A&A Supplement series, Vol. 128, February II 1998, 29-33
Received February 3; accepted June 16, 1997
A.J.M. Piters - J. van Paradijs - J.H.M.M. Schmitt
Send offprint request: J. van Paradijs
Astronomical Institute "Anton Pannekoek" / CHEAF, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands
Physics Department UAH, Huntsville, AL 35899, U.S.A.
Max Planck Institut für Extraterrestrische Physik, Giessenbachstraße 1, 8046 Garching bei München, Germany
We present Walraven photometry and ROSAT All-Sky Survey data for a sample of 173 bright main-sequence stars with spectral types between A8V and G2V. These observations are part of a study of the onset of magnetic surface activity along the main sequence. Values for the effective temperature, surface gravity and interstellar reddening have been obtained from a comparison of the observed Walraven colours with theoretical values. These parameters have been used to derive accurate X-ray surface flux densities.
keywords: stars: activity; fundamental parameters -- X-rays: stars