Wolf-Rayet Mass-loss Rates Research Articles

Driving classical Wolf-Rayet winds: A Γ- and Z-dependent mass-loss

Abstract Classical Wolf-Rayet (WR) stars are at a crucial evolutionary stage for constraining the fates of massive stars. The feedback of these hot, hydrogen-depleted stars dominates their surrounding by tremendous injections of ionizing radiation and kinetic energy. The strength of a WR wind decides the eventual mass of its remnant, likely a massive black hole. However, despite their major influence and importance for gravitational wave detection statistics, WR winds are particularly poorly understood. In this paper, we introduce the first set of hydrodynamically consistent stellar atmosphere models for classical WR stars of both the carbon (C) and nitrogen (N) sequence, i.e. WC and WN stars, as a function of stellar luminosity-to-mass ratio (or Eddington Gamma), and metallicity. We demonstrate the inapplicability of the CAK wind theory for classical WR stars and confirm earlier findings that their winds are launched at the (hot) iron (Fe) opacity peak. For log Z/Z⊙ > −2, Fe is also the main accelerator throughout the wind. Contrasting previous claims of a sharp lower mass-loss limit for WR stars, we obtain a smooth transition to optically thin winds. Furthermore, we find a strong dependence of the mass-loss rates on Eddington Γ, both at solar and sub-solar metallicity. Increases in WC carbon and oxygen abundances turn out to slightly reduce the predicted mass-loss rates. Calculations at subsolar metallicities indicate that below the metallicity of the SMC, WR mass-loss rates decrease much faster than previously assumed, potentially allowing for high black hole masses even in the local universe.

Winds from stripped low-mass helium stars and Wolf-Rayet stars

We present mass-loss predictions from Monte Carlo radiative transfer models for helium (He) stars as a function of stellar mass, down to 2 Msun. Our study includes both massive Wolf-Rayet (WR) stars and low-mass He stars that have lost their envelope through interaction with a companion. For these low-mass He-stars we predict mass-loss rates that are an order of magnitude smaller than by extrapolation of empirical WR mass-loss rates. Our lower mass-loss rates make it harder for these elusive stripped stars to be discovered via line emission, and we should attempt to find them through alternative methods instead. Moreover, lower mass-loss rates will make it less likely that low-mass He stars provide stripped-envelope supernovae (SNe) of type Ibc. We express our mass-loss predictions as a function of L and Z, and not as a function of the He abundance, as we do not consider this physically astute given our earlier work. The exponent of the dM/dt vs. Z dependence is found to be 0.61, which is less steep than relationships derived from recent empirical atmospheric modelling. Our shallower exponent will make it more challenging to produce "heavy" black holes of order 40 Msun, as recently discovered in the gravitational wave event GW 150914, making low metallicity for these types of events even more necessary.

Towards a better understanding of the evolution of Wolf–Rayet stars and Type Ib/Ic supernova progenitors

Hydrogen-deficient Wolf-Rayet (WR) stars are potential candidates of Type Ib/Ic supernova (SN Ib/Ic) progenitors and their evolution is governed by mass loss. Stellar evolution models with the most popular prescription for WR mass-loss rates given by Nugis & Lamers have difficulties in explaining the luminosity distribution of WR stars of WC and WO types and the SN Ic progenitor properties. Here we suggest some improvements in the WR mass-loss rate prescription and discuss its implications for the evolution of WR stars and SN Ib/Ic progenitors. Recent studies on Galactic WR stars clearly indicate that the mass-loss rates of WC stars are systematically higher than those of WNE stars for a given luminosity. The luminosity and initial metallicity dependencies of WNE mass-loss rates are also significantly different from those of WC stars. These factors have not been adequately considered together in previous stellar evolution models. We also find that an overall increase of WR mass loss rates by about 60 per cent compared to the empirical values obtained with a clumping factor of 10 is needed to explain the most faint WC/WO stars. This moderate increase with our new WR mas-loss rate prescription results in SN Ib/Ic progenitor models more consistent with observations than those given by the Nugis & Lamers prescription. In particular, our new models predict that the properties of SN Ib and SN Ic progenitors are distinctively different, rather than they form a continuous sequence.

TYPE Ib/c SUPERNOVAE IN BINARY SYSTEMS. I. EVOLUTION AND PROPERTIES OF THE PROGENITOR STARS

We investigate the evolution of Type Ib/c supernova (SN Ib/c) progenitors in close binary systems, using new evolutionary models that include the effects of rotation, with initial masses of 12 - 25 Msun for the primary components, and of single helium stars with initial masses of 2.8 - 20 Msun. We find that, despite the impact of tidal interaction on the rotation of primary stars, the amount of angular momentum retained in the core at the presupernova stage in different binary model sequences converge to a value similar to those found in previous single star models. This amount is large enough to produce millisecond pulsars, but too small to produce magnetars or long gamma-ray bursts. We employ the most up-to-date estimate for the Wolf-Rayet mass loss rate, and its implications for SN Ib/c progenitors are discussed in detail. In terms of stellar structure, SN Ib/c progenitors in binary systems are predicted to have a wide range of final masses even up to 7 Msun, with helium envelopes of 0.16 - 1.5 Msun. Our results indicate that, if the lack of helium lines in the spectra of SNe Ic were due to small amounts of helium, the distribution of both initial and final masses of SN Ic progenitors should be bimodal. Furthermore, we find that a thin hydrogen layer (0.001 - 0.01 Msun) is expected to be present in many SN Ib progenitors at the presupernova stage. We show that the presence of hydrogen, together with a rather thick helium envelope, can lead to a significant expansion of some SN Ib/c progenitors by the time of supernova explosion. This may have important consequences for the shock break-out and supernova light curve. We also argue that some SN progenitors with thin hydrogen layers produced via Case AB/B transfer might be related to Type IIb supernova progenitors with relatively small radii of about 10 Rsun.

Implications of the metallicity dependence of Wolf-Rayet winds

Aims. Recent theoretical predictions for the winds of Wolf-Rayet stars indicate that their mass-loss rates scale with the initial stellar metallicity in the local Universe. We aim to investigate how this predicted dependence affects the models of Wolf-Rayet stars and their progeny in different chemical environments. Methods. We compute models of stellar structure and evolution for Wolf-Rayet stars for different initial metallicities, and investigate how the scaling of the Wolf-Rayet mass-loss rates affects the final masses, the lifetimes of the WN and WC subtypes, and how the ratio of the two populations vary with metallicity. Results. We find significant effects of metallicity dependent mass-loss rates for Wolf-Rayet stars. For models that include the scaling of the mass-loss rate with initial metallicity, all WR stars become neutron stars rather than black holes at twice the solar metallicity; at lower Z, black holes have larger masses. We also show that our models that include the mass-loss metallicity scaling closely reproduce the observed decrease of the relative population of WC over WN stars at low metallicities.

The formation of black hole low-mass X-ray binaries: Through case B or case C mass transfer?

The formation of low-mass X-ray binaries containing a rather massive ($M \ga 7 M_{\odot}$) black hole is problematic because in most recent stellar evolutionary calculations the immediate progenitors of these black holes (Wolf-Rayet stars) lose so much mass via their stellar wind that their final masses are well below the observed black hole masses. We discuss the recently proposed solution that these binaries are formed through case C mass transfer (i.e. mass transfer after core helium burning is completed), avoiding a long Wolf-Rayet phase and thus significant mass loss. We show that only some of the currently available models for the evolution of massive stars allow this formation channel. We also investigate the effect of the downward revised Wolf-Rayet mass-loss rate as is suggested by observations, and conclude that in that case Wolf-Rayet stars end their lives with significantly higher masses than previously found and may be able to form a black holes.

Radio Continuum Measurements of Southern Early‐Type Stars. II. A Distance‐limited Sample of Wolf‐Rayet Stars

A distance-limited sample of southern Wolf-Rayet stars within 3 kpc of the Sun has been observed with the Australia Telescope Compact Array at 8.64 and 4.80 GHz. Radio continuum flux densities at one or both frequencies were obtained for 10 sources and upper limits for 20; four sources are found to be thermal emitters on the basis of the observed spectral index. Five sources are classified as nonthermal. One source could not be classified. We derive mass-loss rates for the thermal sources. After combining them with all existing radio mass-loss rates of Wolf-Rayet stars in the northern and southern hemisphere, we perform a comparison with mass-loss rates derived from optical emission lines. The two methods lead to consistent results, which suggests either that the assumption of a spherically symmetric, stationary, homogeneous stellar wind is correct or that deviations from this assumption affect both methods in the same way. Wolf-Rayet mass-loss rates are surprisingly uniform across spectral type. We find an average mass-loss rate of 4 × 10-5 M☉ yr-1 for all types observed, except for WC9 stars, which have rates that are lower by at least a factor of 2. An alternative explanation could be partial recombination of helium from He+ to He0 in the radio region, which would lead to a reduced number of free electrons, and therefore reduced radio flux for WC9 stars. Mass-loss rates of 8 × 10-5 M☉ yr-1 for late WN stars favored in recent stellar evolution models disagree with the observations of these subtypes. The results of this survey suggest that ~40% of all Wolf-Rayet stars with measured spectral index are nonthermal emitters at centimeter wavelengths. This percentage is nearly twice as high as that of nonthermal emitters among OB stars and is higher than that previously estimated for WR stars. The nature of the nonthermal emission is still not fully understood. Possible causes of nonthermal emission are discussed. In particular, we speculate that nonthermal emission may arise from an interaction between a thermal WR wind and surrounding material owing to a shell ejected during a previous evolutionary stage or owing the wind of a companion star.

Polarization eclipse model of the Wolf-Rayet binary V444 Cygni with constraints on the stellar radii and an estimate of the Wolf-Rayet mass-loss rate

view Abstract Citations (96) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization Eclipse Model of the Wolf-Rayet Binary V444 Cygni with Constraints on the Stellar Radii and an Estimate of the Wolf-Rayet Mass-Loss Rate St. -Louis, N. ; Moffat, A. F. J. ; Lapointe, L. ; Efimov, Yu. S. ; Shakhovskoj, N. M. ; Fox, G. K. ; Piirola, V. Abstract A comprehensive study is made of the polarization eclipse of V444 Cyg using an extensive data set of multicolor linear polarization observations obtained with emphasis on the eclipse of the WR star by the O-star companion. A model is introduced with an improved treatment of the various occultation effects. A comparison of the model with the observations yields estimates of the O and WR star radii (8.5 +/- 1 and less than 4 solar radii, respectively) as well as of the WR mass-loss rate (0.75 x 10 exp -5 solar mass/yr). Publication: The Astrophysical Journal Pub Date: June 1993 DOI: 10.1086/172751 Bibcode: 1993ApJ...410..342S Keywords: Astronomical Polarimetry; Binary Stars; Stellar Mass Ejection; Stellar Models; Stellar Occultation; Wolf-Rayet Stars; Computational Astrophysics; Electron Scattering; Light Curve; Linear Polarization; Polarization Characteristics; Ubv Spectra; Astrophysics; STARS: BINARIES: ECLIPSING; POLARIZATION; STARS: INDIVIDUAL CONSTELLATION NAME: V444 CYGNI; STARS: WOLF-RAYET full text sources ADS | data products SIMBAD (5) MAST (1)

New Results in Evolution in the Upper HRD

A theoretical interpretation of the observed upper luminosity limit is suggested here. Staritsin (1989) considered the core hydrogen and helium burning stages in a 64 M⊙ star. Mixing in a semi-convection zone in a diffusion approximation (Staritsin, 1987) and mass loss by stellar wind (de Jager et al., 1988) were taken into account. During MS evolution the star looses half of its initial envelope. After MS evolution an intermediate convective zone appears. The hydrogen content in the shell source increases. As a result, the star burns helium in its blue supergiant stage. After the hydrogen content in the envelope has decreased to 10% of its mass, the star looses mass with Wolf-Rayet mass loss rates according to de Jager et al. (1988). The star has a WR character during 5% of its full life time.