EUV activity in late-type stars during the ROSAT WFC All-Sky Survey - II. Comparisons with other observations

Abstract

The extreme ultraviolet (BUV) observations of the ROSAT Wide Field Camera AlI­ Sky Survey have revealed bright and energetic coronae for a sufficiently large number of main-sequence stars that detailed comparisons with other observables can be made. We have detected a large number of energetic BUV flares on stars of different spectral classes, from dF5 to dM6, including flare stars. The BUV emission is well-correlated to several stellar properties such as stellar colour, spectral type, bolometric luminosity, far-infrared photospheric emission, etc. These relations are stronger for the highly active stars like flare stars, and they become somewhat looser for the less active dwarfs. We find that there is a better correlation between the BUV luminosity and stellar rotation period for low-activity stars than for high-activity ones, and this also applies to the RS CVn stars. This is a strong indication of magnetic/active region saturation in these more active stars. We find that there is also a tendency of the L Evv to increase with magnetic field intensity, although there are very few stars with reliable magnetic field strength measurements. These last two correlations support the view that the BUV emission from these stars is regulated by magnetic processes. Our results indicate that the stellar coronae are continuously being heated by small-scale flare events.