Colliding-wind Binary Systems Research Articles

TheChandraHETGS X‐Ray Grating Spectrum of η Carinae

Eta Carinae may be the most massive and luminous star in the Galaxy and is suspected to be a massive, colliding wind binary system. The CHANDRA X-ray observatory has obtained a calibrated, high-resolution X-ray spectrum of the star uncontaminated by the nearby extended soft X-ray emission. Our 89 ksec CHANDRA observation with the High Energy Transmission Grating Spectrometer (HETGS) shows that the hot gas near the star is non-isothermal. The temperature distribution may represent the emission on either side of the colliding wind bow shock, effectively 'resolving' the shock. If so, the pre-shock wind velocities are approximately 700 and 1800 km/s in our analysis, and these velocities may be interpreted as the terminal velocities of the winds from 71 Carinae and from the hidden companion star. The forbidden-to-intercombination line ratios for the He-like ions of S, Si, and Fe are large, indicating that the line forming region lies far from the stellar photosphere. The iron fluorescent line at 1.93 angstroms, first detected by ASCA, is clearly resolved from the thermal iron line in the CHANDRA grating spectrum. The Fe fluorescent line is weaker in our CHANDRA observation than in any of the ASCA spectra. The CHANDRA observation also provides the first high-time resolution lightcurve of the uncontaminated stellar X-ray emission from 77 Carinae and shows that there is no significant, coherent variability during the CHANDRA observation. The 77 Carinae CHANDRA grating spectrum is unlike recently published X-ray grating spectra of single massive stars in significant ways and is generally consistent with colliding wind emission in a massive binary.

Multi-frequency variations of the Wolf-Rayet system HD 193793 (WC7pd+O4-5)

The colliding-wind binary system WR140 (HD193793, WC7pd+O4-5, P=7.94 yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35 short-wavelength high-resolution spectra. An absorption-line radial-velocity solution is obtained from the photospheric lines of the O component, by comparison with a single O star. The resulting orbital parameters, e=0.87 +/- 0.05, w=31 degrees +/- 9 degrees and Ko star=25 +/- 15 km s(-1), confirm the large eccentricity of the orbit, within the uncertainties of previous optical studies. This brings the weighted mean UV-optical eccentricity to e=0.85 +/- 0.04. Occultation of the O-star light by the WC wind and the WC+O colliding-wind region results into orbital modulation of the P-Cygni profiles of the C II, CIV and Si IV resonance lines. Near periastron passage, the absorption troughs of those resonance-line profiles increase abruptly in strength and width, followed by a gradual decrease. In particular, near periastron the blue black-edges of the P-Cygni absorption troughs shift to larger out ow velocities. We discuss that the apparently larger wind velocity and velocity dispersion observed at periastron could be explained by four phenomena: (i) geometrical resonance-line eclipse effects being the main cause of the observed UV spectral variability, enhanced by sightline crossing of the turbulent wind-wind collision zone; (ii) the possibility of an orbital-plane enhanced WC7 stellar wind; (iii) possible common-envelope acceleration by the combined WC and O stellar radiation fields; and (iv) possible enhanced radiatively driven mass loss due to tidal stresses, focused along the orbiting line of centers.

Simultaneous Radio and X‐Ray Observations of the Wolf‐Rayet Star WR 147

We present results of simultaneous ASCA/Very Large Array (VLA) observations of the Wolf-Rayet star WR 147 (AS 431). This WN 8 star is an optical double and may be a WR+OB colliding wind binary system. The new observations place tight constraints on the origin of its X-ray and radio emission. The X-ray emission is due to a multitemperature, optically thin thermal plasma, with the dominant contribution coming from plasma at kT ≈ 1 keV. The absorption column density derived from the X-ray spectrum is NH = 2 × 1022 cm-2, which agrees well with estimates based on the visual extinction but is too large to explain by wind absorption alone. The X-ray temperature structure is consistent with colliding wind shock emission, but the unabsorbed X-ray luminosity Lx = 1032.55 ergs s-1 (0.5-10 keV) is several times smaller than that predicted from colliding wind shock models. The VLA data provide the most complete picture ever obtained of the radio spectral energy distribution of a WR star and consist of near-simultaneous observations at five different wavelengths (1.3, 2, 3.6, 6, and 21 cm). The radio emission consists of a thermal free-free component from the WR wind and a nonthermal component. If the nonthermal emission is due to relativistic particles accelerated by the Fermi mechanism in wind shocks, then the flux is expected to decline at high frequencies according to Sν ∝ ν-0.5. However, the observed falloff is much steeper and cannot be reproduced by a simple power law or by synchrotron models that assume power-law electron energy distributions. A surprising result is that the nonthermal emission can be accurately modeled as synchrotron radiation from relativistic electrons that are nearly monoergetic.

VLA observations of the interacting-wind binary system WR 147

Wolf-Rayet stars are losing mass via their strong winds. Some of them show thermal and non-thermal radio emission and from this group some have companions forming binary systems. It has been suggested that the synchrotron emission arises from the interaction region where the winds of the two stars collide. WR 147 seems to be one of these systems, having a companion and emitting thermal as well as non-thermal radiation. Besides, it has been suggested that it has an anisotropic wind. In the present work we continue studying the WR 147 system by observing it at a radio wavelength of 3.6 cm in two different epochs. We have looked for evidence to support the idea that WR 147 is a colliding-wind binary system. We have also looked for time variations in the source during the time interval of ∼ 1.4 years separating our observations.

Stagnation-point flow in colliding-wind binary systems

The low X-ray temperatures and luminosities of colliding wind binaries compared to theoretical expectations has been a long standing problem. As a potential solution to this problem we present quantitative calculations of the radiation hydrodynamics in colliding wind binary systems, accounting for the effect of two radiation fields on the dynamics of both winds, using a formulation similar to that of Castor et al. (1975, ApJ 195, 157). A more detailed description of this work can be found in Stevens & Pollock (1994, MNRAS in press). Consider only the WR wind being driven under the influence of the WR and O-star radiation fields and motion only on the line-of-centers. Near the WR-star, its own radiation field dominates, and the mass-loss rate will be largely unaffected by the presence of the companion. However, moving towards the O-star, the O-star radiation field becomes a significant fraction of the total flux. As the O-star radiation field is opposed to the WR radiation field the radiative line-force will be diminished and the wind acceleration diminished. This will lead to the WR star wind colliding with the O-star wind at a lower velocity than would be expected from single star models. This mechanism will only be at work in close binary systems. In wide binaries both winds will be at terminal velocity before the other radiation field makes a difference. Results for a sample calculation are shown in Fig 1, using parameters for V444 Cyg from Schmutz et al. (1989, A&A 210, 236). In the absence of any deceleration effects the WR wind velocity at the shock would be ∼ 1300 km s−1, corresponding to kT ∼ 2 keV. The reduction in the WR star velocity at the shock interface caused by the O-star radiation field is predicted to be ∼ x2 leading to kTx ∼ 0.5 keV, in line with X-ray observations.

Magnetic annihilation in colliding-wind binary systems

We investigate the possibility that a stagnation-point magnetic reconnection model may account for the particle acceleration necessary for the generation of nonthermal radio emission in the Wolf-Rayet binary systems exemplified by WR140.

Stagnation-point flow in colliding-wind binary systems

The low X-ray temperatures and luminosities of colliding wind binaries compared to theoretical expectations has been a long standing problem. As a potential solution to this problem we present quantitative calculations of the radiation hydrodynamics in colliding wind binary systems, accounting for the effect of two radiation fields on the dynamics of both winds, using a formulation similar to that of Castor et al. (1975, ApJ 195, 157). A more detailed description of this work can be found in Stevens & Pollock (1994, MNRAS in press). Consider only the WR wind being driven under the influence of the WR and O-star radiation fields and motion only on the line-of-centers. Near the WR-star, its own radiation field dominates, and the mass-loss rate will be largely unaffected by the presence of the companion. However, moving towards the O-star, the O-star radiation field becomes a significant fraction of the total flux. As the O-star radiation field is opposed to the WR radiation field the radiative line-force will be diminished and the wind acceleration diminished. This will lead to the WR star wind colliding with the O-star wind at a lower velocity than would be expected from single star models. This mechanism will only be at work in close binary systems. In wide binaries both winds will be at terminal velocity before the other radiation field makes a difference. Results for a sample calculation are shown in Fig 1, using parameters for V444 Cyg from Schmutz et al. (1989, A&A 210, 236).

Colliding wind bindary systems - Monte Carlo simulations of line-profile variability

We investigate the line-profile variability observed in colliding wind binary systems. In these systems the ultraviolet P Cygni line profiles show marked orbital variability; the nature of which differs between individual lines and binaries. We present results of three-dimensional Monte-Carlo simulations of the radiative transfer and line formation in colliding wind binaries. We present results for an idealized binary system, illustrating some of the factors that will affect line-profile formation in such systems, and the potential complexity of interpreting such variability. In addition, we calculate the expected line-profile variability for the archetypal W-R+O binary V444 Cyg