THE INTERRELATION OF UV AND OPTICAL-ACTIVITY DIAGNOSTICS IN HK-LACERTAE

Optical photometric and UV (IUE) spectroscopic data on the active RS CVn star HK Lac gathered in the years 1979, 1980 and 1987 are studied and compared. The photometry is modelled by representations of 'starspots' and, in general, it is found that the ultraviolet emission lines are stronger at times of spot maximum. The variable interstellar absorption, due to the Doppler motion of the Mg II emission lines from the active stellar component as they move across the interstellar absorption lines, is quantitatively discussed. It is shown that, for HK Lac, the effect produces a double-peaked modulation in the Mg II h + k flux with an amplitude of 4.5 per cent, i.e. considerably smaller than the observed modulation of approximately 40 per cent. Thus we conclude that the variation in the UV line fluxes is stellar, rather than interstellar, in origin. Further analysis shows that a well-defined anticorrelation exists between the IUE emission-line fluxes and the V band, suggestive of an association in longitude between photospheric spots and chromospheric plages. As a corollary to this, a method is suggested for the derivation of the brightness of the unspotted star, and of spot and plage filling factors. We examine in more detail four IUE high-dispersion spectra of Mg II emission features. We fit convolved rotational and Gaussian curves to 'cleaned' k-line profiles, confirming that the star is rotating close to synchronously with its orbital revolution, and probably at high inclination (> 65-degrees). There are indications of regions of localized flux enhancement (plages) and, in at least one case, a plage appears to be located over a similar longitude range to one of the spots derived from the photometry. The Mg II emission lines for HK Lac are considerably wider than would be expected if rotational broadening were the principal mechanism. Multiple Gaussian fits to the profile suggest that the rotational broadening is compounded with a second mechanism, of unknown origin, which results in a width twice that due to rotation alone. Until this mechanism is more clearly understood, caution is suggested in the application of the Doppler imaging technique to the Mg II lines of RS CVn stars.