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Astron. Astrophys. 317, 707-711 (1997)

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VLBI measurement of the size of dMe stars

W. Alef 1, A. O. Benz 2 and M. Güdel 3

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany (walef@mpifr-bonn.mpg.de)
2 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland (benz@astro.phys.ethz.ch)
3 Paul-Scherrer-Institute, CH-5232 Villigen, Switzerland (guedel@astro.phys.ethz.ch)

Received 2 January 1996 / Accepted 13 June 1996


The binary system YY Gem, which consists of two dM1e stars, has been observed during an eclipse using intercontinental Very Long Baseline Interferometry (VLBI) at 1.6 GHz. The stellar emission was at a low, quiescent level at all observing periods. The correlated flux decreases slightly with baseline length, indicating that the source is resolved at the longest baselines. This is confirmed by model fits, which give a FWHM size of 0.94([FORMULA] 0.24) mas ([FORMULA] cm or 2.1 photospheric diameters) for the radio emitting source. A lower limit to the size derived from the lack of observable eclipse effects is consistent with this value. The resulting brightness temperature of [FORMULA] K is compatible with gyrosynchrotron emission. Deviations from circular symmetry are not significant. The loops that trap the radio emitting electrons reaching an altitude of probably more than a stellar radius appear to be distributed isotropically within the limits of the resolution.

Key words: stars: coronae – binaries: eclipsing – YY Gem – radio continuum: stars

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998