Poster Session Program

As JWST uncovers increasingly strong evidence that metal-poor, massive stars in early galaxies dominated reionization, observational constraints on the properties of such stars are more relevant than ever before. However, spectra of individual O- and B-type stars are rare at the low metallicities typical of high-redshift galaxies, leaving models of stellar evolution and ionizing flux poorly constrained by data in this regime. I will discuss our analysis of new medium-resolution optical spectra of OB stars in the local low-metallicity dwarf galaxy NGC 3109. We find evidence of strong mass loss via radiation-driven stellar winds in two O stars, one of which is the hottest, youngest, and most massive star confirmed in the host galaxy to date. Though its spectrum does not meet conventional Wolf-Rayet spectral classification criteria, this metal-poor O If star produces strong He II 4686 emission and its evolutionary status is ambiguous. This work nearly doubles the number of OB stars with measured parameters in NGC 3109, providing a new observational testbed to constrain the stellar astrophysics that drove cosmic reionization and influenced the evolution of the earliest galaxies.

We use COS G140L UV spectroscopy of 8 nearby extremely metal-poor galaxies with optical nebular He II emission to test spectral synthesis models from two independent groups, Charlot & Bruzual and Leitherer et al.. In particular, we test the new Starburst99 models with and without rotation (Hawcroft et al.). Some of the galaxies show high-ionization UV emission lines while others are dominated by stellar-wind P-Cygni like profiles.

In this poster we present preliminary results on the connection between ionized gas and young stars in the Magellanic Clouds using data from the Local Volume Mapper (LVM). The data cubes were reconstructed with 3DCUBEGEN and analyzed with the Data Analysis Pipeline (DAP) and the ionized-region detection code pyHIIextractor. Cubes were processed at multiple spatial resolutions (9, 19, 36, 72, 144, and 288 pc) to assess the applicability of various diagnostic diagrams (e.g., BPT, WHAN, WHaD) for identifying ionization sources. The IFU nature of the data allows us to combine integrated and spatially resolved information, enabling a detailed analysis of the star–gas interplay both within star-forming regions and in zones beyond the ionized gas.

The evolution of massive stars is key to determining the final mass of compact objects, source of gravitational waves. Despite recent advancements in mass-loss prescriptions, details of the evolutionary sequence for the most massive cases remain poorly constrained. Very massive stars (VMS) exhibit a transition to the Wolf-Rayet phase (WNh stars) while still on the main sequence, not due to envelope stripping but due to their proximity to the Eddington limit, also meaning a switch from optically thin to optically thick winds. In this talk, we examine state-of-the-art mass-loss prescriptions for O-type stars (GM23,KK24) and analyse their approach to the O/WNh transition at large Eddington factor (Gamma_e​). We evaluate different transition criteria and establish that Gamma_e=0.5 adequately connects the O new thin winds to the WR thick winds, while also reproducing the observed HRD positions of Galactic WNh stars. Our results indicate that WNh stars can emerge on the main sequence from M_zams∼60 Msun at solar metallicity, with the thin-to-thick switch occurring even earlier at larger initial masses. We compare our findings with previous studies and discuss the potential of extending this framework to lower metallicity environments.

WR 31a (also identified as Hen 3-519) is an evolved massive star that displays a Wolf-Rayet (WR) emission line spectrum and is surrounded by a 1' diameter ring nebula. It may be a post-luminous blue variable (LBV) star that is evolving towards a classical WR star. Multicolor (UBVR) photopolarimetry of WR 31a was obtained over a span of ten nights in early 2007, revealing a "loop" structure in the Stokes Q-U diagram. Such loops often indicate the presence of a binary companion, although one has not yet been observed in WR 31a. We test the binary hypothesis with a set of models and produce constraints on the expected orbital parameters of a possible companion within the context of our assumptions. The prevalence of massive stars in multiple systems is well-established; thus, the potential to detect a binary companion during the post-LBV state of WR 31a adds to the emerging narrative of diverse interactions between massive components as a function of evolutionary stage.

In this talk, I will highlight a new program that significantly enhance the legacy value of the JWST public observations, finding and studying physical properties of a new sample of more than 400 HII galaxies at 3 < z < 10. The sample has been selected from the MAST data archive using NIRcam and NIRSpec at the JWST and it will provide cosmological constraints within the critical redshift range (z ≈ 3 − 10) necessary to restrict the possible evolution of the parameter of the Dark-Energy equation of state, w(z). At the same time, it will help to understand the current tension with the Hubble constant, offering a completely independent measurement of all cosmological parameters.

Massive stars are progenitors of supernovae, neutron stars, and black holes. Rotation is a major driver of internal mixing and angular momentum transport, and drastically changes their evolution. However, constraining rotation from spectroscopy is challenging, as spectral lines often have other broadening beyond rotation, requiring additional velocity fields to match model predictions. The origin of this macroturbulent broadening remains uncertain and unconstrained. We present the analysis of rapid time-series spectroscopy, obtained with the ultra-high resolution ESPRESSO instrument at the VLT for the massive pulsating star ζ Ophiuchi. Using excellent temporal coverage, our analysis reveals that pulsations induce variability in macroturbulence of about 50 km/s , while the average macroturbulent velocity exceeds 100 km/s , consistent with new hydrodynamical predictions. Additionally, the epoch-to-epoch average macroturbulent velocity is systematically lower than that inferred from the stacked spectrum. These results highlight the potential bias in macroturbulence and rotation estimates, and subsequent inference of stellar parameters. We conclude by demonstrating the need to account for pulsations in massive star spectroscopy.

Massive stars shape their surrounding environments and galaxy evolution through their enhanced mass-loss activity, high energy output and eventual fate as supernovae. The next generation of large-scale surveys — LSST, WEAVE, and 4MOST — will enable the detection of the most massive stars and their variability in other galaxies while providing unprecedented spectroscopic datasets. These surveys offer new opportunities to study Yellow Hypergiants (YHGs), a rare class of evolved and unstable stars with fewer than 20 confirmed members in the Milky Way and a handful of candidates in nearby galaxies. The extreme instability of YHGs results in recurring explosive mass-loss episodes and makes them difficult to model. However, in their calm state, they appear very similar to the younger yellow supergiants. If YHGs were detected in distant galaxies, could we reliably recognize them? Can we identify these rare objects from limited survey epochs? I will present a study of well-known YHGs and candidates, examining how their properties correlate with their environments and how classification criteria — currently reliant on long-term monitoring — could be adapted for large surveys, with implications for understanding massive star evolution.

We present an unprecedented look into the infrared spectra of metal-poor O-stars obtained with the NIRspec instrument on board of the JWST. Our sample consists of 13 O-stars in the most massive young star cluster in the SMC, NGC 346, a low-metallicity environment ideal to study conditions that resemble the cosmic noon. The study allows to explore the JWST capabilities in the massive star regime and to develop new, much needed, diagnostics to characterize of the weak winds of the metal poor massive stars. In particular, the NIRspec spectral range includes Br alpha line at 4.05 μm which is expected to be a sensitive indicator to mass-loss in weak winds. The new JWST data are complemented by the HST UV and the VLT MUSE optical spectra. The spectra are analyzed using the stellar atmosphere code PoWR allowing us to determine realistic parameters of O stars and their winds in NGC 346. From a first inspection, we find some discrepancies between the synthetic and the observed spectra. Further studies involving detailed spectral modelling focusing on the infrared range will reveal the suitability of the Br line as a mass-loss indicator and if the models require revision.

One of the biggest unsolved mysteries in the study of massive stars is the sudden weakening of spectroscopic wind emission signatures below a luminosity of log(L/Lsol) = 5.2. This phenomenon was first identified 20 years ago, and has been labeled the 'weak-wind' problem as hydrodynamical simulations of O-type star atmospheres (which match observations above log(L/Lsol) = 5.2) predict mass-loss rates two orders of magnitude higher than those required to reproduce the observed optical and UV spectra. A breakthrough moment is now happening with the detection of highly ionised fine structure emission lines (of [Ne VI], [Ne V] and [O IV]) formed in the stellar wind of late O-type stars with JWST/MIRI. These lines provide strong, independent constraints on the stellar mass-loss rate and terminal wind speed of the 'weak-wind' O9V star 10 Lac. Here I will present our work on MIRI spectroscopy of 10 Lac, and prospects for additional late O-type stars, which will allow us to constrain the true mass-loss rates and terminal wind speeds in the domain of the 'weak-wind' problem.

The SpecpolFlow package is a new, completely Pythonic implementation of software supporting the analysis and interpretation of optical spectropolarimetry from astronomical sources spanning the HR diagram. Its tools enable the normalization and processing of stellar spectra, including measurements of the longitudinal magnetic field, in a streamlined, user-friendly pipeline from telescope to science product. SpecpolFlow's design in a modern, open-source, free-to-use platform supports user accessibility, and the longevity of its analytic tools is actively maintained by its developers. The SpecpolFlow team regularly publishes detailed tutorials that can be used by students, instructors, or experts alike. We announce the official release of SpecpolFlow, available via the pip package manager or GitHub, and highlight key scientific results from the first publications utilizing SpecpolFlow for spectropolarimetry.

I present the results of modeling galaxy emission using collections of HII regions photoionized by massive stars. The HII region models are obtained through Artificial Neuron Networks trained to mimic the results obtained by CPU consuming photoionization codes (Cloudy).

One of the ""holy grails"" in massive star research is to quantify how metallicity (Z) influences the radiation-driven winds that these stars produce during the main sequence. This holds the key to understanding their evolution in low-Z environments and offers insight into the lives of the first stars in the Universe. With surveys like ULLYSES and XShootU now targeting low-Z galaxies such as the Large and Small Magellanic Clouds and probing even lower-Z dwarf galaxies in the Local Group and beyond, the analysis of these low-Z winds is finally possible. In this talk, I will share results from an optical + UV analysis of eleven O-type stars in Local Group dwarf galaxies with SMC-like metallicities or lower. Our findings show that in this Z regime, the modified wind momentum as a function of luminosity aligns with the empirical relation proposed by Backs et al. (2024, A&A, 692, A88), extended into the sub-SMC Z domain. This suggests that, at low luminosity (at log L/L_sun < 5.3) and Z, O stars may lose mass at a lower rate than predicted by theory—implying that main sequence mass loss in such stars is significantly less pronounced compared to equally bright stars at higher metallicities.

Massive binary systems are key in looking for multiplicity since they provide not only information about the orbit of the system but also characterize each component. For this work, we selected a sample of single-lined massive binary systems (SB1), where the primary component is detected in the spectra but not the companion(s), from the database of the OWN Survey. This is a survey dedicated to observe all O and WN stars of the southern hemisphere, and its primary goal was to look for multiplicity. We also look for data of our systems taken for the IACOB and APOGEE surveys. Then, we acquire all possible high resolution data for our sample of massive binary systems, which we combine with our own data and perform several analyses. The systems selected are HD91824, HD94024, HD96622, HD96946, HD101190, HD101191, HD163892 and HD319699. For those whose we can detect the companion, we perform a disentangling of the spectra of each component. Each separated spectrum is suitable for a quantitative spectroscopy study. We also make an abundance study from the high resolution spectra and use the results to upgrade evolution models and predict future evolutionary stages with high precision.

Cyg OB2, one of the most massive and active star-forming regions in the Galaxy, hosts O and B-type stars at various evolutionary stages. Despite extensive studies, the presence of rapidly rotating massive stars (vsini>200 kms−1) has not been confirmed, challenging our understanding of stellar evolution and rotational dynamics. Here, we investigate whether some of the B0 stars previously identified in Cyg OB2 are misclassified and are actually late-0 stars with high rotation, potentially explaining the deficit of fast rotators. We also analyze the impact of rotational broadening on classification accuracy.Our findings suggest that late-O stars (O9.5-O9.7) with vsini>200 kms−1 may be misclassified as rapidly rotating B0 stars. Our spectral reclassification of Cyg OB2's early-B population shows that 20% of B0 stars are actually late-O stars. However, after measuring the projected rotational velocities of the new O-types, the number of fast rotators remains low. Several factors, such as runaways, binary fraction and spin-axis alignment, may contribute to this deficit.

Studying massive stars in low-metallicity environments is crucial for understanding their impact on galaxy formation and chemical enrichment during the early, high-redshift Universe. However, observations of stars in environments with metallicities lower than that of the Small Magellanic Cloud (SMC,Z_SMC ~ 0.2 Z_sol) are scarce. Until recently, massive stars with metallicities below those of the SMC were only known in compact dwarf galaxies, with observations in such distant locations suffering from limited signal-to-noise ratios and spatial resolution. Recently, we discovered three O stars and several early B-type stars in the Magellanic Bridge - a stream of gas and stars linking the two Magellanic Clouds. By combining newly acquired HST UV spectra with archival optical data, we measure the intrinsic iron abundances of these stars for the first time and characterise their wind properties. Our method involves using detailed expanding non-LTE atmosphere models to generate synthetic spectra, which we compare to the optical and UV observations. This led to the discovery of the first nearby massive O star with an iron abundance significantly lower than that of the SMC, reaching as low as 3.6 % Fe_sol.

We present an analysis of seven massive stars in the young, active star-forming region M17 (d = 1.7 kpc) using data from VLT SPHERE. SPHERE is uniquely capable of probing the lower end of the binary mass-ratio due to its ground-breaking extreme adaptive optics and coronagraphic capabilities which allow us to achieve greater contrast ratios than traditional adaptive optics. We utilized SPHERE's high-contrast imaging to resolve milliarcsecond binary systems and detect subsolar mass stellar companions to massive stars. Using SPHERE’s simultaneous dual-band imaging and IFS, we detected over 100 potential companions for the seven target stars and measured the position angle, angular separation, and contrast magnitude for each potential companion. The potential companions had contrast ratios ranging from 5 to 13 mag in the infrared. Previous radial velocity studies in M17 found fewer binary systems than expected. Our study is complementary to previous work in the region. By searching for wide companions to massive stars, we fill in the picture of massive star formation and the effect of multiple systems on the dynamical history of the region.

Understanding the chemical enrichment of galaxies across cosmic time is crucial in the JWST era, particularly for interpreting observations of high-redshift systems. We present a new set of strong-line metallicity calibrations based on the most extensive and high-quality compilation of Te-based abundances to date—over 2000 star-forming galaxies and HII regions with optical auroral line measurements. Unlike previous calibrations derived from a limited dataset, our work fully accounts for observational uncertainties and temperature inhomogeneities, addressing the well-known abundance discrepancy. By incorporating the highest-quality spectroscopic data, we refine empirical metallicity indicators with unprecedented statistical robustness and parameter space coverage. Our results reveal that traditional strong-line methods systematically underestimate metallicities, particularly in metal-poor environments analogous to early galaxies now observed with JWST. These findings are critical for accurately tracing the chemical evolution of the Universe and reconciling empirical datasets with theoretical models of massive star formation and feedback across cosmic time.

Ullyses is a large Director's Discretionary program for the Hubble Space Telescope devoting ~1000 orbits to establish a spectroscopic ultravialet 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.

Classical Wolf-Rayet (WR) stars are hot, evolved massive stars with depleted hydrogen and one of the major sources of ionizing flux. The discovery of high ionization emission lines in high-z galaxies has questioned the origin of He II ionizing radiation as well as its implications for galaxy evolution and cosmic reionization. Current stellar populations fail to reproduce the necessary ionizing fluxes. WR stars at low metallicity could be the missing ingredient for stellar population synthesis models to agree with theoretical insights and observational templates. We have analyzed new HST/COS data of WR stars using updated hydrodynamically consistent 1D PoWR atmospheric models in different low metallicity environments to study the influence on the ionizing flux contribution. In this talk, I will first discuss the basics of 1D PoWR models and compare with other cutting-edge 3D time-dependent, radiation-hydrodynamical simulations. Then, I will present our results including a characterization of the stellar wind properties as well as He II ionizing fluxes of our WR stellar sample, and discuss the implications of our findings in population synthesis models and galaxy evolution.

The multiple Small Magellanic Cloud system HD5980 contains a Wolf-Rayet binary with two very massive components. Both components possess very strong winds and appear to be following a chemically homogeneous evolutionary track which implies that they will still be very massive when they end their lives as supernovae. In 1994 the system underwent an eruptive event similar to those seen in luminous blue variables, shedding close to 0.001 solar mass in the process. Other such eruptions are believed to have occurred in the past and are likely to occur in the future. The eruptive processes combined with intense stellar winds are shaping the circumstellar environment into which the eventual SN ejecta will expand. Given all of these properties and the low-metallicity environment of the SMC HD 5980 provides an excellent prototype for studying the end-stage processes that set the stage for distant, unresolved SN events. In this talk I will review the current status of what we know about this interesting system and discuss possible mechanisms responsible for its eruptive behavior.

IRAS17449+2320 is the first B[e] star with a detected magnetic field. It belongs to a subgroup of peculiar hot B-type stars, called FS CMa stars. These stars are surrounded by a huge amount of gas and dust, whose origin is still unclear. The most accepted scenario is binarity. However, a new scenario -the post-merger- was introduced as a consequence of the detection of the Zeeman splitting in some metal lines in the spectrum of this star. Theoretical models suggest that magnetic fields are created during the merging process and can remain stable on evolutionary timescales. In this presentation, we describe recent work to confirm and characterize the magnetic field of this star, and to refine its physical properties. After analyzing spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaii Telescope, we have confirmed the detection and variability of the star’s magnetic field, and determined its rotational period. We have determined the stellar parameters through synthetic spectra modeling and modeled the magnetic field and its geometry with the Oblique Rotator Model. Identifying more magnetic objects among FS CMa stars is of special interest, as it may lead to better understanding of the merging process.

Detectable surface magnetic fields are a rare phenomenon in massive stars, occurring in less than ten percent of the Galactic O-type star population. The majority of confirmed magnetic O-type stars have strong >1 kG dipolar fields and lie near the ZAMS. It is unclear whether the apparent dearth of magnetic O stars approaching the TAMS is due to magnetic field decay or observational biases. We report a magnetic detection in the evolved (log g=3.5) massive O-type bright giant 63 Oph from high-resolution ESPaDOnS spectropolarimetric observations. We measure longitudinal field strength of ~100 G and set a lower bound on the dipolar field strength of ~ 300 G. We observe Halpha Balmer line variability on the order of ~20 d, which we interpret as the stellar rotation period. A tentative orbital fit to RV measurements implies a low mass companion in an eccentric, ~14 d orbit. Archival K2 halo photometry shows dominant stochastic low-frequency variability. We propose that 63 Oph is a rare transitional object between the strongly magnetic O stars found near the ZAMS and the evolved zeta Ori Aa type magnetic O supergiants. We discuss prospects for identifying similar objects in the IACOB O star sample.

Stellar formation theories have traditionally focused on single star physics, and only recently more systematically include multiplicity aspects that play a crucial role in massive star formation and evolution. Differences in orbital period distributions between low-mass and massive binaries suggest distinct pairing mechanisms, though the connection to their initial formation channels remains unclear. To address this, we analyze over 10,000 O/B/A eclipsing binaries detected by TESS, validating orbital properties and accounting for observational biases. This large dataset enables a consistent mapping of the expected transition region between solar-type stars and high-mass stars, offering key insights into massive star formation. By building a validated analysis framework with TESS data, we further set the stage to leverage BlackGEM survey data to investigate metallicity effects on binary formation and evolution. The work that we present aims to bridge theoretical models and observations, shedding light on the origins and diversity of massive binary systems.

In this work we present a new tehcnique to analyse multi-line polarized profiles. We have use an Artifitial Neuronal Network to efficently produce multi-line profiles using a theoretical approach as polarized raditive transfer codes do. Our results shows that this approach allow to properly infer, from data analysis, the stellar magnetic field configuration.

Here we present a newly detected companion to the Be star, HD 52244, using the Fine Guidance Sensors (FGSs) on the Hubble Space Telescope (HST). In fall 2021, HST became momentarily unavailable to support nominal operations, and we used the operational FGS to carry out a multiplicity survey of 6 Be stars. We were able to resolve a companion to HD 52244, with a separation of 36± 5.5 mas and a position angle of 182 ± 17 with a delta magnitude in the F583W filter of 1.91±0.02 mag. The role that multiplicity plays in the formation of Be stars is widely recognized. This study serves as a proof of concept of the use of FGS for the study of multiplicity among Be stars.

Thorne-Zytkow Objects (TZOs) are theoretical hybrid stars in which a neutron star is at the core of a large, diffuse envelope. It is theorized that TZOs may be formed when a newly formed neutron star receives a “kick” that leads to a collision with its secondary main-sequence companion. Using a moving-mesh hydrodynamics solver integrated into the parallel-code Charm N-body GrAvity solver (ChaNGa), we conduct a parameter study simulating dynamical mergers between neutron stars and a range of massive main sequence stars. Our hydrodynamical simulations explore the stability of the resulting post-merger outcome and how it varies depending on whether the radius of periastron is greater or less than 1. This study further solidifies the viability of TZOs formed through this pathway, with the relevance of this work significantly progressing our limited understanding regarding TZO formation and their role as terminal products of massive binary evolution.

Massive stars serve as natural laboratories for testing variations of fundamental constants, such as the fine-structure constant (𝛼) and the proton-to-electron mass ratio (𝜇). Detecting such variations could provide insights into physics beyond the Standard Model, including dark energy and unification theories. Using high-redshift data from UVES, VLT, and JWST, we analyze spectral features sensitive to α and μ variations, placing new constraints on their cosmological evolution. Our results strengthen tests of fundamental physics and highlight the role of massive stars in probing the stability of physical laws over cosmic time.

Eccentric and asynchronously rotating binary stars experience the effect of a variable gravitational field. The resulting oscillatory motions lead to an induced differential rotation structure (Koenigsberger et al. 2021). In the presence of viscous turbulence, a fraction of the energy involved in the shearing motions can be converted into heat. This tidal shear energy dissipation can be modeled in MESA (Paxton et al. 2011-2019) calculations by injecting the dissipated energy predicted by the TIDES-nvv model (Moreno et al. 2011; Koenigsberger & Estrella-Trujillo 2023) in the outer layers of the perturbed star. The results of such an experiment on the structure of a very massive binary star in a short-period, slightly eccentric orbit show that the equatorial stellar radius, the mass-loss rate and the luminosity all undergo a non-negligible increase as a result of the energy injection. Furthermore, we find that the synchronization timescales for layers closest to the star's core exceed the Main Sequence life of the star. Hence, binary stars on the Main Sequence may not be in true corrotation even if their outer layers appear to be rotating synchronously. We thus conclude that binary stars may appear more luminous and have larger mass-loss rates than their analogous single-star counterparts.

The interpretation of stellar populations of high-z galaxies relies on stellar models. One of the largest source of uncertainties in their predictions comes from the internal transport mechanisms. The GENEC code offers a unique ability to compare the impacts of different treatments of angular momentum transport (AMT). While purely-hydro models with an advecto-diffusive scheme are the best at reproducing the observed nitrogen enrichments of O stars, only magnetic models are successful in reproducing the asteroseismic constraints of lower-mass stars.The apsidal motion rate of eccentric massive binaries offers crucial insights into the interior of the components, as it depends on the apsidal motion constant (AMC), a quantity directly linked to the stars’ density profiles. Earlier studies showed that stellar structure and evolution, notably the density profiles, are strongly impacted by the chosen assumptions for the AMT. I will discuss whether the AMCs of observed systems favor one type of AMT by confronting simulations performed with the two physics to these constraints. I will also investigate the impact of tidal interactions on the AMC with state-of-the-art GENEC simulations including a refined treatment of binarity and tides.

We present an unprecedented look into the infrared spectra of metal-poor O-stars obtained with the NIRspec instrument on board of the JWST. Our sample consists of 13 O-stars in the most massive young star cluster in the SMC, NGC 346, a low-metallicity environment ideal to study conditions that resemble the cosmic noon. The study allows to explore the JWST capabilities in the massive star regime and to develop new, much needed, diagnostics to characterize of the weak winds of the metal poor massive stars. In particular, the NIRspec spectral range includes Br alpha line at 4.05 μm which is expected to be a sensitive indicator to mass-loss in weak winds. The new JWST data are complemented by the HST UV and the VLT MUSE optical spectra. The spectra are analyzed using the stellar atmosphere code PoWR allowing us to determine realistic parameters of O stars and their winds in NGC 346. From a first inspection, we find some discrepancies between the synthetic and the observed spectra. Further studies involving detailed spectral modelling focusing on the infrared range will reveal the suitability of the Br line as a mass-loss indicator and if the models require revision.

The blue supergiants (BSGs) mediate between the main sequence and the late stages of massive stars, which makes them valuable for assessing the physics that govern the diverse evolutionary channels. Here, we explore correlations between the parameters of BSGs in the Galaxy and their variability properties assessed with the Transiting Exoplanet Survey Satellite. We explored the time and frequency domain of the stars by means of various measures, pre-whitening, and modeling of the debated stochastic low-frequency (SLF) variability. A positive trend is found between the light curve amplitudes and log(L/Lo). The less luminous BSGs display frequencies that can be interpreted as rotationally induced, suggesting variability that is driven by a structured wind. The more luminous BSGs are mixed with pulsators of the α Cyg class, and display diverse and/or time-variant photometric properties. We report a significant positive trend between the SLF variability amplitude and log(Teff), highlighting an influential role of the stellar age on the emergence of the ambiguous signal beyond the main sequence. Notably, the α Cyg stars display a suppressed SLF variability, which may mirror their rather advanced stage as post-red supergiants.

The European Extremely Large Telescope (ELT) will be the largest optical -- near infrared (NIR) eye of the world, offering revolutionizing capabilities in terms of sensitivity and spatial resolution. HARMONI will be one of its first generation instruments, consisting of an integral field spectrograph that fully exploits ELT's spatial sampling. In this poster we will explore different lines of massive star research where HARMONI can make relevant contributions.

The Rosette Nebula (RN) is a well-studied HII region, flanked by an active Giant Molecular Cloud, making it an ideal laboratory for studying star formatio. The HII region was formed by the winds from the OB association NGC 2244. We analyze the morphology and kinematics of the region, comparing the ionized and molecular gas, along with dust and the young stellar component. We use a set of new large scale IFU maps from the SDSS Local Volume Mapper, along with stellar atmospheric parameters and radial velocity data from SDSS APOGEE and Milky Way Mapper. We examine the spatial distribution of emission lines (Hα, Hβ, [OIII], [NII], [SII]) and line ratios (Hα/Hβ, [OIII]/Hβ, [NII]/Hα, [SII]/Hα) in relation to the molecular cloud, identifying ionized structures and interaction zones. We also make a comparative analysis of the velocity distribution of Hα, molecular gas, and stars, to understand the influence of stellar associations on gas dynamics and the influence of the local environment in the early evolution of the clusters. Our approach aims to reveal how NGC 2244 have impacted the molecular cloud and shaped the region’s evolution.

Gravitational wave (GW) observations have revealed some events that have been challenging to interpret. Among them is GW190814, a highly asymmetric (q~0.1) merger involving a 2.5Msun compact object. We point towards a potential analog of the isolated binary progenitor for GW190814 in our own Galaxy, the runaway high mass X-ray binary (HMXB) HD 153919, which consists of a 2.5Msun accretor and a ~50Msun donor. We reconstruct its past history using detailed binary evolution models, constrained by observables such as component masses, kinematics, and parent cluster age. We find that early mass transfer during the main sequence is necessary. The outcomes of our models are sensitive to Wolf-Rayet winds, interplay between rotation and mass transfer, and natal kicks. Detailed evolutionary studies of known binaries are required to understand rare outcomes such as GWs in the distant Universe and nearby HMXBs, especially the outliers among them. Additionally, finding more common and rare binary products with surveys like Gaia, eROSITA and LSST will provide a complementary path to constrain uncertainties in massive and binary star evolution, and improving population synthesis simulations to understand their role in the near and far universe.

We present Monte Carlo Radiative Transfer (MCRT) simulations of light and polarization curves from Corotating Interaction Regions (CIRs) in radiatively driven stellar winds and an associated bright surface spot. For single-CIR models, we examine the effects of key parameters affecting the curves, including the optical depth of the wind, the density contrast of the CIR, its colatitude and half-opening angle, the stellar inclination, and the spot luminosity, with some important degeneracies revealed among these parameters. We apply our model to literature mini-HIPPI polarization data and BRITE photometric data for the well-known supergiant star Zeta Puppis, confirming that wind models with CIRs can reproduce the observations, particularly when two CIRs are included. Despite degeneracies in the solutions and constraints related to computational limits, our model succeeds in reproducing the observed variability. An expanded parameter grid and simultaneous fitting of photometric and polarimetric data would improve the model’s diagnostic power and allow for a more global assessment of its validity in interpreting phase variations in massive stars.

The OWN survey, devoted to the determination of the multiplicity status of a sample of southern Galactic O and WN stars, obtained high resolution optical spectroscopy of 212 massive stars with 2-m class telescopes in Argentina and Chile. The observations consist of more than 6000 high quality spectra collected between 2005 and 2022. As a result of the radial velocity analysis of these data, 146 objects showed variations larger than 15 km s-1, the limit we conservatively adopted to consider a star as variable. Reliable radial velocity orbits were derived for 39 double-lined and 31 single-lined systems, many of them analyzed for the first time. Some higher multiplicity systems were also detected. Absolute masses were derived when additional information allowed determination of the orbital inclination. We will present a summary of the observations and main results of the OWN, including a brief description of the observed orbital parameter distribution. In particular, the OWN increased by a factor of three the number of binary systems with periods longer than 30 days for which a reliable orbital solution is available.

The mass loss in massive stars is an important process that determines their future evolution and affects their circumstellar environments. Besides the continuous outflow of matter in the form of stellar winds, massive stars undergo sporadic mass ejections that lead to the formation of circumstellar envelopes. For now it remains unclear at what stage of evolution the first mass ejection occurs and what instabilities lead to it. We present the results of a study of four B-type stars which circumstellar nebulae that have recently been found in the archive of the Wide-field Infrared Survey Explorer (WISE). Two of our objects -- PY Gem and HD253659 -- are eruptive γ Cas variables showing double peaked emission Hα profiles, and during our analysis we found that such nebulae are typical for γ Cas variables. The other two stars -- HD215575 and BD+14 1106 -- have spectra of normal B-type stars on the main sequence. Spectral analysis, numerical modeling (with Tlusty code), as well as their high proper motions argue that these two objects undergone merging in the past.

The Wolf-Rayet (WR) nebula G 2.4+1.4 is the only one surrounding a WO-WR star, WR102, a spectral type characterized by its high abundance of Carbon and Oxygen in its spectrum. In this study, we investigate the star and its circumstellar medium to obtain a better physical understanding of the nebula. We use multiwavelength data (gamma-rays, optical, infrared, and radio) from public archives and surveys, morphological studies and spectral energy distribution (SED) modeling to search for thermal and non-thermal emission signatures. We show that WR102 is a runaway star, moving with at least 60 km/s with respect to its surroundings. Additionally, our modeling of the inner circumstellar nebula suggests a significantly lower total mass than previous estimates, along with a higher dust-to-gas ratio.

The LMC and SMC are superb laboratories for exploring the evolution of massive stars. In recent years we have completed studies of the relative number of WRs and RSGs, the number of WNs vs WCs, and the luminosity functions of yellow and red supergiants for comparison with evolutionary models. For such tests to be meaningful, surveys must identify samples whose completeness limits are well understood. Missing from these comparisons has been the most fundamental: the relative number of unevolved massive stars. Oddly, determining the number of massive O stars as a function of luminosity has proven to be elusive: accurate photometry is needed to identify complete samples for follow-up spectroscopy. Most photometric surveys of the Clouds have been aimed at determining SFR histories, and require going deep, with the result that the many massive stars are saturated. We have therefore undertaken a new ""lite"" UBVRI photometric survey of the massive star content of the LMC and SMC with the Las Campanas Swope 1-meter. We also include H-alpha and O[III] images. This poster will present details of our survey, what we hope to accomplish, and show some of our spectacular images. This work is supported by NSF AST-2307594.

An important aspect, robustly predicted by theory, is that massive young stellar objects (MYSOs) accreting at high accretion rates should bloat or swell up. The theoretical work performing linear stability analysis on rapidly accreting massive protostars predicted that these sources become pulsationally unstable during the bloating phase, causing periodic variability in these sources defined by a period-luminosity (PL) relation. IRAS 19520+2759 (I19520) is a candidate bloated MYSO; to test if I19520 (10^5 L⊙) follows the predicted PL relation, we carried out a variability study of the source using observed and archival data sets. The observed periodic variability, the observed colour trend, and the nature of the variability have been found to be consistent with the pulsational model for a bloated MYSO. For very massive stars (early O-type stars having bolometric luminosity ≥ 10^5 L⊙), the evidence for the presence of a Keplerian disc has still been ambiguous. In the previous sub-mm investigation, a highly collimated outflow and a toroidal structure were found to be associated with I19520. To probe inside the toroid and investigate the possible presence of a Keplerian disc, we performed a high-resolution ALMA study of I19520.

In recent decades, large-area spectroscopic surveys such as RAVE, LAMOST, GALAH, and SDSS/APOGEE-2 have provided vast datasets of stellar spectra encompassing a wide variety of stellar populations. Characterizing these samples is a complex task, often addressed with general-purpose pipelines that perform reasonably well across different types of stars. The determination of basic atmospheric parameters, such as effective temperature, surface gravity, and overall metallicity, typically relies on comparing observed spectra with synthetic models. Although synthetic spectral grids have improved over the years, offering broader and more regular coverage of the parameter space, the reliability of the derived parameters still depends on the spectral type and evolutionary stage of the target, with reduced accuracy at both hot and cool stars. We have developed a procedure to homogeneously determine atmospheric parameters and precise chemical abundances from high-resolution infrared spectra in the APOGEE-2 survey, using a combination of custom-developed and publicly available tools. Our framework has been tested and validated for G-, K-, and M-type main- and pre-main-sequence stars, and extending its applicability to hotter stars (spectral types O, B, A, and F) is the next logical step. Achieving a homogeneous characterization of the upper main sequence is of great importance for a wide range of astronomical fields, including star formation, stellar evolution, and stellar population synthesis.

tau Canis Majoris (CMa) is an intriguing system that has captured astronomers’ attention for more than a century. The two main components Aa and Ab are 117 au apart and contain respectively an O supergiant and an evolved O star. Aa is itself a spectroscopic binary with a 155-days period and a 0.30 eccentricity. We recently detected the SB2 nature of Aa based on STIS spectra, the companion to the O star (Aa1) being a B star (Aa2). Since Hipparcos, we know that there is a 1.3-days period eclipsing binary hidden somewhere in Aa or Ab, but nowhere else. Our analysis finally unravels the mystery: the TESS data reveal that the culprit is Aa2, itself a B+B overcontact binary! Even more spectacular, the 155-days period orbit of Aa shows significant retrograde apsidal motion. We combined the spectroscopic data of tau CMa taken over a century and determined from the associated radial velocities an apsidal motion rate of -0.5 degrees per year. A retrograde apsidal motion necessarily comes from a third companion on an orbit inclined with respect to the 155-days period orbit by at least 40 degrees. Who’s the culprit, Aa2 or Ab or both? Come and see my poster to find it out!

We present the stellar and wind properties of 36 late-O and B supergiants (O9-B8) in the Magellanic Clouds. The parameters are obtained via detailed ultraviolet (HST ULLYSES) and optical (VLT XShootU) spectroscopy using the model atmosphere and radiative transfer code CMFGEN. We explore the dependence of mass loss and wind velocity on metallicity and investigate the existence of the “bi-stability” jump within the context of metal-deficient blue supergiants. We aim to produce an empirical metallicity-dependent mass-loss rate recipe, which can provide an alternative to the commonly employed theoretical recipes in evolutionary and population synthesis models.

We advertise the activities of the IAU G2 massive stars comission.

We honor the contributions to the field of massive stars of colleagues who have passed away since 2022. We apologize for the incomplete list.

The Alma Luminous Star (ALS) catalogue has been a key resource for studying massive stars in the Milky Way. With the arrival of Gaia DR3 and new spectroscopic surveys, we present ALS IV, the first extension incorporating massive stars beyond the Milky Way, specifically from the Large and Small Magellanic Clouds (LMC and SMC). Using a combination of Gaia DR3 data (Luri et al. 2021), ground-based spectroscopy, and other key surveys, we apply rigorous cross-matching and filtering techniques to create a clean and comprehensive sample. ALS IV adds ∼16 000 massive stars from the LMC and SMC, complementing the ∼20 000 Galactic stars from ALS III. This expansion provides new opportunities to explore a wide range of physical properties across different galactic environments, including extinction, variability, and rotational curves, offering new insights into their formation and evolution within the Local Group.

Gaia Data Release 3 astrometric data and reliable radial velocities, where available, are used to identify runaway Galactic Wolf-Rayet stars, and their trajectories are then traced backwards in time in the Galactic potential. Several new runaways are discovered in the Galactic disk on the basis of their fast peculiar motions. Some runaway Wolf-Rayet stars at large distances from the Galactic plane, such as WR 124, are found to have kinematic ages greater than 10 million years, raising questions about the evolutionary scenario for such objects. I highlight which runaways have likely been dynamically ejected from clusters early in their lives and estimate the energies involved.

The modeling of the ionizing spectra of massive stars is crucial to study the physical and chemical properties of HII regions and star-forming galaxies. However, it is not possible to directly detect the exact shapes and intensities of their spectral energy distributions (SED) beyond the Lyman limit. In this poster, I’ll compare the modeled ionizing flux (Q0) of a potentially single O3 V star with the inferred value from H-alpha luminosity of the surrounding HII region with SDSS-V Local Volume Mapper (LVM) data. We also modeled the spatially-resolved LVM observations of strong nebular emission lines using the stellar SED as input for the Cloudy photoionization code.

The detection of non-thermal synchrotron emission from massive stars provide strong evidence of the particle acceleration originating from wind collision in a binary. This aspect of binarity has been explored in the case of two galactic Wolf-Rayet stars, WR 156 and WR 125 using observations from the Giant Metrewave Radio Telescope. This presentation reveals insights into the wind dynamics of the WR 125 binary system and provides clarification on the binarity of WR 156.

We investigate the evolution of red supergiant (RSG) progenitors of core-collapse (CC) supernovae (SNe) with initial masses between 12-20 Msun focusing on the effects of enhanced mass loss due to pulsation-driven instabilities in their envelopes and subsequent dynamical ejections during advanced stages of nuclear burning. Using time-dependent mass loss from detailed MESA stellar evolution models, including a parameterized prescription for pulsation-driven superwinds and time-averaged mass loss rates attributed to resulting shock-induced ejections, we construct the circumstellar medium (CSM) before the SN explosion. We calculate resulting CSM density profiles and column densities considering the acceleration of the stellar wind. Our models produce episodes of enhanced mass loss ~10^-4 - 10^-2 Msun/yr in the last centuries-decades before explosion forming dense CSM (>10^-15 g/cm^3 at distances < 10^15 cm) -- consistent with those inferred from multi-wavelength observations of Type II SNe such as SN~2023ixf, SN~2020ywx, SN~2017hcc, SN~2005ip and SN~1998S. The formation of such dense CS shells, within the explored range of our single star RSG models, provides a natural explanation for observed flash-ionization signatures, X-ray and radio emission, and has important implications for dust formation around Type II SNe.

This study focuses on investigating B5–A5 Galactic supergiants and developing a methodology for accurately determining their fundamental parameters mostly from photometric and spectroscopic data. These stars are highly luminous and can serve as standard candles for measuring extragalactic distances, yet their characteristics remain insufficiently studied. Despite advances in spectroscopy, both temperature and luminosity determination vary from study to study, only small samples have been analyzed so far, and existing spectroscopic criteria require refinement. We analyzed mostly medium-resolution (R = 12,000 − 18,000) spectra of ∼200 B5–A5 Galactic supergiants from the brightest to V ∼ 10 mag. Studying several dozens of absorption lines allowed us to establish correlations between their properties and stellar fundamental parameters. The methodology relies on refined distances from the Gaia mission and spectral energy distributions, ensuring deriving accurate parameters. We present some preliminary results and outline steps toward creating a catalog of observed features and fundamental parameters of the sample stars.

Mass loss strongly shapes the evolution of massive stars, and although large stellar samples across different environments are needed, we still lack them. Moreover, spectroscopy is impractical for thousands of sources at large distances. Using machine learning on Spitzer and Pan- STARRS1 photometry (with Gaia DR3 foreground removal), we classified 1.15 million sources in 26 galaxies (0.07–1.36 Z⊙). About 276,000 (24%) are robust, including 120,000 RSGs. Notably, we identify 21 luminous RSGs, 6 extreme RSGs (log L ≥ 6) in M31, and 159 dusty Yellow Hypergiants, providing rare probes of stellar mass loss. Population trends with metallicity follow expectations but include biases. This catalog, the largest of its kind, offers prime JWST targets and a foundation for follow- up studies of evolved massive stars.