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Astron. Astrophys. 349, 863-872 (1999)


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The active RS Canum Venaticorum binary II Pegasi

III. Chromospheric emission and flares in 1994-1996 *

S.V. Berdyugina, I. Ilyin and I. Tuominen

Astronomy Division, University of Oulu, P.O. Box 3000, FIN-90401 Oulu, Finland

Received 23 April 1999 / Accepted 19 July 1999

Abstract

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 He I D3, Ca II K and Ca II 8498 Å 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 He I  D3 and Ca II K and 8498 Å 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 He I and Ca II K 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 He I 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 He I and Ca II 8498 Å lines but without development in other lines. It probably was a late stage of the flare decay.

Key words: stars: activity – stars: chromospheres – stars: flare – stars: individual: II Peg

* Based on observations collected at the Nordic Optical Telescope (NOT), La Palma, Spain

Send offprint requests to: S.V. Berdyugina (sveta@ukko.oulu.fi)

© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999

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