* Astronomy

Title: New X-ray observations of IQ Aurigae and Alpha2 Canum Venaticorum
Probing the magnetically channelled wind shock model in A0p stars
Authors: J. Robrade and J. H. M. M. Schmitt

Aims. We re-examine the scenario of X-ray emission from magnetically confined/channelled wind shocks (MCWS) for Ap/Bp stars, a model originally developed to explain the ROSAT detection of the A0p star IQ Aur.
Methods. We present new X-ray observations of the A0p stars Alpha2 CVn (Chandra) and IQ Aur (XMM-Newton) and discuss our findings in the context of X-ray generating mechanisms of magnetic, chemically peculiar intermediate mass stars.
Results. The X-ray luminosities of IQ Aur with log LX = 29.6 erg s^-1 and Alpha2 CVn with log LX = 26.0 erg s-1 differ by at least three orders of magnitude, although both are A0p stars. By studying a sample of comparison stars, we find that X-ray emission is preferably generated by more massive objects such as IQ Aur. Besides a strong, cool plasma component, significant amounts of hot (>10 MK) plasma are present during the quasi-quiescent phase of IQ Aur; moreover, diagnostics of the UV sensitive f/i line ratio in He-like O vii triplet point to X-ray emitting regions well above the stellar surface of IQ Aur. In addition we detect a large flare from IQ Aur with temperatures up to  ~100 MK and a peak X-ray luminosity of log LX = 31.5 erg s^-1. The flare, showing a fast rise and e-folding decay time of less than half an hour, originates in a fairly compact structure and is accompanied by a significant metallicity increase. The X-ray properties of IQ Aur cannot be described by wind shocks only and require the presence of magnetic reconnection. This is most evident in the, to our knowledge, first X-ray flare reported from an A0p star.
Conclusions. Our study indicates that the occurrence the of X-ray emission in A0p stars generated by magnetically channelled wind shocks depends on stellar properties such as luminosity, which promote a high mass loss rate, whereas magnetic field configuration and transient phenomena refine their appearance. While we cannot rule out unknown close companions, the X-ray emission from IQ Aur can be described consistently in the MCWS scenario, in which the very strong magnetic confinement of the stellar wind has led to the build-up of a rigidly rotating disk around the star, where magnetic reconnection and centrifugal breakout events occur.

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