Context.
The surface properties of rotating stars can vary from pole to equator, resulting in anisotropic stellar winds which are not included in the currently available evolutionary models.
Aims.
We ...developed a formalism to describe the mass and angular momentum loss of rotating stars which takes into account both the varying surface properties and distortion due to rotation.
Methods.
Adopting the mass-loss recipe for non-rotating stars, we assigned to each point on the surface of a rotating star an equivalent non-rotating star, for which the surface mass flux is given by the recipe. The global mass-loss and angular momentum loss rates are then given by integrating over the deformed stellar surface as appropriate. Evolutionary models were computed and our prescription is compared to the currently used simple mass-loss enhancement recipes for rotating stars.
Results.
We find that mass-loss rates are largely insensitive to rotation for models not affected by the bi-stability jump. For those affected by the bi-stability jump, the increase in mass-loss rates with respect to time is smoothed. As our prescription considers the variation of physical conditions over the stellar surface, the region affected by the bi-stability jump is able to grow gradually instead of the whole star suddenly being affected.
Conclusions.
We have provided an easy to implement and flexible, yet physically meaningful prescription for calculating mass and angular momentum loss rates of rotating stars in a one-dimensional stellar evolution code which compares favourably to more physically comprehensive models
Context.
The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify ...the contribution of binary evolution to the formation of double black holes.
Aims.
A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples, such as the population in 30 Doradus in the Large Magellanic Cloud (LMC).
Methods.
With this purpose in mind, we analysed a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40
M
⊙
, and identified the model systems that potentially evolve into a binary consisting of a black hole and a massive main-sequence star. We then derived the observable properties of such systems, as well as peculiarities of the OB star component.
Results.
We find that ∼3% of the LMC late-O and early-B stars in binaries are expected to possess a black hole companion when stars with a final helium core mass above 6.6
M
⊙
are assumed to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these black holes may be identified in spectroscopic binaries, either by large amplitude radial velocity variations (≳50 km s
−1
) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity (≳10 km s
−1
) and simultaneous rapid rotation of the OB star. The predicted mass ratios are such that main-sequence companions can be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be and X-ray binaries, and known massive black hole binaries supports our conclusion.
Conclusions.
We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general and for the formation of double black hole mergers in particular.
We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution ...far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec × 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than m
F555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C ivλλ1548–51, including 16 O2–3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1–a3), two O supergiants (a5–a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung–Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5
$^{+0.3}_{-0.7}$
Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He ii λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M⊙. The presence of strong He ii λ1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100 M⊙. We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations.
Context. B-type supergiants represent an important phase in the evolution of massive stars. Reliable estimates of their stellar and wind parameters, however, are scarce, especially at mid and late ...spectral subtypes. Aims. We apply the NLTE atmosphere code FASTWIND to perform a spectroscopic study of a small sample of Galactic B-supergiants from B0 to B9. By means of the resulting data and incorporating additional datasets from alternative studies, we investigate the properties of OB-supergiants and compare our findings with theoretical predictions. Methods. Stellar and wind parameters of our sample stars are determined by line profile fitting, based on synthetic profiles, a Fourier technique to investigate the individual contributions of stellar rotation and "macro-turbulence" and an adequate approach to determine the Si abundances in parallel with micro- turbulent velocities. Results. Due to the combined effects of line- and wind-blanketing, the temperature scale of Galactic B-supergiants needs to be revised downward, by 10 to 20%, the latter value being appropriate for stronger winds. Compared to theoretical predictions, the wind properties of OB-supergiants indicate a number of discrepancies. In fair accordance with recent results, our sample indicates a gradual decrease in v_\infty over the bi-stability region, where the limits of this region are located at lower T_{\rm eff} than those predicted. Introducing a distance-independent quantity Q ' related to wind-strength, we show that this quantity is a well defined, monotonically increasing function of T_{\rm eff} outside this region. Inside and from hot to cool, dot M changes by a factor (in between 0.4 and 2.5) which is much smaller than the predicted factor of 5. Conclusions. The decrease in v_\infty over the bi-stability region is not over-compensated by an increase of dot M, as frequently argued, provided that wind-clumping properties on both sides of this region do not differ substantially.
Abstract We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period of P A = 3.2254367 days that forms part of a higher-order multiple system in a ...configuration of (A+B)+C. We derived masses of the Aa+Ab binary of M 1 = 19.02 ± 0.12 and M 2 = 17.50 ± 0.13 M ⊙ , the radii of R 1 = 7.70 ± 0.05 and R 2 = 6.64 ± 0.06 R ⊙ , and temperatures of T 1 = 34,250 ± 500 K and T 2 = 33,750 ± 500 K. From the analysis of the radial velocities, we found a spectroscopic orbit of A in the outer A+B system with P A+B = 195.8 days ( P A+B / P A ≈ 61). In the O − C analysis, we confirmed this orbit and found another component orbiting the A+B system with P AB+C = 2550 days ( P AB+C / P A+B ≈ 13). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, M B ≳ 10.7 M ⊙ . From the light travel time effect fit to the O − C data, we obtained the limit for the mass of the fourth component, M C ≳ 7.3 M ⊙ . These extra components contribute about 20%–30% (increasing with wavelength) to the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 ± 0.06 mag, a reddening of 0.16 ± 0.02 mag, and an R V of 3.2. This work is part of our ongoing project, which aims to calibrate the surface brightness–color relation for early-type stars.
The groundbreaking detection of gravitational waves produced by the inspiralling and coalescence of the black hole (BH) binary GW150914 confirms the existence of ‘heavy’ stellar-mass BHs with masses ...>25 M⊙. Initial characterization of the system by Abbott et al. supposes that the formation of BHs with such large masses from the evolution of single massive stars is only feasible if the wind mass-loss rates of the progenitors were greatly reduced relative to the mass-loss rates of massive stars in the Galaxy, concluding that heavy BHs must form in low-metallicity (Z ≲ 0.25-0.5 Z⊙) environments. However, strong surface magnetic fields also provide a powerful mechanism for modifying mass-loss and rotation of massive stars, independent of environmental metallicity. In this paper, we explore the hypothesis that some heavy BHs, with masses >25 M⊙ such as those inferred to compose GW150914, could be the natural end-point of evolution of magnetic massive stars in a solar-metallicity environment. Using the mesa code, we developed a new grid of single, non-rotating, solar-metallicity evolutionary models for initial zero-age main sequence masses from 40 to 80 M⊙ that include, for the first time, the quenching of the mass-loss due to a realistic dipolar surface magnetic field. The new models predict terminal-age main-sequence (TAMS) masses that are significantly greater than those from equivalent non-magnetic models, reducing the total mass lost by a strongly magnetized 80 M⊙ star during its main-sequence evolution by 20 M⊙. This corresponds approximately to the mass-loss reduction expected from an environment with metallicity Z = 1/30 Z⊙.
The VLT-FLAMES Tarantula Survey Ramirez-Agudelo, O H; Sana, H; de Koter, A ...
Astronomy and astrophysics (Berlin),
4/2017, Volume:
600
Journal Article
Peer reviewed
Context. The Tarantula region in the Large Magellanic Cloud (LMC) contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity ...to test models describing their main-sequence evolution and mass-loss properties. Aims. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. Methods. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model fastwind with the genetic fitting algorithm pikaia to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. The latter has been constrained without taking into account the contribution from macro-turbulent motions to the line broadening. Results. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. The calibration for giants shows a +1kK offset compared to similar Galactic calibrations; a shift of the same magnitude has been reported for dwarfs. The supergiant calibrations, though only based on a handful of stars, do not seem to indicate such an offset. The presence of a strong upturn at spectral type O3 and earlier can also not be confirmed by our data. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by state-of-the-art models. For stars initially more massive than approximately 60M sub(middot in circle), the giant phase already appears relatively early on in the evolution; the supergiant phase develops later. Bright giants, however, are not systematically positioned between giants and supergiants at M sub(init)> or = 25M sub(middot in circle). At masses below 60M sub(middot in circle), the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at M sub(init)< or = 18M sub(middot in circle). Here, stars classified as late OIII and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late OV stars are actually seen. Though we can not exclude that these stars represent a physically distinct group, this behavior may reflect an intricacy in the luminosity classification at late O spectral subtype. Indeed, on the basis of a secondary classification criterion, the relative strength of Siiv to Hei absorption lines, these stars would have been assigned a luminosity class IV or V. Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. This outcome is independent of their projected spin rates. The aforementioned five stars present moderate projected rotational velocities (i.e., nu sub(e) sini< 200kms super(-1)) and hence do not agree with current predictions of rotational mixing in main-sequence stars. They may potentially reveal other physics not included in the models such as binary-interaction effects. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are ~0.3 dex higher than earlier results. For stars brighter than 10 super(5)L sub(middot in circle), that is, in the regime of strong stellar winds, the measured (unclumped) mass-loss rates could be considered to be in agreement with line-driven wind predictions if the clump volume filling factors were f sub(V)~ 1/8 to 1/6.
Context. Extreme ultraviolet (EUV) and X-ray radiation emitted from wind-embedded shocks in hot, massive stars can affect the ionization balance in their outer atmospheres and can be the mechanism ...responsible for producing highly ionized atomic species detected in stellar wind UV spectra. Aims. To allow for these processes in the context of spectral analysis, we have implemented the emission from wind-embedded shocks and related physics into our unified, NLTE model atmosphere/spectrum synthesis code FASTWIND. Methods. The shock structure and corresponding emission is calculated as a function of user-supplied parameters (volume filling factor, radial stratification of shock strength, and radial onset of emission). We account for a temperature and density stratification inside the postshock cooling zones, calculated for radiative and adiabatic cooling in the inner and outer wind, respectively. The high-energy absorption of the cool wind is considered by adding important K-shell opacities, and corresponding Auger ionization rates have been included in the NLTE network. To test our implementation and to check the resulting effects, we calculated a comprehensive model grid with a variety of X-ray emission parameters. Results. We tested and verified our implementation carefully against corresponding results from various alternative model atmosphere codes, and studied the effects from shock emission for important ions from He, C, N, O, Si, and P. Surprisingly, dielectronic recombination turned out to play an essential role for the ionization balance of Oiv/Ov (particularly in dwarfs with T sub(eff)~ 45000 K). Finally, we investigated the frequency dependence and radial behavior of the mass absorption coefficient, kappanu(r), which is important in the context of X-ray line formation in massive star winds. Conclusions. In almost all of the cases considered, direct ionization is of major influence because of the enhanced EUV radiation field, and Auger ionization only affects Nvi and Ovi significantly. The approximation of a radially constant kappanu is justified for r? 1.2 R sub(?) and lambda? 18 A and also for many models at longer wavelengths. To estimate the actual value of this quantity, however, the Heii opacities need to be calculated from detailed NLTE modeling, at least for wavelengths longer than 18 to 20 A, and information on the individual CNO abundances has to be present.
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