Gonzalo Munoz-Sanchez

Context . Mass loss is a key aspect of stellar evolution, particularly in evolved massive stars, yet episodic mass loss remains poorly understood. To investigate this, we need evolved massive stellar populations across various galactic environments. Aims . However, spectral classifications are challenging to obtain in large numbers, especially for distant galaxies. We addressed this by leveraging machine-learning techniques. Methods . We combined Spitzer photometry and Pan-STARRS1 optical data to classify point sources in 26 galaxies within 5 Mpc, and a metallicity range 0.07–1.36 Z _⊙ . Gaia data release 3 (DR3) astrometry was used to remove foreground sources. Classifications are derived using a machine-learning model developed in our previous work. Results . We report classifications for 1 147 650 sources, with 276 657 sources (~24%) being robust. Among these are 120 479 red supergiants (RSGs; ~11%). The classifier performs well even at low metallicities (~0.1 Z _⊙ ) and distances under 1.5 Mpc, with a slight decrease in accuracy beyond ~3 Mpc due to Spitzer ’s resolution limits. We also identified 21 luminous RSGs (log( L/L _⊙ ) ≥ 5.5), 159 dusty yellow hypergiants in M31 and M33, as well as 6 extreme RSGs (log( L/L _⊙ ) ≥ 6) in M31, challenging observed luminosity limits. Class trends with metallicity align with expectations, although biases exist. Conclusions . This catalog serves as a valuable resource for individual-object studies and James Webb Space Telescope target selection. It enables the follow-up on luminous RSGs and yellow hypergiants to refine our understanding of their evolutionary pathways. Additionally, we provide the largest spectroscopically confirmed catalog of extragalactic massive stars and candidates to date, beyond the Clouds, comprising 5273 sources (including ~330 other objects).

Context. The fate of massive stars during the latest stages of their evolution is highly dependent on their mass-loss rate and geometry. The geometry of the mass-loss process can be inferred from the shape of the circumstellar material, which has a significant influence on the evolution of massive stars (between 25 and 40 M _⊙ ), i.e. type II SN progenitors. In this context, post-red supergiants (post-RSGs) offer an excellent opportunity to study mass-loss events. Aims. We aim to investigate the mass-loss history, geometry, and physical conditions of the yellow hypergiant in a post-RSG stage, IRAS 17163−3907 (IRAS 17163, also known as the ‘Fried Egg’ nebula). We place it in context with another famous evolved massive star, the yellow hypergiant IRC+10420. Methods. We combine M -band spectra of the source using high-resolution CRIRES+ spectroscopy, with VLTI/MATISSE mid-infrared interferometry in the L -band, and FORS2 optical spectropolarimetry to probe both the small-scale circumstellar structure and the large-scale dusty environment of IRAS 17163. The interferometric observables were analysed with simple geometric fitting and a more advanced parametric modelling using PMOIRED to extract the morphology of the hot inner shell that was previously reported via radiative transfer modelling. Results. The CRIRES+ spectrum provides the first M -band coverage of IRAS 17163, revealing prominent low-excitation metal lines and hydrogen recombination features, but lacking the pronounced CO absorption seen in IRC+10420. The MATISSE observations reveal the first high angular scales of the source in the L -band and spatially resolve the Br α line-emitting region, which is a factor of two more extended than the continuum emission and hints at a marginally asymmetric and variable ionised wind. FORS2 spectropolarimetry shows intrinsic continuum polarisation and line effects in the Stokes Q parameter across H α , pointing to deviations from perfect spherical symmetry also on larger scales. The interferometry reveals no evidence for a binary companion within the explored parameter space, indicating that the observed clumpy and time-variable mass loss is likely intrinsic to the star rather than companion-driven. Conclusions. Our results demonstrate that IRAS 17163 hosts a dense, structured, and time-variable wind, coexisting with extended dusty shells. Comparison with IRC+10420 highlights the diversity among post-RSG yellow hypergiants, with IRAS 17163 showing an ionised environment without apparent molecular signatures. These findings emphasise the role of clumpy and near-symmetric mass loss in shaping the circumstellar medium of evolved massive stars, with implications for their subsequent evolution and core-collapse supernova progenitor properties.

Mass loss shapes the fate of massive stars; however, the physical mechanism causing it remains uncertain. We present a comprehensive analysis of seven red supergiants, for which we searched evidence of episodic mass loss, in three low-metallicity galaxies: NGC 6822, IC 10, and WLM. Initially, the spectral classification of their optical spectra was refined and compared to previous reported classifications, finding four sources that display spectral variability. We derived the physical properties of five of them using the MARCS atmospheric models corrected for nonlocal thermal equilibrium effects to measure stellar properties from our new near-infrared spectra, such as the effective temperature, surface gravity, metallicity, and microturbulent velocity. Additional empirical and theoretical methods were employed to calculate effective temperatures, finding consistent results. We constructed optical and infrared light curves, discovering two targets in NGC 6822 with photometric variability between 1 and 2.5 mag in amplitude in r and ~0.5 mag in the mid-infrared. Furthermore, we discovered a candidate-dimming event in one of these sources. Periods for three red supergiants were determined using epoch photometry, which were consistent with the empirical estimations from literature period-luminosity relations. Our comprehensive analysis of all the available data for each target provides evidence for episodic mass loss in four red supergiants.

Context. Red supergiants (RSGs) are cool and evolved massive stars exhibiting enhanced mass loss compared to their main sequence phase, affecting their evolution and fate. However, despite recent advances, the theory of the wind-driving mechanism is not well established and the metallicity dependence has not been determined. Aims. We aim to uniformly measure the mass-loss rates of large samples of RSGs in different galaxies with −0.7 ≲ [Z]≲0 to investigate whether there is a potential correlation with metallicity. Methods. We collected photometry from the ultraviolet to the mid-infrared for all our RSG candidates to construct their spectral energy distribution (SED). Our final sample includes 893 RSG candidates in the Small Magellanic Cloud (SMC), 396 in NGC 6822, 527 in the Milky Way, 1425 in M31, and 1854 in M33. Each SED was modelled using the radiative transfer code DUSTY under the same assumptions to derive the mass-loss rate. Results. The mass-loss rates range from approximately 10⁻⁹ M_⊙ yr⁻¹ to 10⁻⁵ M_⊙ yr⁻¹, with an average value of 1.5 × 10⁻⁷ M_⊙ yr⁻¹. We provide a new mass-loss rate relation as a function of luminosity and effective temperature for both the SMC and Milky Way and compare our mass-loss rates with those derived in the Large Magellanic Cloud (LMC). The turning point in the mass-loss rate versus luminosity relation differs by around 0.2 dex between the LMC and SMC. The mass-loss rates of the Galactic RSGs at log(L/L_⊙) < 4.5 are systematically lower than those determined in the other galaxies, possibly due to uncertainties in the interstellar extinction. We find 60–70% of the RSGs to be dusty, while 14% of the LMC and 2% of the SMC RSGs are significantly dusty. The results for M31 and M33 are inconclusive because of significant blending of sources at distances above 0.5 Mpc, given the resolution of Spitzer, which compromises the mid-IR photometry. Conclusions. Overall, we find similar mass-loss rates among the galaxies, indicating no strong correlation with metallicity other than the location of the turning point. More accurate mid-IR photometry is needed to determine the metallicity dependence.

Mass loss in massive stars is crucial to understanding how these stars evolve and explode. Despite increasing evidence of its importance, episodic mass loss remains poorly understood. Here we report the results of an optical spectroscopic survey of evolved massive stars in NGC 6822, IC 10, and IC 1613 conducted as part of the ASSESS project (Episodic Mass Loss in Evolved Massive Stars: Key to Understanding the Explosive Early Universe), which investigated the role of episodic mass loss by targeting stars with infrared excesses indicating a dusty circumstellar environment. We assigned a spectral class to 122 unique sources, the majority of which are dusty. The rate of evolved massive stars was over 60% for the highest-priority targets. We discovered two blue supergiants, one yellow supergiant, and one emission-line object, and confirmed two supernova remnant candidates, a Wolf–Rayet star, and two H II regions. Twenty-eight unique sources were classified as red supergiants (RSGs), 21 of which are new discoveries. In IC 10, we increased the sample of spectroscopically confirmed RSGs from 1 to 17. We used the MARCS models to obtain their surface properties, most importantly the effective temperature, and used spectral energy distribution fitting to obtain the stellar luminosity for 17 of them. The dusty RSGs are cooler, more luminous, more extinguished, and more evolved than the non-dusty ones, in agreement with previous findings. By investigating the optical photometric variability of the RSGs from light curves that cover a period of over a decade, we found that the dusty RSGs are more variable. We further highlight a very extinguished emission-line object, two RSGs that display a significant change in spectral type between two observed epochs, and four dusty K-type RSGs that may have undergone episodic mass loss.

The rate and mechanism of mass loss of red supergiants (RSGs) remain poorly understood. Motivated by a new empirical mass-loss rate prescription based on the largest and most complete sample of RSGs in the Small Magellanic Cloud (SMC), we investigate the impact of various established and recent prescriptions that span a wide range of RSG mass-loss rates on the evolution, the observable properties, and the final outcome of single massive stars. Our results show that higher mass-loss rates result in earlier envelope stripping and shorter RSG lifetimes, particularly for the more luminous stars, leading to a steeper luminosity function and predicting hotter final positions for the supernova (SN) progenitors. None of the considered mass-loss prescriptions is fully consistent with all observational constraints, highlighting persistent uncertainties in deriving and modeling the mass loss of RSGs. Models incorporating new theoretically motivated high mass-loss rates predict overly efficient envelope stripping, conflicting with the observed populations of luminous RSGs and detected SN progenitors, while commonly used moderate and weak rates tend to overestimate the number of luminous RSGs. The enhanced mass-loss rates for luminous RSGs predicted by recent multi-metallicity empirical studies improve the agreement with the observed RSG luminosity function in the SMC and naturally reproduce the updated Humphreys-Davidson limit. However, they also produce an excess of luminous yellow supergiant progenitors, which remain undetected in nearby SNe. We also estimate that binary interactions tend to slightly enhance the formation of luminous RSGs due to mass accretion or merging. Our study examines the impact of RSG mass loss during the late stages of massive stars, and we highlight the significance of using comprehensive observational data, exploring the uncertainties involved, and considering the effects of binary-induced or episodic mass loss.

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Red Supergiants (RSGs) are cool, evolved massive stars in their final evolutionary stage before exploding as a supernova [1, 2]. However, the evolution and fate of the most luminous RSGs is uncertain. Observational evidence for luminous warm, post-RSG objects [3] and the lack of luminous RSGs as supernova progenitors [4] suggest a blueward evolution. Since the 1980s, WOH G64 has been considered the most extreme RSG in the Large Magellanic Cloud given its outstanding size, luminosity, and mass-loss rate [5–7]. Here we report a sudden, yet smooth change in its nature. Time-series photometry and subsequent spectroscopy revealed the most extreme transition ever seen in the optical spectral features of a RSG. We discovered that WOH G64 is a rare, massive symbiotic system formed by a RSG, which transitioned to a Yellow Hypergiant, and a B-star companion. The dramatic transition can be explained by: a) binary interactions partially stripping the envelope, b) the return of WOH G64 to a quiescent state after an outstanding eruption exceeding 30 years, and c) the expulsion of its outer layers due to a pre-supernova superwind phase, indicating its imminent explosion. 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.