Astronomy and Astrophysics, volume 349, 863-872 (1999/9-3)
The active RS Canum Venaticorum binary II Pegasi. III. Chromospheric emission and flares in 1994-1996.
BERDYUGINA S.V., ILYIN I. and TUOMINEN I.
Abstract (from CDS):
We analyse observations of the very active RS CVn-type star II Peg carried out simultaneously in chromospheric and photospheric lines in 1994-1996. We describe the correlation of the strength of the HeI D3, CaIIK and CaII 8498Å emissions with the spot position on the stellar disk. A two-component structure is suggested in the chromosphere of II Peg, similar to that in the corona: nonvariable component (cool plages) with constant contribution to the line emission and variable, active one (hot plages) showing a growth of its activity during 1994-1996. The active component is related to the spots seen in the photosphere. Two subsequent flares on July 19-23, 1995 were observed in the HeI D3 and CaIIK and 8498Å lines showing strong narrow red-shifted emissions. The development of the flares took a few hours and decay lasted several days. At the maximum of the flaring, in addition to the narrow components, broad blue-shifted emissions appeared in HeI and CaIIK and in the cores of many strong absorption lines. The broad components are attributed to the process of the explosive evaporation from the low chromosphere. The amount of energy released in different lines is determined. From the radial velocity curve of the Hei emission the location of the radiating matter is deduced. It appears to be related to the largest active region which is seen in the stellar image. The flare occurred concurrently with the break of the extended group on two well separated spots. On October 26, 1996 another flare was observed in three spectra, as narrow emissions in the HeI and CaII8498Å lines but without development in other lines. It probably was a late stage of the flare decay.