Galactic distribution of WR stars

The recent VIIth Catalogue of Galactic Wolf-Rayet Stars lists 227 Population I WR stars, comprising 127 WN, 87 WC, 10 WN/WC and 3 WO stars. Additional discoveries bring the census to 234 WR stars. A re-determination of the optical photometric distances and the galactic distribution of WR stars shows in the solar neighbourhood a projected surface density of 2.7 WR stars per kpc², a N _WC/N _WN number ratio of 1.3, and a WR binary frequency of 40 %.The galactocentric distance (R _WR) distribution per subtype showsR _WN and R _WC decreasing with WN and WC subtypes. This revised version was published online in September 2006 with corrections to the Cover Date.     

Sample of Wolf-Rayet galaxies from the SLOAN digital sky survey

We have analyzed the spectra of blue compact dwarf galaxies from the SLOAN Digital Sky Survey (SDSS) Data Release 7 and created a sample of 271 galaxies with Wolf-Rayet (WR) spectral features produced by high-velocity stellar winds. A blue WR feature (bump) is a blend of the N V λλ 460.5 and 462.0 nm, N III λλ 463.4 and 464.0 nm, C III λ 465.0 nm, C IV λ 465.8 nm, and He II λ 468.6 nm emission lines. A red WR feature (bump) is the broad C IV λ 580.8 nm emission. The blue WR bump is mainly due to emissions of nitrogen WR (WN) stars, while the red bump is fully produced by emissions of carbon WR (WC) stars. All the sample spectra show the blue WR bumps, whereas the red WR bumps are only identified in 50% of sample spectra. We have derived the numbers of early-type WC stars (WCE) and late-type WN stars (WNL) in the galaxies using the luminosities of single WC and WN stars in the red and blue bumps, respectively. The number of O stars is estimated using the H_β luminosity. The ratio of the overall number of WR stars of different types to the number of all massive stars N(WR)/N(O + WR) decreases with decreasing metallicity, corresponding to the evolution population synthesis models.     

X-ray Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra

We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. These observations are aimed at determining the differences in X-ray properties between massive WR + OB binary systems and putatively single WR stars. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission (including the Fe Kα line complex), characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels.     

First stars. I. Evolution without mass loss

The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M _⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M _⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10⁻⁶ and 10⁻⁹, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.     

A quantitative analysis of the prototype [WCL] star CPD-56° 8032

We present a detailed, quantitative study of the standard [WC10] Wolf-Rayet central star CPD-56^o 8032 based on new high resolution AAT UCLES observations and the Hillier (1990) WR standard model. Our analysis of CPD-56^o 8032 gives the wind properties (T _*=34500K, lg (L/L )=3.8, lg (M/M a^–1)=–5.4,v =225 km s^–1) and chemistry (C/He=0.5, O/He=0.1, by number), the latter suggesting an intimate relationship with the Ovi PN central stars and the PG 1159-035 objects. A comparison between the wind properties of CPD-56^o 8032 and Sk-66^o 40 (WN 10) indicates that low excitation, low wind velocity WR winds are common to both low mass PN central stars (WC sequence) and high mass post-LBV's (WN sequence).     

The effect of massive binaries on stellar populations and supernova progenitors

We compare our latest single and binary stellar model results from the Cambridge stars code to several sets of observations. We examine four stellar population ratios: the number of blue to red supergiants, the number of Wolf–Rayet stars to O supergiants, the number of red supergiants to Wolf–Rayet stars and the relative number of Wolf–Rayet subtypes, WC to WN stars. These four ratios provide a quantitative measure of nuclear burning lifetimes and the importance of mass loss during various stages of the stars' lifetimes. In addition, we compare our models to the relative rate of Type Ib/c to Type II supernovae to measure the amount of mass lost over the entire lives of all stars. We find reasonable agreement between the observationally inferred values and our predicted values by mixing single and binary star populations. However, there is evidence that extra mass loss is required to improve the agreement further, to reduce the number of red supergiants and increase the number of Wolf–Rayet stars.     

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.     

Massive stellar populations in Wolf-Rayet galaxies

The analysis of the long-slit spectral observations of 40 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over two orders of magnitudes from Z_⊙/50 to 2Z_⊙ are presented. We derive the number of O stars from the luminosity of the Hβ emission line, the number of early carbon Wolf-Rayet stars (WCE) from the luminosity of the red bump (broad CIV λ5808 emission) and the number of late nitrogen Wolf-Rayet stars (WNL) from the luminosity of the blue bump (broad emission near λ4650). We identified some of weak WR emission lines, most often the N III λ4512 and Si III λ4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. A new technique for deriving the number of WNL stars (WN7–WN8) from the N III λ4512 and the number of WN9–WN11 from Si III λ4565 emission lines has been proposed. This technique is potentially more precise than the blue bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. We find that the fraction of WR stars relative to all massive stars increases with increasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue and red bumps derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models.     

New Wolf–Rayet central stars of planetary nebulae identified on the AAO/UKST Hα Survey

The central stars of two of the new planetary nebulae found during scans of the AAO/UKST H α Survey of the Milky Way have been found to exhibit Wolf–Rayet (WR) emission features. One (PMR 1) is an early-type star of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC] star which, depending on the classification scheme used, is either intermediate in class between [WC9] and [WC10] or the sole member of the [WC10] class. Both stars exhibit unusual spectral features which may be attributed to enhanced nitrogen in their atmospheres and could be indicative of unusual stellar evolution.     

The Demarcation Point from E-AGB Stars to TP-AGB Stars in HR Diagram for Medium-mass Stars

Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested.     

Photometric variability of WC9 stars

Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of  ∼ 0.1 μm  . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level (  σ∼ 0.005– 0.01  mag).     

Properties of massive stars in four clusters of the VVV survey

The evolution of massive stars is only partly understood. Observational constraints can be obtained from the study of massive stars located in young massive clusters. The ESO Public Survey “VISTA Variables in the Vía Lácteá (VVV)” discovered several new clusters hosting massive stars. We present an analysis of massive stars in four of these new clusters. Our aim is to provide constraints on stellar evolution and to better understand the relation between different types of massive stars. We use the radiative transfer code CMFGEN to analyse K-band spectra of twelve stars with spectral types ranging from O and B to WN and WC. We derive the stellar parameters of all targets as well as surface abundances for a subset of them. In the Hertzsprung–Russell diagram, the Wolf–Rayet stars are more luminous or hotter than the O stars. From the log(C/N)–log(C/He) diagram, we show quantitatively that WN stars are more chemically evolved than O stars, WC stars being more evolved than WN stars. Mass loss rates among Wolf–Rayet stars are a factor of 10 larger than for O stars, in agreement with previous findings.     

Spectral variability of LBV star V 532 (Romano’s star)

We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness difference of ΔB ≈ 2.3 ^m . The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the bona fide WNL stars.     

WR 38/38a and the ratio of total-to-selective extinction in Carina

A reanalysis of the (seemingly very distant) open cluster Shorlin 1, the group of stars associated with WR 38 and WR 38a, is made on the basis of existing UBV and JHK _s observations for cluster members. The 2MASS observations, in particular, imply a mean cluster reddening of E _B−V =1.45±0.07 and a distance of 2.94±0.12 kpc. The reddening agrees with the UBV results provided that the local reddening slope is described by E _U−B /E _B−V =0.64±0.01, but the distance estimates in the 2MASS and UBV systems agree only if the ratio of total-to-selective extinction for the associated dust is R=A _V /E _B−V =4.0±0.1. Both results are similar to what has been obtained for adjacent clusters in the Eta Carinae region by similar analyses, which suggests that “anomalous” dust extinction is widespread through the region, particularly for groups reddened by relatively nearby dust. Dust associated with the Eta Carinae complex itself appears to exhibit more “normal” qualities. The results have direct implications for the interpretation of distances to optical spiral arm indicators for the Galaxy at ℓ=287°–291°, in particular the Carina arm here is probably little more than ∼2 kpc distant, rather than 2.5–3 kpc distant as implied in previous studies. Newly-derived intrinsic parameters for the two cluster Wolf-Rayet stars WR 38 (WC4) and WR 38a (WN5) are in good agreement with what is found for other WR stars in Galactic open clusters, which was not the case previously.     

On the Formation of Massive Primordial Stars

We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 M _⊙. The high accretion rate used in the models (M _⊙ = 4.4 x 10^-3 M_⊙ yr^-1) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the in falling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M _⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 M _⊙. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions. This revised version was published online in September 2006 with corrections to the Cover Date.     

Stellar population of the irregular galaxy IC 10

Based on our observations with the 6-m BTA telescope at the Special Astrophysical Observatory, the Russian Academy of Sciences, and archival Hubble Space Telescope images, we have performed stellar photometry for several regions of the irregular galaxy IC 10, a member of the Local Group. Distance moduli with a median value of m − M = 24.47, D = 780 ± 40 kpc, have been obtained by the TRGB method for several regions of IC 10. We have revealed 57 star clusters with various masses and ages within the fields used. Comparison of the Hertzsprung-Russell diagrams for star clusters in IC 10 with theoretical isochrones has shown that this galaxy has an enhanced metallicity, which probably explains the high ratio of the numbers of carbon and nitrogen Wolf-Rayet stars (WC/WN). The size of the galaxy’s thick disk along its minor axis is 10′.5 and a more extended halo is observed outside this disk.     

Effect of the star limb darkening law on the probabilities of discovering detached close binary stars as eclipsing variables

The probabilities of discovering detached close binary (type DM) stars as eclipsing variables are calculated as a function of the mass of the main component, mass ratio, major semiaxis, and angle of inclination of the orbit. The case of total limb darkening (hypothesis “D”) is examined. This is compared with earlier results for uniformly bright stellar disks (hypothesis “U”). Based on data from Svechnikov and Kuznetsova’s Catalog of Approximate Photometric and Absolute Elements of Eclipsing Variables, the spatial density of stars of this type in the neighborhood of the sun is estimated to be ≈ 460 · 10 ⁻⁶ pc⁻³. __________ Translated from Astrofizika, Vol. 49, No. 1, pp. 151–169 (February 2006).     

Simulating H2O Maser Emission in the Mass Outflows from Evolved Stars

The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H₂O maser emission from the circumstellar envelopes of long period variable stars. The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H₂O maser emission. As is observed in H₂O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H₂O maser regions are in conditions of n_H2 = 10⁶ − 10⁸ cm⁻³, assuming a fractional abundance of 10⁻⁴; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate based dust particles can produce the observed data on small (10-mas) angular scales. This revised version was published online in September 2006 with corrections to the Cover Date.     

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.     

Magellanic Cloud WC/WO Wolf–Rayet stars – II. Colliding winds in binaries

A search for evidence of colliding winds is undertaken among the four certain Magellanic Cloud WC/WO spectroscopic binaries found in the companion Paper I, as well as among two Galactic WC/WO binaries of very similar subtype. Two methods of analysis, which allow the determination of orbital inclination and parameters relating to the shock cone from spectroscopic studies of colliding winds, are attempted. In the first method, Lührs' spectroscopic model is fitted to the moderately strong C  iii 5696-Å excess line emission arising in the shock cone for the stars Br22 and WR 9. The four other systems show only very weak C  iii 5696-Å emission. Lührs' model follows well the mean displacement of the line in velocity space, but is unable to reproduce details in the line profile and fails to give a reliable estimate of the orbital inclination. In the second method, an alternative attempt is also made to fit the variation of more global quantities, full width at half-maximum and radial velocity of the excess emission, with phase. This method also gives satisfactory results in a qualitative way, but shows numerical degeneracy with orbital inclination. Colliding wind effects on the very strong C  iv 5808-Å Wolf–Rayet emission line, present in all six binaries, are also found to behave qualitatively as expected. After allowing for line enhancement in colliding wind binaries, it now appears that all Magellanic Cloud WC/WO stars occupy a very narrow range in spectral subclass: WC4/WO3.