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.     

Non-thermal emission processes in massive binaries

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons, the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars. Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet binary—will be considered in details. Finally, strategies for future developments in this field will be discussed.     

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.     

Modelling the spectra of colliding winds in the Wolf–Rayet WC7+O binaries WR 42 and WR 79

We have obtained complete phase coverage of the WC7+O binaries WR 42 = HD 97152 and WR 79 = HD 152270 with high signal-to-noise ratio (S/N), moderate-resolution spectra. Remarkable orbital phase-locked profile variations of the C  iii λ 5696 line are observed and interpreted as arising from colliding wind effects. Within this scenario, we have modelled the spectra using a purely geometrical model that assumes a cone-shaped wind–wind interaction region which partially wraps around the O star. Such modelling holds the exciting promise of revealing a number of interesting parameters for WR+O binaries, such as the orbital inclination, the streaming velocity of material in the interaction region and the ratio of wind momentum flux. Knowledge of these parameters in turn leads to the possibility of a better understanding of WR star masses, mass-loss rates and wind region characteristics.     

Wind Ionization Structure of the Short-Period Eclipsing LMC Wolf-Rayet Binary BAT99-129: Preliminary Results

BAT99-129 is a rare, short-period eclipsing Wolf-Rayet binary in the Large Magellanic Cloud. We present here medium-resolution NTT/EMMI spectra that allow us to disentangle the spectra of the two components and find the orbital parameters of the binary. We also present VLT/FORS1 spectra of this binary taken during the secondary eclipse, i.e. when the companion star passes in front of the Wolf-Rayet star. With these data we are able to extract, for the first time in absolute units for a WR + O binary, the sizes of the line emitting regions.Based on observations obtained at the La Silla and Paranal Observatories, European Southern Observatory (Chile).     

First wavelet analysis of emission line variations in Wolf-Rayet stars

The quantification of stochastic substructures seen propagating away from the centers of emission lines of Wolf-Rayet (WR) stars is extended using the powerful, objective technique of wavelet analysis. Results for the substructures in one WR star so far show that the scaling laws between (a) flux and velocity dispersion and (b) lifetime and flux, combined with (c) their mass spectrum, strongly support the hypothesis that we are seeing the high mass tail-end distribution of full-scale supersonic compressible turbulence in the winds. This turbulence sets in beyond a critical radius from the star and shows remarkable similarity to the hierarchy of cloudlets seen in giant molecular clouds and other components of the ISM.The velocity dispersion is larger on average for substructures (interpreted as density enhanced turbulent eddies) propagating towards or away from the observer, suggesting that the turbulence is anisotropic. This is not surprising, since the most likely force which drives the windand the ensuing turbulence alike, radiation pressure, is directed outwards in all directions from the star. It is likely that a similar kind of turbulence prevails in the winds of all hot stars, of which those of WR stars are the most extreme.The consequences of clumping in winds are numerous. One of the most important is the necessary reduction in the estimate of the mass-loss rates compared to smooth outflow models.     

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.     

Non-thermal emission in Wolf–Rayet stars: are massive companions required?

We examine the radio spectral indices of 23 Wolf–Rayet (WR) stars to identify the nature of their radio emission. We identify nine systems as non-thermal emitters. In seven of these systems the non-thermal emission dominates the radio spectrum, while in the remaining two it is of comparable strength to the thermal, stellar wind emission, giving 'composite' spectra. Among these nine systems, seven have known spectroscopic or visual binary companions. The companions are all massive O or early B-type stars, strongly supporting a connection between the appearance of non-thermal emission in WR stars and the presence of a massive companion. In three of these binaries, the origin of non-thermal emission in a wind-collision region between the stars has been well established in earlier work. The binary systems that exhibit only thermal emission are all short‐period systems where a wind-collision zone is deep within the opaque region of the stellar wind of the WR star. To detect non-thermal emission in these systems requires optically thin lines of sight to the wind-collision region.     

Eclipsing Binaries in Open Clusters

The study of detached eclipsing binaries in open clusters can provide stringent tests of theoretical stellar evolutionary models, which must simultaneously fit the masses, radii, and luminosities of the eclipsing stars and the radiative properties of every other star in the cluster. We review recent progress in such studies and discuss two unusually interesting objects currently under analysis. GV Carinae is an A0 m + A8 m binary in the Southern open cluster NGC 3532; its eclipse depths have changed by 0.1 mag between 1990 and 2001, suggesting that its orbit is being perturbed by a relatively close third body. DW Carinae is a high-mass unevolved B1 V + B1 V binary in the very young open cluster Collinder 228, and displays double-peaked emission in the centre of the Hα line which is characteristic of Be stars. We conclude by pointing out that the great promise of eclipsing binaries in open clusters can only be satisfied when both the binaries and their parent clusters are well-observed, a situation which is less common than we would like.     

A 3D dynamical model of the colliding winds in binary systems

We present a three-dimensional (3D) dynamical model of the orbital-induced curvature of the wind–wind collision region in binary star systems. Momentum balance equations are used to determine the position and shape of the contact discontinuity between the stars, while further downstream the gas is assumed to behave ballistically. An Archimedean spiral structure is formed by the motion of the stars, with clear resemblance to high-resolution images of the so-called 'pinwheel nebulae'. A key advantage of this approach over grid or smoothed particle hydrodynamic models is its significantly reduced computational cost, while it also allows the study of the structure obtained in an eccentric orbit. The model is relevant to symbiotic systems and γ-ray binaries, as well as systems with O-type and Wolf–Rayet stars.
As an example application, we simulate the X-ray emission from hypothetical O+O and WR+O star binaries, and describe a method of ray tracing through the 3D spiral structure to account for absorption by the circumstellar material in the system. Such calculations may be easily adapted to study observations at wavelengths ranging from the radio to γ-ray.     

Spectral analyses of [WCL]-type central stars

The optical spectra of the seven late-type Wolf-Rayet central stars NGC-40 (PNG 120.0+09.8, subtype [WC8]), He 2-99 (PNG 309.0–04.2, [WC9]), BD+30°3639 (PN G 064.7+05.0, [WC9]), CPD-56°8032 (PNG 332.9–09.9, [WC11]), He2-113 (PNG 321.0+03.9, [WC11]), M4-18 (PNG 146.7+07.6, [WC11]) and K2-16 (PNG 352.9+11.4, [WC11]) are analyzed by means of spherically expanding model atmospheres. The NLTE simulations account for the elements hydrogen, helium, carbon and oxygen. As main results effective temperature, element abundances and final velocity of the wind are determined for each star. Assuming distances or luminosities for the objects, also the stellar radii and the mass-loss rates can be fixed. The results of these analyses establish empirical constraints for the evolutionary status of WC-type central stars as post-AGB objects and provide input for modeling their planetary nebulae.     

Colliding Winds in Binaries: Observations

Colliding winds in binaries are discussed mainly from an observational point of view. Collisions are especially energetic in the case of hot, luminous stars, which drive strong, fast winds. Emphasis is therefore devoted to binaries containing Wolf-Rayet stars. The subject is divided up into (1) continuum radiation (X-ray and non-thermal radio from the hot bow shock head, IR from dust formed in some WC + O binaries far downstream in the collision shock cone) and (2) line radiation (optical and UV, both from various regions downstream from the bow shock head). The latter is particularly useful in providing constraints on the velocity field and hence ultimately the geometry of the wind collision and the binary system itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

A spectrophotometric study of the Wolf-Rayet star HD 50896

New spectrophotometric observations of the Wolf-Rayet system HD 50896 are presented and interpreted in terms of its binary nature. The lines of N V, N IV and C IV show moderate variations, which can be explained using a binary model with a compact companion. He n λ4686 appears to arise from a larger region compared to other lines. The distortion caused by the wind can partly explain its flux variations. The emission fluxes of He I lines are generally constant indicating their non-participation in the orbit. The results are compared with other Wolf-Rayet binaries (V444 Cyg and CQ Cep) and the evolutionary status is discussed.     

Circumstellar shocks: colliding stellar winds

In this paper I will review some recent developments in the field of circumstellar shocks, particularly as they relate to colliding stellar winds. I shall review the basic physics of colliding winds and shocks, and discuss recent developments in hydrodynamic modelling of colliding winds. I shall also report on recent X-ray observations of shock emission in Wolf-Rayet binary systems where high resolution X-ray spectra of colliding wind shock emission is being seen. I will discuss the occurrence of colliding winds to such diverse systems as Wolf-Rayet binaries, pre-main sequence binaries, symbiotic stars as well as the Galactic center object IRS 7, where recent results on interacting winds are yielded insight into the structure of winds in general.     

X-ray spectroscopy of stars

Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.     

WR 146 – observing the OB-type companion

We present new radio and optical observations of the colliding-wind system WR 146 aimed at understanding the nature of the companion to the Wolf–Rayet (WR) star and the collision of their winds. The radio observations reveal emission from three components: the WR stellar wind, the non-thermal wind–wind interaction region and, for the first time, the stellar wind of the OB companion. This provides the unique possibility of determining the mass-loss rate and terminal wind velocity ratios of the two winds, independent of distance. Respectively, these ratios are 0.20±0.06 and 0.56±0.17 for the OB-companion star relative to the WR star. A new optical spectrum indicates that the system is more luminous than had been believed previously. We deduce that the 'companion' cannot be a single, low-luminosity O8 star as suggested previously, but is either a high-luminosity O8 star, or possibly an O8+WC binary system.     

Eclipsing Binaries in Local Group Galaxies

A review is presented of the progress that has been made in the last 3 years towards quantifying the properties of high-mass detached and semi-detached eclipsing binaries in Local Group galaxies. Comparisons between these observational results on masses, radii, temperatures and luminosities for stars in detached binaries and evolution models for single stars at the appropriate metallicity are found to be very good. New evolution models for interacting binaries passing through case A mass exchange are being calculated, and indicate a requirement for some mass loss to find agreement with the observational data. The observational data on such semi-detached systems show similar properties to those in the Milky Way galaxy. The directly-determined distances to all these eclipsing binaries are proving to be most valuable for strengthening the distance scale amongst the Local Group galaxies.     

Photometric analysis of 8 newly discovered short-period eclipsing binaries at Astronomical Observatory at Kolonica Saddle

We present photometric analysis of 8 short-period eclipsing binaries discovered and observed at Astronomical Observatory at Kolonica Saddle between 2007 and 2010 with different instruments. We determined their orbital periods and performed photometric analysis of their light curves. We found that 3 systems are detached binaries, 4 systems are over-contact binaries of W UMa type and one system is semi-detached with the secondary component filling its Roche lobe. Light curves of 2 systems exhibit asymmetries, explained by spot(s) on the surface of the components.     

Wavelet analysis of Wolf-Rayet emission line variability:

A wavelet analysis technique is developed and used to study variable spectral features on top of broad emission lines in Wolf-Rayet stars. A decomposition of the signal in discrete structures allows one to look for scaling laws. Results from the study of two Wolf-Rayet stars suggest we are seeing the upper tail-end of a scaled distribution. This provides a test for turbulence models, and sets new constraints on theoretical wind models.