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Astron. Astrophys. 321, 177-188 (1997)

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1. Introduction

The magnetic structure of stellar coronae has become of increasing interest recently, since observations of magnetically-active stars have shown that the surface fields at least are organised into areas of spots (Stauffer & Hartmann 1987; Baliunas 1991; O'Dell & Collier Cameron 1993; Baliunas et al. 1995). The role of the magnetic field in governing the rotational spin-down of young stars through the action of a hot, magnetically-channelled wind makes it even more important to understand how the coronal field is structured, since angular momentum loss happens principally along open field lines. Areas of the stellar surface that are covered in closed loops are not believed to contribute significantly. Hence the fraction of the surface at any given rotation rate that is covered in closed field is relevant to the spin-down of the star. The extent of this "dead zone" was examined by Mestel & Spruit (1987) for a simple configuration with a dipole field at the stellar equator and open field at the poles. They found that the extent of the closed-field region increases with the strength of the dynamo-generated magnetic field, but decreases if the rotation rate is so high that centrifugal forces affect the pressure balance significantly. In reality, the magnetic structures will be more complex than this. Observations of stellar prominences now show that on a number of rapidly-rotating stars, closed field regions in which prominences are embedded extend out well into the stellar corona, filling a significant fraction of the coronal volume. These prominences form preferentially at the co-rotation radius (where centrifugal forces balance gravity) and move outwards over several days (see for example Jeffries 1993; Collier Cameron & Woods 1992 and references therein). Their contribution to the total angular momentum loss depends on the height above the surface at which the magnetic field can no longer support them and they are expelled.

In order to understand how the coronal field evolves, we need to examine the forces that govern the equilibria that are available to coronal loops. While finding an equilibrium does not guarantee that it will be stable, the absence of an equilibrium is an important constraint on the form of the magnetic field. In particular, we are interested in the role that rotation might play in determining the types of equilibria that are available at any given rotation rate. Jardine & Collier Cameron (1991) (hereafter [JC]) have calculated magnetostatic loop equilibria for an isothermal plasma on rotating stars. They showed that there is a maximum plasma pressure inside the loop beyond which no equilibrium exists. In their models, the maximum height of the loop depends mainly on the strength and scale of the external field within which the loop is embedded. For a dynamo-generated field, this is probably dependent on the rotation rate of the star. The effect of including an energy equation has also been investigated by Collier Cameron (1988). In these models, it is assumed that the loop shape is given by a dipole and the form of the cross-sectional loop area is prescribed as a function of distance along the loop. Here we consider loops that are embedded in an equatorial arcade and investigate the temperature and pressure structure inside these loops. We also look at the loop shapes and the maximum loop heights and widths.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998