Spectral images and multi-component models of the chromospheres

The spatial structure of the chromospheres of AR Lacertae was determined by applying a spectral-imaging technique to high-resolution IUE spectra of the Mg II k emission line (2795.528 A). An interactive, iterative fitting procedure was applied that can fit the sum of up to 5 gaussian-emission components to the observed profile. The primary constraints on the multi-component fits are the radial velocities of both stars and the constant interstellar absorption equivalent width. Secondary constraints are the rough constancy of the fluxes and widths of the global stellar emission components. Residual emission is fit with additional components. If these components move across the line profile on a rotational timescale, then they are interpreted as emission from discrete regions (plages). The relative wavelengths and the widths of these additional components are used to determine the position and size of the plage regions on the stellar surface. Given the size and location of the plages, their surface fluxes can be determined. Using the low-resolution IUE/SWP spectra, the fluxes of the plages are separated by using the rotational and eclipse modulation of the observed flux