Peredur M. Williams

BULK VELOCITIES, CHEMICAL COMPOSITION, AND IONIZATION STRUCTURE OF THE X-RAY SHOCKS IN WR 140 NEAR PERIASTRON AS REVEALED BY THE CHANDRA GRATINGS

The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastron passage of its 8 yr eccentric binary orbit in early 2001 as the two stars made their closest approach. Both stars have powerful supersonic stellar winds that crash into each other between the stars to produce X-rays. Chandra grating observations were made when the X-rays were at their peak, making WR 140 the brightest hot-star X-ray source in the sky and giving the opportunity to study the velocity profiles of lines, all of which were resolved and blueshifted before periastron. In the general context of shock physics, the measurements constrain the flow of hot gas and where different ions were made. The brightness of lines relative to the strong continuum in conjunction with plasma models gives interim abundance estimates for eight different elements in WC-type material including an Ne/S ratio in good agreement with earlier long-wavelength measurements. The lower velocity widths of cool ions imply a plasma that was not in equilibrium, probably due to the collisionless nature of the shock transitions and the slow character of both the postshock energy exchange between ions and electrons and subsequent ionization. Electron heat conduction into fast-moving preshock gas was absent, probably suppressed by the magnetic field involved in WR 140s synchrotron emission. After periastron, the spectrum was weaker due mainly to absorption by cool Wolf-Rayet star material.

Radio emission models of colliding-wind binary systems

We present calculations of the spatial and spectral distribution of the radio emission from a wide WR+OB colliding- wind binary system based on high-resolution hydrodynamical simulations and solutions to the radiative transfer equation. We account for both thermal and synchrotron radio emission, free-free absorption in both the unshocked stellar wind envelopes and the shocked gas, synchrotron self-absorption, and the Razin eect. To calculate the synchrotron emission several simplifying assumptions are adopted: the relativistic particle energy density is a simple fraction of the thermal particle energy density, in equipartition with the magnetic energy density, and a power-law in energy. We also assume that the magnetic field is tangled such that the resulting emission is isotropic. The applicability of these calculations to modelling radio images and spectra of colliding-wind systems is demonstrated with models of the radio emission from the wide WR+OB binary WR 147. Its synchrotron spectrum follows a power-law between 5 and 15 GHz but turns down to below this at lower and higher frequencies. We find that while free-free opacity from the circum-binary stellar winds can potentially account for the low-frequency turnover, models that also include a combination of synchrotron self-absorption and Razin eect are favoured. We argue that the high- frequency turn down is a consequence of inverse-Compton cooling. We present our resulting spectra and intensity distributions, along with simulated MERLIN observations of these intensity distributions. From these we argue that the inclination of the WR 147 system to the plane of the sky is low. We summarise by considering extensions of the current model that are important for models of the emission from closer colliding wind binaries, in particular the dramatically varying radio emission of WR 140.

High-resolution X-ray imaging of the colliding wind shock in WR 147

We analyze new high-resolution Chandra X-ray images of the Wolf-Rayet binary system WR147. This system contains a WN8 star with an early-type companion located 0:6 00 to its north, and is the only known early- type binary with a separation on the sky large enough for the wind-wind collision between the stars to currently be resolved at X-ray energies. The 5 ksec Chandra HRC-I image provides the rst direct evidence for spatially extended X-ray emission in an early-type binary system. The X-ray emission peaks close to the position of the radio bow shock and north of the WN8 star. A deeper X-ray image is needed to accurately determine the degree of spatial extension, to exactly align the X-ray and optical/radio frames, and to determine whether part of the detected X-ray emission arises in the individual stellar winds. Simulated X-ray images of the wind-wind collision have a FWHM consistent with the data, and maximum likelihood ts suggest that a deeper observation may also constrain the inclination and wind momentum ratio of this system. However, as the WR wind dominates the colliding wind X-ray emission it appears unlikely that _ M OB and v1OB can be separately determined from X-ray observations. We also note an inconsistency between numerical and analytical estimates of the X-ray luminosity ratio of the stronger and weaker wind components, and conclude that the analytical results are in error.

Recurrent dust formation by WR 48a on a 30‐year time‐scale★

We present infrared photometry of the WC8 Wolf–Rayet system WR 48a observed with telescopes at the European Southern Observatory, the South African Astronomical Observatory and the Anglo Australian Telescope between 1982 and 2011, which show a slow decline in dust emission from the previously reported outburst in 1978–79 until about 1997, when significant dust emission was still evident. This was followed by a slow rise, accelerating to reach and overtake the first (1978) photometry, demonstrating that the outburst observed in 1978–79 was not an isolated event, but that they recur at intervals of 32+ years. This suggests that WR 48a is a long-period dust maker and colliding-wind binary. The locus of WR 48a in the (H − L), K colour–magnitude diagram implies that the rate of dust formation fell between 1979 and about 1997 and then increased steadily until 2011. Superimposed on the long-term variation are secondary (‘mini’) eruptions in (at least) 1990, 1994, 1997, 1999 and 2004, characteristic of relatively brief episodes of additional dust formation. Spectra show evidence for an Oe or Be companion to the WC8 star, supporting the suggestion that WR 48a is a binary system and indicating a system luminosity consistent with the association of WR 48a and the young star clusters Danks 1 and Danks 2. The range of dust formation suggests that these stars are in an elliptical orbit having e ∼ 0.6. The size of the orbit implied by the minimum period, together with the WC wind velocity and likely mass-loss rate, implies that the post-shock WC wind is adiabatic throughout the orbit – at odds with the observed dust formation. A similar conflict is observed in the ‘pinwheel’ dust-maker WR 112.

FIRST VISUAL ORBIT FOR THE PROTOTYPICAL COLLIDING-WIND BINARY WR 140

Wolf-Rayet (WR) stars represent one of the final stages of massive stellar evolution. Relatively little is known about this short-lived phase and we currently lack reliable mass, distance, and binarity determinations for a representative sample. Here we report the first visual orbit for WR 140 (= HD193793), a WC7+O5 binary system known for its periodic dust production episodes triggered by intense colliding winds near periastron passage. The Infrared-Optical Telescope Array and Center for High Angular Resolution Astronomy interferometers resolved the pair of stars in each year from 2003 to 2009, covering most of the highly eccentric, 7.9 year orbit. Combining our results with the recently improved double-line spectroscopic orbit of Fahed et al., we find the WR 140 system is located at a distance of 1.67 ± 0.03 kpc, composed of a WR star with M WR = 14.9 ± 0.5 M_☉ and an O star with M O = 35.9 ± 1.3 M_☉. Our precision orbit yields key parameters with uncertainties ~6× smaller than previous work and paves the way for detailed modeling of the system. Our newly measured flux ratios at the near-infrared H and Ks bands allow a spectral energy distribution decomposition and analysis of the component evolutionary states.

A Westerbork Synthesis Radio Telescope 1400 and 350 MHz Continuum Survey of the Cygnus OB2 Association, in Search of Hot Massive Stars*

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13 and 55. The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2. We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin. We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

Dust Formation around WC Stars

The presence of heated circumstellar dust around WC type Wolf-Rayet stars is reviewed in terms of the episodic or persistent condensation of carbon grains in their stellar winds. At present, 19 WC stars are known to form dust persistently and 7 episodically. Infrared spectroscopy suggests that the grains are amorphous rather than graphitic carbon. The properties of the stars which make dust episodically are reviewed in terms of processes in colliding winds in WR binary systems and the possible applicability of these processes to the stars which make dust persistently in examined.

Near-infrared H2 emission from Herbig-Haro objects. I - A survey of low excitation objects

A survey for H/sub 2/ 1-0 S(1) emission in 16 Herbig-Haro (HH) objects and three exciting stars for HH objects is reported. Eleven HH objects which show low-excitation optical spectra exhibit H/sub 2/ emission. One object (HH 43) is more than twice as bright as any previously reported HH object. In addition, spectra in the range 1.6-2.55 microns are reported for HH 43 and HH 120, and a 2.0-2.55 micron spectrum is presented for HH 26. The spectra yield estimates of the H/sub 2/ density and temperature ranges in these objects. The role of ultraviolet H/sub 2/ emission-line fluorescence in HH 43 with respect to cascading among excited vibrational states of the ground electronic state is discussed. Models which may account for the combined ultraviolet, optical, and near-IR spectra of HHs are briefly analyzed. 35 references.

High-resolution infrared molecular hydrogen images and optical images of Herbig-Haro object 43

An IR image of HH 43 in the 1-0 S(1) emission line of molecular hydrogen has been obtained at high spatial resolution. Comparison of this image with optical CCD images obtained at comparable resolution in the emission lines of H-alpha and forbidden line S II reveals a very strong spatial correlation between the IR and optical emission. It is suggested that dissociative J-shocks, in which H2 can reform in the postshock gas, are present in HH 43. 14 references.

Suzaku monitoring of the Wolf-Rayet binary WR 140 around periastron passage: An approach for quantifying the wind parameters

Suzaku observations of the Wolf-Rayet (W-R) binary WR 140 (WC7pd+O5.5fc) were made at four different times around periastron passage in 2009 January. The spectra changed in shape and flux with the phase. As periastron approached, the column density of the low-energy absorption increased, which indicates that the emission from the wind-wind collision plasma was absorbed by the dense W-R wind. The spectra can be mostly fitted with two different components: a warm component with kBT = 0.3-0.6 keV and a dominant hot component with kBT ∼ 3 keV. The emission measure of the dominant, hot component is not inversely proportional to the distance between the two stars. This can be explained by the O star wind colliding before it has reached its terminal velocity, leading to a reduction in its wind momentum flux. At phases closer to periastron, we discovered a cool plasma component in a recombining phase, which is less absorbed. This component may be a relic of the wind-wind collision plasma, which was cooled down by radiation, and may represent a transitional stage in dust formation.