Stars with Magnetic Winds

The picture of X-ray emission from stars with winds becomes more complex once magnetic fields are involved. Hybrid models invoking winds disturbed in large-scale magnetospheres have been developed to explain both radio and X-ray emission from early-type stars and have, in particular, been used to describe the observation of hard X-rays from some hot stars that cannot be explained by shocks forming in ra-diatively driven winds. Direct observational evidence for the existence of magnetic fields on early-type stars is scarce and mostly restricted to the class of chemically peculiar stars, often showing field strengths of a few kG [24]. In "normal," i.e., nonchemically peculiar stars, the strong rotational broadening of their photospheric lines has in most cases prevented the detection of fields with direct spectropolarime-try or Zeeman splitting, indicating that they have much weaker fields, if any. Magnetic fields are often held responsible for the observed cyclic variability of the wind properties usually observed in the UV as well as for the non-thermal radio emission seen in some O stars. Although traditionally magnetic fields on hot stars are believed to be fossil remnants of the star formation process, numerical modeling has shown that dynamos may actually operate in their convective cores [5]. The major difficulty the theory has to face is the transport of magnetic flux throughout the thick radiative layer to the stellar surface where such fields are accessible to observations.

Hybrid models for X-ray emission from early-type stars involving winds and magnetic fields come in two flavors as far as the mechanism for X-ray production is concerned. Both approaches assume an originally dipolar field channeling the stellar wind outward along its lines of force. In the first approach, X-ray emission is the result of magnetic reconnection in a current sheet that forms in the equatorial plane where gas pressure opens the field lines [26]. This scenario can explain the observed nonthermal radio emission of magnetic chemically peculiar stars, and yields expected X-ray luminosities of logLx [ergs-1] ~31 - 34. These predictions include in particular the range of X-ray luminosities observed from OBA stars.

In the second approach, the magnetically channeled wind is forced down to the equatorial plane, where the winds from the two hemispheres collide and a quasista-tionary large-scale shock is formed [3]. In this so-called magnetically confined wind shock (MCWS) model, temperatures exceed 107 K, radiative cooling of the heated plasma goes along with comparatively hard X-ray emission, and X-ray luminosities even higher than in instability-driven wind shocks can be achieved. Depending on the relative amount of wind and magnetic field energy, the wind may open up the

confined by the magnetic field. Variations of the scenario predict that the field lines open up near the equator under the influence of the wind pressure, and additional X-rays are produced in the outflow region

field lines near the magnetic equator beyond a critical radius. X-ray photons may be emitted both from closed magnetic field structures (see Fig. 10.21 representing the original MCWS scenario) or from the open field equatorial outflow at large radii. The cooling material accumulates in a disk near the magnetic equator and may alter the observable X-ray spectrum by absorption.

The MCWS model was originally devised to explain the X-ray emission of the Ap star IQ Aur, but it has also been successfully applied to the young O star 01 Ori C [2]. 01 OriC is the only O star with a directly measured magnetic field (1000 G). It shows a high-amplitude periodic variation of its X-ray intensity, which is extraordinary for an early-type star. Figure 10.22 displays the lightcurve of 01 Ori C observed with the ROSAT HRI. Overlaid on the data is the variation predicted by the MCWS model for different viewing angles of the magnetosphere and cooling disk. A similar modulation of the X-ray emission following the period of its magnetic field variations has also been reported from the late-B star HD 133880 with a magnetic field of «2.5 kG. Although observed to be a rather rare phenomenon, these rotational modulations of the X-ray emission from early-type stars cannot be explained in terms of purely wind-driven X-ray production.

Magnetic fields would also naturally explain the X-ray flaring on early-type stars. Large X-ray flares were observed with ROSAT and XMM-Newton on the chemically peculiar star a OriE (spectral type B2 Vp), which has an inferred polar magnetic field in excess of 10 kG. However, whether these flares originate from the magnetic B star or from an as yet unseen late-type companion is subject of some debate

Fig. 10.22 Rotationally modulated X-ray emission from 01 OriC observed by the ROSAT HRI and predictions by the MWCS model for different viewing angles (Fig. 2 of [2])

Fig. 10.22 Rotationally modulated X-ray emission from 01 OriC observed by the ROSAT HRI and predictions by the MWCS model for different viewing angles (Fig. 2 of [2])

at the moment. We finally note that X-ray emission has recently been detected from the famous Babcock's star, the noncompact star with the largest hitherto measured magnetic field. These new observational findings are very intriguing and may in fact point at a closer than previously assumed relationship between the X-ray emission from early- and late-type stars.

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