Mass loss from Wolf-Rayet stars

The observed times of minimum light derived from the photometry of the Wolf-Rayet eclipsing binary stars CQ Cep and V444 Cyg are used to estimate the mass-loss rate of the Wolf-Rayet components in several modes of mass-loss and mass-exchange.     

Mass loss from Wolf-Rayet stars

Mass loss rates for 9 LMC WR stars are determined using IUE, UV, and visible spectrophotometric observations. A good correlations of mass loss rate with effective temperature and luminosity is indicated by the data, in agreement with the theoretical predictions.     

Initial mass functions of Wolf-Rayet stars

On the basis of evolutionary tracks on the HR diagram the lower limit of initial mass functions for Wolf-Rayet stars are estimated. The lower limit to the initial masses of the Wolf-Rayet stars seems to be 20M and in this respect there is no significant difference between the WN and WC stars.     

Unsteady mass outflow from Wolf-Rayet stars

We show that hydrostatically equilibrium models for the thin photospheres of helium stars based on new opacities κ_R (OPAL and OP) can be constructed only for masses M<5M _⊙. The parameter Г=κL/4πGMc, defined as the ratio of light pressure to gravity, exceeds a critical value of 1.0 for larger masses, which must result in mass outflow under light pressure. This mass limit matches the observed lower limit for the masses of Wolf-Rayet stars (M _WR>5M _⊙)), which is an additional argument that the Wolf-Rayet stellar cores are actually helium stars. By solving the equation of radiative transfer in extended atmospheres, we construct a semiempirical model for a WN5 star (M _WN5=10M _⊙)) with a helium core and an expanding envelope, whose physical and geometric parameters are known mainly from light-curve solution for the eclipsing binary V444 Cyg (WN5+06): outflow rate $\dot M \approx 1.0 \times 10^{ - 5} M_ \odot yr^{ - 1} $ , terminal velocity V _∞≈2000 km s^?1, and expanding-envelope optical depth τ_env≈25. The temperature at the outer boundary of the photosphere of a helium star surrounded by such an envelope is approximately 130 kK higher than that in the absence of an envelope, being T _ph≈240 kK. Because of the high temperatures, the absorption coefficients at the corresponding photospheric levels are smaller than those in models with no envelope; therefore, the photosphere turns out to be in hydrostatic equilibrium and stable against light pressure (Г_max≈0.9). As a way out of this conflicting situation (an expanding envelope together with a hydrostatically equilibrium photosphere), we propose a model of discrete mass outflow, which is also supported by the observed cloudy structure of the envelopes in this type of stars. To quantitatively estimate parameters of the nonuniform outflow model requires detailed gas-dynamical calculations.     

Wolf-Rayet stars

This paper reviews the current status of knowledge regarding the basic physical and chemical properties of Wolf-Rayet stars; their overall mass loss and stellar wind characteristics and current ideas about their evolutionary status. WR stars are believed to be the evolved descendents of massive O-type stars, in which extensive mass loss reveals successive stages of nuclear processed material: WN stars the products of interior CNO-cycle hydrogen burning, and WC and WO stars the products of interior helium burning. Recent stellar evolution models, particularly those incorporating internal mixing, predict results which are in good accord with the different chemical compositions observationally inferred for WN, WC and WO stars. WR stars exhibit the highest levels of mass loss amongst earlytype stars: mass loss rates, typically, lie in the range [1–10]×10⁻⁵ M _⊙yr⁻¹. Radiation pressure-driven winds incorporating multi-scattering in high ionisation-stratified winds may cause these levels, but additional mechanisms may also be needed.     

Wolf-Rayet stars

Summary Recent literature on Population I Wolf-Rayet star research extending from the Milky Way to blue compact dwarf galaxies is reviewed, broken down into inventory, basic parameters and galactic distribution, atmospheres, binaries, intrinsic variability, mass loss, enrichment and evolution. Also the incidence of Wolf-Rayet stars with variable non-thermal radio emission, excess X-ray fluxes, and episodic/periodic IR excesses is reviewed. These phenomena appear to be associated with wind-wind interaction in wide long-period WR+OB binaries and with wind-compact object interaction in WR+c binaries, with orbit sizes of the order of magnitude of the WR radio photosphere sizes or larger.     

Infrared excess in Wolf-Rayet stars

On the basis of continuum energy distributions in the wavelength region 3200–11 000 Å, effective temperature for 14 Wolf-Rayet stars are estimated by comparing with Kurucz (1979) model atmospheres. The continuum energy distribution curve shows strong infrared excess emissions above of 5000 Å in every star.     

Observations of southern Wolf-Rayet stars

The Wolf-Rayet star HD 90657 is found to be a spectroscopic binary with a WN5+O6 spectrum and a period of 6.456 days. Preliminary orbital elements show an elliptic orbit and a mass ratioM _WR/M _o0.5. Evidence is presented for the Wolf-Rayet atmosphere being accelerated outwards and not being spherically symmetric.Operated under an agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), the Universidad de Buenos Aires and the Comisión Nacional de Estudios Geoheliofísicos.     

The evolution of massive stars with mass loss

The evolution of massive stars is investigated in the phases of hydrogen and helium burning, taking into account the mass-loss due to light pressure in optically thick media. The evolution in the stage of hydrogen burning near the Main Sequence occurs without mass loss. The large inverse density gradient appears in the outer layers of a 30 M star after it goes into the domain of red super-giants in the helium-burning stage. This effect appears as a consequence of an excess of luminosity of the star the ciritical one in sufficiently extensive outer layer, where convection is not so effective. In this way, the conditions for outflow of matter are formed. The sequence of selfconsistent models is constructed, with the core in hydrostatic equilibrium and hydrodynamically outflowing envelope. The amount of mass loss is not a given parameter, but it is found during the calculations as a characteristic number of the problem. The amount of mass loss is very high, of the order of 0.5M yr, the velocity of the flow is 20 km s^–1. The star loses about 7.2M during 15 yr. The amount of mass loss must rapidly decrease or finish altogether when matter near the hydrogen-burning layer begins to flow out, and a transformation of stellar structure must occur.The evolution of a 9M star is calculated. The density in the envelope of this star is sufficiently large and the outer convective zone, which develops on the red giant stage, prevents the outflow of matter. The intensive mass outflow from such star can take place at the carbon burning, or heavier element burning stages. The formation of infrared stars and Wolf-Rayet stars can be possibly explained by such a mechanism of mass loss, so that the infrared stage must precede the Wolf-Rayet stage.     

The Wolf-Rayet stars at high (spatial) resolution

The Wolf-Rayet stars represent an advanced stage of evolution of the most massive stars. Their next immediate stadium will be supernova explosion. The most striking property of this very rare but exceptionally hot and bright objects is their extreme mass loss, of the order of 10^-5 solar mass per year. In turn of evolution before and during the Wolf-Rayet phase such stars eject a lot of matter (∼ 10 M_⊙) with velocity up to 3000 km/s that surrounds the min the form of gas and dust. In the case of binary Wolf-Rayet star such expanding envelope may interact with a companion (usually hot OB star) wind forming a tail extended for ∼ 100 AU. This spectacular phenomenon as well as some other connected with Wolf-Rayet stars that can be studied with high spatial resolution instruments (both astrometric and imaging) are reviewed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Narrow-band photometry of Wolf-Rayet stars and planetary nebulae

We present observational data in the (16)(9)(25)-photometric system for 11 Wolf-Rayet stars and 7 planetary nebulae. The results show anomalous (16)(9)-indices for these emission objects. Six W-R stars show possible variations in the strength of the HeII line at 6560 Å, perhaps none in the strength of the HeI line at 10830 Å. In spite of these misconstrued (16) and (9)-indices, most WC stars are separated from WN stars in the (16)(9)-array; and PN, in this diagram lie far apart from all kind of stellar objects. The results, yielded by the (25)-index, indicate that the HeI and HeII lines at 10830 Å and 10124 Å, respectively, are probably well suited for photometric examination in W-R stars, PN and other peculiar objects.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Observational properties of population I Wolf-Rayet stars

The classification, numbers, physical parameters, energy distributions, and ring nebulae of Pop. I Wolf-Rayet (WR) stars are reviewed. The numbers of known WR stars in the Galaxy, in the LMC and in the SMC are 195, 114, and 9, respectively.     

The interplay of mass loss and core overshooting in massive stars

In this study analytic models are used in an attempt to constrain the overshooting parameter in massive stars. It is found that core overshooting up to of order 20% is required to explain the non-existence of red supergiants withM _bol<–10. Furthermore, a critical mass of 60M marks the upper mass limit for stars that can undergo a red supergiant phase. The analysis employed clearly demonstrates an important interplay between mass loss and core overhooting-highlighting the difficulty of establishing the uniqueness of a given set of results.     

Properties of Wolf-Rayet stars in eclipsing binary systems

The results of investigations of a number of eclipsing Wolf-Rayet binaries are presented. The ‘core’ radiuses, the ‘core’ temperatures and masses of WR stars in the eclipsing WR+OB binary systems V 444 Cyg, CX Cep, CQ Cep, and CV Ser are obtained (see Table I). The results obtained from the light curves analysis of the V 444 Cyg in the range λλ2460 Å-3.5μ give strong evidence for the Beals (1944) model of WR phenomenon. The chromospheric-coronal effects in the WN5 extended atmosphere are not observed up to a distance ofr?20R _⊙. In the Hertzsprung—Russell diagram all the WR stars lie on the left side from the main sequence between the main sequence and the sequence of uniform helium stars (see Figure 9). Their locations are close to those of the helium remnants formed as a result of mass exchange in massive close binary systems. The period variations in the systems V 444 Cyg and CQ Cep have been discovered and a reliable value of the mass loss rateM=10^?5 M _⊙yr^?1 is obtained, for the two WR stars. The results of the photometric and spectroscopic investigations of the WR stars with low mass companions (post X-ray binary stage?) are presented too (see Table II). The masses of the companions are (1–2)M _⊙, their optical luminosity is ～10³⁶, erg s^?1 which implies that these companions cannot be the normal stars. It is possible that these companions are neutron stars accreting from the stellar wind of the WR stars. Low values of the X-ray luminosities of such WR stars with low mass companions imply that the accretion of matter in such systems is distinct from the accretion process in classical X-ray binary systems. It is noted also that the parameters of low massive companions coupled with WR stars are close to those of helium stars.     

Evidence for mass loss in visual binary stars

Summary The present status of the observational evidence about mass loss in visual binary and multiple systems is reviewed. Particular emphasis is placed on the importance of mass-loss rates measures in these stars, as a systematic study would provide useful constraints in two fields. First, as mass-outflows processes are not modified by the interaction of the two stars, and distances, masses, and evolutionary stages are usually fairly good known, an absolute stellar scale for can more easily and accurately be derived for these systems. Second, rates are essential to study the detailed orbital evolution of wide pairs, providing clues to their origin. Detailed numerical simulations including mass loss have been carried out. It is suggested that mass loss may explain in part the recent statistics on separation of wide visual binaries as a function of spectral type and age.     

Very massive stars: Evolution with mass loss

The evolution of mass-losing very massive stars in the 500–10000M _⊙ range has been investigated for two different initial compositions, (X, Z)=(0.8,0.0) and (X, Z)=(1.0,0.0). The evolutionary tracks are governed by two opposing factors which are the increase in the mean molecular weight in the convective core and the effect of mass loss. Conservative evolution of stars with massM?10000M _⊙ is similar to that of massive stars (20–100M _⊙), always moving to lower effective temperatures. For low values of the standard mass loss parameterN (50?N?200) the two opposing factors are almost in balance and the star is forced to move in a series of loops. For higher mass loss rates the loops disappear. In the 10000M _⊙ case no loops are observed and the tracks always move to higher effective temperatures. For a given mass loss rate the transition between right and left moving tracks occurs at higher masses the lower is the mass loss rate.     

Rocket observations of mass loss from hot stars

Rocket observations have shown that the far-ultraviolet resonance lines have P-Cygni profiles in the spectra of many hot stars, including of and Wolf-Rayet stars and OB supergiants. Velocity shifts as high as–300- km sec^–1 have been measured for the short-wavelength edges of some of the lines. Estimates of the rates of mass loss range from 10^–8 to 10^–6 M year^–1.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Very massive stars: Evolution with mass loss

In a previous paper (Klapp, 1983) we have described the evolution of metal-free mass-losing very massive stars (VMS) in the 500–10 000M range. The present paper concerns to the nucleosynthesis and mass loss aspects of the hydrogen- and helium-burning phase. Through radiation driven winds, the star losses 20–40% of its mass as helium and 1% as carbon and oxygen. The results show that VMS cannot produce the cosmological helium abundance without overproducing heavier elements. The high oxygen yield of VMS makes them the best candidates for producing an oxygen overabundance in old Population II stars.     

2MASS and WISE data analysis of Galactic Wolf-Rayet stars

We collected almost all Galactic Wolf-Rayet (hereafter WR) stars found so far from the literature. 578 WR stars are gathered in this paper. 2MASS counterparts with good quality magnitudes in all JHK bands are listed for 364 WR stars. In addition, WISE counterparts for these sources are also identified. It is found that free-free emission is the main dominant source for the infrared excess in most WR stars up to 3.4 μm. However at the longer wavelengths the thermal radiation is dominant. In addition, WR stars in Clusters of the Galactic center region have the strong infrared excess in the near infrared due to the dust thermal emission from the strong star forming activity in the Galactic center region. For some WR stars with the WC spectral type, in particular, with WCd type, the dust thermal emission is important radiation source while many WR stars with the WC spectral type have the near infrared flux enhancement from the broad line emission in the K band. It is also shown that many single WC stars with different spectral sub-types have different locations in the near infrared two-color diagram, in particular, WC6 and WC9d stars can be separated respectively from other spectral type stars while single WN stars with different spectral sub-types can not be separated in the near infrared two-color diagram.