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Astron. Astrophys. 330, 559-568 (1998)

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Mg II h&k line diagnostics of IM Pegasi *

K. Oláh 1, D. Marik 1, E.R. Houdebine 2, R.C. Dempsey *** , 3 and E. Budding 4

1 Konkoly Observatory of the Hungarian Academy of Sciences, H-1525 Budapest, Hungary (e-mail: olah@ogyalla.konkoly.hu, dmarik@ogyalla.konkoly.hu)
2 Space Science Department, ESA-ESTEC Postbus 299, 2200 AG Noordwijk, and Sterrenkundig Instituut, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 Astronomy Programs, Computer Sciences Corporation, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
4 Central Institute of Technology, P.O. Box 40740, Upper Hutt, Wellington, New Zealand; Carter Observatory, P.O. Box 2909,
Wellington C.1., New Zealand

Received 18 December 1996 / Accepted 8 October 1997


We have analysed the Mg II h&k resonance lines of IM Peg (KII-III) on 14 high resolution IUE LWR spectra and compared the results coming from visual spectroscopic and [FORMULA] photometric data. The `clean' spectra (free from interstellar absorption) were modelled using a convolution of the rotational profile with a Gaussian profile, that represents macroturbulence and broadenings of other origin, the former dominating.

Significant variations in the h&k line fluxes and widths were found throughout the orbital phase range. A line broadening effect is found at maximum spot visibility both in the k and h lines. The Mg II line widths increase linearly with the flux of the transition region lines as a function of rotational phase. Anticorrelations as a function of phase were found between the Mg II line fluxes and the higher temperature lines. The k line is broader than the h line, correlated with a slope greater than unity, in keeping with a greater radiation transfer effect in the k line, and a non-optically thin medium at all phases. We observe a systematic asymmetry in the ratio of the h to k line profiles at all phases. The spectral diagnostics thus suggest the appearance of a plage region on the stellar disc.

We obtained new orbital elements of the IM Peg system, deriving a small, but non-zero eccentricity.

Key words: stars: individual: IM Peg – stars: activity – stars: chromospheres – stars: binaries: spectroscopic – stars: late-type

* The authors would like to dedicate this paper to their late referee, Dr. P.B. Byrne.
** Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.
*** Guest Investigator with the International Ultraviolet Explorer satellite, which is sponsored and operated by the National Aeronautics and Space Administration, by the the Science Research Council of the United Kingdom, and by the European Space Agency.

Send offprint requests to: K. Oláh

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998