Thin-shell mixing in radiative wind-shocks and the Lx  ∼  Lbol scaling of O-star X-rays

Owocki, Stan; Sundqvist, J. O.; Cohen, David H.; Gayley, K. G.

doi:10.1093/mnras/sts599

Cited by 69 publications

(110 citation statements)

References 43 publications

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“…This is significantly lower than the ∼ 10 −7 value expected for single O-type stars. This is in agreement with an expected steeper decline of the X-ray luminosity for the rather weak winds of single B-type star with respect to those of O-type stars (Berghoefer et al 1997;Owocki et al 2013).…”

Section: The Starssupporting

confidence: 88%

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X-ray study of bow shocks in runaway stars

Becker

Valle

Romero

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Massive runaway stars produce bow shocks through the interaction of their winds with the interstellar medium, with the prospect for particle acceleration by the shocks. These objects are consequently candidates for non-thermal emission. Our aim is to investigate the X-ray emission from these sources. We observed with XMM-Newton a sample of 5 bow shock runaways, which constitutes a significant improvement of the sample of bow shock runaways studied in X-rays so far. A careful analysis of the data did not reveal any X-ray emission related to the bow shocks. However, X-ray emission from the stars is detected, in agreement with the expected thermal emission from stellar winds. On the basis of background measurements we derive conservative upper limits between 0.3 and 10 keV on the bow shocks emission. Using a simple radiation model, these limits together with radio upper limits allow us to constrain some of the main physical quantities involved in the non-thermal emission processes, such as the magnetic field strength and the amount of incident infrared photons. The reasons likely responsible for the non-detection of non-thermal radiation are discussed. Finally, using energy budget arguments, we investigate the detectability of inverse Compton X-rays in a more extended sample of catalogued runaway star bow shocks. From our analysis we conclude that a clear identification of non-thermal X-rays from massive runaway bow shocks requires one order of magnitude (or higher) sensitivity improvement with respect to present observatories.

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Section: The Starssupporting

confidence: 88%

“…The latter value is in full agreement with the X-ray emission from a single O-type star (see e.g. Owocki et al 2013). …”

Section: The Starssupporting

confidence: 86%

X-ray study of bow shocks in runaway stars

Becker

Valle

Romero

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…3) throughout the orbit, the shocks will be relatively smooth and we cannot appeal to thin-shell mixing, or oblique shock angles, caused by corrugated shock interfaces as a mechanism to reduce the intrinsic X-ray luminosity as these effects are typically associated with radiative shocks − see, e.g. , Owocki et al (2013), andKee et al (2014). Furthermore, although an under-estimate in the observationally inferred column density 5 would allow larger mass-loss rates, the over-prediction of the X-ray luminosity (which scales with the mass-loss rate) is unavoidable unless mass-loss rates are reduced.…”

Section: Constraining Wind Mass-loss Ratesmentioning

confidence: 99%

The 2.35 year itch of Cygnus OB2 #9

Parkin

Pittard

Nazé

et al. 2014

A&A

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Context. The wind-wind collision in a massive star binary system leads to the generation of high temperature shocks that emit at X-ray wavelengths and, if particle acceleration is effective, may exhibit non-thermal radio emission. Cyg OB2#9 is one of a small number of massive star binary systems in this class. Aims. X-ray and radio data recently acquired as part of a project to study Cyg OB2#9 are used to constrain physical models of the binary system, providing in-depth knowledge about the wind-wind collision and the thermal, and non-thermal, emission arising from the shocks. Methods. We use a 3D, adaptive mesh refinement simulation (including wind acceleration, radiative cooling, and the orbital motion of the stars) to model the gas dynamics of the wind-wind collision. The simulation output is used as the basis for radiative transfer calculations considering the thermal X-ray emission and the thermal/non-thermal radio emission. Results. The flow dynamics in the simulation show that wind acceleration (between the stars) is inhibited at all orbital phases by the opposing star's radiation field, reducing pre-shock velocities below terminal velocities. To obtain good agreement with the X-ray observations, our initial mass-loss rate estimates require a down-shift by a factor of ∼7.7 to 6.5 × 10 −7 M yr −1 and 7.5 × 10 −7 M yr −1 for the primary and secondary star, respectively. Furthermore, the low gas densities and high shock velocities in Cyg OB2 #9 are suggestive of unequal electron and ion temperatures, and the X-ray analysis indicates that an immediately post-shock electron-ion temperature ratio of 0.1 is also required. The radio emission is dominated by non-thermal synchrotron emission. A parameter space exploration provides evidence against models assuming equipartition between magnetic and relativistic energy densities. However, fits of comparable quality can be attained with models having stark contrasts in the ratio of magnetic-to-relativistic energy densities. Both X-ray and radio lightcurves are largely insensitive to viewing angle. The variations in X-ray emission with orbital phase can be traced back to an inverse relation with binary separation and pre-shock velocity. The radio emission also scales with pre-shock velocity and binary separation, but to positive powers (i.e. not inversely). The radio models also reveal a subtle effect whereby inverse Compton cooling leads to an increase in emissivity as a result of the synchrotron characteristic frequency being significantly reduced. Finally, using the results of the radio analysis, we estimate the surface magnetic field strengths to be ≈0.3−52 G.

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“…These values are totals for the two components, treating each one as adhering to the relation for single stars. This canonical scaling for single stars is known to have significant dispersion, and its extension much into the B spectral class is recognized as dubious (e.g., Cohen et al 1997;Nazé 2009;Owocki et al 2013). …”

Section: Expected X-ray Propertiesmentioning

confidence: 99%

An X-Ray Study of Two B+B Binaries: AH Cep and CW Cep

Ignace

Hole

Oskinova

et al. 2017

ApJ

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AH Cep and CW Cep are both early B-type binaries with short orbital periods of 1.8 d and 2.7 d, respectively. All four components are B0.5V types. The binaries are also double-lined spectroscopic and eclipsing. Consequently, solutions for orbital and stellar parameters make the pair of binaries ideal targets for a study of the colliding winds between two B stars. Chandra ACIS-I observations were obtained to determine X-ray luminosities. AH Cep was detected with an unabsorbed X-ray luminosity at a 90% confidence interval of (9 − 33) × 10L Bol , relative to the combined Bolometric luminosities of the two components. While formally consistent with expectations for embedded wind shocks, or binary wind collision, the near-twin system of CW Cep was a surprising non-detection. For CW Cep, an upper limit was determined with L X /L Bol < 10 −8 , again for the combined components. One difference between these two systems is that AH Cep is part of a multiple system. The X-rays from AH Cep may not arise from standard wind shocks nor wind collision, but perhaps instead from magnetism in any one of the four components of the system. The possibility could be tested by searching for cyclic X-ray variability in AH Cep on the short orbital period of the inner B stars.

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Thin-shell mixing in radiative wind-shocks and the Lx ∼ Lbol scaling of O-star X-rays

Cited by 69 publications

References 43 publications

X-ray study of bow shocks in runaway stars

X-ray study of bow shocks in runaway stars

The 2.35 year itch of Cygnus OB2 #9

An X-Ray Study of Two B+B Binaries: AH Cep and CW Cep

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