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Astron. Astrophys. 339, 840-845 (1998)

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1. Introduction

All activity at the surface of cool stars is almost certainly due to magnetic fields: the magnetic field plays a key role in our understanding of this activity and the coronal X-ray emission. These fields are required first to confine the hot X-ray emitting coronal gas, and second, they are also believed to be relevant for the coronal heating process heating the coronal plasma to X-ray emitting temperatures (Schmitt 1994).

The Yohkoh coronal X-ray images plainly show that the brightness of the corona, and hence the strength of coronal heating, is generally much stronger in the region of the strong magnetic field (active regions). This simple observation alone implies that coronal heating is a magnetic phenomenon. In terms of practical observations, we might expect the bright coronal substructures in active regions to be rooted in places that stand out in some way in photospheric magnetograms. If so, the magnetic field at these sites might shed light on the coronal heating process (Falconer et al. 1997).

Coronal heating is widespread among cool main-sequence stars, and this heating depends ultimately on mechanical energy associated with convective flows. There are two principal ways by which convection can generate mechanical energy. One is direct, in the form of acoustic waves that inevitably occur when compressible material is forced into motion. The second is indirect and relies on operation of a dynamo to give rise to magnetic fields. Mullan and Fleming (1996) suggested that the quiet coronal regions may be heated acoustically, but that the active coronal regions are heated magnetically.

We use the Raymond-Smith model to fit the soft X-ray spectrum, and find a remarkable correlation of the coronal temperatures with the photospheric magnetic field strength for single stars, which suggest that the corona is heated magnetically. Thus we can estimate the magnetic field strength from the X-ray spectrum.

In Sect. 2 we discuss the data used in our analysis and in particular discuss the criteria used to determine the properties of the program stars. Sect. 3 contains our basic results; a comparison between the coronal temperatures and magnetic field strength is specifically presented, statistical evidence for sample stars is also presented, and finally the correlation between the coronal temperatures and the magnetic field strength is discussed. In the last section, the main conclusions are summarized.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998