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Astron. Astrophys. 323, 121-138 (1997)


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X-ray emission from Ap-Bp stars: a magnetically confined wind-shock model for IQ Aur *

J. Babel 1 and T. Montmerle 2

1 Institut für Astronomie und Astrophysik, Universität Kiel, Germany (babel@astrophysik.uni-kiel.de)
2 CEA, Service d'Astrophysique, Centre d'Etudes de Saclay, France (montmerle@sapvxg.saclay.cea.fr)

Received 22 November 1996 / Accepted 19 December 1996

Abstract

We present the results of a ROSAT -PSPC pointed observation on the hot A0p star IQ Aur (HD 34452). The X-ray luminosity obtained is [FORMULA] ([FORMULA]) and the plasma temperature is [FORMULA]. Since A and B stars have no known magnetic activity, the only possibility is to invoke a wind origin for this emission. On IQ Aur, the expected mass loss rate driven by stellar radiation is small, around 10-10 - 10-11 [FORMULA] yr-1 with [FORMULA] km s-1, so that the kinetic energy flux of the wind is not much larger than the observed X-ray emission. This implies that the X-ray emission from IQ Aur must arise from a very efficient process. These constraints, together with the large observed magnetic field of IQ Aur, lead us to propose that the confinement of the wind by the magnetic field leads to a collision from the wind components of the two hemispheres in the closed magnetosphere, leading to a strong shock. In this model the magnetic field confines the wind and also affects the mass loss rate.

We propose a self-consistent approach for the X-ray emission of IQ Aur, using radiatively driven wind models based on the stellar parameters of IQ Aur and including the effect of magnetic confinement of the wind. We also model the whole postshock region. We show that our shock model is able to satisfy the constraints on the observed X-ray luminosity and temperature. The model also leads to the formation of a disk at the magnetic equator corotating up to [FORMULA]. We show that ambipolar diffusion of hydrogen in the disk or current sheet formation due to equipartition between the disk and the magnetic field might play a significant role in emptying the magnetosphere.

We discuss the interplay between mass loss and particle diffusion. Our computations suggest that the onset of a wind on IQ Aur is very recent, so that the abundance anomalies at the stellar surface have not yet been removed by the outflow, or that the wind exhibits transient phases due to the mutual feedback between the wind and abundance anomalies.

Finally, we point out that our wind-shock model provides a very convenient framework to explain the radio emission of Ap-Bp stars. It has been shown that the emission mechanism is optically thick gyrosynchrotron, but instead of (or in addition to) the previously invoked acceleration by magnetic reconnection in current sheets, we propose that the electrons are accelerated by second-order Fermi acceleration mechanism ("stochastic" acceleration) often invoked for the acceleration of solar flare flare particles. We show that electrons accelerated by the wind shock easily reach the required energies for radio emission in the GHz band throughout the magnetosphere.

Given the success of our model in explaining IQ Aur, we think that it has a fairly general application to magnetic Ap-Bp stars. The wind-shock model also provides a unified explanation for both the X-rays and the radio emission from these stars.

Key words: X-rays: stars – stars: mass loss – stars: magnetic fields – stars: chemically peculiar – stars: HD 34452

* Based on pointed observation made with the ROSAT satellite and on observations made at the Observatoire de Haute Provence (France).

Send offprint requests to: J. Babel

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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