arXiv subjects

N. Langer

Publications and source records attributed to N. Langer.

At least 37 records · Page 2Linked to original sources

arXiv2025

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

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 analysed OBe stars with well-determined binary characteristics and orbital parameters are required. We investigate 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 analyse the first nine epochs of spectroscopic observations obtained over approximately three months in 2023. Radial velocities (RVs) of all stars are measured. Applying commonly-used binarity criteria we classify objects as binaries, binary candidates, and apparently single (RV stable) objects. We further inspect the spectra for double-lined spectroscopic binaries and cross-match with catalogues of X-ray sources and photometric binaries. We classify 14 OBe stars as binaries, and an additional 11 as binary candidates. The two Of?p stars are apparently single. Two more objects are most likely currently interacting binaries. Without those, the observed binary fraction for the OBe sample (78 stars) is f_OBe_obs=0.18+/-0.04 (f_obs_cand=0.32+/-0.05 including candidates). This 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 have indeed fundamentally different binary properties than OB stars. We find no evidence for OBe binaries with massive compact companions, in contrast to expectations from binary population synthesis. 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.

astro-ph.SR↗

arXiv2025

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

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 range 6 to 30 solar masses. The observed binary fraction for the B5-9 supergiants is 25+/-6 % (10+/-4 %) and 5+/-2 % (0 %) for the A-F stars, using a radial velocity (RV) variability threshold of 5 kms (10 kms) 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$^{+9}_{-7}$% for the AF stars, for the orbital periods up to 10$^{3.5}$day and mass-ratios (q) in the range 0.1 < q < 1. The large stellar radii of these supergiant stars prevent short orbital periods but we demonstrate that this effect alone cannot explain our results. We assess 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 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.

astro-ph.SR↗

arXiv2024

Red supergiant stars in binary systems II. Confirmation of B-type companions of red supergiants in the Small Magellanic Cloud using Hubble ultra-violet spectroscopy

Red supergiant stars (RSGs) represent the final evolutionary phase of the majority of massive stars and hold a unique role in testing the physics of stellar models. Eighty eight RSGs in the Small Magellanic Cloud (SMC) were recently found to have an ultra-violet excess that was attributed to a B-type companion. We present follow-up Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) ultra-violet (1700 -- 3000\,Å) spectroscopy for 16 of these stars to investigate the nature of the UV excess and confirm the presence of a hot companion. In all cases we are able to confirm that the companion is a main-sequence B-type star based on the near-UV continuum. We determine effective temperatures, radii and luminosities from fitting the UV continuum with TLUSTY models and find stellar parameters in the expected range of SMC B-type stars. We display these results on a Hertzsprung--Russell diagram and assess the previously determined stellar parameters using UV photometry alone. From this comparison we conclude that UV photometric surveys are vital to identify such companions and UV spectroscopy is similarly vital to characterise the hot companions. From a comparison with IUE spectra of 32 Cyg, a well known RSG binary system in the Galaxy, four targets display evidence of being embedded in the wind of the RSG, like 32 Cyg, although none to the more extreme extent of VV Cep. The ages of six targets, determined via the stellar parameters of the hot companions, are found to be in tension with the ages determined for the RSG. A solution to this problem could be binary mass-transfer or red straggler stars.

astro-ph.SR↗

arXiv2024

The IACOB project XIII. Helium enrichment in O-type stars as a tracer of past binary interaction

There is increasing evidence that single-star evolutionary models are inadequate to reproduce all observational properties of massive stars. Binary interaction has emerged as a key factor in the evolution of a significant fraction of massive stars. In this study, we investigate the helium ($Y_{\mathrm He}$) and nitrogen ($ε_{\mathrm N}$) surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities $v\sin(i)\leq150{\mathrm km}\cdot{\mathrm s}^{-1}$. We found a subsample ($\sim20\%$ of the total, and $\sim80\%$ of the stars with $Y_{\mathrm He}\geq0.12$) with a $Y_{\mathrm He}$ and $ε_{\mathrm N}$ combined pattern unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.

astro-ph.SR↗

arXiv2024

Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY: Uncovering A Long Period Binary Desert

Wolf-Rayet stars (WRs) are one of the final evolutionary stages of massive stars and immediate progenitors of stellar-mass black holes. Their multiplicity forms an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Galactic WRs (WNs and WCs) at short as well as long orbital periods, in contradiction with evolutionary model predictions. In this work, we employed infrared interferometry using the $K$-band instrument GRAVITY at the VLTI to investigate the multiplicity of WRs at long periods and explore the nature of their companions. We present a survey of 39 Galactic WRs, including 11 WN, 15 WC and 13 H-rich WN (WNh) stars. We detected wide companions with GRAVITY for only four stars: WR 48, WR 89, WR 93 and WR 115. Combining with spectroscopic studies, we arrived at multiplicity fractions of $f^{\rm WN}_{\rm obs} = 0.55\pm0.15$, $f^{\rm WC}_{\rm obs} = 0.40\pm0.13$ and $f^{\rm WNh}_{\rm obs} = 0.23\pm0.12$. In addition, we also found other features in the GRAVITY dataset such as (i) a diffuse extended component in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78 and WR 110. Our survey reveals a lack of intermediate (few 100s d) and long- (few years to decades) period WR systems. The 200-d peak in the period distributions of WR+OB and BH+OB binaries predicted by Case B mass-transfer binary evolution models is not seen in our data. The rich companionship of their O-type progenitors in this separation range suggest that the WR progenitor stars expand and interact with their companions, most likely through unstable mass-transfer, resulting in either a short-period system or a merger.

astro-ph.SR↗

arXiv2024

An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud: implications for mass loss and black hole masses at low metallicity

In order to predict the black hole mass distributions at high redshift, we need to understand whether very massive single stars ($M>40$ M$_\odot$) at low metallicity $Z$ lose their hydrogen-rich envelopes, like their metal-rich counterparts, or whether a binary companion is required to achieve this. To test this, we undertake a deep spectroscopic search for binary companions of the seven apparently single Wolf-Rayet (WR) stars in the Small Magellanic Cloud (SMC; $Z \simeq 1/5 Z_\odot$). For each of them, we acquired six high-quality VLT-UVES spectra spread over 1.5 years. By using the narrow N V lines in these spectra, we monitor radial velocity (RV) variations to search for binary motion. We find low RV variations between 6 and 23 km/s for the seven WR stars, with a median standard deviation of $5$ km/s. Our Monte Carlo simulations imply probabilities below ~5% for any of our target WR stars to have a binary companion more massive than ~5 M$_\odot$ at orbital periods of less than a year. We estimate that the probability that all our target WR stars have companions with orbital periods shorter than 10 yr is below ~10$^{-5}$, and argue that the observed modest RV variations may originate from intrinsic atmosphere or wind variability. Our findings imply that metal-poor massive stars born with $M \gtrsim 40$ M$_\odot$ can lose most of their hydrogen-rich envelopes via stellar winds or eruptive mass loss, which strongly constrains their initial mass - black hole mass relation. We also identify two of our seven target stars (SMC AB1 and SMC AB11) as runaway stars with a peculiar radial velocity of ~80 km/s. Moreover, with all five previously detected WR binaries in the SMC exhibiting orbital periods of below 20 d, a puzzling absence of intermediate-to-long-period WR binaries has emerged, with strong implications for the outcome of massive binary interaction at low metallicity.

astro-ph.SR↗

arXiv2024

Rotational synchronisation of B-type binaries in 30 Doradus

The spin evolution of stars in close binary systems can be strongly affected by tides. We investigate the rotational synchronisation of the stellar components for 69 SB1 systems and 14 SB2 B-type systems in the 30 Doradus region of the Large Magellanic Cloud using observations from the VFTS and BBC surveys. Their orbital periods range from a few to a few hundred days, while estimated primary masses for these systems are in the range 5-20 Msun with mass ratio ranges of approximately q=0.03-0.5 and q=0.6-1.0 for the SB1 and SB2 systems, respectively. Projected rotational velocities of the stellar components have been compared with their synchronous velocities derived from the orbital periods. We find that effectively all systems with orbital period of more than 10 days must be asynchronous, whilst all the systems with periods of less than 3 days are likely synchronised. In terms of the stellar fractional radius (r), our results imply that all systems with r<0.1 are asynchronous, with those having r>0.2 probably being synchronised. For the apparently synchronised systems our results are more consistent with synchronisation at the mean orbital angular velocity rather than with that at periastron.

astro-ph.SR↗

arXiv2023

Sulfur isotope ratios in the Large Magellanic Cloud

Sulfur isotope ratios have emerged as a promising tool for tracing stellar nucleosynthesis, quantifying stellar populations, and investigating the chemical evolution of galaxies. While extensively studied in the Milky Way, in extragalactic environments they remain largely unexplored. We focus on investigating the sulfur isotope ratios in the Large Magellanic Cloud (LMC) to gain insights into sulfur enrichment in this nearby system and to establish benchmarks for such ratios in metal-poor galaxies. We conducted pointed observations of CS and its isotopologues toward N113, one of the most prominent star-formation regions in the LMC, utilizing the Atacama Pathfinder EXperiment 12~m telescope. We present the first robust detection of C$^{33}$S in the LMC by successfully identifying two C$^{33}$S transitions on a large scale of $\sim$5 pc. Our measurements result in an accurate determination of the $^{34}$S/$^{33}$S isotope ratio, which is 2.0$\pm$0.2. Our comparative analysis indicates that the $^{32}$S/$^{33}$S and $^{34}$S/$^{33}$S isotope ratios are about a factor of 2 lower in the LMC than in the Milky Way. Our findings suggest that the low $^{34}$S/$^{33}$S isotope ratio in the LMC can be attributed to a combination of the age effect, low metallicity, and star formation history.

astro-ph.GA↗

arXiv2023

The B-type Binaries Characterisation Programme II. VFTS 291: A stripped star from a recent mass transfer phase

Recent studies of massive binaries with putative black hole companions have uncovered a phase of binary evolution that has not been observed before, featuring a bloated stripped star that very recently ceased transferring mass to a main-sequence companion. In this study, we focus on the candidate system VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude velocity ($K_1=93.7\pm0.2$ km s$^{-1}$). Through our analysis of the disentangled spectra of the two components, together with dynamical and evolutionary arguments, we identify a narrow-lined star of ~1.5-2.5 $M_\odot$ dominating the spectrum, and an early B-type main-sequence companion of $13.2\pm1.5$ $M_\odot$. The low mass of the narrow-lined star, and the high mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the narrow-lined star being bloated and stripped of its hydrogen-rich envelope, sharing many similarities with other recently discovered stripped stars. Our finding is supported by our detailed binary evolution models, which indicate that the system can be well explained by an initial configuration consisting of an 8.1 $M_\odot$ primary with an 8 $M_\odot$ companion in a 7 d orbital period. While some open questions remain, particularly concerning the surface helium enrichment of the stripped star and the rotational velocity of the companion, we expect that high-resolution spectroscopy may help reconcile our estimates with theory. Our study highlights the importance of multi-epoch spectroscopic surveys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.

astro-ph.SR↗

arXiv2023

The young massive SMC cluster NGC 330 seen by MUSE III. Stellar parameters and rotational velocities

The origin of initial rotation rates of stars, and how a star's surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the 35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). We develop a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multi-band Hubble Space Telescope photometry and Multi Unit Spectroscopic Explorer spectroscopy. This allows us to homogeneously constrain the physical parameters of over 250 B and Be stars, brighter than mF814W = 18.8 mag. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B stars. The rotational velocities vary as a function of the star's position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. The rotational velocities of the 35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the bulk of the B stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different vsini-characteristics of NGC 330 compared to younger populations can be understood in this framework.

astro-ph.SR↗

arXiv2023

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.

astro-ph.SR↗

arXiv2023

A helium-burning white dwarf binary as a supersoft X-ray source

Type Ia supernovae are cosmic distance indicators, and the main source of iron in the Universe, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed and suggested as Type Ia supernovae progenitors. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years, also including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as due to helium burning near the surface of the accreting white dwarf. The properties of our system provides evidence for extended pathways towards Chandrasekhar mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae, within this scenario.

astro-ph.HE↗

arXiv2023

Reverse Algols and hydrogen-rich Wolf-Rayet stars from very massive binaries

Massive star feedback affects the evolution of galaxies, where the most massive stars may have the largest impact. The majority of massive stars are born as members of close binary systems. Here, we investigate detailed evolutionary models of very massive binaries (30$\dots$90$M_{\odot}$) with Large Magellanic Cloud (LMC) metallicity. We identify four effects defying the conventional knowledge of binary evolution, which are all related to the proximity of the models to the Eddington limit. We find that the majority of systems undergo mass transfer during core hydrogen burning. During the ensuing nuclear timescale evolution, many mass donors remain more massive than their companions (``reverse Algols''), and nuclear timescale mass transfer may be interrupted or absent altogether. Furthermore, due to the elevated luminosity-to-mass ratio, many of the core-hydrogen burning donors may develop Wolf-Rayet-type winds, at luminosities where single stars would not. We identify observational counterparts of very massive reverse Algol binaries in the LMC, and discuss their contribution to the observed hydrogen-rich Wolf-Rayet stars. We argue that an understanding of very massive Algol systems is key to predicting the advanced evolution of very massive binaries, including their ability to evolve into observable gravitational wave sources.

astro-ph.SR↗

arXiv2022

Hubble Space Telescope survey of Magellanic Cloud star clusters. Photometry and astrometry of 113 clusters and early results

In the past years, we have undertaken an extensive investigation of LMC and SMC star clusters based on HST data. We present photometry and astrometry of stars in 101 fields observed with the WFC/ACS, UVIS/WFC3 and NIR/WFC3 cameras. These fields comprise 113 star clusters. We provide differential-reddening maps and illustrate various scientific outcomes that arise from the early inspection of the photometric catalogs. In particular, we provide new insights on the extended main-sequence turn-off (eMSTO) phenomenon: i) We detected eMSTOs in two clusters, KMHK361 and NGC265, which had no previous evidence of multiple populations. This finding corroborates the conclusion that the eMSTO is a widespread phenomenon among clusters younger than ~2 Gyr. ii) The homogeneous color-magnitude diagrams (CMDs) of 19 LMC clusters reveal that the distribution of stars along the eMSTO depends on cluster age. iii) We discovered a new feature along the eMSTO of NGC1783, which consists of a distinct group of stars going on the red side of the eMSTO in CMDs composed of ultraviolet filters. Furthermore, we derived the proper motions of stars in the fields of view of clusters with multi-epoch images. Proper motions allowed us to separate the bulk of bright field stars from cluster members and investigate the internal kinematics of stellar populations in various LMC and SMC fields. As an example, we analyze the field around NGC346 to disentangle the motions of its stellar populations, including NGC364 and BS90, young and pre-MS stars in the star-forming region associated with NGC346, and young and old field stellar populations of the SMC. Based on these results and the fields around five additional clusters, we find that young SMC stars exhibit elongated proper-motion distributions that point toward the LMC, thus bringing new evidence for a kinematic connection between the LMC and SMC.

astro-ph.SR↗

arXiv2022

The spins of stripped B stars support magnetic internal angular momentum transport

In order to predict the spins of stellar remnants we need to understand the evolution of the internal rotation of stars, and to identify at which stage the rotation of the contracting cores of evolved stars decouples from their expanding envelopes. The donor stars of mass transferring binaries lose almost their entire envelope and may thus offer a direct view on their core rotation. After the mass transfer event they contract and fade rapidly, although they are well observable when caught in the short-lived B-star phase. The B-type primary of the galactic binary system LB-1, which was originally suggested to contain a massive black hole, is nicely explained as a stripped star accompanied by a fainter Be star. The narrow absorption lines in the primary's spectrum signify extremely slow rotation, atypical of B-type main-sequence stars. Here we investigate the evolution of mass donors in generic grids of detailed binary evolution models, where both stars include differential rotation, internal angular momentum transport, and spin-orbit coupling. Whereas the mass gainers are typically spun-up during the mass transfer, we find that the spins of the stripped donor models depend sensitively on the employed mechanism for internal angular momentum transport. Purely hydrodynamic transport cannot explain the observed slow rotation, while models including magnetic angular momentum transport are able to reproduce the observed rotation of LB-1 and similar stars, independent of the initial rotation rate. In such models the spin of the white dwarfs that emerge at the end of the evolution is independent of the mass stripping. We find evidence that the mass transfer in LB-1 was moderately non-conservative.

astro-ph.SR↗

arXiv2022

The VLT-FLAMES Tarantula Survey: Observational evidence for two distinct populations of massive runaway stars in 30 Doradus

Two main scenarios have been proposed for origin of massive runaway stars -- dynamical ejection or release from a binary at the first core collapse -- but their relative contribution remains debated. Using two large spectroscopic campaigns towards massive stars in 30 Doradus, we aim to provide observational constraints on the properties of the O-type runaway population in the most massive active star-forming region in the Local group. We use RV measurements of the O-type star populations in 30 Doradus obtained by the VLT-FLAMES Tarantula Survey and the Tarantula Massive Binary Monitoring to identify single and binary O-type runaways. We discuss their rotational properties and qualitatively compare observations with expectations of ejection scenarios. We identify 23 single and one binary O-type runaway objects, most of them outside the main star-forming regions in 30 Doradus. We find an overabundance of rapid rotators (vsini > 200km/s) among the runaway population, providing an explanation of the overabundance of rapidly rotating stars in the 30 Doradus field. Considerations of the projected rotation rates and runaway line-of-sight (los) velocities reveal a conspicuous absence of rapidly rotating (vsini > 210k/ms), fast moving (v_{los} > 60km/s) runaways, and suggest the presence of two different populations of runaway stars: a population of rapidly-spinning but slowly moving runaways and a population of fast moving but slowly rotating ones. These are detected with a ratio close to 2:1 in our sample. We argue that slowly moving but rapidly spinning runaways result from binary ejections, while rapidly moving but slowly spinning runaways could result from dynamical ejections. Given that detection biases will more strongly impact the slow-moving population, our results suggest that the binary evolution scenario dominates the current massive runaway population in 30 Doradus.

astro-ph.SR↗

arXiv2022

Searching for compact objects in the single-lined spectroscopic binaries of the young Galactic cluster NGC 6231

Recent evolutionary computations predict that a few percent of massive OB stars in binary systems should have a dormant BH companion. Despite several reported X-ray quiet OB+BH systems over the last couple of years, finding them with certainty remains challenging. These have great importance as they can be gravitational wave (GW) source progenitors, and are landmark systems in constraining supernova kick physics. This work aims to characterise the hidden companions to the single-lined spectroscopic binaries (SB1s) in the B star population of the young open Galactic cluster NGC 6231 to find candidate systems for harbouring compact object companions. With the orbital solutions for each SB1 previously constrained, we applied Fourier spectral disentangling to multi-epoch optical VLT/FLAMES spectra of each target to extract a potential signature of a faint companion, and to identify newly disentangled double-lined spectroscopic binaries (SB2s). For targets where the disentangling does not reveal any signature of a stellar companion, we performed atmospheric and evolutionary modelling on the primary to obtain constraints on the unseen companion. Seven newly classified SB2 systems with mass ratios down to near 0.1 were identified. From the remaining targets, for which no faint companion could be extracted from the spectra, four are found to have companion masses in the predicted mass ranges of neutron stars (NSes) and BHes. Two of these have companion masses between 1 and 3.5 $M_{\odot}$, making them potential hosts of NSes (or lower mass main sequence stars). The other two are between 2.5 to 8 $M_{\odot}$ and 1.6 and 26 $M_{\odot}$, respectively, and so are identified as candidates for harbouring BH companions. However, unambiguous identification of these systems as X-ray quiet compact object harbouring binaries requires follow up observations.

astro-ph.SR↗

arXiv2022

A census of OBe stars in nearby metal-poor dwarf galaxies reveals a high fraction of extreme rotators

The Early Universe, together with many nearby dwarf galaxies, is deficient in heavy elements. The evolution of massive stars in such environments is thought to be affected by rotation. Extreme rotators amongst them tend to form decretion disks and manifest themselves as OBe stars. We use a combination of U B, GAIA, Spitzer, and Hubble Space Telescope photometry to identify the complete populations of massive OBe stars - one hundred to thousands in number - in five nearby dwarf galaxies. This allows us to derive the galaxy-wide fractions of main sequence stars that are OBe stars (f_OBe), and how it depends on absolute magnitude, mass, and metallicity (Z). We find f_OBe = 0.22 in the Large Magellanic Cloud (0.5 Z_Sun), increasing to f_OBe = 0.31 in the Small Magellanic Cloud (0.2 Z_Sun). In the so far unexplored metallicity regime below 0.2 Z_Sun, in Holmberg I, Holmberg II, and Sextans A, we also obtain high OBe star fractions of 0.27, 0.27, and 0.27, respectively. These high OBe star fractions, and the strong contribution in the stellar mass range which dominates the production of supernovae, shed new light on the formation channel of OBe stars, as well as on the preference of long-duration gamma-ray bursts and superluminous supernovae to occur in metal-poor galaxies.

astro-ph.GA↗