Ups!... I did it again:unveiling the hidden companion in Upsilon Sagittarii, a unique binary system at a second mass transfer stage

Upsilon Sagittarii is a hydrogen-deficient binary that has been suggested to be in its second stage of mass transfer, after the primary has expanded to become a helium supergiant following core helium exhaustion. A tentative identification of the faint companion in the ultraviolet led to mass estimates of both components that made the helium star in Upsilon Sagittarii a prototypical immediate progenitor of a type Ib/c supernova. However, no consistent model for the complex spectrum has been achieved, casting doubt on this interpretation. In this study, we provide for the first time a composite spectral model that fits the ultraviolet data, and clearly identifies the companion as a rapidly rotating, slowly moving ≈ 7 M☉ B-type star, unlike previously suggested. The stripped helium supergiant is less luminous than previous estimates, and with an estimated mass of &lt; 1 M☉ is ruled out as a core-collapse supernova progenitor. We provide a detailed binary evolution scenario that explains the temperature and luminosity of the two components as well as the very low gravity (log g ≈ 1) and extreme hydrogen deficiency of the primary (atmospheric mass fraction XH, 1 ≈ 0.001). The best-fitting model is an intermediate-mass primary (MZAMS,1 ≈ 5 M☉) with an initial orbital period of a few days, and a secondary that appears to have gained a significant amount of mass despite its high rotation. We conclude that Upsilon Sagittarii is a key system for testing binary evolution processes, especially envelope stripping and mass accretion.</p

An optimal envelope ejection efficiency for merging neutron stars

We use the rapid binary stellar evolution code \texttt{binary_c} to estimate the rate of merging neutron stars with numerous combinations of envelope ejection efficiency and natal kick dispersion. We find a peak in the local rate of merging neutron stars around $\alpha \approx 0.3$$-$$0.4$, depending on the metallicity, where $\alpha$ is the efficiency of utilising orbital energy to unbind the envelope. The peak height decreases with increasing electron-capture supernova kick dispersion $\sigma_\mathrm{ECSN}$. We explain the peak as a competition between the total number of systems that survive the common-envelope phase increasing with $\alpha$ and their separation, which increases with $\alpha$ as well. Increasing $\alpha$ reduces the fraction of systems that merge within a time shorter than the age of the Universe and results in different mass distributions for merging and non-merging double neutron stars. This offers a possible explanation for the discrepancy between the Galactic double neutron star mass distribution and the observed massive merging neutron star event GW190425. Within the $\alpha$$-$$\sigma_\mathrm{ECSN}$ parameter space that we investigate, the rate of merging neutron stars spans several orders of magnitude up to more than $1\times 10^{3} \, \mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$ and can be higher than the observed upper limit or lower than the observed lower limit inferred thus far from merging neutron stars detected by gravitational waves. Our results stress the importance of common-envelope physics for the quantitative prediction and interpretation of merging binary neutron star events in this new age of gravitational wave astronomy.Comment: 16 pages, 10 figures, this is a pre-copyedited, author-produced version of an article accepted for publication in MNRA

ASASSN-15lh: a superluminous ultraviolet rebrightening observed by Swift and Hubble

We present and discuss ultraviolet and optical photometry from the Ultraviolet/Optical Telescope and X-ray limits from the X-Ray Telescope on Swift and imaging polarimetry and ultraviolet/optical spectroscopy with the Hubble Space Telescope of ASASSN-15lh. It has been classified as a hydrogenpoor superluminous supernova (SLSN I) more luminous than any other supernova observed. ASASSN- 15lh is not detected in the X-rays in individual or coadded observations. From the polarimetry we determine that the explosion was only mildly asymmetric. We find the flux of ASASSN-15lh to increase strongly into the ultraviolet, with a ultraviolet luminosity a hundred times greater than the hydrogen-rich, ultraviolet-bright SLSN II SN 2008es. We find objects as bright as ASASSN-15lh are easily detectable beyond redshifts of ∼4 with the single-visit depths planned for the Large Synoptic Survey Telescope. Deep near-infrared surveys could detect such objects past a redshift of ∼20 enabling a probe of the earliest star formation. A late rebrightening – most prominent at shorter wavelengths – is seen about two months after the peak brightness, which is itself as bright as a superluminous supernova. The ultraviolet spectra during the rebrightening are dominated by the continuum without the broad absorption or emission lines seen in SLSNe or tidal disruption events and the early optical spectra of ASASSN-15lh. Our spectra show no strong hydrogen emission, showing only Lyα absorption near the redshift previously found by optical absorption lines of the presumed host. The properties of ASASSN-15lh are extreme when compared to either SLSNe or tidal disruption events

Effects of winds on the leftover hydrogen in massive stars following Roche-lobe overflow

We find that applying a theoretical wind mass-loss rate from Monte Carlo radiative transfer models for hydrogen-deficient stars results in significantly more leftover hydrogen following stable mass transfer through Roche-lobe overflow than when we use an extrapolation of an empirical fit for Galactic Wolf-Rayet stars, for which a negligible amount of hydrogen remains in a large set of binary stellar evolution computations. These findings have implications for modelling progenitors of Type Ib and Type IIb supernovae. Most importantly, our study stresses the sensitivity of the stellar evolution models to the assumed mass-loss rates and the need to develop a better theoretical understanding of stellar winds

A massive helium star with a sufficiently strong magnetic field to form a magnetar

Magnetars are highly magnetized neutron stars; their formation mechanism is unknown. Hot helium-rich stars with spectra dominated by emission lines are known as Wolf-Rayet stars. We observe the binary system HD 45166 using spectropolarimetry, finding that it contains a Wolf-Rayet star with a mass of 2 solar masses and a magnetic field of 43 kilogauss. Stellar evolution calculations indicate that this component will explode as a type Ib or IIb supernova, and the strong magnetic field favors a magnetar remnant. We propose that the magnatized Wolf-Rayet star formed by the merger of two lower mass helium stars.Comment: Published in Science on the 18 August 2023. Radial velocities, spectra, and software available in: https://zenodo.org/record/8042656 ESO press release: www.eso.org/public/news/eso231

A massive helium star with a sufficiently strong magnetic field to form a magnetar

Magnetars are highly magnetized neutron stars, the formation mechanism of which is unknown. Hot helium-rich stars with spectra dominated by emission lines are known as Wolf-Rayet stars. We observed the binary system HD 45166 using spectropolarimetry and reanalyzed its orbit using archival data. We found that the system contains a Wolf-Rayet star with a mass of 2 solar masses and a magnetic field of 43 kilogauss. Stellar evolution calculations indicate that this component will explode as a supernova, and that its magnetic field is strong enough for the supernova to leave a magnetar remnant. We propose that the magnetized Wolf-Rayet star formed by the merger of two lower-mass helium stars

Binarity at LOw Metallicity (BLOeM) The multiplicity properties and evolution of BAF-type supergiants

\ua9 The Authors 2025. Given the uncertain evolutionary status of blue supergiant stars, their multiplicity properties hold vital clues to better understand their origin and evolution. As part of The Binarity at LOw Metallicity (BLOeM) campaign in the Small Magellanic Cloud, we present a multi-epoch spectroscopic survey of 128 supergiant stars of spectral type B5–F5, which roughly correspond to initial masses in the 6–30 M range. The observed binary fraction for the B5–9 supergiants is 25 \ub1 6% (10 \ub1 4%) and 5 \ub1 2% (0%) for the A–F stars, which were found using a radial-velocity (RV) variability threshold of 5 km s−1 (10 km s−1) as a criterion for binarity. Accounting for observational biases, we find an intrinsic multiplicity fraction of less than 18% for the B5–9 stars and 8+−97% for the AF stars, for the orbital periods up to 103.5 days and mass ratios (q) in the 0.1 &lt; q &lt; 1 range. The large stellar radii of these supergiant stars prevent short orbital periods, but we demonstrate that this effect alone cannot explain our results. We assessed the spectra and RV time series of the detected binary systems and find that only a small fraction display convincing solutions. We conclude that the multiplicity fractions are compromised by intrinsic stellar variability, such that the true multiplicity fraction may be significantly smaller. Our main conclusions from comparing the multiplicity properties of the B5–9- and AF-type supergiants to that of their less evolved counterparts is that such stars cannot be explained by a direct evolution from the main sequence. Furthermore, by comparing their multiplicity properties to red supergiant stars, we conclude that the AF supergiant stars are neither progenitors nor descendants of red supergiants

Binarity at LOw Metallicity (BLOeM) Multiplicity properties of Oe and Be stars

\ua9 The Authors 2025. Context. Rapidly rotating classical OBe stars have been proposed as the products of binary interactions, and the fraction of Be stars with compact companions implies that at least some are. However, to constrain the interaction physics spinning up the OBe stars, a large sample of homogeneously analyzed OBe stars with well-determined binary characteristics and orbital parameters are required. Aims. We investigated the multiplicity properties of a sample of 18 Oe, 62 Be, and two Of?p stars observed within the BLOeM survey in the Small Magellanic Cloud. We analyzed the first nine epochs of spectroscopic observations obtained over approximately three months in 2023. Methods. Radial velocities (RVs) of all stars were measured using cross-correlation based on different sets of absorption and emission lines. Applying commonly used binarity criteria, we classified objects as binaries, binary candidates, and apparently single (RV stable) objects. We further inspected the spectra for double-lined spectroscopic binaries and cross-matched with catalogs of X-ray sources and photometric binaries. Results. We classify 14 OBe stars as binaries, and an additional 11 as binary candidates. The two Of?p stars are apparently single. We find two more objects that are most likely currently interacting binaries. Without those, the observed binary fraction for the remaining OBe sample of 78 stars is fOBeobs = 0.18 \ub1 0.04 (fOBeobs+cand = 0.32\ub10.05 including candidates). This binary fraction is less than half of that measured for OB stars in BLOeM. Combined with the lower fraction of SB2s, this suggests that OBe stars do indeed have fundamentally different present-day binary properties than OB stars. We find no evidence for OBe binaries with massive compact companions, in contrast to expectations from binary population synthesis. Conclusions. Our results support the binary scenario as an important formation channel for OBe stars, as post-interaction binaries may have been disrupted or the stripped companions of OBe stars are harder to detect. Further observations are required to characterize the detected binaries, their orbital parameters, and the nature of their companions

Binarity at LOw Metallicity (BLOeM): A spectroscopic VLT monitoring survey of massive stars in the SMC

\ua9 The Authors 2024.Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z⊙). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P ≲ 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960- 4570 \uc5 resolving power R = 6200; typical signal-to-noise ratio(S/N) ≈70- 100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 ≲ Teff/kK ≲ 45 and 3.7 &lt; log L/L⊙ &lt; 6.1 and initial masses of 8 ≲ Mini ≲ 80 M⊙. The sample comprises 159 O-type stars, 331 early B-type (B0- 3) dwarfs and giants (luminosity classes V- III), 303 early B-type supergiants (II- I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries

Binarity at LOw Metallicity (BLOeM)

Given the uncertain evolutionary status of blue supergiant stars, their multiplicity properties hold vital clues to better understand their origin and evolution. As part of The Binarity at LOw Metallicity (BLOeM) campaign in the Small Magellanic Cloud, we present a multi-epoch spectroscopic survey of 128 supergiant stars of spectral type B5–F5, which roughly correspond to initial masses in the 6–30 M⊙ range. The observed binary fraction for the B5–9 supergiants is 25 ± 6% (10 ± 4%) and 5 ± 2% (0%) for the A–F stars, which were found using a radial-velocity (RV) variability threshold of 5 km s−1 (10 km s−1) as a criterion for binarity. Accounting for observational biases, we find an intrinsic multiplicity fraction of less than 18% for the B5–9 stars and 8−7+9% for the AF stars, for the orbital periods up to 103.5 days and mass ratios (q) in the 0.1 < q < 1 range. The large stellar radii of these supergiant stars prevent short orbital periods, but we demonstrate that this effect alone cannot explain our results. We assessed the spectra and RV time series of the detected binary systems and find that only a small fraction display convincing solutions. We conclude that the multiplicity fractions are compromised by intrinsic stellar variability, such that the true multiplicity fraction may be significantly smaller. Our main conclusions from comparing the multiplicity properties of the B5–9- and AF-type supergiants to that of their less evolved counterparts is that such stars cannot be explained by a direct evolution from the main sequence. Furthermore, by comparing their multiplicity properties to red supergiant stars, we conclude that the AF supergiant stars are neither progenitors nor descendants of red supergiants