Photospheric and chromospheric active regions on three single-lined RS CVn binaries

Abstract

A monitoring of three active RS CVn binaries has been performed with medium resolution spectroscopy with the aim of investigating the behavior of chromospheric and photospheric inhomogeneities. Surface temperature, as recovered from line-depth ratios (LDRs), allowed us to map the photospheric spots, while the H α emission has been used as an indicator of chromospheric inhomogeneities. We have found that the rotational modulation of the H α emission is always in anti-phase with the temperature wave, i.e. at the time of our observations active regions at chromospheric and photospheric levels are closely spatially associated in these active stars. The residual H α profiles, obtained as the difference between the observed spectra and non-active templates, are well reproduced by a two Gaussian fitting. The broad emission component, responsible for the wide emission wings in near all the spectra, is often blue-shifted with respect to the center of the stellar disk. The narrow H α emission displays a phase-dependent variation in all stars and is anti-correlated with the photospheric diagnostics, while the broad one displays no or little rotational modulation. We suggest that the broad emission component is mainly related to physical phenomena, like micro-flaring or strong chromospheric velocity fields, occurring all over the star disk, while the central narrow emission is more affected by chromospheric plages. We have also detected a modulation of the intensity of the $\ion{He}{i}$ D 3 line with the star rotation, suggesting surface features also in the upper chromosphere of these stars.