Emmanouil Zapartas

Aims. We aim to understand the progenitor channels of core-collapse supernovae (CCSNe) via a statistical study of the ages of their environments. Methods. We compiled a large and minimally biased sample of 128 CCSNe discovered by un-targeted wide-field transient surveys and with archival VLT/MUSE integral-field-unit spectroscopy. We measured the local H α luminosity within a 300 pc aperture centered on the SN explosion site as an empirical proxy for the environmental age. Results. We find that the mean local H α luminosities are ordered as II(P) ≈ IIb ≲ Ib < Ic. The differences between Types II(P), IIb, and Ib, if any, are very small. Type Ic SNe are located in clearly younger environments than the other types. Conclusions. Our result suggests that Type Ic SNe have much younger and more massive progenitors than the other CCSN types and likely originate from a distinct progenitor channel. The distinction between Types II(P), IIb, and Ib is insensitive to progenitor mass and mainly due to differences in binary separation. In contrast, Type Ic SNe predominantly require much higher-mass progenitors accompanied by close companions with large mass ratios and/or much stronger stellar wind, the latter of which depends sensitively on progenitor mass.

Context. The study of cold molecular hydrogen gas (hereafter molecular gas) provides crucial insights into its interplay with star-forming regions. However, the connection between molecular gas turbulence and the sites of massive star (≳8–10 M _⊙ ) explosions as core-collapse supernovae (CCSNe) remains unexplored. Aims. We measure for the first time the turbulence of molecular gas in environments of CCSNe, with the aim of constraining the nature of their progenitors. Methods. In order to reach spatial resolutions of giant molecular cloud sizes (~100 pc), we collected ALMA carbon monoxide J = 2 → 1 spectral line (~230.54 GHz) observations (as a tracer of molecular gas) at the locations of 33 nearby CCSNe (Results. We found that CCSNe prefer molecular gas regions with a high velocity dispersion compared to the average of their host galaxies. Conclusions. For CCSN progenitors, this observational evidence supports their increased formation in regions of high densities and/or their binary nature.

Mass transfer between nondegenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct-impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk–star coupling model can produce more-massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also the core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk–star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

Understanding supernova (SN) progenitors remains a major challenge in astrophysics, as it involves untangling the complex interplay between stellar physics (e.g. evolution, binarity, and explosion) and environments (e.g. metallicity and the star formation rate). To address this, we present relative frequencies of core-collapse SNe (CCSNe) as a function of metallicity using two complementary samples: (i) all literature SNe that have associated host galaxy parameters (absolute magnitudes, stellar masses, and/or oxygen abundances); and (ii) SNe classified between 2019 and 2024 with host magnitude information, including distance-limited subsamples within 50 Mpc and 100 Mpc. We find that CCSNe from the literature sample are associated with luminous galaxies, reflecting both the higher stellar content of such systems and the selection biases inherent to targeted surveys. In contrast, the distance-limited subsamples provide a less biased view, showing that hydrogen-rich SNe (SNe II) are more commonly found in lower-luminosity galaxies than stripped-envelope SNe (SESNe). Comparisons between the literature sample and the distance-limited subsamples indicate that trends derived from global measurements remain consistent. For the SESNe-to-SNe II ratios, we confirm a slight increase with metallicity, reflecting a higher fraction of SESNe in metal-rich environments. Comparison with theoretical predictions shows that models that include either binary interactions or rotation can broadly reproduce the observed trends, although degeneracies remain and no single scenario uniquely explains the data. Overall, our results provide observational constraints on massive-star evolution and highlight the key role of metallicity and binarity in shaping the observed diversity of CCSNe.

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).

Type IIn supernovae (SNe IIn) are hydrogen-rich explosions embedded in dense circumstellar medium, which gives rise to their characteristic narrow hydrogen emission lines. The nature of their progenitors and pre-explosion mass loss remains, however, poorly understood. This study analyzes the local stellar environments of a volume-limited sample ( z < 0.02) of 31 SNe IIn using high-resolution Hubble Space Telescope imaging. The environments are found to be highly diverse and are classified into three categories: inside star-forming regions (Class 1), outside but near star-forming regions (Class 2), and regions with no obvious star formation (Class 3). Bright SNe IIn ( M _peak < −19.5 mag) predominantly occur in Class 1 environments, supporting the origin of very massive progenitors, while the faint SNe IIn ( M _peak < −15.5 mag) are associated with the Class 2 and 3 environments, consistent with the less massive progenitors. SNe IIn with intermediate peak magnitudes (−19.5 mag < M _peak < −15.5 mag) appear in all three types of environments. Furthermore, directly detected SN IIn progenitors are systematically brighter and/or bluer than the youngest stellar populations in their environments, implying a nonquiescent pre-explosion state or past binary interactions. These results point to multiple progenitor channels of SNe IIn, spanning a wide range of masses, evolutionary stages, and potential binary interaction histories.

Mass transfer between non-degenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically-motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk-star coupling model can produce more massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk-star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

We present a systematic comparison between the tidal secular evolution timescales predicted by the direct numerical method and those given by the commonly used semianalytic prescriptions implemented in 1D hydrostatic binary evolution codes. Our study focuses on binary systems with intermediate- to high-mass primaries ( M ₁ = 5–50 M _⊙ ), companion masses between 1.4 M _⊙ and 10 M _⊙ , and orbital periods ranging from 0.5–50 days. Before mass transfer, both approaches predict synchronization and orbital decay timescales that agree within ∼2 orders of magnitude and typically exceed the stellar main-sequence lifetime, implying negligible tidal impact on secular orbital evolution. However, the implied dissipation channels differ, and the differences become more pronounced once mass transfer begins. To test the theoretical predictions against observations, we apply both approaches to the well-characterized PSR J0045–7319 system, which has an orbital decay timescale of 0.5 Myr. The numerical solution reveals strong resonances with internal gravity waves, bringing the predicted orbital period change rate close to the observed value. In contrast, the semianalytic prescriptions predict orbital decay timescales longer than the Hubble time. These results suggest that for population studies, modestly calibrated parameterized equations may suffice, but for individual systems, reliable interpretation requires direct numerical approaches.

Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceed μ ≈ 1000. Nine such sources were identified in JWST NIRCam imaging of a single galaxy at redshift z = 0.94 known as the “Warhol arc,” which is mirror imaged by the galaxy cluster MACS J0416.1−2403. Here we present the discovery of two coincident and well-characterized microlensing events at the same location followed by a third event observed in a single filter approximately 18 months later. The events can be explained by microlensing of a binary star system consisting of a red supergiant ( T ≈ 4000 K) and a B-type ( T ≳ 13,000 K) companion star. The timescale of the coincident microlensing events constrains the estimated projected source-plane size to tens of astronomical units. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses M 1 init = 23 . 6 − 4.3 + 5.3 M _⊙ and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curves in all filters gives a relatively small transverse velocity of ∼50 km s ⁻¹ . This requires the dominant velocity component of several hundreds of kilometers per second to be roughly parallel to the microcaustic. An alternative possibility would be that the three microlensing events correspond to unrelated stars crossing distinct microcaustics, but this would imply a highly elevated rate of events at their common position, even though no underlying knot is present at the location.

With over two dozen detections in the Milky Way, double neutron stars (DNSs) provide a unique window into massive binary evolution. We use the POSYDON binary population synthesis code to model DNS populations and compare them to the observed Galactic sample. By tracing their origins to underlying single and binary star physics, we place constraints on the detailed evolutionary stages leading to DNS formation. Our study reveals a bifurcation within the well-known common envelope (CE) formation channel for DNSs, which naturally explains an observed split in the orbital periods of the Galactic systems. The two subchannels are defined by whether the donor star has a helium core (Case B mass transfer) or a carbon-oxygen core (Case C) at the onset of the CE, with only the helium core systems eventually merging due to gravitational-wave-modulated orbital decay. We find that across different treatments of the CE phase, the formation of DNSs through both subchannels requires either a generous core definition of ≃30% H-fraction or a high CE ejection efficiency of α _CE ≳ 1.2. By testing different supernova kick velocity models, we find that galactic DNSs are best reproduced using a prescription that favors low velocity kicks (≲50 km s ⁻¹ ), in agreement with previous studies. Furthermore, our models indicate that merging DNSs are born from a stripped progenitor with a median pre-supernova envelope mass ∼0.2 M _⊙ . Our results highlight the value of detailed evolutionary models for improving our understanding of exotic binary star formation.