3. Line emission from the Red star
The line emission, on the Doppler images of Fig. 3, is stronger on the side of the red star facing the white dwarf and further it weakens towards the equator of the red star, indicative of screening by the disc. The He I maps show a relative strength similar to that seen in diffuse nebulae (weaker in 4388 Å, stronger in 4472 Å; Kaler 1976). We measured the velocity locations of the peak intensities in the Doppler images using Gaussian fitting. There may be a systematic shift towards the L1 point with higher-ionization potential (see Table 1). As a consistency test for the properties of the "spots" being realistic, the velocity widths of the irradiated sites are indeed less than the rotational broadening of the companion star, km s-1 (1), which is obtained from the relation
where , and km s-1 (Wolf et al. 1998). The measured relative shifts between the wavelength-dependent irradiated sites are with respect to zero velocity (binary's centre), therefore any uncertainties in the system parameters do not alter our conclusion.
Table 1. Red star emission
In the past, similar emission has been interpreted as irradiation of the inner side of the red star by soft X-ray photons emitted by the boundary layer (Harlaftis & Marsh 1996 and references therein). The Roche lobe maps may suggest that there is temperature foreshortening or that the shadow cast by the disc on the companion star decreases with higher energy photons (Mg II, He I, He II). Indeed, the disc thickness may hinder efficient irradiation around L1 relative to the polar regions.
In this way, the red star emission can also be used to measure the thickness of the disc as seen by the soft X-rays which excite the line emission, independently of X-ray data. From the values in Table 1, the L1 region may mainly be clear of emission around 50 km s-1 (or 2 pixels) from the L1. This corresponds to a Roche lobe height of %, or , where is the binary separation (assuming q=0.6 and use of Table 3-1 in Kopal 1959). Therefore, there is potential to probe the vertical structure of the disc with higher quality data. Moreover, disc contamination and small-scale blurring caused by the Doppler tomography (which assumes isotropic emission from the orbital plane) are significant at that level and only improvement of the Doppler tomography technique combined with higher-resolution data will clarify better the ionization zones on the Roche lobe and the vertical disc height.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999