The chromospherically active binary star EI Eridani II. Long-term Doppler imaging

Abstract

Data from 11 years of continuous spectroscopic observations of the active RS CVn-type binary star EI Eridani - gained at NSO/McMath-Pierce, KPNO/Coude Feed and during the MUSICOS 98 campaign - were used to obtain 34 Doppler maps in three spectroscopic lines for 32 epochs, 28 of which are independent of each other. Various parameters are extracted from our Doppler maps: average temperature, fractional spottedness, and longitudinal and latitudinal spot-occurrence functions. We find that none of these parameters show a distinct variation nor a correlation with the proposed activity cycle as seen from photometric long-term observations. This suggests that the photometric brightness cycle may not necessarily be due to just a cool spot cycle. The general morphology of the spot pattern remains persistent over the whole period of 11 years. A large cap-like polar spot was recovered from all our images. A high degree of variable activity was noticed near latitudes of approx. 60-70 degrees where the appendages of the polar spot emerged and dissolved.