The behaviour of the excess CaII H & K and H_epsilon emissions in chromospherically active binaries

Abstract

In this work we analyze the behaviour of the excess Ca~{\sc ii} H \& K and H$\hat{I}\mu$ emissions in a sample of 73 chromospherically active binary systems (RS CVn and BY Dra classes), of different activity levels and luminosity classes. This sample includes the 53 stars analyzed by Fernández-Figueroa et al. (1994) and the observations of 28 systems described by Montes et al. (1995c). By using the spectral subtraction technique (subtraction of a synthesized stellar spectrum constructed from reference stars of spectral type and luminosity class similar to those of the binary star components) we obtain the active-chromosphere contribution to the Ca~{\sc ii} H \& K lines in these 73 systems. We have determined the excess Ca~{\sc ii} H \& K emission equivalent widths and converted them into surface fluxes. The emissions arising from each component were obtained when it was possible to deblend both contributions. We have found that the components of active binaries are generally stronger emitters than single active stars for a given effective temperature and rotation rate. A slight decline of the excess Ca~{\sc ii} H \& K emissions towards longer rotation periods, P${}_{\mathrm{rot}}$, and larger Rossby numbers, R${}_0$, is found. When we use R${}_0$ instead of P${}_{\mathrm{rot}}$ the scatter is reduced and a saturation at R${}_0$ $\approx$~0.3 is observed. A good correlation between the excess Ca~{\sc ii}~K and H$\hat{I}\mu$ chromospheric emission fluxes has been found. The correlations obtained between the excess Ca~{\sc ii}~K emission and other activity indicators, (C~{\sc iv} in the transition region, and X-rays in the corona) indicate that the exponents of the power-law relations increase with the formation temperature of the spectral features.

17 pages in uuencoded compressed postscript file including text, tables, and figures. The paper is also available through http://www.ucm.es/OTROS/Astrof/dpto_preprints.html Request hard copy of preprint to [email protected]. Accepted for publication in 'Astronomy and Astrophysics'