Effect of the Coriolis Force on the Hydrodynamics of Colliding Wind Binaries

Using fully three-dimensional hydrodynamic simulations, we investigate the effect of the Coriolis force on the hydrodynamic and observable properties of colliding wind binary systems. To make the calculations tractable, we assume adiabatic, constant velocity winds. The neglect of radiative driving, gravitational deceleration, and cooling limit the application of our models to real systems. However, these assumptions allow us to isolate the effect of the Coriolis force, and by simplifying the calculations, allow us to use a higher resolution (up to 640^3) and to conduct a larger survey of parameter space. We study the dynamics of collidng winds with equal mass loss rates and velocities emanating from equal mass stars on circular orbits, with a range of values for the ratio of the wind to orbital velocity. We also study the dynamics of winds from stars on elliptical orbits and with unequal strength winds. Orbital motion of the stars sweeps the shocked wind gas into an Archimedean spiral, with asymmetric shock strengths and therefore unequal postshock temperatures and densities in the leading and trailing edges of the spiral. We observe the Kelvin-Helmholtz instability at the contact surface between the shocked winds in systems with orbital motion even when the winds are identical. The change in shock strengths caused by orbital motion increases the volume of X-ray emitting post-shock gas with T > 0.59 keV by 63% for a typical system as the ratio of wind velocity to orbital velocity decreases to V_w/V_o = 2.5. This causes increased free-free emission from systems with shorter orbital periods and an altered time-dependence of the wind attenuation. We comment on the importance of the effects of orbital motion on the observable properties of colliding wind binaries.Comment: 12 pages, 17 figures, accepted for publication in Ap

Chandra Observations of WR147 Reveal a Double X-ray Source

We report the first results from deep X-ray observations of the Wolf-Rayet binary system WR147 with the Chandra HETG. Analysis of the zeroth order data reveals that WR147 is a double X-ray source. The northern counterpart is likely associated with the colliding wind region, while the southern component is certainly identified with the WN star in this massive binary. The latter is the source of high energy X-rays (including the Fe K_alpha complex at 6.67 keV) whose production mechanism is yet unclear. For the first time, X-rays are observed directly from a WR star in a binary system.Comment: 15 pages, 3 figures, To Appear in The Astrophysical Journal Letter

XMM-Newton Observations Reveal Very High X-ray Luminosity from the Carbon-rich Wolf-Rayet Star WR 48a

We present XMM-Newton observations of the dusty Wolf-Rayet star WR 48a. This is the first detection of this object in X-rays. The XMM-Newton EPIC spectra are heavily absorbed and the presence of numerous strong emission lines indicates a thermal origin of the WR 48a X-ray emission, with dominant temperature components at kT_cool approx. 1 keV and kT_hot approx. 3~keV, the hotter component dominating the observed flux. No significant X-ray variability was detected on time scales < 1 day. Although the distance to WR 48a is uncertain, if it is physically associated with the open clusters Danks 1 and 2 at d ~ 4 kpc, then the resultant X-ray luminosity L_X ~ 10^(35) ergs/s makes it the most X-ray luminous Wolf-Rayet star in the Galaxy detected so far, after the black-hole candidate Cyg X-3. We assume the following scenarios as the most likely explanation for the X-ray properties of WR 48a: (1) colliding stellar winds in a wide WR+O binary system, or in a hierarchical triple system with non-degenerate stellar components; (2) accretion shocks from the WR 48a wind onto a close companion (possibly a neutron star). More specific information about WR48a and its wind properties will be needed to distinguish between the above possibilities.Comment: 10 pages, 3 figures, 1 table, accepted for publication in The Astrophysical Journal Letter

Chandra HETG Observations of the Colliding Stellar Wind System WR 147

We present an extended analysis of deep Chandra HETG observations of the WR+OB binary system WR 147 that was resolved into a double X-ray source (Zhekov & Park, 2010, ApJ, 709, L119). Our analysis of the profiles of strong emission lines shows that their centroids are blue-shifted in the spectrum of the northern X-ray source. We find no suppressed forbidden line in the He-like triplets which indicates that the X-ray emitting region is not located near enough to the stars in the binary system to be significantly affected by their UV radiation. The most likely physical picture that emerges from the entire set of HETG data suggests that the northern X-ray source can be associated with the colliding stellar wind region in the wide WR+OB binary system, while the X-rays of its southern counterpart, the WN8 star, are result from stellar wind shocking onto a close companion (a hypothesized third star in the system).Comment: 22 pages, 6 figures, 2 Tables; accepted for publication in The Astrophysical Journa

X-ray Variability in the Young Massive Triple theta2 Ori A

Massive stars rarely show intrinsic X-ray variability. The only O-stars credited to be intrinsically variable are theta1 Ori C due to effects from magnetic confinement of its wind, and theta2 Ori A suspected of similar activity. Early Chandra observations have shown that the most massive star system in the Orion Trapezium Cluster, theta2 Ori A, shows rapid variability on time scales of hours. We determine X-ray fluxes and find that the star shows very strong variability over the last 5 years. We observed a second large X-ray outburst in November 2004 with the high resolution transmission grating spectrometer on-board Chandra. In the low state X-ray emissivities indicate temperatures well above 25 MK. In the high state we find an extended emissivity distribution with high emissivities in the range from 3 MK to over 100 MK. The outburst event in stellar terms is one of the most powerful ever observed and the most energetic one in the ONC with a lower total energy limit of 1.5x10^37 ergs. The line diagnostics show that under the assumption that the line emitting regions in the low states are as close as within 1 -- 2 stellar radii from the O-star's photosphere, whereas the hard states suggest a distance of 3 -- 5 stellar radii. The two outbursts are very close to the periastron passage of the stars. We argue that the high X-ray states are possibly the result of reconnection events from magnetic interactions of the primary and secondary stars of the spectroscopic binary. Effects from wind collisions seem unlikely for this system. The low state emissivity and R-ratios strengthen the predicament that the X-ray emission is enhanced by magnetic confinement of the primary wind. We also detect Fe fluorescence indicative of the existence of substantial amounts of neutral Fe in the vicinity of the X-ray emission.Comment: 11 pages, 8 figures, accepted for publication in The Astrophysical Main Journa

Collision of two identical hypersonic stellar winds in binary systems

We investigate the hydrodynamics of two identical hypersonic stellar winds in a binary system. The interaction of these winds manifests itself in the form of two shocks and a contact surface between them. We neglect the binary rotation and assume that the gas flow ahead of the shocks is spherically symmetrical. In this case the contact surface that separates the gas emanated from the different stars coincides with the midplane of the binary components. In the shock the gas is heated and flows away nearly along the contact surface. We find the shock shape and the hot gas parameters in the shock layer between the shock and the contact surface.Comment: 19 pages, 3 figures, accepted for publication in Ap

In Hot Pursuit of the Hidden Companion of Eta Carinae: An X-ray Determination of the Wind Parameters

We present X-ray spectral fits to a recently obtained Chandra grating spectrum of Eta Carinae, one of the most massive and powerful stars in the Galaxy and which is strongly suspected to be a colliding wind binary system. Hydrodynamic models of colliding winds are used to generate synthetic X-ray spectra for a range of mass-loss rates and wind velocities. They are then fitted against newly acquired Chandra grating data. We find that due to the low velocity of the primary wind (~500 km/s), most of the observed X-ray emission appears to arise from the shocked wind of the companion star. We use the duration of the lightcurve minimum to fix the wind momentum ratio at 0.2. We are then able to obtain a good fit to the data by varying the mass-loss rate of the companion and the terminal velocity of its wind. We find that Mdot ~ 1e-5 Msol/yr and v ~ 3000 km/s. With observationally determined values of ~500-700 km/s for the velocity of the primary wind, our fit implies a primary mass-loss rate of Mdot ~ 2.5e-4 Msol/yr. This value is smaller than commonly inferred, although we note that a lower mass-loss rate can reduce some of the problems noted by Hillier et al. (2001) when a value as high as 1e-3 Msol/yr is used. The wind parameters of the companion are indicative of a massive star which may or may not be evolved. The line strengths appear to show slightly sub-solar abundances, although this needs further confirmation. Based on the over-estimation of the X-ray line strengths in our model, and re-interpretation of the HST/FOS results, it appears that the homunculus nebula was produced by the primary star.Comment: 12 pages, 7 figures, accepted by A&

A dozen colliding wind X-ray binaries in the star cluster R136 in the 30Doradus region

We analyzed archival Chandra X-ray observations of the central portion of the 30 Doradus region in the Large Magellanic Cloud. The image contains 20 X-ray point sources with luminosities between $5 \times 10^{32}$ and $2 \times 10^{35}$ erg s$^{-1}$ (0.2 -- 3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectral type O stars as optical counterparts. Nine of these are within $\sim 3.4$pc of R136, the central star cluster of NGC2070. We derive an empirical relation between the X-ray luminosity and the parameters for the stellar wind of the optical counterpart. The relation gives good agreement for known colliding wind binaries in the Milky Way Galaxy and for the identified X-ray sources in NGC2070. We conclude that probably all identified X-ray sources in NGC2070 are colliding wind binaries and that they are not associated with compact objects. This conclusion contradicts Wang (1995) who argued, using ROSAT data, that two earlier discovered X-ray sources are accreting black-hole binaries. Five of the eighteen brightest stars in R136 are not visible in our X-ray observations. These stars are either single, have low mass companions or very wide orbits. The resulting binary fraction among early type stars is then unusually high (at least 70%).Comment: 23 pages, To appear in August in Ap

X-rays from Colliding Stellar Winds: the case of close WR+O binary systems

We have analysed the X-ray emission from a sample of close WR+O binaries using data from the public Chandra and XMM-Newton archives. Global spectral fits show that two-temperature plasma is needed to match the X-ray emission from these objects as the hot component (kT > 2 keV) is an important ingredient of the spectral models. In close WR+O binaries, X-rays likely originate in colliding stellar wind (CSW) shocks driven by the massive winds of the binary components. CSW shocks in these objects are expected to be radiative due to the high density of the plasma in the interaction region. Opposite to this, our analysis shows that the CSW shocks in the sample of close WR+O binaries are adiabatic. This is possible only if the mass-loss rates of the stellar components in the binary are at least one order of magnitude smaller than the values currently accepted. The most likely explanation for the X-ray properties of close WR+O binaries could be that their winds are two-component flows. The more massive component (dense clumps) play role for the optical/UV emission from these objects, while the smooth rarefied component is a key factor for their X-ray emission.Comment: MNRAS, accepted for publication (Feb 6, 2012); 13 pages, 6 figures, 3 table

X-ray Emission from Nitrogen-Type Wolf-Rayet Stars

We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR) stars obtained in a limited survey aimed at establishing the X-ray properties of WN stars across their full range of spectral subtypes. None of the detected stars is so far known to be a close binary. We report Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6), and an XMM-Newton detection of WR79a (WN9ha). These observations clearly demonstrate that both WNE and WNL stars are X-ray sources. We also discuss Chandra archive detections of the WN6h stars WR 20b, WR 24, and WR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). The X-ray spectra of all WN detections show prominent emission lines and an admixture of cool (kT 2 keV) plasma. The hotter plasma is not predicted by radiative wind shock models and other as yet unidentified mechanisms are at work. Most stars show X-ray absorption in excess of that expected from visual extinction (Av), likely due to their strong winds or cold circumstellar gas. Existing data suggest a falloff in X-ray luminosity toward later WN7-9 subtypes, which have higher Lbol but slower, denser winds than WN2-6 stars. This provides a clue that wind properties may be a more crucial factor in determining emergent X-ray emission levels than bolometric luminosity.Comment: 42 pages, 5 tables, 10 figure