X-Shooting ULLYSES: Massive stars at low metallicity

Context Massive stars (>8 M⊙) play an important role in galactic evolution at all cosmic ages. A deeper understanding of the behaviour of mass loss in low metallicity environments is therefore required. This behaviour largely determines the path of a massive star throughout its life, and its final fate. A better understanding would allow us to predict the evolution of massive stars in the early Universe better. Aims We investigated the theoretical bi-stability jump, which predicts an increase in the mass-loss rates below Teff ≈25–21 kK. We further constrained the photospheric and wind parameters of a sample of late-O and B supergiants in the Large Magellanic Cloud. Methods We used the 1D non-local thermal equilibrium radiative transfer model CMFGEN in a grid-based approach and a fine-tuned spectroscopic fitting procedure that allowed us to determine the stellar and wind parameters of each star. We applied this method to ultra-violet data from the ULLYSES programme and to complementary optical data from the XShootU collaboration. We also used evolutionary models to obtain the evolutionary masses, and we compared them to the spectroscopic masses we derived. Results We derived physical parameters and wind properties of 16 late-O and B supergiants that span a wide temperature range of Teff ≈12–30 kK, surface gravity range of log (g/cm s−2) ≈1.8–3.1, and mass-loss rate range of Ṁ ˙≈ 10−7.6−10−5.7 M⊙ yr−1. We also compared our results to previous studies that attempted to investigate the metallicity dependence of the wind properties. Conclusions The photospheric and wind properties we derived are consistent with those of multiple previous studies. The evolutionary and spectroscopic masses for most of our sample are consistent within the uncertainties. Our results do not reproduce a bi-stability jump in any temperature range, but rather a monotonic decrease in the mass-loss rate at lower temperatures. We obtain a relation of the wind terminal velocity to effective temperature for supergiants in the Large Magellanic Cloud of ν∞/km s−1 = 0.076(±0.011)Teff/K − 884(±260). The mass-loss rates we derived disagree with the mass-loss rates predicted by any of the numerical recipes. This is also the case for the ratio of the terminal wind velocity to the escape velocity ν∞/νesc, and we derived the relation ν∞/νesc = 4.1(±0.8) log (Teff/K) −16.3(± 3.5). The wind parameters depend on the metallicity, based on a comparison with a previous study of the Small Magellanic Cloud, and the modified wind momentum-luminosity relation is log DmomLMC = 1.39(±0.54)log(Lbol/L⊙) + 20.4(±3.0).

Our limited understanding of the winds of massive stars hampers our ability to predict their evolution. We currently often rely on mass-loss rate prescriptions that show strong features that lack a firm empirical confirmation, such as the bi-stability jump. This bi-stability jump is a pronounced increase in mass loss in the decreasing temperature regime T_ ̊m eff ∼28-21,kK, i.e., spectral types B1-B2. Different prescriptions also give different metallicity dependences; these sometimes include a dependence on effective temperature, luminosity or both, that may also differ among prescriptions. Although recent papers have compared empirical results to prescriptions, a large observational sample of B stars for which the wind has been scrutinised over different metallicity ranges is still lacking. Through the modelling of both ultraviolet (from the ULLYSES programme) and optical (from the XShootU programme) high-resolution spectra, we determined the stellar and wind parameters, including detailed clumping parameters, of 24 SMC B stars ranging in T_ eff from 13 to 29,kK. By combining this sample with LMC studies, we compared the wind behaviour of B stars in two different metallicity regimes. We compared our results to common mass-loss rate prescriptions to test features present in these models and their metallicity dependence. We have used the model atmosphere code fastwind and the genetic algorithm code Kiwi-GA to fit the UV and optical spectra simultaneously. This allows us to determine wind properties including clumping behaviour. The moderate to strong metallicity trends present in the mass-loss prescriptions (dot M ∝ Z^ 0.41-1.4 ) explored here overestimate the empirical metallicity dependence in the B-star regime, which appears very weak. Similarly to a previous study of B supergiants in the LMC, we do not find a sudden increase in mass-loss rate at approximately spectral type B1, but only a weak temperature trend. As in this LMC study, we show that on average around 40% of the wind mass is located in the wind medium between the clumps. Using other studies from this paper series, we compiled a sample of 80+ O and B stars in the SMC and LMC. From a comparison we find a clear difference in O- and B-type metallicity dependence. The apparent lack of a bi-stability jump in the B-star regime and a very weak metallicity dependence for the same stars offers new empirical constraints to theoretical models of line-driven winds. As differences between these models are large (reaching orders of magnitude in parts of parameter space) such constraints are much needed. More studies exploring the mass-loss rates of B stars and cooler objects will be helpful to our understanding of hot star winds.

Context. The star cluster R136 inside the Large Magellanic Cloud hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact. Aims. Our goal is to observationally constrain the stellar and wind properties of the massive stars in R136, in particular the wind-structure parameters related to wind clumping. Methods. We simultaneously analyse optical and ultraviolet spectroscopy of 53 O-type and three WNh-stars using the Fastwind model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium. Results. We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities, and clumping characteristics and compare them to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities. Conclusions. We confirm a cluster age of 1.0–2.5 Myr and derived an initial stellar mass of ≥250 M⊙ for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of fcl = 29 ± 15. We find tentative trends in the wind-structure parameters as a function of the mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduce both quantities. The prescription of Krtička & Kubát (2018) matches the observed mass-loss rates best.

Hot, massive stars are known to host unstable, radiation-driven outflowing winds, giving rise to dense clumps of material which severely affect the diagnostic techniques used to derive wind properties of massive stars. Most of the current diagnostic models account for wind inhomogeneities by assuming a one-component medium consisting of optically thin clumps, and maintaining a smooth velocity-field. However, this neglects important light-leakage effects through porous channels in-between the clumps. These light-leakage effects have recently been incorporated in the stellar atmosphere modelling code FASTWIND, and here we will present quantitative mass-loss results from a combined Ultraviolet-Optical wind analysis of O-supergiants in the Galaxy. Using a genetic-algorithm fitting-approach, we systematically investigate the impact the wind physics has on derived stellar and wind parameters, and how this depends on metallicity and spectral type. We compare our findings with earlier results (which do not take into account such light-leakage effects), to standard mass-loss rates usually included in evolution model studies of massive stars, and with theoretical predictions of clumping properties. We will also present the first systematic empirical constraints on the new wind parameters, associated with light-leakage, and compare these with theoretical predictions.

Context. Winds of massive stars have density inhomogeneities (clumping) that may affect the formation of spectral lines in different ways, depending on their formation region. Most of previous and current spectroscopic analyses have been performed in the optical or ultraviolet domain. However, massive stars are often hidden behind dense clouds rendering near-infrared observations necessary. It is thus inevitable to compare the results of such analyses and the effects of clumping in the optical and the near-infrared, where lines share most of the line formation region. Aims. Our objective is to investigate whether a spectroscopic analysis using either optical or infrared observations results in the same stellar parameters with comparable accuracy, and whether clumping affects them in different ways. Methods. We analyzed optical and near-infrared observations of a set of massive O stars with spectral types O4-O9.5 and all luminosity classes. We used Fastwind model atmospheres with and without optically thin clumping. We first studied the differences in the stellar parameters derived from the optical and the infrared using unclumped models. Based on a coarse model grid, different clumping stratifications were tested. A subset of four linear clumping laws was selected to study the differences in the stellar parameters derived from clumped and unclumped models, and from the optical and the infrared wavelength regions. Results. We obtain similar stellar parameters in the optical and the infrared, although with larger uncertainties in the near-infrared, both with and without clumping, albeit with some individual deviating cases. We find that the inclusion of clumping improves the fit to Hα or He II 4686 in the optical for supergiants, as well as that of Brγ in the near-infrared, but it sometimes worsens the fit to He II 2.18 μm. Globally, there are no significant differences when using the clumping laws tested in this work. We also find that the high-lying Br lines in the infrared should be studied in more detail in the future. Conclusions. The infrared can be used for spectroscopic analyses, giving similar parameters as from the optical, though with larger uncertainties. The best fits to different lines are obtained with different (linear) clumping laws, indicating that the wind structure may be more complex than adopted in the present work. No clumping law results in a better global fit, or improves the consistency between optical and infrared stellar parameters. Our work shows that the optical and infrared lines are not sufficient to break the dichotomy between the mass-loss rate and clumping factor.

Context. Red supergiant (RSG) stars exhibit significant mass loss by means of a slow, dense wind. They are often considered to be the more massive counterparts of Asymptotic Giant Branch (AGB) stars. While AGB mass loss is related to their strong pulsations, the RSG are often only weakly variable. This raises the question of whether their wind-driving mechanism and the dust composition of the wind are the same. Aims. We study the conditions at the base of the wind by determining the dust composition of a sample of RSG. The dust composition is assumed to be sensitive to the density, temperature, and acceleration at the base of the wind. We compare the derived dust composition with the composition measured in AGB star winds. Methods. We compile a sample of 27 RSG infrared spectra (ISO-SWS) and supplement these with photometric measurements to derive the full spectral energy distribution (SED). These data are modelled using a dust radiative-transfer code, taking into account the optical properties of the relevant candidate materials to search for correlations between mass-loss rate, density at the inner edge of the dust shell, and stellar parameters. Results. We find strong correlations between the dust composition, mass-loss rate, and the stellar luminosity, roughly in agreement with the theoretical dust condensation sequence. We identify the need for a continuous (near-)IR dust opacity and tentatively propose amorphous carbon, and we note significant differences with AGB star winds in terms of the presence of PAHs, absence of “the” 13 μm band, and a lack of strong water bands. Conclusions. Dust condensation in RSG is found to experience a freeze-out process that is similar to that in AGB stars. Together with the positive effect of the stellar luminosity on the mass-loss rate, this suggests that radiation pressure on dust grains is an important ingredient in the driving mechanism. Still, differences with AGB stars are manifold and thus the winds of RSG should be studied individually in further detail.

<i>Aims. <i/>We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem).<i>Methods. <i/>Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), <i>ζ<i/> Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code.<i>Results. <i/>The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in C iv 1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived ( yr<sup>-1<sup/>). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory (), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log . We have estimated the carbon abundance for the stars of our sample and concluded that its value cannot be reduced to sufficiently small values to solve the <i>weak wind problem<i/>. Upper limits on were established for all objects using lines of different ions: P v 1118, 1128, C iii , N v 1239, 1243, Si iv 1394, 1403, and N iv <i>λ<i/>1718. All the values obtained are in disagreement with theoretical predictions, bringing support to the reality of <i>weak winds<i/>. Together with C iv 1548, 1551, the use of N v 1239, 1243 results in the lowest mass-loss rates: the upper limits indicate that must be <i>less<i/> than about -1.0 dex . Upper mass-loss rate limits obtained for other transitions are also low: they indicate that must be <i>less<i/> than about (-0.5 <i>±<i/> 0.2) dex . We studied the behavior of the H<i>α<i/> line with different mass-loss rates. For two stars, only models with very low 's provide the best fit to the UV and optical spectra. We also explored ways to fit the observed spectra with the theoretical mass-loss rates. By using large amounts of X-rays, we could reduce the predicted wind emission to the observed levels. However, unrealistic X-ray luminosities had to be used (log ). The validity of the models used in our analyses is discussed.

We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have a lower mass-loss rate, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.

Mass loss through stellar winds plays a dominant role in the evolution of massive stars. In particular, the mass-loss rates of very massive stars ($\gt 100\, M_{\odot}$) are highly uncertain. Such stars display Wolf–Rayet spectral morphologies (WNh), whilst on the main sequence. Metal-poor very massive stars are progenitors of gamma-ray bursts and pair instability supernovae. In this study, we extended the widely used stellar wind theory by Castor, Abbott &amp; Klein from the optically thin (O star) to the optically thick main-sequence (WNh) wind regime. In particular, we modify the mass-loss rate formula in a way that we are able to explain the empirical mass-loss dependence on the Eddington parameter (Γe). The new mass-loss recipe is suitable for incorporation into current stellar evolution models for massive and very massive stars. It makes verifiable predictions, namely how the mass-loss rate scales with metallicity and at which Eddington parameter the transition from optically thin O star to optically thick WNh star winds occurs. In the case of the star cluster R136 in the Large Magellanic Cloud we find in the optically thin wind regime $\dot{M} \propto \Gamma _{\rm e}^{3}$, while in the optically thick wind regime $\dot{M} \propto 1/ (1 - \Gamma _{\rm e})^{3.5}$. The transition from optically thin to optically thick winds occurs at Γe, trans ≈ 0.47. The transition mass-loss rate is $\log \dot{M}~(\mathrm{M}_{\odot } \, \mathrm{yr}^{-1}) \approx -4.76 \pm 0.18$, which is in line with the prediction by Vink &amp; Gräfener assuming a volume filling factor of $f_{\rm V} = 0.23_{-0.15}^{+0.40}$.

view Abstract Citations (357) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Terminal Velocities and the Bistability of Stellar Winds Lamers, Henny J. G. L. M. ; Snow, Theodore P. ; Lindholm, Douglas M. Abstract The terminal velocity, υ∞, of the stellar winds of 117 stars of spectral types 0 through F has been measured from the spectra in the new Atlas of Ultraviolet P Cygni Profiles by Snow et al. For the 68 stars with the most accurate determination of υ∞ we studied the relation between υ∞ and the photospheric escape velocity, V/esc. The ratio υ/∞/ decreases from 2.58±0.20 in O-stars to 0.72±0.15 in A and F stars. There is a clear discontinuity in this ratio near Teff ≃ 21,000 K at spectral type B1, where the ratio drops by almost a factor 2 from υ∞/υesc ≃ 2.6 at the high-temperature side υ∞/υesc ≃ 1.3 at the low-temperature side, and possibly a smaller discontinuity near Teff ≃ 10,000 K. The discontinuity near 21,000 K is due to the change in the nature of the lines that produce the radiation pressure. At Teff ≃ 21,000 K the radiation pressure is due to high- ionization lines which are largely optically thick, whereas at Teff ≃ 21,000 K the radiation pressure is due to very large numbers of low-ionization lines, which are more optically thin, Near Teff ≃ 21,000 K the winds can exist in two states: winds with high mass-loss rates and low velocities, and winds with low mass-loss rates and high velocities. This is the bistability of stellar winds for stars with Teff ≃ 21,000 K. We present evidence that the bistability may produce a jump in the mass-loss rates of the stars near Teff ≃ 21,000 K with the higher mass-loss rates at the low temperature side of the jump. We briefly discuss the possible role of bistability in the formation of disks around B[e]-supergiants. The observed values of υ/∞ are used to derive empirical values of the force multiplier parameter αemp that can be used to predict the terminal velocity of any early-type star with an accuracy of better than 5% with the method described in Appendix B, if the stellar parameters are known. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176575 Bibcode: 1995ApJ...455..269L Keywords: STARS: EARLY-TYPE; STARS: MASS LOSS; STARS: SUPERGIANTS; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (117)

It is yet well understood how mass-loss rates from evolved stars depend on metallicities. With a half of the solar metallicity and the distance of only 50 kpc, the evolved stars of the Large Magellanic Cloud (LMC) are an ideal target for studying mass loss at low metallicity. We have obtained spectra of red-supergiants in the LMC, using the Hershel Space Observatory, detecting CO thermal lines fro J=6–5 up to 15–14 lines. Modelling CO lines with non-LTE Radiative transfer code suggests that CO lines intensities can be well explained with high gas-to-dust ratio, with no obvious reduction in mass-loss rate at the LMC. We conclude that the luminosities of the stars are primary factors on mass-loss rates, rather than the metallicity.

Context: Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium. Aims: Attempt to, for the first time, spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud. Methods: ALMA was used to observe 2 OH/IR stars and 4 carbon stars in the LMC in the CO J= 2-1 line. Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C-stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. Gas-to-dust ratios are derived that make the predicted velocities agree with the observed ones. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with "extreme" C-stars that have properties more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than derived from the dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC. Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear similar. This is in agreement with dynamical dust-driven wind models.

In order to determine the composition of the dust in the circumstellar\nenvelopes of oxygen-rich asymptotic giant branch (AGB) stars we have computed a\ngrid of modust radiative-transfer models for a range of dust compositions,\nmass-loss rates, dust shell inner radii and stellar parameters. We compare the\nresulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec\nLarge Magellanic Cloud (LMC) sample, finding good overall agreement for stars\nwith a mid-infrared excess. We use these models to fit a sample of 37 O-rich\nAGB stars in the LMC with optically thin circumstellar envelopes, for which\n5$-$35-$\\mu$m Spitzer infrared spectrograph (IRS) spectra and broadband\nphotometry from the optical to the mid-infrared are available. From the\nmodelling, we find mass-loss rates in the range $\\sim 8\\times10^{-8}$ to\n$5\\times10^{-6}$ M$_{\\odot}\\ \\mathrm{yr}^{-1}$, and we show that a grain\nmixture consisting primarily of amorphous silicates, with contributions from\namorphous alumina and metallic iron provides a good fit to the observed\nspectra. Furthermore, we show from dust models that the AKARI [11]$-$[15]\nversus [3.2]$-$[7] colour-colour diagram, is able to determine the fractional\nabundance of alumina in O-rich AGB stars.\n

Growth in body mass in pups of the subantarctic fur seal Arctocephalus tropicalis was studied at Amsterdam Island, southern Indian Ocean during the rearing period between December 1995 and August 1996. We examined the change in the rates of mass gain and mass loss with pup age to explain the overall change in mass throughout the rearing period, when mothers alternate foraging trips at sea and attendance periods ashore. In the absence of their mothers, pups fast and lose mass. During the maternal attendance period ashore pups suck and gain mass. There was no significant difference in the rate of growth in mass between male and female pups. The rate of loss in mass during a fasting event was positively related to the initial body mass but negatively related to the age and duration of the fast of the pup. The rate of mass loss was best related to pup age by a quadratic relationship. The rate of mass loss increased until pups were 120 days old and then decreased until the end of the study period. The rate of gain in mass following a maternal foraging trip was positively related to the initial pup mass but negatively related to the foraging trip duration, and we found that the rate of mass gain decreased linearly with the age of the pup. The concomitant decrease in the rate of mass loss and mass gain for pups &gt; 120 days old allowed pups to maintain a positive growth until they were 220 days old. After 220 days of age the rate of mass loss exceeded the rate of mass gain and the pups start to lose mass until weaning.

The photon-scattering winds of M giants absorb parts of the chromospheric emission lines and produce self-reversed spectral features in high-resolution Hubble Space Telescope (HST)/GHRS spectra. These spectra provide an opportunity to assess fundamental parameters of the wind, including flow and turbulent velocities, the optical depth of the wind above the region of photon creation, and the star’s mass-loss rate. This paper is the last paper in the series “GHRS Observations of Cool, Low-Gravity Stars”; the last several have compared empirical measurements of spectral emission lines with models of the winds and mass loss of K giants and supergiants. We have used the Sobolev with Exact Integration radiative transfer code, along with simple models of the outer atmosphere and wind, to determine and compare the wind characteristics of the two M-giant stars, γ Cru (M3.5III) and μ Gem (M3IIIab), with previously derived values for low-gravity K-stars. The analysis specifies the wind parameters and calculates line profiles for the Mg ii resonance lines, in addition to a range of unblended Fe ii lines. Our line sample covers a large range of wind opacities and, therefore, probes a range of heights in the atmosphere. Our results show that μ Gem has a slower and more turbulent wind than γ Cru. Also, μ Gem has a weaker chromosphere, in terms of surface flux, with respect to γ Cru. This suggests that μ Gem is more evolved than γ Cru. Comparing the two M giants in this work with previously studied K-giant and supergiant stars (α Tau, γ Dra, and λ Vel) reveals that the M giants have slower winds than the earlier giants, but exhibit higher mass-loss rates. Our results are interpreted in the context of the winds being driven by Alfvén waves.