4. Discussion and conclusions
VLBA observations have spatially resolved YZ CMi and the data could be fitted with a circular gaussian of a FWHP of 0.98 0.2 mas. The radio corona extent is cm above the photosphere (the phospheric radius is assumed to be cm, Pettersen1980). For AD Leo, which is closer and has a larger photosphere, but was observed at a much weaker flux level, the corona was not resolved and we set a robust upper limit of cm above the photosphere (see Sect. 3.3).
4.1. The brightness temperature
For YZ CMi (see Table 1) we obtain a mean brightness temperature K, while for AD Leo we set an upper limit of K. These mean values are smaller than previously reported for YZ CMi by Benz & Alef(1991) and AD Leo at 18 cm (Jackson et al. 1989). The lower values at 3.6 cm are still consistent with a non-thermal spectrum from gyrosynchrotron but do not formally exclude thermal processes. However the significant circular polarisation found during the observations strongly argues for a gyrosychrotron emission mechanism.
The derived extent of the coronae above the photosphere of the dMe stars is compared to the Sun in Table 3. The solar value refers to stereoscopic measurements of the thermal gyroresonance emission of active regions. The average value at 10-14 GHz reported by Aschwanden et al.(1995) has been used.
Table 3. Luminosity and coronal size of Sun and dMe stars at 3.6 cm wavelength.
The experience from the solar radio emission makes it clear that the observed radio size is only a lower limit of the size of the stellar corona. Nevertheless, these results indicate that the observed active dMe stars have much larger active coronae than the Sun. It might be that such dMe stars posess systems of closed loops reaching heights in excess of a stellar diameter. One way to realize such extended coronae is by large distances between footpoints as possibly seen in the case of UV Cet B (Benz et al.1998). This indicates either that active regions are very large, or that active loops preferentially connect different active regions.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999