arXiv subjects

D. Pauli

Publications and source records attributed to D. Pauli.

At least 19 recordsLinked to original sources

arXiv2026

Potsdam Wolf-Rayet stellar atmosphere grids of OB-type stars: I. Spectroscopic temperature diagnostics across metallicity

Massive hot stars are among the major ionizing sources in the Universe. The ionizing flux of a star strongly depends on its temperature, which is best measured spectroscopically using lines of different ionization stages of the same element. For the quantitative spectral analysis of O-type stars, helium lines are commonly employed, and for B-type stars, lines of silicon and magnesium are used. At low metallicity, many of the diagnostic metal lines disappear. We conduct a systematic theoretical analysis of stellar atmosphere models of OB stars to study the effect of metallicity on key temperature diagnostic lines and how this impacts spectral classification. We computed large grids of state-of-the-art non-LTE stellar atmosphere models at four different metallicities, ranging from solar to 1/31 solar. We presented equivalent widths of selected diagnostic lines in the temperature-gravity plane and investigated their dependence on metallicity. Using Galactic stellar templates of O- and B-type stars, we established equivalent width ratios for spectral classification and applied them to our models. For the hottest stars in our model grids, He I lines become detectable at lower metallicity due to the reduced back-warming in the atmosphere, opening new possibilities for their classification. Metal lines used for the analysis of B-type stars are mostly absent in the spectra of stars with metallicity below that of the Small Magellanic Cloud, except Si lines, challenging current classification schemes. Our stellar atmosphere grids reveal that the decrease in metallicity causes many metal lines to disappear, leaving different sensitivities of He I lines the only temperature indicators for B-type stars in the optical. The ionizing flux of OB-type stars can be partially trapped within the stellar wind. As a result, at lower metallicity, a larger fraction of stars can emit hard ionizing radiation.

astro-ph.SR↗

arXiv2026

Multiplicity of Massive Stars at Low Metallicity: Early Results from the BLOeM Campaign

Massive stars at low metallicity (Z) play a central role in shaping the high-redshift Universe, yet their multiplicity remains poorly constrained. The Binarity at Low Metallicity (BLOeM) campaign is a two-year survey of 929 stars in the Small Magellanic Cloud with the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument at ESO's Very Large Telescope, providing the first large-scale spectroscopic monitoring of massive stars at low Z (1/5 solar). Analysis of the initial nine epochs reveals high intrinsic binary fractions (>70%) on the main sequence and a steep decline in evolved objects. Analysis of the full dataset will yield orbital solutions, identify black-hole companions, and allow a derivation of the initial mass function for single and binary stars at low Z.

physics.gen-ph↗

arXiv2026

Southern massive stars at high angular resolution: WR 25 is a massive hierarchical triple system

WR 25 is a massive colliding-wind binary in the Carina nebula comprising a WN6ha primary with an O5 companion in an eccentric 208-d orbit. Recent spectroscopic analysis estimates the total binary mass to approach $100\,M_\odot$, and a primary-to-secondary mass ratio of $q= M_1/M_2\approx2$. The presence of additional spectroscopic signatures from a third, intruder star was also noted, making it a candidate hierarchical triple system. In this study, we present a VLTI/PIONIER interferometric observation of WR 25, spatially resolving all three components for the first time. For the inner WN6ha + O5 binary, we find an angular separation of $1.68\pm0.02$ milliarcseconds (mas). Leveraging the fortunate timing of the VLTI observation, which was obtained when the two components were passing the line of nodes, we determined the semi-major axis $a=3.11\pm0.20$ au. Subsequently, the newly constrained total dynamical binary mass is $93\pm18\,M_\odot$, with a primary mass $M_1=62\pm13\, M_\odot$ and secondary mass $M_2=31\pm7\, M_\odot$. We detect the tertiary with an angular separation of $27.69\pm0.02$ mas from the primary, with a chance alignment probability lower than $10^{-4}$. Using newly obtained brightness ratios between all components, we revisit archival spectroscopic data of WR 25 to disentangle spectra for individual components and derive their stellar parameters. The tertiary, which has a spectral type O7, is coeval with the inner binary and has an evolutionary mass $M_3=25.6^{+2.8}_{-2.3}\,M_\odot$. Based on simulations, we estimate the tertiary period to be in the range 19 - 82 yr. The newly confirmed triple nature of WR 25 makes it an important benchmark system to measure accurate dynamical masses of the inner binary and potentially the tertiary, to calibrate stellar evolution and atmosphere models, and to study its formation and stability as a hierarchical triple system.

astro-ph.SR↗

arXiv2026

Building a Roadmap for Hubble science into the 2030s: Crucial UV spectroscopy of Oe stars in nearby galaxies

Hubble's unique COS G130M+G160M and STIS E140M UV spectral capabilities are essential for characterizing and understanding fundamental properties of main-sequence O-type emission-line (Oe) stars. These are fast rotators, and some are believed to be spun up in binaries. UV medium resolution observations of these stars are crucial for understanding massive binaries and their role in galaxy evolution. Oe stars are more prevalent at low metallicity, where they are highly under-studied, but UV spectra of these stars at all metallicities are needed. Observations of these stars in the 2030's with Hubble are particularly important in the era of ultra wide-field IFU optical and transient astronomy surveys. Ultimately, these observations will inform future UV observations with the Habitable Worlds Observatory.

astro-ph.IM↗

arXiv2026

First measurement of wind line formation regions in an early O-type star

Massive stars with their strong ionizing radiation and strong stellar winds are the key feedback agents of the universe. Stellar winds of massive stars are often measured by fitting resonance lines in the UV using non-LTE stellar atmosphere models. So far, the line formation regions of these lines have not been measured empirically, preventing a comparison to the model's structures. We aim to conduct the first measurement of the resonance line formation regions in an early-type eclipsing binary in the SMC, namely AzV 75. We employ TESS and ASAS-SN photometry in combination with radial velocity measurements from multi-epoch HST UV spectra to derive the ephemeris. We examine the intensity changes in the C IV and N V resonance lines in the UV and combine them with a light-curve analysis to estimate the region in the wind where these lines are formed. AzV 75 has an orbital period P=165.66d, eccentricity e=0.42, mass ratio q=0.72, and inclination i=85.77{\deg}. With this orbital configuration, no secondary eclipse is expected. We report that the optically thick UV resonance lines exhibit flattening and shortening of the absorption trough, and weakening of their emission features, as they approach the phase of the expected secondary eclipse, while the continuum UV flux appears to remain unaffected. We illustrate that this can be explained by the primary's optically thick wind eclipsing the secondary star. The C IV and N V resonance line formation regions in the primary star extend up to 316 Rsol. The measured extend of the formation regions of resonance lines in a stellar wind are important benchmarks for 1D as well as 3D non-LTE stellar atmosphere models. A first comparison to 1D-stellar atmosphere models indicates that a classical beta-law with an exponent of beta=0.5 instead of beta=0.8 might be favoured for the primary star's velocity field.

astro-ph.SR↗

arXiv2026

X-Shooting ULLYSES: Massive stars at low metallicity XV. On the metallicity dependence of B-supergiant mass-loss rates

Context. For stellar evolution models we rely on mass-loss rate prescriptions that show features that lack empirical confirmation, such as the bi-stability jump. This jump is an increase in mass loss in the decreasing temperature regime Teff 28-21 kK. Although papers compared empirical results to prescriptions,a large observational sample of B stars for which the wind has been scrutinised over different metallicities is still lacking. Aims. By modelling of both ultraviolet (ULLYSES) and optical (XShootU) spectra, we determined the stellar and wind parameters, of 24 SMC B stars ranging in Teff from 13 to 29 kK. By combining this sample with LMC studies, we compared the wind behaviour of B stars in two different metallicity regimes. We compared our results to common mass-loss rate prescriptions to test features present in these models and their metallicity dependence. Methods. We have used the model atmosphere code fastwind and the genetic algorithm code Kiwi-GA to fit the UV and optical spectra simultaneously. This allows us to determine wind properties including clumping behaviour. Results. The metallicity trends present in the mass-loss prescriptions (Z^(0.41-1.4)) explored here overestimate the empirical metallicity dependence in the B-star regime, which appears very weak. We do not find an increase in mass-loss rate at approximately spectral type B1. We show that on average 40% of the wind mass is located in the wind medium between the clumps. We compiled a sample of 80+ O and B stars in the SMC and LMC. From a comparison we find a clear difference in O- and B-type metallicity dependence. Conclusions. The lack of a bi-stability jump in the B-star regime and a weak metallicity dependence for the same stars offers new empirical constraints to models of line-driven winds. As differences between these models are large (reaching orders of magnitude) such constraints are much needed.

astro-ph.SR↗

arXiv2026

The drastic impact of Eddington-limit induced mass ejections on massive star populations

Massive stars are the key engines of the Universe. However, their evolution and thus their ionizing feedback are still not fully understood. One of the largest gaps in current stellar evolution calculations is the lack of a model for the mass ejections that occur when the stars reach the Eddington limit, such as during an Luminous Blue Variable (LBV) phase. We aim to remedy this situation by providing a physically motivated and empirically calibrated method applicable in any 1D stellar evolution code to approximate the effect of such mass loss on stellar evolution. We employ the 1D stellar evolution code MESA, in which we implement a new mass-loss prescription that is acting when stellar models inflate too much when reaching the Eddington limit. Synthetic massive-star stellar populations using calculated grids of single-star models with this mass loss prescription are compared with the observed populations in the Large and Small Magellanic Clouds. In combination with already computed grids of binary evolution models, we investigate the impact of binarity on our predictions. Our single-star models reproduce key features of the observed stellar populations, namely (i) the absence of stars located beyond the Humphreys-Davidson limit, (ii) an upper limit of RSG luminosities, (iii) the faintest observed single WR stars, (iv) the absolute number of O-stars, WRs, and RSGs, (v) WO stars in low metallicity environments, and (vi) the positions of LBV stars in the HRD. Our binary population explains at the same time the 70% binary fraction of O-stars and the 40% binary fraction of WR stars. However, our synthetic population also has caveats, such as an overproduction of bright H-free WN stars. Our results show that the effect of Eddington-limit induced mass ejections on the structure and evolution of massive stars can remove tension between predicted and observed massive star populations.

astro-ph.SR↗

arXiv2025

Binarity at LOw Metallicity (BLOeM): Projected rotational velocities

The Binarity at LOw Metallicity (BLOeM) survey is an ESO large programme designed to obtain multi-epoch spectroscopy for 929 massive stars in the Small Magellanic Cloud (SMC). It will provide binary fractions and orbital configurations of binary systems, and search for dormant black-hole binary candidates (OB+BH). Here we present projected rotational velocities (vsini) of all sources and, using the multiplicity properties presented in previous papers, we derive the vsini distributions of apparent single stars, single lined spectroscopic (SB1) binaries, and SB2 systems. We identify a locus in the Hertzsprung-Russell diagram where rotational velocities decrease significantly; we interpret this feature as broadly corresponding to the terminal-age main sequence. The main sequence cohort is distinguished by a broad range of vsini values, but with a strong peak in the distribution in the range 30-60 km/s, close to the resolution limit of 30 km/s. Sources in this low vsini peak are distributed throughout the main sequence, and are also present in the SB1 sample, though less prominent than in the single star distribution. A preliminary analysis of the lowest vsini cohort, that includes SB1 systems, implies that roughly one third may be nitrogen rich and we speculate that this cohort is a mix of pristine single stars, long period binaries, and merger products. The SB2 systems appear to be mostly short period binaries in synchronous rotation and have vsini estimates distributed around a mean value of approximately 140 km/s . Higher vsini sources are also present in the single and SB1 systems, all of which have tail to higher vsini values, consistent with tidal and mass-transfer effects. The supergiants, with a few exceptions, have low vsini, the bulk of these systems being essentially unresolved at current spectral resolution.

astro-ph.SR↗

arXiv2025

Hidden massive eclipsing binaries in red supergiant systems: The hierarchical triple system KQ Puppis and other candidates

The majority of massive stars are part of binary systems that may interact during their evolution. However, not many RSGs are known binaries, and only a few have constrained orbital parameters. We search the available TESS photometry for eclipsing companions of RSGs. We focus on the best candidate, VV Cephei type binary KQ Pup, which is made up of a RSG, KQ Pup A, and a B-type companion, KQ Pup B (orbital period of 26 yr). We use photometry, spectroscopy, and newly taken interferometric data with VLTI-GRAVITY. Using TESS, we discovered eclipses with a period of $17.2596 \: \rm d$, associated with KQ Pup B, making it a Ba+Bb binary. The detection of the hydrogen Br$\gamma$ line with VLTI-GRAVITY enabled us to track the orbital motion of the Ba+Bb pair relative to A and determine the astrometric orbit of A+B. The dynamical masses agree with independent estimates from asteroseismology and evolutionary models. The results give a mass of $ \sim 10 \: \rm M_{\odot} $ for the RSG KQ Pup A and $ \sim 14 \: \rm M_{\odot} $ for the sum of the hot components Ba+Bb. We determined an orbital parallax of $\pi = 1.24^{+0.05}_{-0.04}\, \rm mas $, which is the first such parallax measurement for a RSG. KQ Pup represents a unique demonstration of mass transfer mechanism in wide eccentric RSG systems. The variability of Balmer emission lines and the detection of Br$\gamma$ are a strong signature of accretion to Ba+Bb near periastron. With the RSG filling its Roche lobe only by $\sim 70\%$ at periastron, the mass transfer is instead driven by accretion from its extended atmosphere via the Wind Roche Lobe Overflow. The accretion disk dissipates by apastron. Overall, we discovered that several previously assumed RSG binaries host eclipsing inner systems, corresponding to $\sim 10 \%$ of all known Galactic RSG binaries. This suggests that many of the other RSG binaries may also be hierarchical triples.

astro-ph.SR↗

arXiv2025

HR6819: a puffed-up stripped star system challenging stable mass transfer theory

HR6819 is the first system with a puffed-up low mass stripped star and a classical Be star whose nature has been confirmed by optical interferometry. It shows the most extreme mass ratio (15.7 +/- 1.1), the lowest stripped star mass (0.270 +/- 0.056 Msun), and one of the shortest orbital periods (40.3266 +/- 0.0016 days) among similar post-interaction binaries. These properties make HR6819 a unique test case for binary interaction physics, in particular the efficiency of mass transfer onto the Be progenitor required to reach such an extreme mass ratio. We reconstruct the possible evolutionary history of the system with grids of MESA simulations spanning mass transfer efficiencies from fully to fifty percent conservative. We show that stable Roche lobe overflow cannot simultaneously reproduce the observed orbital period and extreme mass ratio: the maximum ratio achievable is ~11.5 at ~40 days, even in the fully conservative case. Furthermore, the observed luminosities of both components exceed those expected from their model masses; the luminosity of the stripped star would be consistent with a ~0.7 Msun mass, over twice its dynamical mass. Our results demonstrate that the post-interaction properties of HR6819 cannot be explained by stable mass transfer under standard assumptions.

astro-ph.SR↗

arXiv2025

A high fraction of close massive binary stars at low metallicity

At high metallicity, a majority of massive stars have at least one close stellar companion. The evolution of such binaries is subject to strong interaction processes, heavily impacting the characteristics of their life-ending supernova and compact remnants. For the low-metallicity environments of high-redshift galaxies constraints on the multiplicity properties of massive stars over the separation range leading to binary interaction are crucially missing. Here we show that the presence of massive stars in close binaries is ubiquitous, even at low metallicity. Using the Very Large Telescope, we obtained multi-epoch radial velocity measurements of a representative sample of 139 massive O-type stars across the Small Magellanic Cloud, which has a metal content of about one fifth of the solar value. We find that 45% of them show radial velocity variations which demonstrate that they are members of close binary systems, and predominantly have orbital periods shorter than one year. Correcting for observational biases indicates that at least 70[+11:-6]% of the O stars in our sample are in close binaries, and that at least 68[+7:-8]% of all O stars interact with a companion star during their lifetime. We found no evidence supporting a statistically significant trend of the multiplicity properties with metallicity. Our results indicate that multiplicity and binary interactions govern the evolution of massive stars and determine their cosmic feedback and explosive fates.

astro-ph.SR↗

arXiv2025

X-Shooting ULLYSES: Massive stars at low metallicity. XIII. Putting the bi-stability jump to the test in the LMC

We aim to investigate the theoretical bi-stability jump, which predicts an increase in mass-loss rates below 21 kK. We further aim to constrain the photospheric and wind parameters of a sample of 16 LMC late-O and B supergiants. We utilise the 1D, non-LTE radiative transfer model CMFGEN in a grid-based approach and subsequent fine-tuned spectroscopic fitting procedure to determine the stellar and wind parameters of each star. We apply this method to ultra-violet data from the ULLYSES programme and complementary optical data from the XShootU collaboration. We also utilise evolutionary models to obtain the evolutionary masses and compare them to our derived spectroscopic masses. We derive physical parameters and wind properties of 16 late-O and B supergiants that span a wide $T_{eff}$ range of 12-30 kK, surface gravity range $\log{g/cm~s^{-2}}$ of 1.8-3.1, and a mass-loss rate range of $10^{-7.6}-10^{-5.7}M_{\odot}yr^{-1}$. We also compare our results to previous studies spectroscopic studies of LMC OB stars. We find that our derived photospheric and wind properties are consistent with multiple previous studies. For most of our sample, we find that the evolutionary masses and spectroscopic masses are consistent. Our results do not reproduce a bi-stability jump in any temperature range, but rather a monotonic decrease in mass-loss rate at lower temperatures. We obtain a terminal wind velocity-effective temperature relation for LMC supergiants. We find that our derived mass-loss rates do not agree with predictions from any of the numerical recipes. This is also the case for the ratio of the terminal wind velocity to the escape velocity $v_{\infty}/v_{esc}$, and we derive a $v_{\infty}/v_{esc}$-$T_{eff}$ relation. We find that wind properties are metallicity dependent from a comparison with a previous SMC study, and we obtain a new modified wind momentum-luminosity relation.

astro-ph.SR↗

arXiv2025

Binarity at LOw Metallicity (BLOeM): Bayesian inference of natal kicks from inert black hole binaries

Context. The emerging population of inert black hole binaries (BHBs) provides a unique opportunity to constrain black hole (BH) formation physics. These systems are composed of a stellar-mass BH in a wide orbit around a non-degenerate star with no observed Xray emission. Inert BHBs allow for narrow constraints to be inferred on the natal kick and mass loss during BH-forming core-collapse events. Aims. In anticipation of the upcoming BLOeM survey, we aim to provide tight constraints on BH natal kicks by exploiting the full parameter space obtained from combined spectroscopic and astrometric data to characterize the orbits of inert BHBs. Multi-epoch spectroscopy from the BLOeM project will provide measurements of periods, eccentricities, and radial velocities for inert BHBs in the SMC, which complements Gaia astrometric observations of proper motions. Methods. We present a Bayesian parameter estimation framework to infer natal kicks and mass loss during core-collapse from inert BHBs, accounting for all available observables, including the systemic velocity and its orientation relative to the orbital plane. The framework further allows for circumstances when some of the observables are unavailable, such as for the distant BLOeM sources which preclude resolved orbits. Results. With our new framework, we are able to distinguish between BH formation channels, even in the absence of a resolved orbit. In cases when the pre-explosion orbit can be assumed to be circular, we precisely recover the parameters of the core-collapse, highlighting the importance of understanding the eccentricity landscape of pre-explosion binaries, both theoretically and observationally. Treating the near-circular, inert BHB, VFTS 243, as a representative of the anticipated BLOeM systems, we constrain the natal kick to less than 27 km/s and the mass loss to less than 2.9 Msun within a 90% credible interval.

astro-ph.SR↗

arXiv2025

New empirical mass-loss recipe for UV radiation line-driven winds of hot stars across various metallicities

The winds of massive stars remove a significant fraction of their mass, strongly impacting their evolution. As a star evolves, the rate at which it loses mass changes. In stellar evolution codes, different mass-loss recipes are employed for different evolutionary stages. The choice of the recipes is user-dependent and the conditions for switching between them are poorly defined. Focusing on hot stars, we aim to produce a physically motivated, empirically calibrated mass-loss recipe suitable for a wide range of metallicities. We want to provide a ready-to-use universal recipe that eliminates the need for switching between recipes for hot stars during stellar evolution calculations. We compile a sample of hot stars with reliable stellar and wind parameters in the Galaxy and the Magellanic Clouds. The sample is used to determine the dependence of the mass-loss rate on the basic stellar parameters. We find that independent of evolutionary stage and temperature, the wind mass-loss rate is a function of the electron-scattering Eddington parameter ($\Gamma_e$) and metallicity (Z), being in line with expectations of radiation-driven wind theory. Our derived scaling relation provides an adequate ($\Delta$log($\dot{M}$/(M$_\odot$/yr)) = 0.43) and broadly applicable mass-loss recipe for hot stars. The newly derived mass-loss recipe covers nearly the entire parameter space of hot stars with UV radiation-driven winds and eliminates the need for interpolation between mass-loss formulae at different evolutionary stages when applied in stellar evolution models. Examples of stellar evolution calculations using our new recipe reveal that the predictions on the ionizing fluxes and final fates of massive stars, especially at low metallicity, differ significantly from models that use the standard mass-loss rates, impacting our understanding of stellar populations at low metallicity and in the young Universe.

astro-ph.SR↗

arXiv2025

Comprehensive UV and Optical spectral analysis of Cygnus X-1: Stellar and wind parameters, abundances, and evolutionary implications

We present a comprehensive analysis of Cyg X-1, utilizing archival high-resolution UV and optical spectra in conjunction with sophisticated atmospheric models. Notably, this is the first investigation to simultaneously analyze UV and optical spectra of Cyg X-1 along with incorporating X-rays to constrain the stellar and wind parameters. Our analysis yields notably lower masses for both the donor (29$\,M_\odot$) and the black hole (12.7 to 17.8$\,M_\odot$, depending on inclination), and confirms that the donor's radius is close to reaching the inner Lagrangian point. We find super-solar Fe, Si, and Mg abundances (1.3-1.8 times solar) at the surface of the donor star, while the total CNO abundance remains solar despite evidence of CNO processing (N enrichment, O depletion) and He enrichment. This abundance pattern is distinct from the surrounding Cyg OB3 association. We observe a clear difference in wind parameters between X-ray states: $v_\infty \approx 1200\,\mathrm{km\,s}^{-1}$ and $\dot{M} \approx 3\times 10^{-7}\,M_\odot\,\mathrm{yr}^{-1}$ in the high/soft state, increasing to $v_\infty\lesssim 1800\,\mathrm{km\,s}^{-1}$ and $\dot{M}\lesssim 5\times 10^{-7}\,M_\odot\,\mathrm{yr}^{-1}$ in the low/hard state. The observed X-ray luminosity is consistent with wind-fed accretion. Evolutionary models show that Cyg X-1 will undergo Roche lobe overflow in the near future. Under a fully conservative mass accretion scenario, our models predict a future binary black hole merger for Cyg X-1 within $\sim5$ Gyr. Our comprehensive analysis provides refined stellar and wind parameters of the donor star in Cyg X-1, highlighting the importance of using advanced atmospheric models and considering X-ray ionization and wind clumping. The observed abundances suggest a complex formation history involving a high initial metallicity.

astro-ph.SR↗

arXiv2025

Coupling hydrodynamics with comoving frame radiative transfer. III. The wind regime of early-type B hypergiants

Context. B hypergiants (BHGs) are important for understanding high-mass stellar evolution. While they are in a similar parameter space of B supergiants (BSGs), some BHGs are known to be luminous blue variables (LBVs). Their spectra with absorption and emission features resemble Of/WNh stars. Yet, their wind physics and their evolutionary connections are not clear. Aims. In this study, we aim to understand (i) the atmospheric and wind structure, (ii) the wind-launching and -driving mechanisms, and (iii) the spectrum formation of early-type BHGs. As an observational prototype, we use zet1 Sco (B1.5Ia+). Methods. Using the atmosphere code PoWRhd, we calculated the first hydrodynamically consistent models at the BHG domain. They give insights into the radiative driving of the calculated wind regimes and enable us to study the influence of clumping and X-rays on the resulting wind structure. Results. Our consistent model reproduces the main spectral features of zet1 Sco. The obtained mass-loss rate is higher than that of similar spectral type BSGs. However, the wind optical depth of BHGs is way below unity, making them less of a transition type. To reproduce zet1 Sco's spectrum, we needed low clumping with subsonic onset. The wind has a shallow-gradient velocity profile, deviating from the beta law, and is mainly driven by Fe III opacity. Conclusions. Our study suggests that despite more mass loss, early-type Galactic BHGs have winds relatively similar to BSGs. Their winds are not thick enough to characterize them as "transition-type" stars, unlike Of/WNh, implying that emission features arise more easily in cooler than in hotter stars. The spectral BHG appearance is likely connected to atmospheric inhomogeneities below the sonic point. To reach an appearance similar to LBVs, BHGs need to be either closer to the Eddington limit or have higher wind clumping than inferred for zeta1 Sco.

astro-ph.SR↗

arXiv2025

Binarity at LOw Metallicity (BLOeM): Enhanced multiplicity of early B-type dwarfs and giants at $Z=0.2\,{\rm Z}_\odot$

Early B-type stars ($M_i=8-15$ M$_\odot$) are frequently in multiple systems, as evidenced by spectroscopic campaigns in the Milky Way (MW) and the Large Magellanic Cloud (LMC). Previous studies have shown no strong metallicity dependence in the close-binary (a>10 au) fraction or orbital-period distributions between the MW's solar metallicity (Z$_\odot$) and that of the LMC (Z=0.5 Z$_\odot$). However, similar analyses in more metal-poor environments are still scarce. We focus on 309 early B-type stars (luminosity classes III-V) from the Binarity at LOw Metallicity campaign in the Small Magellanic Cloud (SMC, Z=0.2 Z$_\odot$) using VLT/FLAMES multi-epoch spectroscopy. By applying binary detection criteria consistent with previous works, we identify 153 stars (91 SB1, 59 SB2, 3 SB3) exhibiting significant radial-velocity (RV) variations, resulting in an observed multiplicity fraction of $f^{obs}_{mult}=50\pm3\%$. Using Monte Carlo simulations to account for observational biases, we infer an intrinsic close-binary fraction of $f_{mult}=80\pm8\%$. A Markov chain Monte Carlo analysis of the peak-to-peak RV distribution ($\Delta{\rm RV}_{\rm max}$) confirms a high multiplicity fraction of $f_{mult}=78\pm5\%$. These findings suggest a possible anti-correlation between metallicity and the fraction of close B-type binaries, with the SMC multiplicity fraction significantly exceeding previous measurements in the LMC and MW. The enhanced fraction of close binaries at SMC's low metallicity may have broad implications for massive-star evolution in the early Universe. More frequent mass transfer and envelope stripping could boost the production of exotic transients, stripped supernovae, gravitational-wave progenitors, and sustained UV ionising flux, potentially affecting cosmic reionisation. Theoretical predictions of binary evolution under metal-poor conditions will provide a key test of our results.

astro-ph.SR↗

arXiv2025

Binarity at LOw Metallicity (BLOeM): Multiplicity of early B-type supergiants in the Small Magellanic Cloud

The blue supergiant (BSG) domain contains a large variety of stars whose past and future evolutionary paths are still highly uncertain. Since binary interaction plays a crucial role in the fate of massive stars, investigating the multiplicity among BSGs helps shed light on the fate of such objects. We aim to estimate the binary fraction of a large sample of BSGs in the Small Magellanic Cloud within the Binarity at LOw Metallicity (BLOeM) survey. In total, we selected 262 targets with spectral types B0-B3 and luminosity classes I-II. This work is based on spectroscopic data collected by the GIRAFFE instrument, mounted on the Very Large Telescope, which gathered nine epochs over three months. Our spectroscopic analysis for each target includes the individual and peak-to-peak radial velocity measurements, an investigation of the line profile variability, and a periodogram analysis to search for possible short- and long-period binaries. By applying a 20 km s$^{-1}$ threshold on the peak-to-peak radial velocities above which we would consider the star to be binary, the resulting observed spectroscopic binary fraction for our BSG sample is 23 $\pm$ 3$\%$. In addition, we derived reliable orbital periods for 41 spectroscopic binaries and potential binary candidates, among which there are 17 eclipsing binaries, including 20 SB1 and SB2 systems with periods of less than 10 days. We reported a significant drop in the binary fraction of BSGs with spectral types later than B2 and effective temperatures less than 18 kK, which could indicate the end of the main sequence phase in this temperature regime. We found no metallicity dependence in the binary fraction of BSGs, compared to existing spectroscopic surveys of the Galaxy and Large Magellanic Cloud.

astro-ph.SR↗