million-model photoionization grid. Simple stellar population models from Starburst25 are employed as input.

In this talk I will present the grid of photoionization models and important applications in the context of JWST and large-scale surveys.
Our understanding of massive stars in high-redshift (z>1) galaxies remains incomplete. For example, many high-z galaxies and nearby, low-metallicity analogs exhibit strong He II emission, indicating large quantities of photons with energies >54.4 eV that conventional methods cannot explain.

Recent studies show that we must account for binary evolution and non-solar abundance patterns to explain the distinct spectra of high-z galaxies observed by JWST. However, the treatment of these properties varies drastically between models, with extreme cases featuring pure Wolf-Rayet populations or metal-free Population III stars.

I will present the first results from a comparison of multiple models with different treatments of binary evolution, including BPASS and a novel set of stripped star models, with rest-UV-optical spectra of local analogs and galaxies at Cosmic Noon (z~2–3).

This type of investigation can provide insights into which aspects of binary evolution are most important to reproduce observations and help identify priorities in ongoing efforts to improve models. By constraining the properties of massive stars at high-z, we can learn about the processes at play in high-z galaxies and massive star evolution more broadly.
Clusters of massive stars present an exciting nexus of stellar and high-energy astrophysics. In this talk, I will bridge the scales from simulations of individual stellar winds to those of clusters, and show how their interactions and collective effects evolve over time.

Young, dense clusters such as Westerlund 1 (Wd 1) have a collective wind driven primarily by their WR stars. Recent JWST observations have revealed cool, dusty outflows from RSGs in Wd 1 with unprecedented resolution, but the origin of this material is as yet unclear.

I will present 3DHD simulations of an RSG interacting with a cluster wind, and synthetic dust maps that correspond well with the observed fluxes and morphologies of the outflows. I will discuss implications for massive star feedback and measuring RSG mass-loss rates (Larkin+ subm.).

I will also show simulations detailing the effects of successive SNe in a dense cluster (Larkin+ in prep), and how collective wind interactions differ for older and less compact clusters such as Cyg OB2 (Vieu, Larkin+ 24).
Future gravitational-wave observing runs with ground-based detectors will rapidly expand the gravitational wave (GW) catalog, providing a larger observed population of double compact object (DCO) mergers (binary black holes, binary neutron stars, and black hole–neutron star pairs) to study massive binary star evolution.

By using population synthesis models to simulate the evolution of large populations of massive star binaries and comparing them to observations, we gain insight into the progenitors, formation mechanisms, and expected rates and properties of DCOs. However, uncertainties persist in isolated massive binary star evolution.

We present a new framework for analyzing GW progenitors, focusing on intermediate evolutionary stages that impact DCO formation. Moving beyond predicted merger rates, our method quantifies survival fractions across key evolutionary stages, revealing major bottlenecks in dominant DCO formation channels.

We find that the limiting evolutionary stage factors vary by up to ~90% across stellar evolution models, highlighting the critical role of intermediate-stage uncertainties. Our results aim to provide new constraints on binary evolution physics and improve predictions of DCO populations.

Friday 19

Sesión 8 | Session Chair: Paul Crowther

Looking to the future

Despite significant recent advancements, largely driven by new instrumentation, many fundamental questions about massive stars remain unresolved. Fortunately, both ground-based observatories and space missions continue to be developed, paving the way for further progress.

For instance, large-scale spectroscopic surveys targeting extensive samples of massive stars in the Milky Way and the Magellanic Clouds will provide valuable insights into the role of metallicity. Meanwhile, massive-star (magneto-)asteroseismology is already offering an unprecedented view into the interior structure and transport mechanisms of these stars, with the upcoming launch of PLATO set to propel this field even further.

Additionally, future facilities, such as space-based multi-wavelength spectropolarimetry, will enable detailed studies of (magnetized) circumstellar environments, stellar winds, and mass loss. In this overview talk, I will discuss the planned new facilities relevant to massive star research and the groundbreaking discoveries anticipated from them.
After 35 years of operations, Hubble is expected to remain at the forefront of astrophysical discovery, thanks to its unique high-sensitivity imaging and spectroscopic ultraviolet-optical (UVO) capabilities, which will not be matched until the Habitable Worlds Observatory is operational. Hubble’s scientific productivity is still on the rise. Hubble’s systems and instruments have redundancy and a high probability of delivering transformative science well into the 2030s. As exemplified by the ULLYSES (UV Legacy Library of Young Stars as Essential Standards) program, Hubble’s UVO coverage is indispensable to characterize stars of all masses, stellar populations, and star formation in the nearby universe. Moreover, synergies between Hubble and existing and upcoming space- and ground-based facilities are opening up new areas of stellar astrophysics. Hubble is already supporting time-domain and multi-messenger astrophysics through rapid follow-ups of high-energy transients, which are expected to dramatically increase in number with Rubin and Roman online. In this talk, we will provide an update on the Hubble mission and an outlook of the ground-breaking science the observatory will be pursuing through 2030 and beyond, with a focus on the advances enabled by the observatory in the area of massive stars.
their physical properties. Hubble's systems and instruments have redundancy and a high probability of delivering transformative science well into the 2030s. I will give an overview of the status of the mission, and look ahead to how its capabilities will continue to push the frontier of massive-star research, both on its own, and in synergy with JWST and other facilities.
Rotation, angular momentum transport, and rotational mixing crucially affect the evolution of high-mass stars and their systems, yet diagnosing these processes without spatial resolution remains challenging.

We introduce Polstar, a small explorer mission (SMEX) space telescope concept for ultraviolet spectropolarimetry (115–2200 Å) at R = 20,000 (15 km/s). By leveraging polarimetry to probe spatial geometry and magnetic fields, Polstar aims to investigate the impact of rotation—from rapid to critical—on both magnetic and non-magnetic massive stars, and to study mass transfer in massive binaries, including the identification of stripped core stars.

If selected, Polstar is projected for a 2031 launch and will observe roughly 200 stars, while its high-resolution UV capabilities will also support a Guest Observer program for broader astrophysical discoveries.
Massive stars internal structure has remained poorly constrained. Uncertainties include (effective) core masses, rotation profile and chemical transport. Through pulsation mode identification, asteroseismology offers a unique probe into stellar interiors.

Spectroscopic line profile monitoring is desirable to complement photometric approach. Ground-based campaigns are however complicated by the day/night cycles, weather gaps, and the need to combine multi-site data to overcome these.

In this context, we designed CubeSPEC, a 12U ESA-demonstrator with a compact échelle spectrograph and an aperture of ~200 cm². Planned for launch in Summer 2026, CubeSPEC will provide high-cadence, space-based high-resolution spectroscopy of a handful of β-Cephei stars.

In this talk, I'll introduce CubeSPEC and discuss the observational strategy of the mission as validated through home-made simulated line profile variations time-series reflecting the response of the instrument and the satellite orbital properties, carrying frequency analysis and mode identification, and considering various cadence scenarios. By assessing the retrievability of pulsation modes in β-Cephei, we demonstrate CubeSPEC's potential to contribute to massive-star asteroseismology.