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


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VLBI observations of two single dMe stars: spatial resolution and astrometry

M.R. Pestalozzi 1, A.O. Benz 2, J.E. Conway 1 and M. Güdel 3

1 Onsala Space Observatory, 439 92 Onsala, Sweden
2 Institute of Astronomy, ETH, 8092 Zürich, Switzerland
3 Paul-Scherrer-Insitute, 5232 Villigen, Switzerland

Received 5 July 1999 / Accepted 24 September 1999

Abstract

We report on 3.6 cm VLA and VLBA observations of YZ CMi and AD Leo, two nearby dMe stars. We resolve YZ CMi and can fit a circular symmetrical gaussian component of FWHP of 0.98 [FORMULA]0.2 mas, corresponding to an extent of the corona above the photosphere of [FORMULA] cm or 0.7 [FORMULA] [FORMULA] ([FORMULA] refers to the photospheric radius). We obtain an estimate of the brightness temperature of [FORMULA] K, which is consistent with that expected from gyrosynchrotron emission. For AD Leo the emitting region is unresolved. We therefore set a conservative upper limit to its diameter of 1.8 times the photosphere diameter, which leads to an extent of the corona above the photosphere of [FORMULA]cm or [FORMULA] [FORMULA]. We compare the radio emitting dMe stars with measured sizes with the Sun and conclude that these active stars have much more extended coronal radio emission than the Sun. The VLBA position of YZ CMi has been found to differ by 32 mas from the positions calculated from the Hipparcos catalogue. The discrepancy is caused by large errors in the listed proper motion. An improved value is given.

Key words: radio continuum: stars – stars: coronae – astrometry – stars: individual:

Send offprint requests to: M.R. Pestalozzi (michele@oso.chalmers.se)

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999

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