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Journal ArticleDOI

Wind Inhomogeneities in Wolf-Rayet Stars. II. Investigation of Emission-Line Profile Variations

TL;DR: In this article, a phenomenological model was introduced to simulate line-profile variations (LPVs) in emission lines from a clumped wind, and the authors investigated the effects on the LPVs of local velocity gradients, optical depths, various numbers of discrete wind elements, and a statistical distribution in the line flux from individual elements.
Abstract: We present high-resolution spectroscopic monitoring of the line-profile variations (LPVs) in the He II λ5411 emission line of four Wolf-Rayet (WR) stars of the WN sequence (HD 96548, HD 191765, HD 192163, and HD 193077) and in the C III λ5696 emission line of five WR stars of the WC sequence (HD 164270, HD 165763, HD 192103, HD 192641, and HD 193793). The LPVs are shown to present systematic patterns: they all consist of a number of relatively narrow emission subpeaks that tend to move from the line centers toward the line edges. We introduce a phenomenological model that depicts WR winds as being made up of a large number of randomly distributed, radially propagating, discrete wind emission elements (DWEEs). This working model is used to simulate LPV patterns in emission lines from a clumped wind. General properties of the LPV patterns are analyzed with the help of novel numerical tools (based on multiscale, wavelet analysis), and simulations are compared to the data. We investigate the effects on the LPVs of local velocity gradients, optical depths, various numbers of discrete wind elements, and a statistical distribution in the line flux from individual elements. We also investigate how the LPV patterns are affected by the velocity structure of the wind and by the extension of the line-emission region (LER). Eight of the stars in our sample are shown to possess strong similarities in their LPV patterns, which can all be explained in terms of our simple model of local wind inhomogeneities. We find, however, that a very large number (104) of DWEEs must be used to account for the LPV. Large velocity dispersions must occur within DWEEs, which give rise to the ξ~100 km s-1 line-of-sight velocity dispersions. We find evidence for anisotropy in the velocity dispersion within DWEEs with σvr~4σvθ, where σvr and σvθ are the velocity dispersions in the radial and azimuthal directions, respectively. We find marginal evidence for optical depth effects within inhomogeneous features, with the escape probability being slightly smaller in the radial direction. The kinematics of the variable features reveals lower than expected radial accelerations, with 20 r−1)β, with v∞ the terminal wind velocity. The mean duration of subpeak events, interpreted as the crossing time of DWEEs through the LER, is found to be consistent with a relatively thin LER. As a consequence, the large emission-line broadening cannot be accounted for by the systematic radial velocity gradient from the accelerating wind. Rather, emission-line broadening must be dominated by the large "turbulent" velocity dispersion σvr suggested by the LPV patterns. The remaining WR star in our sample (HD 191765) is shown to present significant differences from the others in its LPV pattern. In particular, the associated mean velocity dispersion is found to be especially large (ξ~350 km s-1, compared to ξ~100 km s-1 in other stars). Accordingly, the LPV patterns in HD 191765 cannot be satisfactorily accounted for with our model, requiring a different origin.
Citations
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Journal ArticleDOI
TL;DR: The VIIth catalogue of galactic Population-I Wolf-Rayet stars as mentioned in this paper provides improved coordinates, spectral types and bv photometry of known WR stars and adds 71 new WR stars to the previous catalogue, including 15 WNL and 11 WCL stars within 30 pc of the Galactic Center.

711 citations

Journal ArticleDOI
TL;DR: In this paper, the authors discuss various aspects of radiation-driven mass loss, both from the theoretical and the observational side, focusing on the winds from OB-stars, and discuss the problems of measuring mass loss rates from weak winds and the potential of the NIR Br α -line as a tool to enable a more precise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model.
Abstract: Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively if it is to be successfully included in broader astrophysical applications such as galactic and cosmic evolution and ionization. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on developments in the past decade, concentrating on the winds from OB-stars, with some excursions to the winds from Luminous Blue Variables (including super-Eddington, continuum-driven winds), winds from Wolf–Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After recapitulating the 1-D, stationary standard model of line-driven winds, extensions accounting for rotation and magnetic fields are discussed. Stationary wind models are presented that provide theoretical predictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results from corresponding campaigns (including the VLT-flames survey of massive stars) with theoretical predictions, featuring the mass loss-metallicity dependence. Subsequently, we concentrate on two urgent problems, weak winds and wind-clumping, that have been identified from various diagnostics and that challenge our present understanding of radiation driven winds. We discuss the problems of “measuring” mass-loss rates from weak winds and the potential of the NIR Br α -line as a tool to enable a more precise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model. Wind-clumping, conventionally interpreted as the consequence of a strong instability inherent to radiative line-driving, has severe implications for the interpretation of observational diagnostics, since derived mass-loss rates are usually overestimated when clumping is present but ignored in the analyses. Depending on the specific diagnostics, such overestimates can amount to factors of 2 to 10, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present-day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars. Such emission is thought to originate both from magnetically confined winds and from non-magnetic winds, in the latter case related to the line-driven instability and/or clump-clump collisions. We highlight as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping.

594 citations


Cites methods from "Wind Inhomogeneities in Wolf-Rayet ..."

  • ...Physical models of such lpv interpreted in terms of clumps generated by the line-driven instability have been provided by Dessart and Owocki (2002a,b), where thla ter study extends the Lepine and Moffat (1999) wavelet formalism to modeling lpv from hy- drodynamic simulations....

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  • ...Lepine and Moffat (1999) monitored the line-profile variations in the HeII emission lines of WN stars and in CIII emission lines of WC stars, and studied these variations by means of a detailed wavelet analysis....

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Journal ArticleDOI
TL;DR: In this article, the authors performed a pilot study of mass loss predictions for late-type Wolf-Rayet (WR) stars as a function of metal abundance, over a range between 10 −5 ≤ (Z/Z ○. ) ≤ 10.
Abstract: We have performed a pilot study of mass loss predictions for late-type Wolf-Rayet (WR) stars as a function of metal abundance, over a range between 10 -5 ≤ (Z/Z ○. ) ≤ 10. We find that the winds of nitrogen-rich Wolf-Rayet stars are dominated by iron lines, with a dependence of mass loss on Z similar to that of massive OB stars. For more evolved, carbon-rich, WR stars the wind strength is also found to be dependent on the Fe abundance, so that they depend on the chemical environment of the host galaxy, but with a mass loss metallicity dependence that is less steep than for OB stars. Our finding that WR mass loss is Z-dependent is a new one, with important consequences for black hole formation and X-ray population studies in external galaxies. A further finding of our study is that the Z dependence of C-rich WR stars becomes weaker at melallicities below Z/Z ○. ≤ 1/10, and mass loss no longer declines once the metal abundance drops below (Z/Z ○. ) ≃ 10 -3 . This is the result of an increased importance of radiative driving by intermediate mass elements, such as carbon. In combination with rapid rotation and/or proximity to the Eddington limit - likely to be relevant for massive Population III stars - this effect may indicate a role for mass loss in the appearance and evolution of these objects, as well as a potential role for stellar winds in enriching the intergalactic medium of the early Universe.

470 citations


Cites background from "Wind Inhomogeneities in Wolf-Rayet ..."

  • ...One reason is the discovery that WR stars are highly clumped (e.g. Moffat & Robert 1994, Lepine & Moffat 1999), which has resulted in a downward revision of the observed WR mass-loss rates (e.g. Hamann & Koesterke 1998)....

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Journal ArticleDOI
TL;DR: In this article, the authors study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization and determine the occurrence and properties of these convection regions as function of the stellar parameters.
Abstract: Context. We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. Aims. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. Methods. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. Results. We find the iron convection zone (FeCZ) to be more prominent fo r lower surface gravity, higher luminosity and higher initi al metallicity. It is absent for luminosities below about 10 3.2 L⊙, 10 3.9 L⊙, and 10 4.2 L⊙ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variabil ity. Conclusions. The confirmation of all three trends for the FeCZ as function o f stellar parameters by empirical microturbulent velociti es argues for a physical connection between sub-photospheric convective motions and small scale stochastic velocities i n the photosphere of O- and B-type stars. We further suggest that clumping in the inner parts of the winds of OB stars could be caused by the same mechanism, and that magnetic fields produced in the FeCZ coul d appear at the surface of OB stars as diagnosed by discrete absorption components in ultraviolet absorption lines.

359 citations


Cites background from "Wind Inhomogeneities in Wolf-Rayet ..."

  • ...This may relate to the very strong clumping found observationally in Wolf-Rayet winds (Lépine & Moffat 1999; Marchenko et al. 2006), and may be required for an understanding of the very high mass-loss rates of Wolf-Rayet stars (Eichler et al. 1995; Kato & Iben 1992; Heger & Langer 1996)....

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  • ...So far this is best documented for Wolf-Rayet (WR) stars, where line variability on time scales of minutes to hours is thought to constitute direct evidence of outflows that are clumped already in the acceleration zone near the base of the wind (Lépine & Moffat 1999)....

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Journal ArticleDOI
TL;DR: In this paper, the Potsdam Wolf-Rayet (PoWR) model was generalized in the formal integral to account for clumps that are not necessarily optically thin.
Abstract: Context. The mass-loss rate is a key parameter of massive stars. Adequate stellar atmosphere models are required for spectral analyses and mass-loss determinations. Present models can only account for the inhomogeneity of stellar winds in the approximation of small-scale structures that are optically thin. Compared to previous homogeneous models, this treatment of “microclumping” has led to reducing empirical mass-loss rates by factors of two to three. Further reductions are presently discussed in the literature, with far-reaching consequences e.g. for stellar evolution and stellar yields. Aims. Stellar wind clumps can be optically thick in spectral lines. We investigate how this “macroclumping” influences the radiative transfer and the emergent line spectra and discuss its impact on empirical mass-loss rates. Methods. The Potsdam Wolf-Rayet (PoWR) model atmosphere code is generalized in the “formal integral” to account for clumps that are not necessarily optically thin. The stellar wind is characterized by the filling factor of the dense clumps and by their average separation. An effective opacity is obtained by adopting a statistical distribution of clumps and applied in the radiative transfer. Results. Optically thick clumps reduce the effective opacity. This has a pronounced effect on the emergent spectrum. Our modeling for the O-type supergiant ζ Puppis reveals that the optically thin Hα line is not affected by wind porosity, but that the P v resonance doublet becomes significantly weaker when macroclumping is taken into account. The reported discrepancies between resonance-line and recombination-line diagnostics can be resolved entirely with the macroclumping modeling without downward revision of the mass-loss rate. In the case of Wolf-Rayet stars, we demonstrate for two representative models that stronger lines are typically reduced by a factor of two in intensity, while weak lines remain unchanged by porosity effects. Conclusions. Mass-loss rates inferred from optically thin emission, such as the Hα line in O stars, are not influenced by macroclumping. The strength of optically thick lines, however, is reduced because of the porosity effects. Therefore, neglecting the porosity in stellar wind modeling can lead to underestimating empirical mass-loss rates.

249 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors proposed to use the idea of super levels first pioneered by Anderson to improve the performance of the non-LTE radiative transfer code of Hillier stars with stellar winds.
Abstract: Extensive modifications to the non-LTE radiative transfer code of Hillier have been made in order to improve the spectroscopic analysis of stars with stellar winds. The main improvement to the code is the inclusion of blanketing due to thousands of overlapping lines. To implement this effect, we have used the idea of super levels first pioneered by Anderson. In our approach, levels with similar excitation energies and levels are grouped together. Within this group, we assume that the departure coefficients are identical. Only the population (or equivalently, the departure coefficient) of the super level need be solved in order to fully specify the populations of the levels within a super level. Our approach is a natural extension of the single-level LTE assumption, and thus LTE is recovered exactly at depth. In addition to the line blanketing modifications, the code has been improved significantly in other regards. In particular, the new code incorporates the effect of level dissolution, the influence of resonances in the photoionization cross sections, and the effect of Auger ionization. Electron scattering with a thermal redistribution can be considered, although it is normally treated coherently in the comoving frame (which still leads to redistribution in the observer's frame). Several example calculations are described to demonstrate the importance of line blanketing on spectroscopic analysis. We find that the inclusion of blanketing modifies the strengths of some optical CNO lines in Wolf-Rayet (W-R) stars by factors of 2-5. In particular, the strengths of the WC classification lines C III λ5696 and C IV λ5805 are both increased because of iron blanketing. This should help alleviate problems found with nonblanketed models, which were incapable of matching the strengths of these lines. We also find that, in the UV (1100-1800 A), the influence of Fe is readily seen in both emission and absorption. The emission is sensitive to the iron abundance and should allow, for the first time, Fe abundances to be deduced in W-R stars. The improvements made to our code should greatly facilitate the spectroscopic analysis of stars with stellar winds. We will be able to determine the importance and influence of line blanketing, as well as of several other effects that have been included in the new code. It will also allow us to better determine W-R star parameters, such as luminosity, elemental abundances, wind velocity, and mass-loss rate. With future application to related objects, such as novae and supernovae, our new code should also improve our understanding of these objects with extended outflowing atmospheres.

1,115 citations


Additional excerpts

  • ...Recently, some attempts have been made to relax certain assumptions of the standard model (Hillier 1996, and references therein), and spectral analysis using clumped wind models has been attempted (Schmutz 1997 ; Hillier & Miller 1998)....

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Journal ArticleDOI
TL;DR: In this paper, numerical radiation-hydrodynamics simulations of the nonlinear evolution of instabilities in radiatively driven stellar winds have been performed and the results show a strong tendency for the unstable flow to form rather sharp rarefactions in which the highest speed material has very low density.
Abstract: Numerical radiation-hydrodynamics simulations of the nonlinear evolution of instabilities in radiatively driven stellar winds have been performed. The results show a strong tendency for the unstable flow to form rather sharp rarefactions in which the highest speed material has very low density. The qualitative features of the model agree well with the reqirements of displaced narrow absorption components in UV lines.

643 citations


Additional excerpts

  • ...This might be explained with shocks propagating toward the star, in the wind rest frame, like the ““ reverse shocks ÏÏ found by Owocki et al. (1988) in one-dimensional simulations of radiatively driven winds....

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Journal ArticleDOI
TL;DR: In this article, it is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles.
Abstract: It is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles. These estimators are used to determine terminal velocities, v(infinity), for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars, the values are typically 15-20 percent smaller than the extreme violet edge velocities, v(edge), while for WR stars v(infinity) = 0.76 v(edge) on average. New mass-loss rates for WR stars which are thermal radio emitters are given, taking into account the new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. The relationships between v(infinity), the surface escape velocities, and effective temperatures are examined. 67 refs.

362 citations

Journal ArticleDOI

356 citations


Additional excerpts

  • ...On the other hand, the He II j5411 line in the WN stars is likely not to be optically thin, which means that the estimated size and location of the LER will be biased by the fact that optically thick lines tend to be naturally rounded (Castor 1970)....

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