Astron. Astrophys. 353, 569-574 (2000)

1. Introduction

Hot coronae are the enigmatic link between cool, convective stars and their environment. While most of this hot plasma ( K) is contained by magnetic fields, some may escape due to overpressure as a hot stellar wind along open field lines. The extent of intense coronal emissions in X-rays or radio waves thus yields a lower limit to the size of closed magnetic loops, i.e. the size of the stellar magnetosphere.

Radio emission from solar active regions at 3 cm wavelength originates from altitudes above the photosphere between 5 to cm, or 0.006 - 0.015 , (e.g. Aschwanden et al.1995). It is generally attributed to thermal gyroresonance emission. The size of the radio image of the Sun increases at longer wavelengths. In soft X-rays and EUV line emissions, coronal loops reach cm (0.5 ) (e.g. Sturrock et al. 1996). Magnetic loops in excess of cm (2 ) have been observed in metric U bursts (Labrum & Stewart 1970). Coronograph observations in white light, however, do not regularly see loops of that size, suggesting that they are shortlived. In general, large magnetic loops are confined to low latitudes and, consequently, the radio image of the Sun is more extended in the equatorial direction.

Very Long Baseline Interferometry (VLBI) has made it possible to study the size and shape of main-sequence stellar coronae . At 3.6 cm, Benz et al.(1998) have resolved the dMe star UV Cet B at an higher and variable flux level into two components separated by cm (4.4 ). One of the components was spatially resolved with the size of about the stellar photosphere. The orientation of the two sources was found to lie along the probable axis of rotation, strongly suggestive of coronal enhancements extending at least cm (2.1 ) above the poles. Thus, UV Cet B differs considerably from the Sun in size and shape of the radio corona.

More VLBI observations of dMe stars have been reported at 18 cm wavelength. For YZ CMi, Benz & Alef(1991) found an upper limit of cm, suggesting an extent of a circular corona above the photosphere of less than cm (0.74 ). Two size measurements of AD Leo by Benz et al.(1995) suggest a coronal extent of less than cm (1.0 ) and cm (2.6 ) above the photosphere, assuming a circular, concentric shape. A small total size of less than cm for EQ Peg (Benz et al.1995) refers to an observation during a totally polarized flare, and an extremely large size of cm was reported for the dMe close binary YY Gem (Alef et al. 1997).

The radio coronae of active, rapidly rotating dMe stars have been modeled to determine the emission process. It is generally agreed (Güdel 1994; White et al. 1989) that weakly polarized radio emission is produced by the gyrosynchrotron mechanism of a population of mildly relativistic electrons. This mechanism, however, cannot account for polarizations exceeding about 50% which may originate from a coherent process. Circular polarizations of up to 80% have been reported for YZ CMi at 20 cm and 18 cm (Lang & Wilson 1986; Benz & Alef1991).

Here we report on the results of VLBA experiments of YZ CMi and AD Leo at 3.6 cm. These are well known, nearby, young radio stars close to zero-age main sequence (ZAMS). Some of their general properties are listed in Table 1.

Table 1. Summary of the results from the observations and fits for YZ CMi and AD Leo. The sizes of the radio emission are labeled with the name of the procedure used to obtain them (UVFIT in AIPS, gaussfit outside AIPS). The distances are taken from the Hipparcos and Gliese catalogues, respectively, and the optical diameters are from Pettersen (1980). For the AIPS fit of scan 2 we constrained the program to fit a circular gaussian because of the small number of visibilities. We also did a fit for AD Leo but it did not converge (see text).

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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