MG II h&k line diagnostics of IM Pegasi

Abstract

We have analysed the Mgii 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 UBV(RI)_C 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 Mgii 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 Mgii 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. The authors would like to dedicate this paper to their late referee, Dr. P.B. Byrne.