Further observations of Capella in the He I 10830 Å line: the activity cycle and role of binarity effects
M.M. Katsova 1 and
A.G. Shcherbakov 2
Received 6 December 1996 / Accepted 29 August 1997
High dispersion CCD spectra of the spectroscopic binary Capella in the infrared He I 10830 Å line were obtained with the Shajn 2.6-m telescope of the Crimean Astrophysical Observatory during 1989-1994 over several orbital periods. An analysis of new data confirms our earlier conclusions: i) the main (narrow) He I 10830 Å line is formed in the outer atmosphere of the primary, cooler component, which is a G6 giant, as follows from the dependence of the radial velocity curves of the He I and photospheric lines on the orbital period; and ii) the equivalent width of the He I line varies with the 104-day orbital period of Capella's binary. The mean level of the equivalent width values changes from year to year.
We show that the amplitude of variations of the equivalent width as a function of the orbital phase becomes lower when the mean level of the equivalent width is higher. The long-term variability of the helium absorption is reliably determined from uniform CCD observations in 1985-1994; this variability is associated with an activity cycle with a period of 6 years (this cycle is traced through the whole set of our data since 1980). During a season when the activity level of the Capella system was high, we were able for the first time to identify the fraction of the He I absorption formed in the chromosphere of the active, hotter F9 giant.
All these helium data together with the EUV results of Dupree & Brickhouse (1995) on the high-temperature Fe XX-Fe XX III lines can be explained using a model with an enhanced magnetized stellar wind that originates from a local region on the active F9 giant, and then forms a shock wave in the outer layers of the corona of the quiescent G6 giant.
Key words: stars: activity stars: atmospheres binaries: spectroscopic Capella stars: late-type
© European Southern Observatory (ESO) 1998
Online publication: December 16, 1997