Wolf-Rayet Stars Research Articles

Tidal Spin-up of Black Hole Progenitor Stars

Gravitational wave observations indicate the existence of merging black holes (BHs) with high spin (a ≳ 0.3), whose formation pathways are still an open question. A possible way to form those binaries is through the tidal spin-up of a Wolf–Rayet (WR) star by its BH companion. In this work, we investigate this scenario by directly calculating the tidal excitation of oscillation modes in WR star models, determining the tidal spin-up rate, and integrating the coupled spin–orbit evolution for WR–BH binaries. We find that, for short-period orbits and massive WR stars, the tidal interaction is mostly contributed by standing gravity modes, in contrast to Zahn’s model of traveling waves, which is frequently assumed in the literature. The standing modes are less efficiently damped than traveling waves, meaning that prior estimates of tidal spin-up may be overestimated. We show that tidal synchronization is rarely reached in WR–BH binaries, and the resulting BH spins have a ≲ 0.4 for all but the shortest-period (P orb ≲ 0.5 day) binaries. Tidal spin-up in lower-mass systems is more efficient, providing an anticorrelation between the mass and spin of the BHs, which could be tested in future gravitational wave data. Nonlinear damping processes are poorly understood but may allow for more efficient tidal spin-up. We also discuss a new class of gravito-thermal modes that appear in our calculations.

From Dust to Nanodust: Resolving Circumstellar Dust from the Colliding-wind Binary Wolf-Rayet 140

Wolf-Rayet (WR) 140 is the archetypal periodic dust-forming colliding-wind binary that hosts a carbon-rich WR (WC) star and an O-star companion with an orbital period of 7.93 yr and an orbital eccentricity of 0.9. Throughout the past few decades, multiple dust-formation episodes from WR 140 have been observed that are linked to the binary orbit and occur near the time of periastron passage. Given its predictable dust-formation episodes, WR 140 presents an ideal astrophysical laboratory to investigate the formation and evolution of dust in the hostile environment around a massive binary system. In this paper, we present near- and mid-infrared (IR) spectroscopic and imaging observations of WR 140 with Subaru/SCExAO+CHARIS, Keck/NIRC2+PyWFS, and Subaru/Cooled Mid-Infrared Camera and Spectrograph taken between 2020 June and September that resolve the circumstellar dust emission linked to its most recent dust-formation episode in 2016 December. Our spectral energy distribution analysis of WR 140's resolved circumstellar dust emission reveals the presence of a hot (T d ∼ 1000 K) near-IR dust component that is co-spatial with the previously known and cooler (T d ∼ 500 K) mid-IR dust component composed of 300–500 Å sized dust grains. We attribute the hot near-IR dust emission to the presence of nano-sized (nanodust) grains and suggest they were formed from grain–grain collisions or the rotational disruption of the larger grain size population by radiative torques in the strong radiation field from the central binary. Lastly, we speculate on the astrophysical implications of nanodust formation around colliding-wind WC binaries, which may present an early source of carbonaceous nanodust in the interstellar medium.

Chemical Abundances of a Sample of Oxygen-dominated Galaxies

We spectroscopically analyzed a sample of 85 Sloan Digital Sky Survey compact, oxygen-dominated galaxies located at redshift z ∼ 0.001–0.350, selected because of their large equivalent width of [O iii]λ5007 (larger than 200 Å). These galaxies might be considered as extreme emission-line galaxies due to their strong [O iii]λ5007 emission line. We detected high-ionization lines, even those related to the presence of Wolf–Rayet stars, in almost all the galaxies studied. We obtained physical properties (electron density and temperature) as well as chemical abundances by using the direct method based on the electron temperature. In this analysis, we obtained three different measurements of T(high): the usual one, T([O iii]), but also that of T([S iii]) and T([Ar iii]) for five and three of the galaxies, respectively. Further, we established a new calibration for T([S iii]). We determined oxygen, nitrogen, sulfur, neon, argon, iron, and chlorine abundances when possible, and we compared to the results of other late-type, low-metallicity galaxies, such as blue compact dwarfs, Ims, and green peas. From such a detailed study, we can conclude that the majority of the galaxies in this sample have similar metallicities to the SMC (about ) ≈ 8 dex), and that only 12% of the galaxies are extremely metal-poor, with abundances lower than 7.7 dex. Also, a comparison with some chemical evolution models as well as a brief discussion on the chemical evolution with time is considered.

Microquasar Cyg X-3 – a unique jet-wind neutrino factory?

ABSTRACT The origin of astrophysical neutrinos is one of the most debated topics today. Perhaps the most robust evidence of neutrino counterpart comes from supermassive black holes in active galactic nuclei associated with strongly collimated outflows, or jets, that can accelerate particles to relativistic energies and produce neutrinos through hadronic interactions. Similar outflows can also be found from X-ray binaries, or ‘microquasars’, that consist of a neutron star or a stellar-mass black hole accreting matter from a non-degenerate companion star. In some cases, these systems can accelerate particles up to GeV energies implying an efficient acceleration mechanism in their jets. Neutrino production in microquasar jets can be expected with suitable conditions and a hadronic particle population. Microquasar Cyg X-3 is a unique, short orbital period X-ray binary hosting a Wolf–Rayet companion star with a strong stellar wind. The interaction of the dense stellar wind with a relativistic jet leads to particle collisions followed by high-energy gamma-ray and potentially neutrino emission. Here, using the 10-yr neutrino candidate sample of the IceCube neutrino observatory, we find that the events with the highest spatial association with Cyg X-3 occur during short-lived high-energy gamma-ray flaring periods indicating the possible astrophysical nature of these events.

The magnetic, spectroscopic, and photometric variability of the Wolf–Rayet star WR 55

ABSTRACT Studies of magnetic fields in the most evolved massive stars, the Wolf–Rayet stars, are of special importance because they are progenitors of certain types of supernovae. The first detection of a magnetic field of the order of a few hundred gauss in the WN7 star WR 55, based on a few FORS2 low-resolution spectropolarimetric observations, was reported in 2020. In this work, we present new FORS2 observations allowing us to detect magnetic and spectroscopic variability with a period of 11.90 h. No significant frequencies were detected in TESS and ASAS-SN photometric observations. Importantly, magnetic field detections are achieved currently only in two Wolf–Rayet stars, WR 6 and WR 55, both showing the presence of corotating interacting regions.

Hydrogen-free Wolf-Rayet stars: Helium stars with envelope-inflation structure and rotation

Context. Observations have shown that the effective temperature of hydrogen-free Wolf-Rayet (WR) stars is considerably lower than that of the standard model, which means that the radius of the observed H-free WR stars is several times larger than that estimated by the standard model. The envelope inflation structure (EIS) caused by the radiation luminosity being close to the Eddington luminosity in the iron opacity peak region of H-free WR stars may be the key to resolving the radius problem of H-free WR stars. Aims. We study the structure and evolution of helium (He) stars with the EIS and discuss the influence of rotation on these He stars. We aim to explain the radius problem of H-free WR stars observed in the Milky Way (MW) and the Large Magellanic Cloud (LMC) through the He stars. Methods. Using the Modules for Experiments in Stellar Astrophysics code, we compute the evolution of He stars with and without MLT++ prescriptions, and discuss their effects on the EIS. We calculated the evolution of He stars using a new mass-loss rate formula and three different relative rotational velocities and compared our results with observations on Hertzsprung–Russell diagrams. Results. The EIS has different effects on the structure and evolution of He stars with different masses. Due to the luminosity well below the Eddington limit, low-mass He stars with an initial mass of less than 12 M⊙ do not produce EIS with or without the MLT++ prescription. High-mass He stars with an initial mass exceeding 12 M⊙ and without the MLT++ prescription produce the EIS. Since the EIS is Eddington factor Γ-dependent, its radius increases with the increase in metallicity and decreases with rotational velocity increase. For rotating low-mass He stars, since the rotational mixing timescale is smaller than the evolutionary timescale, rotational mixing can increase the lifetime and allow He stars to evolve into WC stars during the helium giant phase. For rotating high-mass He stars, since rotation increases the mass-loss rate, the radius of the EIS decreases as rotational velocity increases. The rotation-decay timescale of rapidly rotating He stars is very short, and the rapidly rotating He stars only appear within the first one-tenth of their lifetime, which is consistent with the observations of WR stars. Conclusions. The low-luminosity (log(L/L⊙)≤5.2) H-free WR stars in the MW and the LMC can be explained by the helium giant phase in low-mass He stars, the high XC and XO in WC stars can only evolve through low-mass He stars with a rapid rotation. High-mass He stars with the EIS can explain H-free WR stars with a luminosity exceeding 105.7 L⊙ and an effective temperature above 104.7 K in the MW. They can also explain H-free WR stars on the right-hand side of the He zero-age main sequence in the LMC. High-mass stars with the EIS evolve into WO stars at the final evolution stage, and the shorter lifetime fraction is consistent with the small number of observed WO stars.

A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars

Context. Massive stars are powerful cosmic engines that have a huge impact on their surroundings and host galaxies. The majority of massive stars will interact with a companion star during their evolution. The effects of this interaction on their end-of-life products are currently poorly constrained. In the phases immediately preceding core-collapse, massive stars in the Galaxy with Mi ≳ 20 M⊙ may appear as classical Wolf-Rayet (WR) stars. The multiplicity properties of the WR population are thus required to further our understanding of stellar evolution at the upper-mass end. Aims. As the final contribution of a homogeneous radial velocity (RV) survey, this work aims to constrain the multiplicity properties of northern Galactic late-type nitrogen-rich Wolf-Rayet (WNL) stars. We compare their intrinsic binary fraction and orbital period distribution to the carbon-rich (WC) and early-type nitrogen-rich (WNE) populations from previous works. Methods. We obtained high-resolution spectra of the complete magnitude-limited sample of 11 Galactic WNL stars with the Mercator telescope on the island of La Palma. We used cross-correlation with a log-likelihood framework to measure relative RVs and flagged binary candidates based on the peak-to-peak RV dispersion. By using Monte Carlo sampling and a Bayesian framework, we computed the three-dimensional likelihood and one-dimensional posteriors for the upper period cut-off (log PmaxWNL), power-law index (πWNL), and intrinsic binary fraction (fintWNL). Results. Adopting a threshold C of 50 km s−1, we derived fobsWNL = 0.36 ± 0.15. Our Bayesian analysis produces fintWNL = 0.42−0.17+0.15, πWNL = −0.70−1.02+0.73 and log PmaxWNL = 4.90−3.40+0.09 for the parent WNL population. The combined analysis of the Galactic WN population results in fintWN = 0.52−0.12+0.14, πWN = −0.99−0.50+0.57 and log PmaxWN = 4.99−1.11+0.00. The observed period distribution of Galactic WN and WC binaries from the literature is in agreement with what is found. Conclusions. The period distribution of Galactic WN binaries peaks at P ∼ 1–10 d and that of the WC population at P ∼ 5000 d. This shift cannot be reconciled by orbital evolution due to mass loss or mass transfer. At long periods, the evolutionary sequence O (→LBV) → WN → WC seems feasible. The high frequency of short-period WN binaries compared to WC binaries suggests that they either tend to merge, or that the WN components in these binaries rarely evolve into WC stars in the Galaxy.

UM 462, a local green pea galaxy analogue under the MUSE magnifying glass

Context. Stellar feedback in high-redshift galaxies plays an important, if not dominant, role in the re-ionisation epoch of the Universe. Because of their extreme star formation (SF), the relatively closer green pea (GP) galaxies are postulated as favorite local laboratories, and analogues to those high-redshift galaxies. However, at their typical redshift of z ∼ 0.2, the most intimate interaction between stars and the surrounding interstellar medium cannot be disentangled. Detailed studies of blue compact dwarf (BCD) galaxies sharing properties with GP galaxies are necessary to anchor our investigations on them. Aims. We want to study in detail UM 462, which is a BCD with emission line ratios and equivalent widths, stellar mass, and metallicity similar to those observed in GP galaxies, and thus it is ideally suited as a corner stone and reference galaxy. Methods. We use high-quality optical integral field spectroscopy data obtained with MUSE on the ESO Very Large Telescope. Results. The electron density (ne) and temperature (Te) were mapped. Median Te decreases according to the sequence [S III] → [N II] → He I. Furthermore, Te([S III]) values are ∼13 000 K, and uniform within the uncertainties over an area of ∼20″ × 8″ (∼1.4 kpc × 0.6 kpc). The total oxygen abundance by means of the direct method is 12 + log(O/H) ∼ 8.02 and homogenous all over the galaxy within the uncertainties, which is in stark contrast with the metallicities derived from several strong line methods. This result calls for a systematic study to identify the best strategy to determine reliable metallicities at any location within a galaxy. The strong line ratios used in the BPT diagrams and other ratios tracing the ionisation structure were mapped. They are compatible with plasma ionised by massive hot stars. However, there is a systematic excess in the [O I]/Hα ratio, suggesting an additional mechanism or a complex relative configuration of gas and stars. The velocity field for the ionised gas presents receding velocities in the east and approaching velocities in the west and south-west with velocity differences of Δv ∼ 40 km s−1, but it is not compatible with simple rotation. The most striking feature is a velocity stratification in the area towards the north with redder velocities in the high ionisation lines and bluer velocities in the low ionisation lines. This is the only area with velocity dispersions clearly above the MUSE instrumental width, and it is surrounded by two ∼1 kpc-long structures nicknamed ‘the horns’. We interpret the observational evidence in that area as a fragmented super-bubble fruit of the stellar feedback and it may constitute a preferred channel through which Lyman continuum photons from the youngest generation of stars can escape. The galaxy luminosity is dominated by a young (i.e. ∼6 Myr) stellar population that contributes only 10% to the stellar mass, as derived from the modelling of the stellar continuum. The most recent SF seems to propagate from the outer to the inner parts of the galaxy, and then from east to west. We identified a supernova remnant and Wolf-Rayet stars – as traced by the red bump – that support this picture. The direction of the propagation implies the presence of younger Wolf-Rayet stars at the maximum in Hα. These may be detected by deep observations of the blue bump (not covered here). Conclusions. The ensemble of results exemplifies the potential of 2D detailed spectroscopic studies of dwarf star-forming galaxies at high spatial resolution as a key reference for similar studies on primeval galaxies.

Science in Images: Before exploding as supernovae, massive Wolf-Rayet stars spew gas and dust into space, seeding the formation of future stellar and planetary systems.

Science in Images: Before exploding as supernovae, massive Wolf-Rayet stars spew gas and dust into space, seeding the formation of future stellar and planetary systems.

Cosmic Ray Enrichment with 22Ne in Young Massive Star Clusters

—We developed the hypothesis that the 22Ne overabundance in cosmic rays (CRs) is generated in massive star clusters which contain populations of Wolf–Rayet stars. Winds of Wolf–Rayet stars are considered to have high content of 22Ne. We present a model of cosmic ray enrichment with 22Ne in young massive star clusters, adding isotopic yields from supernovae and considering the acceleration efficiency during the lifetime of the stars. The impact of the parameters (the initial mass function in the cluster, rotation velocity, black hole cut-off mass) is discussed. The luminosity of the sources is found to be sufficient to provide the observed CR flux. The local source hypothesis is investigated.

Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity

Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M1, 2 ≈ 32 M⊙. Both stars have weak stellar winds with mass-loss rates of log Ṁ/(M⊙ yr−1) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with Teff ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with Teff ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity.

Integral Field Spectroscopy of the cometary starburst galaxy NGC 4861

ABSTRACT Using the PMAS Integral Field Unit on the Calar Alto 3.5 m telescope, we observed the southern component (Markarian 59) of the ‘cometary’ starburst galaxy NGC 4861. Mrk 59 is centred on a giant nebula and concentration of stars 1 kpc in diameter. Strong $\rm H\,\alpha$ emission points to a star-formation rate (SFR) at least 0.47 $\rm M_{\odot }\, yr^{-1}$. Mrk 59 has a very high [O iii] $\rm \lambda 5007/H\,\beta$ ratio, reaching 7.35 in the central nebula, with a second peak at a star-forming hotspot further north. Fast outflows are not detected but nebular motion and galaxy rotation produce relative velocities up to 40 km $\rm s^{-1}$. Spectral analysis of different regions with ‘Fitting Analysis using Differential evolution Optimization’ (FADO) finds that the stars in the central and ‘spur’ nebulae are very young, $\rm \le 125~Myr$ with a large $\rm \lt 10~Myr$ contribution. Older stars ($\rm \sim 1~Gyr$) make up the northern disk component, while the other regions show mixtures of 1 Gyr age with very young stars. This and the high specific SFR $\rm \sim 3.5~Gyr^{-1}$ imply a bimodal star formation history, with Mrk 59 formed in ongoing starbursts fuelled by a huge gas inflow, turning the galaxy into an asymmetric ‘green pea’ or blue compact dwarf. We map the He ii λ4686 emission, and identify a broad component from the central nebula, consistent with the emission of ∼300 Wolf–Rayet stars. About a third of the He ii λ4686 flux is a narrow line emitted from a more extended area covering the central and spur nebulae, and may have a different origin.

Chandra X-ray measurement of gas-phase heavy element abundances in the central parsec of the galaxy

ABSTRACT Elemental abundances are key to our understanding of star formation and evolution in the Galactic Centre. Previous work on this topic has been based on infrared (IR) observations, but X-ray observations have the potential of constraining the abundance of heavy elements, mainly through their K-shell emission lines. Using 5.7 Ms Chandra observations, we provide the first abundance measurement of Si, S, Ar, Ca, and Fe, in four prominent diffuse X-ray features located in the central parsec of the Galaxy, which are the manifestation of shock-heated hot gas. A two-temperature non-equilibrium ionization spectral model is employed to derive the abundances of these five elements. In this procedure, a degeneracy is introduced due to uncertainties in the composition of light elements, in particular, H, C, and N. Assuming that the hot gas is H-depleted but C- and N-enriched, as would be expected for a standard scenario in which the hot gas is dominated by Wolf–Rayet star winds, the spectral fit finds a generally subsolar abundance for the heavy elements. If, instead, the light elements had a solar-like abundance, the heavy elements have a fitted abundance of ∼1–2 solar. The α/Fe abundance ratio, on the other hand, is mostly supersolar and insensitive to the exact composition of the light elements. These results are robust against potential biases due to either a moderate spectral signal-to-noise ratio or the presence of non-thermal components. Implications of the measured abundances for the Galactic Centre environment are addressed.

Evidence for non-thermal X-ray emission from the double Wolf-Rayet colliding-wind binary Apep

Context. Massive colliding-wind binaries (CWBs) can be non-thermal sources. The emission produced in their wind-collision region (WCR) encodes information of both the shock properties and the relativistic electrons accelerated in them. The recently discovered system Apep, a unique massive system hosting two Wolf-Rayet stars, is the most powerful synchrotron radio emitter among the known CWBs. It is an exciting candidate in which to investigate the non-thermal processes associated with stellar wind shocks. Aims. We intend to break the degeneracy between the relativistic particle population and the magnetic field strength in the WCR of Apep by probing its hard X-ray spectrum, where inverse-Compton (IC) emission is expected to dominate. Methods. We observed Apep with NuSTAR for 60 ks and combined this with a reanalysis of a deep archival XMM-Newton observation to better constrain the X-ray spectrum. We used a non-thermal emission model to derive physical parameters from the results. Results. We detect hard X-ray emission consistent with a power-law component from Apep. This is compatible with IC emission produced in the WCR for a magnetic field of ≈105–190 mG, corresponding to a magnetic-to-thermal pressure ratio in the shocks of ≈0.007–0.021, and a fraction of ∼1.5 × 10−4 of the total wind kinetic power being transferred to relativistic electrons. Conclusions. The non-thermal emission from a CWB is detected for the first time in radio and at high energies. This allows us to derive the most robust constraints so far for the particle acceleration efficiency and magnetic field intensity in a CWB, reducing the typical uncertainty of a few orders of magnitude to just within a factor of a few. This constitutes an important step forward in our characterisation of the physical properties of CWBs.

Constraints on the Binarity of the WN3/O3 Class of Wolf–Rayet Stars* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

The WN3/O3 Wolf–Rayet (WR) stars were discovered as part of our survey for WRs in the Magellanic Clouds. The WN3/O3s show the emission lines of a high-excitation WN star and the absorption lines of a hot O-type star, but our prior work has shown that the absorption spectrum is intrinsic to the WR star. Their place in the evolution of massive stars remains unclear. Here we investigate the possibility that they are the products of binary evolution. Although these are not WN3+O3 V binaries, they could still harbor unseen companions. To address this possibility, we have conducted a multiyear radial velocity study of six of the nine known WN3/O3s. Our study finds no evidence of statistically significant radial velocity variations, and allows us to set stringent upper limits on the mass of any hypothetical companion star: for probable orbital inclinations, any companion with a period less than 100 days must have a mass <2M ⊙. For periods less than 10 days, any companion would have to have a mass <1M ⊙. We argue that scenarios where any such companion is a compact object are unlikely. The absorption lines indicate a normal projected rotational velocity, making it unlikely that these stars evolved with the aid of a companion star that has since merged. The modest rotation also suggests that these stars are not the result of homogenous evolution. Thus it is likely that these stars are a normal but short-lived stage in the evolution of massive stars.

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. In this work, we investigated detailed evolutionary models of very massive binaries (30−90 M⊙) 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 understanding 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.

Comparison the Massive Star Formation Theorem of Collision and Accretion

Massive star (M≥10 M⊙) is so luminous that the radiation pressure has negative effects to the formation itself. There are several mainstream theories could explain how massive star form in this particular surrounding. The collision theory and accretion would be discussed in this paper. Compared to the collision case, star is more likely to form massive by the accretion process. The simulation of collision case seems to work well. However, there is no evidence that astrophysicists have been found in the observation that supports this theory. The paper summarizes the simulation of two mainstream theories and the limitation for the research of high-mass star formation. The paper also proposes a possible method by analyzing the geometry of Wolf-Rayet star. With the development of models of gas distribution, hydrodynamical and mathematics work, it is necessary to calculate the gas structure in order to speculate the history of protostar in the center. These results could also provide reference and guidance for investigation of star formation.

Stripped-envelope stars in different metallicity environments

Stripped-envelope stars can be observed as Wolf-Rayet (WR) stars or as less luminous hydrogen-poor stars with low mass-loss rates and transparent winds. Both types are potential progenitors of Type I core-collapse supernovae (SNe). We used grids of core-collapse models obtained from single helium stars at different metallicities to study the effects of metallicity on the transients and remnants these stars produce. We characterised the surface and core properties of our core-collapse models and investigated their ‘explodability’ using three criteria. In the cases where explosions are predicted, we estimated the ejecta mass, explosion energy, nickel mass, and neutron star (NS) mass. Otherwise, we predicted the mass of the resulting black hole (BH). We constructed a simplified population model and find that the properties of SNe and compact objects depend strongly on metallicity. The ejecta masses and explosion energies for Type Ic SNe are best reproduced by models with Z = 0.04 that exhibit strong winds during core helium burning. This implies that either their mass-loss rates are underestimated or that Type Ic SN progenitors experience mass loss through other mechanisms before exploding. The distributions of ejecta masses, explosion energies, and nickel mass for Type Ib SNe are not well reproduced by progenitor models with WR mass loss, but are better reproduced if we assume no mass loss in progenitors with luminosities below the minimum WR star luminosity. We find that Type Ic SNe become more common as metallicity increases, and that the vast majority of progenitors of Type Ib SNe must be transparent-wind stripped-envelope stars. We find that several models with pre-collapse CO masses of up to ∼30 M⊙ may form ∼3 M⊙ BHs in fallback SNe. This may have important consequences for our understanding of SNe, binary BH and NS systems, X-ray binary systems, and gravitational wave transients.

Exploring the influence of different velocity fields on Wolf–Rayet star spectra

ABSTRACT Given their strong stellar winds, Wolf–Rayet (WR) stars exhibit emission line spectra that are predominantly formed in expanding atmospheric layers. The description of the wind velocity field $\upsilon (r)$ is therefore a crucial ingredient in the spectral analysis of WR stars, possibly influencing the determination of stellar parameters. In view of this, we perform a systematic study by simulating a sequence of WR-star spectra for different temperatures and mass-loss rates using β-type laws with 0.5 ≤ β ≤ 20. We quantify the impact of varying $\upsilon (r)$ by analysing diagnostic lines and spectral classifications of emergent model spectra computed with the Potsdam Wolf–Rayet (PoWR) code. We additionally cross-check these models with hydrodynamically consistent – hydro – model atmospheres. Our analysis confirms that the choice of the β exponent has a strong impact on WR-star spectra, affecting line widths, line strengths, and line profiles. In some parameter regimes, the entire range of WR subtypes could be covered. Comparison with observed WR stars and hydro models revealed that values of β ≳ 8 are unlikely to be realized in nature, but a range of β values needs to be considered in spectral analysis. UV spectroscopy is crucial here to avoid an underestimation of the terminal velocity $\upsilon _\infty$. Neither single- nor double-β descriptions yield an acceptable approximation of the inner wind when compared to hydro models. Instead, we find temperature shifts to lower T2/3 when employing a hydro model. Additionally, there are further hints that round-lined profiles seen in several early WN stars are an effect from non-β velocity laws.

Line luminosities of Galactic and Magellanic Cloud Wolf–Rayet stars

ABSTRACT We provide line luminosities and spectroscopic templates of prominent optical emission lines of 133 Galactic Wolf–Rayet (WR) stars by exploiting Gaia DR3 parallaxes and optical spectrophotometry, and provide comparisons with 112 counterparts in the Magellanic Clouds. Average line luminosities of the broad blue (He ii λ4686, C iii λλ4647,51, N iii λλ4634,41, and N v λλ4603,20) and yellow (C iv λλ5801,12) emission features for WN, WN/C, WC, and WO stars have application in characterizing the WR populations of star-forming regions of distant, unresolved galaxies. Early-type WN stars reveal lower line luminosities in more metal-poor environments, but the situation is less clear for late-type WN stars. LMC WC4–5 line luminosities are higher than their Milky Way counterparts, with line luminosities of Magellanic Cloud WO stars higher than Galactic stars. We highlight other prominent optical emission lines, N iv λλ3478,85 for WN and WN/C stars, O iv λλ3403,13 for WC and WO stars, and O vi λλ3811,34 for WO stars. We apply our calibrations to representative metal-poor and metal-rich WR galaxies, IC 4870 and NGC 3049, respectively, with spectral templates also applied based on a realistic mix of subtypes. Finally, the global blue and C iv λλ5801,12 line luminosities of the Large Magellanic Clouds or LMCs (Small Magellanic Clouds) are 2.6 × 1038 erg s−1 (9 × 1036 erg s−1) and 8.8 × 1037 erg s−1 (4 × 1036 erg s−1), respectively, with the cumulative WR line luminosity of the Milky Way estimated to be an order of magnitude higher than the LMC.