Wolf-Rayet Stars Research Articles

A machine-learning photometric classifier for massive stars in nearby galaxies

Context. Mass loss is a key parameter in the evolution of massive stars. Despite the recent progress in the theoretical understanding of how stars lose mass, discrepancies between theory and observations still hold. Moreover, episodic mass loss in evolved massive stars is not included in models, and the importance of its role in the evolution of massive stars is currently undetermined. Aims. A major hindrance to determining the role of episodic mass loss is the lack of large samples of classified stars. Given the recent availability of extensive photometric catalogs from various surveys spanning a range of metallicity environments, we aim to remedy the situation by applying machine-learning techniques to these catalogs. Methods. We compiled a large catalog of known massive stars in M 31 and M 33 using IR (Spitzer) and optical (Pan-STARRS) photometry, as well as Gaia astrometric information, which helps with foreground source detection. We grouped them into seven classes (Blue, Red, Yellow, B[e] supergiants, luminous blue variables, Wolf-Rayet stars, and outliers, e.g., quasi-stellar objects and background galaxies). As this training set is highly imbalanced, we implemented synthetic data generation to populate the underrepresented classes and improve separation by undersampling the majority class. We built an ensemble classifier utilizing color indices as features. The probabilities from three machine-learning algorithms (Support Vector Classification, Random Forest, and Multilayer Perceptron) were combined to obtain the final classification. Results. The overall weighted balanced accuracy of the classifier is ∼83%. Red supergiants are always recovered at ∼94%. Blue and Yellow supergiants, B[e] supergiants, and background galaxies achieve ∼50 − 80%. Wolf-Rayet sources are detected at ∼45%, while luminous blue variables are recovered at ∼30% from one method mainly. This is primarily due to the small sample sizes of these classes. In addition, the mixing of spectral types, as there are no strict boundaries in the features space (color indices) between those classes, complicates the classification. In an independent application of the classifier to other galaxies (IC 1613, WLM, and Sextans A), we obtained an overall accuracy of ∼70%. This discrepancy is attributed to the different metallicity and extinction effects of the host galaxies. Motivated by the presence of missing values, we investigated the impact of missing data imputation using a simple replacement with mean values and an iterative imputer, which proved to be more capable. We also investigated the feature importance to find that r − i and y − [3.6] are the most important, although different classes are sensitive to different features (with potential improvement with additional features). Conclusions. The prediction capability of the classifier is limited by the available number of sources per class (which corresponds to the sampling of their feature space), reflecting the rarity of these objects and the possible physical links between these massive star phases. Our methodology is also efficient in correctly classifying sources with missing data as well as at lower metallicities (with some accuracy loss), making it an excellent tool for accentuating interesting objects and prioritizing targets for observations.

The Diverse Properties of Type Icn Supernovae Point to Multiple Progenitor Channels

We present a sample of Type Icn supernovae (SNe Icn), a newly discovered class of transients characterized by their interaction with H- and He-poor circumstellar material (CSM). This sample is the largest collection of SNe Icn to date and includes observations of two published objects (SN 2019hgp and SN 2021csp) and two objects not yet published in the literature (SN 2019jc and SN 2021ckj). The SNe Icn display a range of peak luminosities, rise times, and decline rates, as well as diverse late-time spectral features. To investigate their explosion and progenitor properties, we fit their bolometric light curves to a semianalytical model consisting of luminosity inputs from circumstellar interaction and radioactive decay of 56Ni. We infer low ejecta masses (≲2 M ⊙) and 56Ni masses (≲0.04 M ⊙) from the light curves, suggesting that normal stripped-envelope supernova (SESN) explosions within a dense CSM cannot be the underlying mechanism powering SNe Icn. Additionally, we find that an estimate of the star formation rate density at the location of SN 2019jc lies at the lower end of a distribution of SESNe, in conflict with a massive star progenitor of this object. Based on its estimated ejecta mass, 56Ni mass, and explosion site properties, we suggest a low-mass, ultra-stripped star as the progenitor of SN 2019jc. For other SNe Icn, we suggest that a Wolf–Rayet star progenitor may better explain their observed properties. This study demonstrates that multiple progenitor channels may produce SNe Icn and other interaction-powered transients.

Stellar population in the vicinity of the H ii region G331.03-00.15

We present a multi-wavelength study of the H ii region G331.03-00.15, with the aim of identifying the population of massive stars associated with it. The distributions of ionised gas and heated dust both exhibit similar spherical forms, while the bordering photo-dissociating region follows a very ring-like distribution, indicating the presence of abundant neighbouring molecular material being photo-dissociated. There is only one high-mass star catalogued in the region, namely, the WR star 1051-67L. Based on an energy analysis of the detected radio continuum emission, we deduced that additional high-mass stars could be interacting with G331.03-00.15. We conducted a search of massive star candidates by applying a series of colour criteria to a sample of highly reddened infrared (IR) point sources projected over the radio continuum morphology, with the additional condition that they be located at the same distance assumed for the region. Fourteen candidate sources were selected and classified using low-resolution, near-infrared (NIR) spectroscopic data in the H and K bands, obtained with Gemini/FLAMINGOS-2. We identified one massive star and classified it as an O7 V star. Adding the contribution from this new star to the total ionising radiation available, we concluded that it would be possible for the H ii region to be powered solely by these two massive stars. However, we do not rule out the possibility that there could be more early-type stars that remain undetected in this highly obscured part of the Galaxy. Additionally, we searched for primary tracers of star-forming activity and identified several class I and II candidate young stellar objects (YSOs), positioned over the dense clouds and clumps of molecular material found in the area. The presence of several maser species located in the densest cloud are also indicative of massive star formation taking place in the vicinity of G331.03-00.15.

Stellar feedback in M 83 as observed with MUSE

Context. Energy and momentum injected by young, massive stars into the surrounding gas play an important role in regulating further star formation and in determining the galaxy’s global properties. Before supernovae begin to explode, stellar feedback consists of two main processes: radiation pressure and photoionisation. Aims. We study pre-supernova feedback and constrain the leakage of Lyman continuum (LyC) radiation in a sample of ∼4700 H II regions in the nearby spiral galaxy M 83. We explore the impact that the galactic environment and intrinsic physical properties (metallicity, extinction, and stellar content) have on the early phases of H II region evolution. Methods. We combined VLT/MUSE observations of the ionised gas with young star cluster physical properties derived from HST multiwavelength data. We identified H II regions based on their Hα emission, and cross-matched the sample with planetary nebulae and supernova remnants to assess contaminant sources and identify evolved H II regions. We also spectroscopically identified Wolf-Rayet (WR) stars populating the star-forming regions. We estimated the physical properties of the H II regions (luminosity, size, oxygen abundance, and electron density). For each H II region, we computed the pressure of ionised gas (Pion) and the direct radiation pressure (Pdir) acting in the region, and investigated how they vary with galactocentric distance, with the physical properties of the region, and with the pressure of the galactic environment (PDE). For a subset of ∼500 regions, we also investigated the link between the pressure terms and the properties of the cluster population (age, mass, and LyC flux). By comparing the LyC flux derived from Hα emission with the one modelled from their clusters and WRs, we furthermore constrained any escape of LyC radiation (fesc). Results. We find that Pion dominates over Pdir by at least a factor of 10 on average over the disk. Both pressure terms are strongly enhanced and become almost comparable in the central starburst region. In the disk (R ≥ 0.15 Re), we observe that Pdir stays approximately constant with galactocentric distance. We note that Pdir is positively correlated with an increase in radiation field strength (linked to the negative metallicity gradient in the galaxy), while it decreases in low extinction regions, as is expected if the amount of dust to which the momentum can be imparted decreases. In addition, Pion decreases constantly for increasing galactocentric distances; this trend correlates with the decrease in extinction – indicative of more evolved and thus less compact regions – and with changes in the galactic environment (traced by a decrease in PDE). In general, we observe that H II regions near the centre are underpressured with respect to their surroundings, whereas regions in the rest of the disk are overpressured and hence expanding. We find that regions hosting younger clusters or those that have more mass in young star clusters have a higher internal pressure, indicating that clustered star formation likely plays a dominant role in setting the pressure. Finally, we estimate that only 13% of H II regions hosting young clusters and WR stars have fesc ≥ 0, which suggests that star formation taking place outside young clusters makes a non-negligible contribution to ionising H II regions.

Signatures of spin precession and nutation in isolated black-hole binaries

The spin precession of binary black holes (BBHs) that originate from isolated high-mass binary stars is determined by the interplay of phenomena such as tides, winds, accretion, common-envelope evolution, natal kicks, and stellar core-envelope coupling. In previous work, we identified regions of the parameter space that may produce BBHs with large misalignments from natal kicks and high spin magnitudes from three mechanisms - tides, accretion, or inheritance via minimal core-envelope coupling. Here, we explore the spin precession of such BBHs using five parameters that describe the amplitude and frequency with which the orbital angular momentum precesses and nutates about the total angular momentum, modulating the gravitational-wave emission. Precession is generally possible for sufficiently strong natal kicks provided at least one of the black holes is spinning. Nutation is a consequence of spin-spin coupling and depends on the three spin-up mechanisms. Tidal synchronization can leave a distinct correlation between the aligned effective spin and the nutation frequency, but does not produce large nutations. When a black hole accretes $\gtrsim 20\%$ of its companion's envelope, the precession frequency and amplitude are large. A much smaller amount of accretion, e.g., $\approx 2\%$, is needed to provide a large precession frequency and amplitude when the accretor is a Wolf-Rayet (WR) star. The inheritance of high natal WR spins ($\gtrsim 5\%$ of their maximum breakup value) via minimal core-envelope coupling is the most promising mechanism for producing nutating BBHs, implying that a measurement of nutation from gravitational-wave observations may suggest isolated-binary origin with minimal core-envelope coupling.

Inside the core of a young massive star cluster: 3D MHD simulations

ABSTRACT Young massive star clusters inhabit regions of star formation and play an essential role in the galactic evolution. They are sources of both thermal and non-thermal radiation, and are effective cosmic ray accelerators. We present the 3D magnetohydrodynamic modelling of the plasma flows in a young compact cluster at the evolutionary stage comprising multiple interacting supersonic winds of massive OB and WR stars. The modelling allows studying the partitioning of the mechanical energy injected by the winds between the bulk motions, thermal heating, and magnetic fields. Cluster-scale magnetic fields reaching the magnitudes of ∼300 $\mu$G show the filamentary structures spreading throughout the cluster core. The filaments with the high magnetic fields are produced by the Axford–Cranfill-type effect in the downstream of the wind termination shocks, which is amplified by a compression of the fields with the hot plasma thermal pressure in the central part of the cluster core. The hot (∼a few keV) plasma is heated at the termination shocks of the stellar winds and compressed in the colliding post-shock flows. We also discuss a possible role of the thermal conduction effects on the plasma flow, and analyse temperature maps in the cluster core and the diffuse thermal X-ray emission spectra. The presence of high cluster-scale magnetic fields supports the possibility of high-energy cosmic ray acceleration in clusters at the given evolutionary stage.

A wind environment and Lorentz factors of tens explain gamma-ray bursts X-ray plateau

Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with initial Lorentz factors in the order of a few hundreds. Many GRBs display an early X-ray light-curve plateau, which was not theoretically expected and therefore puzzled the community for many years. Here, we show that this observed signal is naturally obtained within the classical GRB fireball model, provided that the initial Lorentz factor is rather a few tens, and the expansion occurs into a medium-low density wind. The range of Lorentz factors in GRB jets is thus much wider than previously thought and bridges an observational gap between mildly relativistic jets inferred in active galactic nuclei, to highly relativistic jets deduced in few extreme GRBs. Furthermore, long GRB progenitors are either not Wolf-Rayet stars, or the wind properties during the final stellar evolution phase are different than at earlier times. Our model has predictions that can be tested to verify or reject it in the future, such as lack of GeV emission, lack of strong thermal component and long (few seconds) variability during the prompt phase characterizing plateau bursts.

Massive young stellar objects in the Local Group spiral galaxy M 33 identified using machine learning

ABSTRACT We present a supervised machine learning classification of stellar populations in the Local Group spiral galaxy M 33. The Probabilistic Random Forest (PRF) methodology, previously applied to populations in NGC 6822, utilizes both near and far-IR classification features. It classifies sources into nine target classes: young stellar objects (YSOs), oxygen, and carbon-rich asymptotic giant branch stars, red giant branch, and red super-giant stars, active galactic nuclei, blue stars (e.g. O-, B-, and A-type main sequence stars), Wolf–Rayet stars, and Galactic foreground stars. Across 100 classification runs the PRF classified 162 746 sources with an average estimated accuracy of ∼86 per cent, based on confusion matrices. We identified 4985 YSOs across the disc of M 33, applying a density-based clustering analysis to identify 68 star forming regions (SFRs) primarily in the galaxy’s spiral arms. SFR counterparts to known H ii regions were recovered with ∼91 per cent of SFRs spatially coincident with giant molecular clouds identified in the literature. Using photometric measurements, as well as SFRs in NGC 6822 with an established evolutionary sequence as a benchmark, we employed a novel approach combining ratios of [Hα]/[24 μm] and [250 μm]/[500 μm] to estimate the relative evolutionary status of all M 33 SFRs. Masses were estimated for each YSO ranging from 6–27M⊙. Using these masses, we estimate star formation rates based on direct YSO counts of 0.63M⊙ yr−1 in M 33’s SFRs, 0.79 ± 0.16M⊙ yr−1 in its centre and 1.42 ± 0.16M⊙ yr−1 globally.

Higher differentiability of solutions for a class of obstacle problems with non standard growth conditions

We are going to prove a higher differentiability result for the solutions to a class of variational obstacle problems of the formmin⁡{∫ΩF(x,Dω)dx:ω∈Kψ(Ω)} where the function F satisfies non standard growth conditions of the type ν|z|p≤F(x,z)≤L(1+|z|q) with 1<p<q and 0<ν<L; in particular we will consider the subquadratic case: 1<p<q<2.Moreover Ω⊂Rn,n>2 is a bounded open domain, the function ψ:Ω→[−∞,+∞) is called obstacle and it belongs to the Sobolev class W1,p(Ω), p>1 and Kψ(Ω) is the set of admissible functions, i.e.Kψ(Ω)={ω∈u0+W01,p(Ω):ω≥ψa.e.inΩ} where u0 is a fixed boundary value. The main result consists in showing thatDψ∈Wloc1,r(Ω)⇒(1+|Du|2)p−24Du∈Wloc1,2(Ω) under the conditions:qp<1+1n−1r,1<p<q<2<n<r.

First 3D radiation-hydrodynamic simulations of Wolf-Rayet winds

Context. Classical Wolf-Rayet (WR) stars are direct supernova progenitors undergoing vigorous mass loss. Understanding the dense and fast outflows of such WR stars is thus crucial for understanding advanced stages of stellar evolution and the dynamical feedback of massive stars on their environments, and for characterizing the distribution of black hole masses. Aims. In this paper, we develop the first time-dependent, multidimensional, radiation-hydrodynamical models of the extended optically thick atmospheres and wind outflows of hydrogen-free classical WR stars. Methods. A flux-limiting radiation hydrodynamics approach is used on a finite volume mesh to model WR outflows. The opacities are described using a combination of tabulated Rosseland mean opacities and the enhanced line opacities expected within a supersonic flow. Results. For high-luminosity models, a radiation-driven, dense, supersonic wind is launched from deep subsurface regions associated with peaks in the Rosseland mean opacity. For a model with lower luminosity, on the other hand, the Rosseland mean opacity is not sufficient to sustain a net-radial outflow in the subsurface regions. Instead, what develops in this case, is a "standard" line-driven wind launched from the optically thin regions above an extended, moderately inflated, and highly turbulent atmosphere. We thus find here a natural transition from optically thick outflows of classical WR stars to optically thin winds of hot, compact subdwarfs; in our simulations, this transition occurs approximately at a luminosity that is ~40% of the Eddington luminosity. Because of the changing character of the wind-launching mechanism, this transition is also accompanied by a large drop (on the low-luminosity end) in the average mass-loss rate. Since the subsurface opacity peaks are further associated with convective instabilities, the flows are highly structured and turbulent, consisting of coexisting regions of outflowing, stagnated, and even pockets of infalling gas. Typical velocity dispersions in our 3D models are high, 100–300 km s−1, but the clumping factors are rather modest, fc1 ≡ 〈ρ2〉/〈ρ〉2 ~ 2. We further find that, while the low-density gas in our simulations is strongly radiation-driven, the overdense structures are, after their initial launch, primarily advected outward by ram-pressure gradients. This inefficient radiative acceleration of dense "clumps" reflects the inverse dependence of line driving on mass density and leads to a general picture wherein high-density gas parcels move significantly slower than the mean and low-density wind material.

WR 63: a multiple system (O+O) + WR?

ABSTRACT The spectrum of the Wolf–Rayet (WR) star WR 63 contains spectral lines of two different O stars that show regular radial velocity (RV) variations with amplitudes of ∼160 and ∼225 km s−1 on a ∼4.03 d period. The light curve shows two narrow eclipses that are 0.2 mag deep on the same period as the RV changes. On the other hand, our data show no significant RV variations for the WR spectral lines. Those findings are compatible with WR 63 being a triple system composed of two non-interacting late-O stars orbiting a WR star on a period longer than 1000 d. The amplitude of the WR spectral line-profile variability reaches 7–8 per cent of the line intensity and seems related to a 0.04 mag periodic photometric variation. Large wind density structures are a possible origin for this variability, but our data are not sufficient to verify this. Our analysis shows that, should the three stars be bound, they would be coeval with an age of about 5.9 ± 1.4 Myr. The distance to the O stars is estimated to be $3.4\, \pm \, 0.5$ kpc. Their dynamical masses are 14.3 ± 0.1 and 10.3 ± 0.1 M⊙. Using rotating single-star evolutionary tracks, we estimate their initial masses to be 18 ± 2 and 16 ± 2 M⊙ for the primary and the secondary, respectively. Regular spectral monitoring is required in the future to detect RV variations of the WR star that would prove that it is gravitationally bound to the close O+OB system and to determine its mass.

The integrated radio spectrum of G2.4+1.4

ABSTRACT The Galactic source G2.4+1.4 is an optical and radio nebula containing an extreme Wolf–Rayet star. At one time, this source was regarded as a supernova remnant, because of its apparent non-thermal radio spectrum, although this was based on limited observations. Subsequent observations instead supported a flat, optically thin thermal radio spectrum for G2.4+1.4, and it was identified as a photoionized, mass-loss bubble, not a supernova remnant. Recently, however, it has been claimed that this source has a non-thermal integrated radio spectrum. I discuss the integrated radio flux densities available for G2.4+1.4 from a variety of surveys, and show that it has a flat spectrum at gigahertz frequencies (with a spectral index α of 0.02 ± 0.08, where flux density S scales with frequency ν as S ∝ ν−α).

X-ray properties of early-type stars in the Tarantula Nebula from T-ReX

ABSTRACT We reassess the historical LX/LBol relation for early-type stars from a comparison between T-ReX, the Chandra ACIS X-ray survey of the Tarantula Nebula in the Large Magellanic Cloud (LMC), and contemporary spectroscopic analysis of massive stars obtained primarily from VLT/FLAMES, VLT/MUSE, and HST/STIS surveys. For 107 sources in common (some host to multiple stars), the majority of which are bolometrically luminous (40 per cent exceed 106L⊙), we find an average log LX/LBol = −6.90 ± 0.65. Excluding extreme systems Mk 34 (WN5h+WN5h), R140a (WC4+WN6+), and VFTS 399 (O9 IIIn+?), plus four WR sources with anomalously hard X-ray components (R130, R134, R135, Mk 53) and 10 multiple sources within the spatially crowded core of R136a, log LX/LBol = −7.00 ± 0.49, in good agreement with Galactic OB stars. No difference is found between single and binary systems, nor between O, Of/WN, and WR stars, although there does appear to be a trend towards harder X-ray emission from O dwarfs, through O (super)giants, Of/WN stars, and WR stars. The majority of known OB stars in the Tarantula are not detected in the T-ReX point source catalogue, so we have derived upper limits for all undetected OB stars for which log LBol/L⊙ ≥ 5.0. A survival analysis using detected and upper limit log LX/LBol values indicates no significant difference between luminous O stars in the LMC and the Carina Nebula. This analysis suggests that metallicity does not strongly influence LX/LBol. Plasma temperatures for single, luminous O stars in the Tarantula ($\overline{kT_{m}}=1.0$ keV) are higher than counterparts in Carina ($\overline{kT_{m}}=0.5$ keV).

Neutrino Emission from Luminous Fast Blue Optical Transients

Mounting evidence suggests that luminous fast blue optical transients (LFBOTs) are powered by a compact object, launching an asymmetric and fast outflow responsible for the radiation observed in the ultraviolet, optical, infrared, radio, and X-ray bands. Proposed scenarios aiming to explain the electromagnetic emission include an inflated cocoon, surrounding a jet choked in the extended stellar envelope. Alternatively, the observed radiation may arise from the disk formed by the delayed merger of a black hole with a Wolf–Rayet star. We explore the neutrino production in these scenarios, i.e., internal shocks in a choked jet and interaction between the outflow and the circumstellar medium (CSM). If observed on axis, the choked jet provides the dominant contribution to the neutrino fluence. Intriguingly, the IceCube upper limit on the neutrino emission inferred from the closest LFBOT, AT2018cow, excludes a region of the parameter space otherwise allowed by electromagnetic observations. After correcting for the Eddington bias on the observation of cosmic neutrinos, we conclude that the emission from an on-axis choked jet and CSM interaction is compatible with the detection of two track-like neutrino events observed by the IceCube Neutrino Observatory in coincidence with AT2018cow, and otherwise considered to be of atmospheric origin. While the neutrino emission from LFBOTs does not constitute the bulk of the diffuse background of neutrinos observed by IceCube, the detection prospects of nearby LFBOTs with IceCube and the upcoming IceCube-Gen2 are encouraging. Follow-up neutrino searches will be crucial for unraveling the mechanism powering this emergent transient class.

A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars

Context. Most massive stars reside in multiple systems that will interact over the course of their lifetime. This has important consequences on their future evolution and their end-of-life products. Classical Wolf-Rayet (WR) stars represent the final end stages of stellar evolution at the upper-mass end. While their observed multiplicity fraction is reported to be ∼0.4 in the Galaxy, their intrinsic multiplicity properties and the distributions of their orbital parameters remain insufficiently constrained to provide a reliable anchor to compare to evolutionary predictions. Aims. As part of a homogeneous, magnitude-limited (V ≤ 12) spectroscopic survey of northern Galactic WR stars, this paper aims to establish the observed and intrinsic multiplicity properties of the early-type nitrogen-rich WR population (WNE), including estimates of the multiplicity fraction and the shape of their orbital period distribution. Additionally, we compare these with the properties of the carbon-rich WR population (WC) stars obtained in the first paper of this series. Methods. We obtained high-resolution spectroscopic time series of the complete magnitude-limited sample of 16 WNE stars observable with the 1.2 m Mercator telescope at La Palma, typically providing a time base of about two to eight years. We measured relative radial velocities (RVs) using cross-correlation and used RV variations to flag binary candidates. Using an updated Monte Carlo method with a Bayesian framework, we calculated the three-dimensional likelihood for the intrinsic binary fraction (fintWNE), the maximum period (log Pmax), and the power-law index for the period distribution (π) for the WNE population with Pmin fixed at 1 d. We also used this updated method to re-derive multiplicity parameters for the Galactic WC population. Results. Adopting a peak-to-peak RV variability threshold of 50 km s−1 as a criterion, we classify 7 of the 16 targets as binaries. This results in an observed multiplicity fraction (fobsWNE) of 0.44 ± 0.12. Assuming flat priors, we derive the best-fit multiplicity properties fintWNE = 0.56−0.15+0.20, log Pmax = 4.60−0.77+0.40, and π = −0.30−0.53+0.55 for the parent WNE population. We explored different mass-ratio distributions and note that they did not change our results significantly. For the Galactic WC population from Paper I, we re-derive fintWC = 0.96−0.22+0.04, log Pmin = 0.75−0.60+0.26, log Pmax = 4.00−0.34+0.42, and π = 1.90−1.25+1.26. Conclusions. The derived multiplicity parameters for the WNE population are quite similar to those derived for main-sequence O binaries but differ from those of the WC population. The significant shift in the WC period distribution towards longer periods is too large to be explained via expansion of the orbit due to stellar winds, and we discuss possible implications of our results. Analysis of the WNL population and further investigation of various evolutionary scenarios is required to connect the different evolutionary phases of stars at the upper-mass end.

Radio observations of massive stars in the Galactic centre: The Quintuplet cluster

We present high-angular-resolution radio continuum observations of the Quintuplet cluster, one of the most emblematic massive clusters in the Galactic centre. Data were acquired in two epochs and at 6 and 10 GHz with the Karl G. Jansky Very Large Array. With this work we have quadrupled the number of known radio stars in the cluster. Even though the uncertainty of the measured spectral indices is relatively high, we tentatively classify the 30 detected stars. Eleven have spectral indices consistent with thermal emission from ionised stellar winds, ten have flat to inverted spectral indices indicative of non-thermal emission arising in colliding winds in binaries, and the nine remaining sources cannot be easily classified because of large uncertainties or extremely positive values of the spectral index. The mean mass-loss rate estimated for Wolf-Rayet stars agrees with previous work. Regarding variability, remarkably we find a significantly higher fraction of variable stars in the Quintuplet cluster (∼30%) than in the Arches cluster (&lt; 15%), probably because the Quintuplet cluster is older. Our determined stellar wind mass-loss rates are in good agreement with theoretical models. Finally, we show that the radio luminosity function can be used as a tool to constrain the age and the mass function of a cluster.

Long-term X-ray spectral variation of the Wolf–Rayet binary WR 102-1 in the Galactic bulge: evidence for wind distortion in the binary

ABSTRACT WR 102-1 was detected by Suzaku as a conspicuous point source in the 6.7 keV intensity map of the central region of the Milky Way. The source was suggested as a possible Wolf–Rayet binary based on its X-ray and infrared spectral characteristics. The iron line emission is expected to originate in the Wolf–Rayet star’s dynamic stellar wind when colliding the companion’s mild stellar wind. Here, we report the result of a long-term X-ray monitoring of WR 102-1 since 1998 using archival data of ASCA, XMM–Newton, Chandra, Suzaku, and Swift to reveal variations of the iron K-emission line and the circumstellar absorption. Consequently, we have detected significant redshifts of the iron K-emission line from the XMM–Newton observation in 2003 March and the Suzaku observation in 2006 September. Furthermore, when the redshift was observed, which suggests that the Wolf–Rayet star was in front of the companion star, the circumstellar absorption values were smaller than other periods. These results appear contrary to the expectation if the Wolf–Rayet’s stellar wind is spherically symmetric, but may be understood if the Wolf–Rayet star’s stellar wind is significantly distorted due to the rapid orbital motion near the periastron.

The Extreme Scarcity of Dust-enshrouded Red Supergiants: Consequences for Producing Stripped Stars via Winds

Quiescent mass loss during the red supergiant (RSG) phase has been shown to be far lower than prescriptions typically employed in single-star evolutionary models. Importantly, RSG winds are too weak to drive the production of Wolf-Rayet (WR) stars and stripped-envelope supernovae (SE-SNe) at initial masses of roughly 20–40 M ⊙. If single stars are to make WR stars and SE-SNe, this shifts the burden of mass loss to rare dust-enshrouded RSGs (DE-RSGs), objects claimed to represent a short-lived, high-mass-loss phase. Here, we take a fresh look at the purported DE-RSGs. By modeling the mid-IR excesses of the full sample of RSGs in the Large Magellanic Cloud, we find that only one RSG has both a high mass-loss rate ( ≥ 10−4 M ⊙ yr−1) and a high optical circumstellar dust extinction (7.92 mag). This RSG is WOH G64, and it is the only one of the 14 originally proposed DE-RSGs that is actually dust enshrouded. The rest appear to be either normal RSGs without strong IR excess, or lower-mass asymptotic giant branch stars. Only one additional object in the full catalog of RSGs (not previously identified as a DE-RSG) shows strong mid-IR excess. We conclude that if DE-RSGs do represent a pre-SN phase of enhanced in single stars, it is extremely short-lived, only capable of removing ≤2 M ⊙ of material. This rules out the single-star post-RSG pathway for the production of WR stars, luminous blue variables, and SE-SNe. Single-star models should not employ -prescriptions based on these extreme objects for any significant fraction of the RSG phase.

Spectra of Cosmic-Ray Sodium and Aluminum and Unexpected Aluminum Excess

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, , e ±, and nuclei (H–Si, Fe), which resulted in a number of breakthroughs. The most recent AMS-02 result is the measurement of the spectra of CR sodium and aluminum up to ∼2 TV. Given their low solar system abundances, a significant fraction of each element is produced in fragmentations of heavier species, predominantly Ne, Mg, and Si. In this paper, we use precise measurements of the sodium and aluminum spectra by AMS-02 together with ACE-CRIS and Voyager 1 data to test their origin. We show that the sodium spectrum agrees well with the predictions made with the GalProp-HelMod framework, while the aluminum spectrum shows a significant excess in the rigidity range from 2–7 GV. In this context, we discuss the origin of other low-energy excesses in Li, F, and Fe found earlier. The observed excesses in Li, F, and Al appear to be consistent with the local Wolf-Rayet stars hypothesis, invoked to reproduce anomalous 22Ne/20Ne, 12C/16O, and 58Fe/56Fe ratios in CRs, while excess in Fe is likely connected with a past supernova activity in the solar neighborhood. We also provide updated local interstellar spectra (LIS) of sodium and aluminum in the rigidity range from a few megavolts to ∼2 TV. Our calculations employ the self-consistent GalProp-HelMod framework, which has proved to be a reliable tool in deriving the LIS of CR , e −, and nuclei Z ≤ 28.

Modeling the Optical to Ultraviolet Polarimetric Variability from Thomson Scattering in Colliding-wind Binaries

Massive-star binaries are critical laboratories for measuring masses and stellar wind mass-loss rates. A major challenge is inferring viewing inclination and extracting information about the colliding-wind interaction (CWI) region. Polarimetric variability from electron scattering in the highly ionized winds provides important diagnostic information about system geometry. We combine for the first time the well-known generalized treatment of Brown et al. for variable polarization from binaries with the semianalytic solution for the geometry and surface density CWI shock interface between the winds based on Cantó et al. Our calculations include some simplifications in the form of inverse-square law wind densities and the assumption of axisymmetry, but in so doing they arrive at several robust conclusions. One is that when the winds are nearly equal (e.g., O+O binaries) the polarization has a relatively mild decline with binary separation. Another is that despite Thomson scattering being a gray opacity, the continuum polarization can show chromatic effects at ultraviolet wavelengths but will be mostly constant at longer wavelengths. Finally, when one wind dominates the other, as, for example, in WR+OB binaries, the polarization is expected to be larger at wavelengths where the OB component is more luminous and generally smaller at wavelengths where the WR component is more luminous. This behavior arises because, from the perspective of the WR star, the distortion of the scattering envelope from spherical is a minor perturbation situated far from the WR star. By contrast, the polarization contribution from the OB star is dominated by the geometry of the CWI shock.