Stellar mergers produce more massive, rejuvenated (strongly magnetic) stars, with potentially peculiar properties, and can be detected as luminous red novae. Using a grid of detailed 1D binary evolution models, we aim to determine which binary systems are likely to merge and at what evolutionary stage. This will tell us more about the merger products, and might help us understand some of the trends found in observed single- and multiple-star populations.

We use the RIOTS4 sample of SMC field OB stars to determine the origin of massive runaways in this low-metallicity galaxy using Gaia proper motions, together with stellar masses obtained from RIOTS4 data. These data allow us to estimate the relative contributions of stars accelerated by the dynamical ejection vs binary supernova mechanisms, since dynamical ejection favors faster, more massive runaways, while SN ejection favors the opposite trend. In addition, we use the frequencies of classical OBe stars, high-mass X-ray binaries, and non-compact binaries to discriminate between the mechanisms. Our results show that the dynamical mechanism dominates by a factor of 2 – 3. This also implies a significant contribution from two-step acceleration that occurs when dynamically ejected binaries are followed by SN kicks. We update our published quantitative results from Gaia DR2 proper motions with new data from DR3.

Long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe) are expected to result from massive star deaths. However to date, there has been no direct observational measurement of their cloud collapse timescales nor progenitor lifetimes to help constrain their mass. Our analyses of z 2 LGRB afterglow spectra and Hubble Space Telescope images find a higher fraction of host galaxies that are interacting, have a close companion, and/or may have experienced a recent galaxy ‘fly by’ as compared to the general z 2 galaxy population. A smaller set of z 2 SLSNe suggests a similar result. Under the hypothesis that galaxy interactions induce cloud collapse and star formation near their closest approach, we explore measurements of the host and companion galaxy velocities and separations at the time of the LRGB/SLSN event as a direct physical means to measure the timescale of cloud collapse plus progenitor star lifetime.

Empirical constraints are master keys for testing theoretical evolutionary model predictions. In massive stars, the region in the Hertzsprung-Russell diagram (HRD) in which the Blue Supergiants (BSGs) are located sets several important constraints to the models, and in particular to the theoretical end of the main sequence (MS). So far, we are missing from a full quantitative spectroscopic analysis (QSA) of a sample of BSGs large enough to be statistically significant and without observational biases. We present results from a QSA of a sample of ∼700 Galactic BSGs for which we have high-resolution multi-epoch optical spectra.

We report recent ESPaDOnS and HARPSpol spectropolarimetric observations from our ongoing magnetic survey of the brightest twenty-five classical Cepheids. Stokes V magnetic signatures are detected in eight of fifteen targets observed to date. The Stokes V profiles show a diversity of morphologies with weak associated longitudinal field measurements of order 1 G. Many of the Stokes V profiles are difficult to interpret in the context of the normal Zeeman effect. They consist of approximately unipolar single or double lobe(s) of positive or negative circular polarization. We hypothesize that these unusual signatures are due to the Zeeman effect modified by atmospheric velocity or magnetic field gradients. In contrast, the Stokes V profiles of Polaris and MY Pup appear qualitatively similar to the complex magnetic signatures of non-pulsating cool supergiants, possibly due to the low pulsation amplitudes of these two stars.

A fundamental question for theories of massive star formation is whether OB stars can form in isolation. We assess the contribution of any in-situ OB star formation by using 210 field OB stars in the Small Magellanic Cloud (SMC) from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4). We search for tiny, sparse clusters around our target OB stars using cluster-finding algorithms. Employing statistical tests, we compare these observations with random-field data sets. We find that ∼5% of our target fields do show evidence of higher central stellar densities, implying the presence of small clusters. This frequency of small clusters is low and within errors, it is also consistent with the field OB population being composed entirely of runaway and walkaway stars. Assuming this small cluster fraction is real, it implies that some OB stars may form in highly isolated conditions. The low frequency could be caused by these clusters evaporating on a short timescale. However, another interpretation is that the low fraction of small clusters is observed because these form rarely, or not at all, implying a higher cluster lower-mass limit and generally consistent with a relationship between maximum stellar mass (mmax) and the cluster mass (Mcl).

In 2020, HR 6819 was reported to be a triple system containing the closest black hole to Earth. However, these results were contested, with an alternative explanation of a post-interaction binary suggested. Using new integral field spectroscopic and interferometric data, we have been able to determine the true nature of this exotic source.

When a star is rapidly rotating, it deviates from spherical symmetry causing non-uniform distributions of the surface gravity and temperature across the surface. These three-dimensional effects lead to an inclination dependence of many observable spectroscopic parameters, however this is often neglected when analyzing rapidly rotating systems. Using spamms, we generate synthetic spectra that account for the 3D geometry of the system and fit them with 1D models to investigate how much the 3D effects can change the derived stellar parameters. We show that these 3D effects can lead to observed temperature differences of thousands of kelvin for the same star viewed at different inclinations, and a systematic underestimation of the helium abundance.

We discovered a nebula in the low-metallicity (Z=0.1 ) nearby ( Mpc) dwarf galaxy NGC 4068, which reveals broad Hα line profile and unusual emission line fluxes in its spectrum. The object also shows significant nitrogen overabundance, not typical for metal-poor environment. We assumed that the nebula could be ionized by an evolved massive star with , Wolf-Rayet or Blue Supergiant, and built models of the nebula using Cloudy and CMFGEN codes. Our models successfully reproduce the optical emission spectrum of the object, including the peculiar [SII]/[NII] ratio and the presence of the HeII λ4686 line.

The galactic binary star LB-1 contains a recently stripped B-type star. Comparing its properties to detailed binary star models shows that tidal braking and magnetic torques lead to low surface rotational velocities in the stripped donors after Roche-lobe overffiow. Models without magnetic torques cannot reproduce the observed low surface rotation.

We present the results of our long-term RTT-150 photometric and spectroscopic observations Be optical counterpart of the High-mass X-ray binary IGR J21343+4738.

During the last years we have carried out different studies in the Cygnus OB2 association based on new spectroscopic data and benefiting from the unprecedented Gaia astrometry. They include membership, chemical and structure studies, that allowed us to discern for the first time ever between two stellar groups separated by several hundred parsecs within the association and find at least two star-forming bursts at ∼3 and ∼5 Myr. Using these studies as a template and combining upcoming spectroscopic WEAVE data and the expected accuracy that Gaia will reach in the Cygnus-X area (DR3 and forthcoming releases), we will be able to perform the deepest multi-dimensional study ever done before in a massive star-forming complex. The results of this project will lead to an important improvement of our knowledge of star formation and evolution of star-forming regions and clusters, including our understanding of the dynamics and kinematics of OB associations and stellar groups.

The current angular momentum (AM) transport models fail to reproduce asterosieimic observations. One of the best candidates to explain this discrepancy is the magnetic field in radiative zones with its various possible topologies, for instance axisymmetric toroidal magnetic field. If such azimuthal field is strong enough, the Tayler’s instability could occur which induces a magnetic torque that allows a very efficient transport of AM and could trigger dynamo action in radiative layers. If such important field does not emerge at the surface, spectropolarimetry is blind. In this case, the only way to detect and characterise the field is by using magneto-asteroseismology. It consists in searching for the characteristic signatures of magnetic field in the observed frequency spectra of stellar oscillations.

The study of the multiplicity of massive stars gives hints of their formation processes and their evolution path. Optical interferometry is mandatory to fulfill our knowledge of their multiplicity by probing the separation gap between 1 and 50 mas. We demonstrated the capability of the new interferometric instrument MIRC-X, located at the CHARA array, to study a large sample of more than 120 (H < 7.5) O-type stars. We observed 29 O-type star systems, including a couple of systems in average atmospheric conditions around a magnitude of H = 7.5. Out of these 29 systems, we detected 18 companions in 16 different systems, resulting in a multiplicity fraction fm = 16/29 = 0.55, and a companion fraction of fc = 18/29 = 0.62. We observed for the first time 11 of these detected companions. This study concludes that a large survey on more than 120 Northern O-type stars is possible with MIRC-X.

Rotation is one of the important parameters affecting the evolution and final fate of massive stars but the origin of fast rotators remains unclear (imprint of the star formation process, result of binary interactions). In this work, we aim at investigating the binary status, photometric variability, and runaway status of a statistically meaningful sample of Galactic fast-rotating O stars. We perform a comprehensive multi-epoch analysis of new high-quality spectroscopic observations gathered by the IACOB and OWN surveys. Notably, we find that the total percentage of spectroscopic binaries in the investigated sample range between 25 and 40%, in agreement with previous finding for the case of O-type stars with lower projected rotational velocities. On the contrary, the fraction of runaway stars among fast rotators (∼35–50%) is significantly higher than in the case of slow rotators (∼20–30%). By combining all these observational results we will evaluate each scenario about the origin of fast rotators.

The jittering jets explosion mechanism of core collapse supernovae proposes that stochastic convective motion, amplified by instabilities, results in intermittent accretion disk that launches jittering jets that explode the star. We conduct one-dimensional simulations of a wide range of stellar masses and show that the convective motion in the pre-collapse stellar core, when scaled to corresponding three-dimensional simulations, supply sufficient angular momentum fluctuation to form the intermittent accretion disk the launches the jittering jets. The resulting neutron star masses are consistent with observations.

With the upcoming third Gaia data release (DR3), the first Gaia astrometric orbital solutions for binary sources will become available. Potentially, many rarely seen single-degenerate massive binaries with a black hole (OB+BH) will be revealed. Here, we investigate how many OB+BHs are expected to be detected as binaries in Gaia astrometry by using tailored models for the massive star population. We use a method based on the astrometric data to investigate how many OB+BH binaries will be uncovered by Gaia. We estimate that∼200 OB+BHs are detectable among the sources in the second Alma Luminous Star massive star catalogue, either in DR3 or in upcoming data releases. Moreover, we show that BH-formation scenarios could be constrained from the distributions of parameters such as the orbital periods and eccentricities.

We report on a study of 9 nearby primitive galaxies observed by Hubble’s COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. By “primitive galaxies,” we mean galaxies having a low stellar mass, and low gas metallicity, , whether they are local or at high redshift. We find that far-UV spectra of these galaxies show evidence of hard radiation, including X-rays. Following Thuan et al. (2004), we identify these galaxies as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of extremely strong X-radiation. Following Heger et al. (2003), we suggest that the accreting black hold is produced by direct collapse of stars having initial masses greater than 50 . The X-radiation produced by black hole disk directly affects the surrounding interstellar medium, and many of these effects are observable in far-UV spectra.

The Intensity Interferometry technique consists of measuring the spatial coherence (visibility) of an object via its intensity fluctuations over a sufficient range of telescope separations (baselines). This allows us to study the size, shape and morphology of stars with an unprecedented resolution. Cherenkov telescopes have a set of characteristics that coincidentally allow for Intensity Interferometry observations: very large reflective surfaces, sensitivity to individual photons, temporal resolution of nanoseconds and the fact that they come in groups of several telescopes. In the recent years, the MAGIC Collaboration has developed a deadtime-free Intensity Interferometry setup for its two 17 m diameter Cherenkov telescopes that includes a 4-channel GPU-based real-time correlator, 410–430 nm filters and new ways of splitting its primary mirrors into submirrors using Active Mirror Control (AMC). With this setup, MAGIC can operate as a long-baseline optical interferometer in the baseline range 40–90 m, which translates into angular resolutions of 0.5-1 mas. Additionally, thanks to its AMC, it can simultaneously measure the zero-baseline correlation or, by splitting into submirrors, access shorter baselines under 17 m in multiple u-v plane orientations. The best candidates to observe with this technique are relatively small and bright stars, in other words, massive stars (O, B and A types). We will present the science cases that are currently being proposed for this setup, as well as the prospects for the future of the system and technique, like the possibility of large-scale implementation with CTA.

Massive stars play crucial roles in astrophysical settings across cosmic history, and thus it is a fundamental problem to understand whether their formation processes are universal or diverse in various galactic environments. In particular, metallicity is the essential characteristic of cosmic evolution. Our theoretical studies have suggested some degrees of metallicity dependence of massive star formation. In the extremely metal-poor case of , protostellar disks are significantly unstable, and the photoionization feedback is more efficient. We also execute an ALMA survey targeting massive protostars in the Large Magellanic Clouds (LMC) with . We found that the outflow properties of LMC protostars (mass, momentum, energy) are consistent with those of Galactic protostars, suggesting the universality of massive star formation at least in the range of .