Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity ...Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (
Z
). The
Hubble
Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low
Z
in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO’s Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of
Z
. As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low
Z
. To be able to confidently interpret
James Webb
Space Telescope spectra of the first stellar generations, the individual spectra of low-
Z
stars need to be understood, which is exactly where XShootU can deliver.
Abstract
Striking profile variations of the C iii λ5696 emission line are visible amongst the high signal-to-noise ratio, moderate resolution spectra of the 29.7 d WC8d+O8-9IV binary CV Ser (WR 113) ...presented here. Using a significantly revised code, we have modelled these variations assuming the emission originates from the undisturbed WR star wind and a colliding wind shock region that partially wraps around the O star. Changes to the modelling code are chiefly in the form of additional parameters, intended to refine the modelling and facilitate comparison with recent predictions arising from theoretical and hydrodynamical work. This modelling provides measurements of crucial parameters such as the orbital inclination (63.5° ± 2.5°) and thus, together with the RV orbits, the stellar masses (11.7 ± 0.9 M⊙ for the WR star and 33.3 ± 2.0 M⊙ for the O star). We find good agreement with expectations based on theoretical studies and hydrodynamical modelling of colliding wind systems. Moreover, it raises the exciting prospect of providing a reliable method to learn more about WR stellar masses and winds, and for studying the physics of colliding winds in massive stars.
We report the results of a spectroscopic and polarimetric study of the massive, hydrogen-rich WN6h stars R144 (HD 38282 = BAT99-118 = Brey 89) and R145 (HDE 269928 = BAT99-119 = Brey 90) in the Large ...Magellanic Cloud. Both stars have been suspected to be binaries by previous studies (R144: Schnurr et al.; R145: Moffat). We have combined radial-velocity (RV) data from these two studies with previously unpublished polarimetric data. For R145, we were able to establish, for the first time, an orbital period of 158.8 d, along with the full set of orbital parameters, including the inclination angle i, which was found to be i= 38°± 9°. By applying a modified version of the shift-and-add method developed by Demers et al., we were able to isolate the spectral signature of the very faint line companion star. With the RV amplitudes of both components in R145, we were thus able to estimate their absolute masses. We find minimum masses MWRsin3i= 116 ± 33 M⊙ and MOsin3i= 48 ± 20 M⊙ for the WR and the O component, respectively. Thus, if the low-inclination angle were correct, resulting absolute masses of the components would be at least 300 and 125 M⊙, respectively. However, such high masses are not supported by brightness considerations when R145 is compared to systems with known very high masses such as NGC 3603-A1 or WR20a. An inclination angle close to 90° would remedy the situation, but is excluded by the currently available data. More and better data are thus required to firmly establish the nature of this puzzling, yet potentially very massive and important system. As to R144, however, the combined data sets are not sufficient to find any periodicity.
We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6 m Canada–France–Hawaii Telescope ...within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17+19−9M⊙ star with a radius of 7.0+2.4−1.8R⊙ and a relatively low mass loss rate of 1.4+3.1−0.95× 10−9M⊙yr−1. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sini= 15 ± 3 kms−1. This v sini implies a maximum rotational period of 31.5d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes V profiles and longitudinal field. Using a Bayesian analysis of the velocity resolved Stokes V profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of 1680+134−356G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.
We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf–Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This ...survey concludes the decade-long effort of the Montréal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the six crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial velocity (RV) variability in order to identify the short- to intermediate-period (P≲ 200 d) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries is classical, helium-burning WNL star, but rather superluminous, hence extremely massive, hydrogen-burning object, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group.
This study is the second part of a survey searching for large-scale spectroscopic variability in apparently single Wolf-Rayet (WR) stars. In a previous paper (Paper I), we described and characterized ...the spectroscopic variability level of 25 WR stars observable from the northern hemisphere and found 3 new candidates presenting large-scale wind variability, potentially originating from large-scale structures named corotating interaction regions (CIRs). In this second paper, we discuss an additional 39 stars observable from the southern hemisphere. For each star in our sample, we obtained 4-5 high-resolution spectra with a signal-to-noise ratio of ~100 and determined its variability level using the approach described in Paper I. In total, 10 new stars are found to show large-scale spectral variability of which 7 present CIR-type changes (WR 8, WR 44, WR55, WR 58, WR 61, WR 63, WR 100). Of the remaining stars, 20 were found to show small-amplitude changes and 9 were found to show no spectral variability as far as can be concluded from the data on hand. Also, we discuss the spectroscopic variability level of all single galactic WR stars that are brighter than v ~ 12.5, and some WR stars with 12.5 < v <= 13.5, i.e., all the stars presented in our two papers and four more stars for which spectra have already been published in the literature. We find that 23/68 stars (33.8%) present large-scale variability, but only 12/54 stars (~22.1%) are potentially of CIR type. Also, we find that 31/68 stars (45.6%) only show small-scale variability, most likely due to clumping in the wind. Finally, no spectral variability is detected based on the data on hand for 14/68 (20.6%) stars. Interestingly, the variability with the highest amplitude also has the widest mean velocity dispersion.
Using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared, we have obtained repeated adaptive-optics-assisted, near-infrared spectroscopy of the six central ...luminous, Wolf–Rayet (WR) stars in the core of the very young (∼1 Myr), massive and dense cluster R136, in the Large Magellanic Cloud (LMC). We also de-archived available images that were obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph, and extracted high-quality, differential photometry of our target stars to check for any variability related to binary motion. Previous studies, relying on spatially unresolved, integrated, optical spectroscopy, had reported that one of these stars was likely to be a 4.377-d binary. Our study set out to identify the culprit and any other short-period system among our targets. However, none displays significant photometric variability, and only one star, BAT99-112 (R136c), located on the outer fringe of R136, displays a marginal variability in its radial velocities; we tentatively report an 8.2-d period. The binary status of BAT99-112 is supported by the fact that it is one of the brightest X-ray sources among all known WR stars in the LMC, consistent with it being a colliding wind system. Followup observations have been proposed to confirm the orbital period of this potentially very massive system.
We present the results of an intensive photometric and spectroscopic monitoring campaign of the WN4 Wolf-Rayet (WR) star WR 1 = HD 4004. Our broadband V photometry covering a timespan of 91 days ...shows variability with a period of P = 16.9{sup +0.6}{sub -0.3} days. The same period is also found in our spectral data. The light curve is non-sinusoidal with hints of a gradual change in its shape as a function of time. The photometric variations nevertheless remain coherent over several cycles and we estimate that the coherence timescale of the light curve is of the order of 60 days. The spectroscopy shows large-scale line-profile variability which can be interpreted as excess emission peaks moving from one side of the profile to the other on a timescale of several days. Although we cannot unequivocally exclude the unlikely possibility that WR 1 is a binary, we propose that the nature of the variability we have found strongly suggests that it is due to the presence in the wind of the WR star of large-scale structures, most likely corotating interaction regions (CIRs), which are predicted to arise in inherently unstable radiatively driven winds when they are perturbed at their base. We also suggest that variability observed in WR 6, WR 134, and WR 137 is of the same nature. Finally, assuming that the period of CIRs is related to the rotational period, we estimate the rotation rate of the four stars for which sufficient monitoring has been carried out, i.e., v{sub rot} = 6.5, 40, 70, and 275 km s{sup -1} for WR 1, WR 6, WR 134, and WR 137, respectively.
With recent detections of magnetic fields in some of their progenitor O stars, combined with known strong fields in their possible descendant neutron stars, it is natural to search for magnetic ...fields in Wolf-Rayet (WR) stars, despite the problems associated with the presence of winds enhanced by an order of magnitude over those of O stars. We continue our search among a sample of 11 bright WR stars following our introductory study in a previous paper of WR6 = EZ CMa using the spectropolarimeter ESPaDOnS at Canada-France-Hawaii Telescope, most of them in all four Stokes parameters. This sample includes six WN stars and five WC stars encompassing a range of spectral subclasses. Six are medium/long-period binaries and three show corotating interaction regions. We report no definite detections of a magnetic field in the winds in which the lines form (which is about the same distance from the center of the star as it is from the surface of the progenitor O star) for any of the eleven stars. Possible reasons and their implications are discussed. Nonetheless, the data show evidence supporting marginal detections for WR134, WR137, and WR138. According to the Bayesian analysis, the most probable field intensities are B sub(wind) ~ 200, 130, and 80 G, respectively, with a 95.4% probability that the magnetic fields present in the observable parts of their stellar wind, if stronger, does not exceed B super(max) sub(wind) ~ 1900 G, ~ 1500 G, and ~ 1500 G, respectively. In the case of non-detections, we report an average field strength upper limit of B super(max) sub(wind) ~ 500 G.
We present the results of a spectroscopic monitoring campaign of nine presumably single Wolf-Rayet (WR) stars, eight of type WC 9 and one WC 8d. We characterize their variability and search for clues ...to the mechanism responsible for the formation of dust in their wind. For seven out of eight WC 9s, we find a large-scale line-flux variability level of ... > 5-8 per cent. The only WC 8d star is variable at a level more comparable with those associated with wind clumping, ... = 2.2 per cent. The changes take place on a time-scale of days but in many cases, observing over longer time spans resulted in higher line-flux variability levels. The width of the substructures ranges from ~150 to 300 km s super( -1), with the widest structures corresponding to stars with the highest variability amplitude. We searched for periodicities in integrated line quantities for C iii ...5696. Radial velocity changes are typically ~20 km s super( -1) but never exceed 40 km s super( -1) and are anticorrelated with the skewness of the line, strongly suggesting that they do not correspond to a real movement of the star. No periodicity was found in these integrated quantities, except for WR 103. Therefore, a wind-wind collision in a close binary does not seem to be responsible for the short-term variability. We cannot, however, exclude that these stars are intermediate- to long-period binaries. We estimate that for periods up to a few years, the shock-cone resulting from wind collisions would be non-adiabatic and thus unstable. We suggest that this represents a viable mechanism to explain the spectroscopic variability. (ProQuest: ... denotes formulae/symbols omitted.)
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