Wolf-Rayet Stars Research Articles

The X-Ray Luminous Type Ibn SN 2022ablq: Estimates of Preexplosion Mass Loss and Constraints on Precursor Emission

Type Ibn supernovae (SNe Ibn) are rare stellar explosions powered primarily by interaction between the SN ejecta and H-poor, He-rich material lost by their progenitor stars. Multiwavelength observations, particularly in the X-rays, of SNe Ibn constrain their poorly understood progenitor channels and mass-loss mechanisms. Here we present Swift X-ray, ultraviolet, and ground-based optical observations of the Type Ibn SN 2022ablq, only the second SN Ibn with X-ray detections to date. While similar to the prototypical Type Ibn SN 2006jc in the optical, SN 2022ablq is roughly an order of magnitude more luminous in the X-rays, reaching unabsorbed luminosities L X ∼ 4 × 1040 erg s−1 between 0.2–10 keV. From these X-ray observations we infer time-varying mass-loss rates between 0.05 and 0.5 M ⊙ yr−1 peaking 0.5–2 yr before explosion. This complex mass-loss history and circumstellar environment disfavor steady-state winds as the primary progenitor mass-loss mechanism. We also search for precursor emission from alternative mass-loss mechanisms, such as eruptive outbursts, in forced photometry during the 2 yr before explosion. We find no statistically significant detections brighter than M ≈ −14—too shallow to rule out precursor events similar to those observed for other SNe Ibn. Finally, numerical models of the explosion of an ∼15 M ⊙ helium star that undergoes an eruptive outburst ≈1.8 yr before explosion are consistent with the observed bolometric light curve. We conclude that our observations disfavor a Wolf–Rayet star progenitor losing He-rich material via stellar winds and instead favor lower-mass progenitor models, including Roche-lobe overflow in helium stars with compact binary companions or stars that undergo eruptive outbursts during late-stage nucleosynthesis stages.

2019odp -- A massive oxygen-rich Type Ib supernova

Stripped envelope (SE) supernovae are explosions of stars that have somehow lost most of their outer envelopes. We present the discovery and analyse the observations of the supernova 2019odp (a.k.a. ZTF19abqwtfu) covering epochs within days of the explosion to late nebular phases at 360\,d post-explosion. Our observations include an extensive set of photometric observations and low- to medium-resolution spectroscopic observations, both covering the complete observable time range. We analysed the data using analytic models for the recombination cooling emission of the early excess emission and the diffusion of the peak light curve. We expanded on existing methods to derive oxygen mass estimates from nebular phase spectroscopy, and briefly discuss progenitor models based on this analysis. Our spectroscopic observations confirm the presence of He in the supernova ejecta and we thus (re)classify SN 2019odp as a supernova . From the pseudo-bolometric light curve, we estimate a high ejecta mass of $M_ ej 4 - 7 M_ The high ejecta mass, large nebular O I Ca II $ line flux ratio ($1.2 - 1.9$), and an oxygen mass above $ 0.5\, M_ point towards a progenitor with a pre-explosion mass higher than $18\,M_ Whereas a majority of analysed SE supernovae in the literature seem to have low ejecta masses, indicating stripping in a binary star system, SN 2019odp instead has parameters that are consistent with an origin in a single massive star. The compact nature of the progenitor ($ 10\,R_ suggests that a Wolf-Rayet star is the progenitor.

Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY. Uncovering a long-period binary desert

Wolf-Rayet stars (WRs) represent one of the final evolutionary stages of massive stars and are thought to be the immediate progenitors of stellar-mass black holes. Their multiplicity characteristics form 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 both short and long orbital periods, in contradiction with evolutionary model predictions. In this work, we employed long-baseline infrared interferometry to investigate the multiplicity of WRs at long periods and explored the nature of their companions. We present a magnitude-limited ($K<9$; $V<14$) survey of 39 Galactic WRs, including 11 WN, 15 WC, and 13 H-rich WN (WNh) stars. We used the $K$-band instrument GRAVITY at the Very Large Telescope Interferometer (VLTI) in Chile. The sensitivity of GRAVITY at spatial scales of sim 1--200 milliarcseconds and flux contrast of $1<!PCT!>$ allowed an exploration of periods in the range $10^ $ d and companions down to sim 5$\,M_ We carried out a companion search for all our targets, with the aim of either finding wide companions or calculating detection limits. We also explored the rich GRAVITY dataset beyond a multiplicity search to look for other interesting properties of the WR sample. We detected wide companions with VLTI/GRAVITY for only four stars in our sample: WR 48, WR 89, WR 93, and WR 115. Combining our results with spectroscopic studies, we arrived at observed multiplicity fractions of $f^ WN obs WC obs and WNh obs The multiplicity fractions and period distributions of WNs and WCs are consistent in our sample. For single WRs, we placed upper limits on the mass of potential companions down to sim 5$\,M_ for WNs and WCs, and sim 7$\,M_ for WNh stars. In addition, we also found other features in the GRAVITY dataset, such as (i) a diffuse extended component contributing significantly to the $K$-band flux in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data, which constitutes an alternative detection method for close binaries; and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78, and WR 110. Our survey reveals a lack of intermediate- (a few hundred days) and long- (a few years to decades) period WR systems. The 200d 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 suggests 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.

The X-ray re-brightening of GRB afterglow revisited: a possible signature from activity of the central engine

ABSTRACT Long-duration gamma-ray bursts (GRBs) are thought to be from core collapse of massive stars, and a rapidly spinning magnetar or black hole may be formed as the central engine. The extended emission in the prompt emission, flares, and plateaus in X-ray afterglow, are proposed to be as the signature of central engine re-activity. However, the direct evidence from observations of identifying the central engines remains an open question. In this paper, we systemically search for long-duration GRBs that consist of bumps in X-ray afterglow detected by Swift/XRT and find that the peak time of the X-ray bumps exhibit bimodal distribution (defined as ‘early’ and ‘late’ bumps) with division line at $t=7190$ s. Although we cannot rule out that such a bimodality arises from selection effects. We proposed that the long-duration GRBs with an early (or late) bumps may be originated from the fall-back accretion onto a new-born magnetar (or black hole). By adopting Monte Carlo Markov Chain (MCMC) method to fit the early (or late) bumps of X-ray afterglow with the fall-back accretion of magnetar (or black hole), it is found that the initial surface magnetic field and period of magnetars for most early bumps are clustered around $5.88\times 10^{13}$ G and 1.04 ms, respectively. Meanwhile, the derived accretion mass of black hole for late bumps is in the range of $[4\times 10^{-4}, 1.8\times 10^{-2}]\,{\rm M}_{\odot }$, and the typical fall-back radius is distributed range of $[1.04, 4.23]\times 10^{11}$ cm, which is consistent with the typical radius of a Wolf–Rayet star. However, we also find that the fall-back accretion magnetar model is disfavoured by the late bumps, but the fall-back accretion of black hole model cannot be ruled out to interpret the early bumps of X-ray afterglow.

Evidence for Type Ib/c Origin of the Supernova Remnant G292.0+1.8

Circumstellar material (CSM) produced by mass loss from massive stars (≳10 M ⊙) through strong stellar winds or binary stripping provides rich information for understanding progenitors of core-collapse supernova remnants. In this paper we present a grating spectroscopy of a Galactic SNR G292.0+1.8, which is claimed to be a Type Ib/c remnant in a binary system according to recent studies. If G292.0+1.8 had experienced a strong mass-loss via binary interactions before its explosion, an oxygen-rich material produced in the He-burning layer is expected to be observed in the central belt-like structure formed by shock-heated CSM. Using the Reflection Grating Spectrometer onboard XMM-Newton, we detect N vii Lyα line (0.50 keV) for the first time in G292.0+1.8 and find that the abundance ratio of nitrogen to oxygen is significantly lower (N/O = 0.5 ± 0.1) than the solar value. This low N/O suggests that the progenitor of G292.0+1.8 experienced strong mass-loss and ended up as a Wolf–Rayet star exposing the He-burning layer at the presupernova. Comparing our result and the evolution models of single stars and binaries, we conclude that the progenitor of G292.0+1.8 experienced strong enough mass-loss to occur a Type Ib/c supernova. Our finding is another crucial piece of evidence for a stripped-envelope supernova such as Type Ib/c as the origin of G292.0+1.8.

Exploring the boundary between stable mass transfer and L2 overflow in close binary evolution

The majority of massive stars resides in binary systems, which are expected to experience mass transfer during their evolution. However, the conditions under which mass transfer leads to a common envelope, and thus possibly to a merging of both stars, are currently only poorly understood. The main uncertainties arise from the possible swelling of the mass gainer and from angular momentum loss from the binary system during non-conservative mass transfer. We have computed a dense grid of detailed models of stars that accrete mass at constant rates to determine the radius increase that is due to their thermal disequilibrium. While we find that models with an accretion that is faster than the thermal timescale expand in general, this expansion remains quite limited in the intermediate-mass regime even for accretion rates that exceed the thermal timescale accretion rate by a factor of 100. Our models of massive stars expand to extreme radii under these conditions. When the accretion rate exceed the Eddington accretion rate, our models expand rapidly. We derived analytical fits to the radius evolution of our models and a prescription for the boundary between stable mass transfer and L2 overflow for arbitrary accretion efficiencies. We then applied our results to grids of binary models adopting various constant mass-transfer efficiencies and angular momentum budgets. We find that the first parameter affects the outcome of the Roche-lobe overflow more strongly. Our results are consistent with detailed binary evolution models and often lead to a smaller initial parameter space for stable mass transfer than do other recipes in the literature. We used this method to investigate the origin of Wolf-Rayet stars with O star companions in the Small Magellanic Cloud, and we found that the efficiency of the mass transfer process that led to the formation of the Wolf-Rayet star was likely lower than 50%.

Multi-wavelength observations of the luminous fast blue optical transient AT2023fhn. Up to ∼200 days post-explosion

Luminous fast blue optical transients (LFBOTs) are a class of extragalactic transients notable for their rapid rise and fade times, blue colour, and accompanying luminous X-ray and radio emission. Only a handful have been studied in detail since the prototypical example AT\,2018cow. Their origins are currently unknown, but ongoing observations of previous and new events are placing ever stronger constraints on their progenitors. We aim to put further constraints on the LFBOT AT\,2023fhn, and LFBOTs as a class, using information from the multi-wavelength transient light curve, its host galaxy, and local environment. Our primary results were obtained by fitting galaxy models to the spectral energy distribution of AT\,2023fhn's host and local environment, and by modelling the radio light curve of AT\,2023fhn as due to synchrotron self-absorbed emission from an expanding blast wave in the circumstellar medium. We find that neither the host galaxy nor circumstellar environment of AT\,2023fhn are unusual compared with previous LFBOTs, but that AT\,2023fhn has a much lower X-ray to ultraviolet luminosity ratio than previous events. We argue that the variety in ultraviolet-optical to X-ray luminosity ratios among LFBOTs is likely due to viewing angle differences, and that the diffuse, yet young local environment of AT\,2023fhn - combined with a similar circumstellar medium to previous events - favours a progenitor system containing a massive star with strong winds. Plausible progenitor models in this interpretation therefore include the mergers of black holes and Wolf-Rayet stars or failed supernovae.

Binary Yellow Supergiants in the Magellanic Clouds. I. Photometric Candidate Identification

Recent works have constrained the binary fraction of evolved populations of massive stars in local galaxies such as red supergiants and Wolf–Rayet stars, but the binary fraction of yellow supergiants (YSGs) in the Hertzsprung gap remains unconstrained. Binary evolution theory predicts that the Hertzsprung gap is home to multiple populations of binary systems with varied evolutionary histories. In this paper, we develop a method to distinguish single YSGs from YSG plus O- or B-type main-sequence binaries using optical and ultraviolet photometry, and then apply this method to identify candidate YSG binaries in the Magellanic Clouds. After constructing a set of combined stellar atmosphere models, we find that optical photometry is, given typical measurement and reddening uncertainties, sufficient to discern single YSGs from YSG+OB binaries if the OB-star is at least ∼5M ⊙ for T eff,YSG ∼ 4000 K, but requires a ∼20M ⊙ OB star for YSGs up to T eff,YSG ∼ 9000 K. For these hotter YSG temperatures, ultraviolet photometry allows binaries with OB companions as small as ∼7M ⊙ to be identified. We use color–color spaces developed from these models to search for evidence of excess blue or ultraviolet light in a set of ∼1000 YSG candidates in the Magellanic Clouds. We identify hundreds of candidate YSG binary systems and report a preliminary fraction of YSGs that show a blue/UV color excess of 20%–60%. Spectroscopic follow-up is now required to confirm the true nature of this population.

GA-NIFS: an extremely nitrogen-loud and chemically stratified galaxy at z ~ 5.55

ABSTRACT We report the chemical abundance pattern of GS_3073, a galaxy hosting an overmassive active black hole at $z=5.55$, by leveraging observations from JWST/NIRSpec and Very Large Telescope/VIsible Multi-Object Spectrograph. Based on the rest-frame ultraviolet (UV) emission lines, which trace high-density ($\sim 10^5~{\rm cm^{-3}}$) and highly ionized gas, we derive $\rm \log (N/O) = 0.42^{+0.13}_{-0.10}$. At an estimated metallicity of $0.2~Z_{\odot }$, this is the most extreme nitrogen-rich object found by JWST thus far. In comparison, the relative carbon abundance derived is $\rm \log (C/O) = -0.38^{+0.13}_{-0.11}$, which is not significantly higher than those in local galaxies and stars with similar metallicities. We also found potential detection of [Fe vii]$\lambda 6087$ and [Fe xiv]$\lambda 5303$, both blended with [Ca v]. We inferred a range of Fe abundances compatible with those in local stars and galaxies. Overall, the chemical abundance pattern of GS_3073 is compatible with enrichment by supermassive stars with $M_* \gtrsim 1000~M_\odot$, asymptotic giant branch stars, or Wolf–Rayet stars. Interestingly, when using optical emission lines that trace lower density ($\sim 10^3~{\rm cm}^{-3}$) and lower ionization gas, we found a sub-solar N/O ratio, consistent with local galaxies at the same metallicity. We interpret the difference in N/O derived from UV lines and optical lines as evidence for a stratified system, where the inner and denser region is both more chemically enriched and more ionized. Our results suggest that nitrogen loudness in high-z galaxies might be confined to the central, dense, and highly ionized regions of the galaxies, while the bulk of the galaxies evolves more normally.

A trifecta of modelling tools: a Bayesian binary black hole model selection combining population synthesis and galaxy formation models

ABSTRACT Gravitational waves (GWs) have revealed surprising properties of binary black hole (BBH) populations, but there is still mystery surrounding how these compact objects evolve. We apply Bayesian inference and an efficient method to calculate the BBH merger rates in the Shark host galaxies, to determine the combination of COMPAS parameters that outputs a population most like the GW sources from the LIGO, Virgo, and KAGRA (LVK) transient catalogue. For our COMPAS models, we calculate the likelihood with and without the dependence on the predicted number of BBH merger events. We find strong correlations between hyper-parameters governing the specific angular momentum (AM) of mass lost during mass transfer, the mass-loss rates of Wolf–Rayet stars via winds and the chemically homogeneous evolution (CHE) formation channel. We conclude that analysing the marginalized and unmarginalized likelihood is a good indicator of whether the population parameters distribution and number of observed events reflect the LVK data. In doing so, we see that the majority of the models preferred in terms of the population-level parameters of the BBHs greatly overpredict the number of events we should have observed to date. Looking at the smaller number of models that perform well with both likelihoods, we find that those with no CHE, AM loss occurring closer to the donor during the first mass-transfer event, and/or higher rates of mass-loss from Wolf–Rayet winds are generally preferred by current data. We find these conclusions to be robust to our choice of selection criteria.

Variable Stars in M31 Stellar Clusters from the Panchromatic Hubble Andromeda Treasury

Variable stars in stellar clusters can offer key constraints on stellar evolution and pulsation models, utilizing estimates of host cluster properties to constrain stellar physical parameters. We present a catalog of 86 luminous (F814W < 19) variable stars in M31 clusters identified by mining the archival Panchromatic Hubble Andromeda Treasury (PHAT) survey using a combination of statistical analysis of sparse PHAT light curves and difference imaging. We determine the evolutionary phases and initial masses of these variable stars by matching them with theoretical isochrones generated using host cluster properties from the literature. We calculate the probability of PHAT photometry being blended due to the highly crowded nature of cluster environments for each cluster-variable star, using these probabilities to inform our level of confidence in the derived properties of each star. Our 86 cluster-variable stars have initial masses between 0.8 and 67 M ⊙. Their evolutionary phases span the main sequence, more evolved hydrogen- and helium-burning phases, and the post–asymptotic giant branch. We identify numerous candidate variable star types: RV Tauri variables, red supergiants, and slowly pulsating B-type supergiants, along with Wolf–Rayet stars, α Cygni and Mira variables, a classical Cepheid, and a possible superasymptotic giant. We characterize 12 cluster-variable stars at higher confidence based on their difference image quality and lower blending probability. Ours is the first systematic study of variable stars in extragalactic stellar clusters leveraging the superior resolution of the Hubble Space Telescope and demonstrating the unique power of stellar clusters in constraining the fundamental properties of variable stars.

Multistructured Accretion Flow of Sgr A*. II. Signatures of a Cool Accretion Disk in Hydrodynamic Simulations of Stellar Winds

Hydrodynamic simulations of the stellar winds from Wolf–Rayet stars within the Galactic center can provide predictions for the X-ray spectrum of the supermassive black hole Sgr A*. Herein, we present results from updated smooth particle hydrodynamics simulations, building on the architecture of Cuadra et al. and Russell et al., and find that a “cold” (104 K) gas disk forms around Sgr A* with a simulation runtime of 3500 yr. This result is consistent with previous grid-based simulations, demonstrating that a cold disk can form regardless of numerical method. We examine the plasma scenarios arising from an environment with and without this cold disk, by generating synthetic spectra for comparison to the quiescent Fe Kα Sgr A* spectrum from Chandra HETGS, taken through the Chandra X-ray Visionary Program. We find that current and future X-ray missions are unlikely to distinguish between the kinematic signatures in the plasma in these two scenarios. Nonetheless, the stellar wind plasma model presents a good fit to the dispersed Chandra spectra within 1.″5 of Sgr A*. We compare our results to the radiatively inefficient accretion flow (RIAF) model fit to the HETGS spectrum presented in Paper I and find that the Bayesian model evidence does not strongly favor either model. With 9″ angular resolution and high spectral resolution of the X-IFU, NewAthena will offer a clearer differentiation between the RIAF plasma model and hydrodynamic simulations, but only a future X-ray mission with arcsecond resolution will significantly advance our understanding of Sgr A*’s accretion flow in X-rays.

The Sunburst Arc with JWST

We report the detection of a population of Wolf-Rayet (WR) stars in the Sunburst Arc, a strongly gravitationally lensed galaxy at redshift z = 2.37. As the brightest known lensed galaxy, the Sunburst Arc has become an important cosmic laboratory for studying star and cluster formation, Lyman α (Lyα) radiative transfer, and Lyman continuum (LyC) escape. Here, we present the first results of JWST/NIRSpec IFU observations of the Sunburst Arc, focusing on a stacked spectrum of the 12-fold imaged Sunburst LyC-emitting (LCE) cluster. In agreement with previous studies, we find that the Sunburst LCE cluster is a very massive, compact star cluster with Mdyn = (9 ± 1)×106 M⊙. Our age estimate of 4.2–4.5 Myr is much larger than the crossing time of tcross = 183 ± 9 kyr, indicating that the cluster is dynamically evolved and consistent with it being gravitationally bound. We find a significant nitrogen enhancement of the low ionization state interstellar medium (ISM), with log(N/O) = − 0.74 ± 0.09, which is ≈0.8 dex above typical values for H II regions of a similar metallicity in the local Universe. We find broad stellar emission complexes around He IIλ4686 and C IVλ5808 with associated nitrogen emission; this is the first time WR signatures have been directly observed at redshifts above ∼0.5. The strength of the WR signatures cannot be reproduced by stellar population models that only include single-star evolution. While models with binary evolution better match the WR features, they still struggle to reproduce the nitrogen-enhanced WR features. JWST reveals the Sunburst LCE cluster to be a highly ionized proto-globular cluster with low oxygen abundance and extreme nitrogen enhancement that hosts a population of WR stars, likely including a previously suggested population of very massive stars (VMSs), which together are rapidly enriching the surrounding medium.

Mapping Multiphase Metals in Star-forming Galaxies: A Spatially Resolved UV+Optical Study of NGC 5253

We present a pioneering, spatially resolved, multiphase gas abundance study on the blue compact dwarf galaxy NGC 5253, targeting 10 star-forming (SF) clusters inside six far-UV (FUV) Hubble Space Telescope/Cosmic Origins Spectrograph (COS) pointings with cospatial optical Very Large Telescope/MUSE observations throughout the galaxy. The SF regions span a wide range of ages (1–15 Myr) and are distributed at different radii (50–230 pc). We performed a robust absorption-line profile fitting on the COS spectra, covering 1065–1430 Å in the FUV, allowing an accurate computation of neutral-gas abundances for 13 different ions sampling eight elements. These values were then compared with the ionized-gas abundances, measured using the direct method on MUSE integrated spectra inside analog COS apertures. Our multiphase, spatially resolved comparisons find abundances, which are lower in the neutral gas than the ionized gas by 0.22, 0.80, and 0.58 dex for log(O/H), log(N/H), and log(N/O), respectively. We modeled the chemical abundance distributions and evaluated correlations as a function of the radius and age. It was found that, while N, O, and N/O abundances decrease as a function of age in the ionized gas, they increase with age in the neutral gas. No strong correlations for N, O, or N/O were observed as a function of the radius. The N/O and N/H offsets between the phases were found to decrease with age, providing evidence that chemical enrichment happens differentially, first in the ionized-gas phase around 2–5 Myrs (due to N-rich Wolf–Rayet stars) and then mixing out into the cold neutral gas on longer timescales of 10–15 Myr.

EWOCS-II: X-ray properties of the Wolf–Rayet stars in the young Galactic super star cluster Westerlund 1

Context. Wolf–Rayet (WR) stars are massive evolved stars that exhibit particularly fast and dense stellar winds. Although they constitute a very short phase near the end of a massive star’s life, they play a crucial role in the evolution of massive stars and have a substantial impact on their surrounding environment. Aims. We present the most comprehensive and deepest X-ray study to date of the properties of the richest Wolf–Rayet population observed in a single stellar cluster, Westerlund 1 (Wd1). By examining the X-ray signatures of WR stars, we aim to shed light on the hottest plasma in their stellar winds and gain insights into whether they exist as single stars or within binary systems. Methods. This work is based on 36 Chandra observations obtained from the “Extended Westerlund 1 and 2 Open Clusters Survey” (EWOCS) project, plus 8 archival Chandra observations. The overall exposure depth Ms) and baseline of the EWOCS observations extending over more than one year enable us to perform a detailed photometric, colour, and spectral analysis, as well as to search for short- and long-term periodicity. Results. In X-rays, we detect 20 out of the 24 known Wolf–Rayet stars in Wd1 down to an observed luminosity of ~7 × 1029 erg s−1 (assuming a distance of 4.23 kpc to Wd1), with 8 WR stars being detected in X-rays for the first time. Nine stars show clear evidence of variability over the year-long baseline, with clear signs of periodicity. The X-ray colours and spectral analysis reveal that the vast majority of the WR stars are hard X-ray sources (kT≥2.0 keV). The Fe XXV emission line at ~6.7 keV, which commonly originates from the wind–wind collision zone in binary systems, is detected for the first time in the spectra of 17 WR stars in Wd1. In addition the ~6.4 keV fluorescent line is observed in the spectra of three stars, which are among the very few massive stars exhibiting this line, indicating that dense cold material coexists with the hot gas in these systems. Overall, our X-ray results alone suggest a very high binary fraction (≥80%) for the WR star population in Wd1. When combining our results with properties of the WR population from other wavelengths, we estimate a binary fraction of ≥92%, which could even reach unity. This suggests that either all the most massive stars are found in binary systems within Wd1, or that binarity is essential for the formation of such a rich population of WR stars.

Chemical evolution of a young super star cluster at the Sunburst Arc

ABSTRACT Recent observations of high-redshift galaxies have revealed starburst galaxies with excessive amounts of nitrogen, well above that expected in standard evolutionary models. The Sunburst Arc galaxy, particularly its young and massive star cluster, represents the closest ($z=2.4$) and brightest of these as a strongly lensed object. In this work, we study the chemical history of this star cluster to determine the origin of the elevated gas-phase nitrogen using a chemical evolution model. Our model includes the enrichment of OB stars through stellar winds and core-collapse supernovae assuming that massive stars ($M\gt 25$ $\mathrm{ M}_\odot$) collapse directly into black holes at the end of their lives. We fit the model parameters to the observed chemical abundances of the Sunburst Arc cluster: O/H, C/O, and N/O. We find that the observed chemical abundances can be explained by models featuring intense star formation events, characterized by rapid gas accretion and high star formation efficiencies. Additionally, the stellar population contributing to the gas enrichment must exclude Wolf–Rayet stars. These conditions might be present in other nitrogen-rich objects as their similar chemical abundances suggest a common history. As previous studies have proposed the presence of Wolf–Rayet stars in the new nitrogen-rich objects, further research using chemodynamic modeling is necessary to ascertain the true nature of these objects.

The Discovery of Three Galactic Wolf–Rayet Stars

Wolf–Rayet stars (WRs) are evolved massive stars in the brief stage before they undergo core collapse. Not only are they rare, but they also can be particularly difficult to find due to the high extinction in the Galactic plane. This paper discusses the discovery of three new Galactic WRs previously classified as Hα emission stars, but thanks to Gaia spectra, we were able to identify the broad, strong emission lines that characterize WRs. Using the Lowell Discovery Telescope and the DeVeny spectrograph, we obtained spectra for each star. Two are WC9s, and the third is a WN6 + O6.5 V binary. The latter is a known eclipsing system with a 4.4 day period from ASAS-SN data. We calculate absolute visual magnitudes for all three stars to be between −7 and −6, which is consistent with our expectations of these subtypes. These discoveries highlight the incompleteness of the WR census in our local volume of the Milky Way and suggest the potential for future Galactic WR discoveries from Gaia low-dispersion spectra. Furthermore, radial velocity studies of the newly found binary will provide direct mass estimates and orbital parameters, adding to our knowledge of the role that binarity plays in massive star evolution.

Massive stars exploding in a He-rich circumstellar medium. X. Flash spectral features in the Type Ibn SN 2019cj and observations of SN 2018jmt

We present optical and near-infrared observations of two Type Ibn supernovae (SNe), SN\,2018jmt and SN\,2019cj. Their light curves have rise times of about ten days, reaching an absolute peak magnitude of $M_g$(SN\,2018jmt) = $-$19.07 pm 0.37 and $M_V$(SN\,2019cj) = $-$18.94 pm 0.19 mag, respectively. The early-time spectra of SN\,2018jmt are dominated by a blue continuum, accompanied by narrow (600$-$1000 km $) He i lines with the P-Cygni profile. At later epochs, the spectra become more similar to those of the prototypical SN Ibn 2006jc. At early phases, the spectra of SN\,2019cj show flash ionisation emission lines of C iii N iii and He ii superposed on a blue continuum. These features disappear after a few days, and then the spectra of SN\,2019cj evolve similarly to those of SN\,2018jmt. The spectra indicate that the two SNe exploded within a He-rich circumstellar medium (CSM) lost by the progenitors a short time before the explosion. We modelled the light curves of the two SNe Ibn to constrain the progenitor and the explosion parameters. The ejecta masses are consistent with either what is expected for a canonical SN Ib (sim 2 odot $) or for a massive Wolf Rayet star ($&gt;$ sim 4 M$_ odot $), with the kinetic energy on the order of $10^ $ erg. The lower limit on the ejecta mass ($&gt;$ sim 2 M odot $) argues against a scenario involving a relatively low-mass progenitor (e.g. $M_ ZAMS $ sim 10 M$_ odot $). We set a conservative upper limit of sim 0.1 M$_ odot $ for the 56Ni masses in both SNe. From the light curve modelling, we determined a two-zone CSM distribution, with an inner, flat CSM component and an outer CSM with a steeper density profile. The physical properties of SN\,2018jmt and SN\,2019cj are consistent with those expected from the core collapse of relatively massive envelope-stripped stars.

An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud

To predict black hole mass distributions at high redshifts, we need to understand whether very massive single stars (M ≳ 40 M⊙) with low metallicities (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 undertook a deep spectroscopic search for binary companions of the seven known apparently single Wolf-Rayet (WR) stars in the Small Magellanic Cloud (SMC; where Z ≃ 1/5 Z⊙). For each of them, we acquired six high-quality VLT-UVES spectra spread over a time period of 1.5 years. By using the narrow N V lines in these spectra, we monitored radial velocity (RV) variations to search for binary motion. We find low RV variations of 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% that any of our target WR stars have a binary companion more massive than ∼5 M⊙ with 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 ≳ 40 M⊙ 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 RV of ∼80 km/s. Moreover, with all five previously detected WR binaries in the SMC exhibiting orbital periods of less than 20 d, a puzzling absence of intermediate-to-long-period WR binaries has emerged, with strong implications for the outcome of massive binary interactions at low metallicities.

Spectroscopic Observations of Ten Galactic Wolf–Rayet Stars at Bosscha Observatory: Determination of Stellar Parameters and Mass-loss Rates

We present optical spectra of 10 Galactic Wolf–Rayet (WR) stars that consist of five WN and five WC stars. The optical observation was conducted using a low-resolution spectrograph NEO-R1000 (λ/Δλ ∼ 1000) at GAO-ITB RTS (27.94 cm, F/10.0), Bosscha Observatory, Lembang. We implemented stellar atmosphere Postdam Wolf–Rayet (PoWR) grid modeling to derive stellar parameters. The normalized optical spectrum can be used to find the best model from the available PoWR grid, then we could derive stellar temperature and transformation radius. To derive luminosity, stellar radius and color excess, we conducted a Spectral Energy Distribution (SED) analysis with additional data on the near-ultraviolet spectrum from the International Ultraviolet Explorer (IUE) database, and UBV and 2MASS JHK broadband filter data. Additional analysis to derive asymptotic terminal wind velocity was conducted from the P-Cygni profile analysis of the high-resolution IUE ultraviolet spectrum. With previously derived parameters, we could determine the mass loss rate of the WR stars. Furthermore, we compared our results with previous work that used PoWR code and the differences are not more than 20%. We conclude that the PoWR spectral grid is sufficient to derive WR stellar parameters quickly and could provide more accurate initial parameter input to the PoWR program code.