Astron. Astrophys. 346, L73-L75 (1999)

2. Doppler tomography

IP Peg is a well-studied, double-eclipsing dwarf nova, which shows semi-periodic outbursts every 3 months. For details on the observations and data reduction see Harlaftis et al. (1999). Average spectra, in the range 4354-4747 Å, at four characteristic binary phases are displayed in Fig. 1. In addition to the Bowen blend and He II 4686 lines, weak He I lines at 4388, 4471, 4713 Å Mg II 4481 Å and Ti II 4418 Å are also observed (see also trailed spectra in Fig. 2). These lines display a sharp peak at phase 0.5 indicating a component from the companion star. We adopt the binary ephemeris from Wolf et al. (1993) , where is the inferior conjunction of the white dwarf.

Fig. 1. Average spectra of IP Peg in outburst at characteristic binary phases (as marked). The emission lines are sharper at phase 0.5, when the inner side of the red dwarf passes through the line-of-sight. During eclipse, structure on the continuum is significantly suppressed and the emission-line flux is still detectable.

Fig. 2. Trailed spectra over the 4350-7450 Å range scaled, so that the motion of weak lines is revealed (0-10 mJy). All the lines identified in Fig. 1 can be traced here, such as the Ti II line at 4418 Å. The trailed spectra of the Bowen blend are very similar to the He II line. The velocity relative to the line centre along the horizontal axis is plotted and the binary phase along the vertical axis. The intensity scale is 0-10 mJy (greyscale bar).

The trailed spectra over the full wavelength range are shown in Fig. 2 with the aim to display the motion of the weak lines (the intensity scale is adjusted so that He II line appears saturated). The disc and red star emission components are seen in the lines of He I 4388, He I 4472, Mg II 4481, the Bowen blend and the He I 4713. The red star component is the sharp `S'-wave moving from red to blue at phase 0.5. It can also be traced in the He II 4542 and the Ti II 4418 lines. Note that the Mg II `S'-wave component disappears earlier (binary phase 0.7) than that of the neighbouring He I 4472 (binary phase 0.75).

We reconstruct the Doppler images of the emission lines using the trailed spectra (Marsh and Horne 1988). A Doppler image is the reconstruction of the emission line distribution in velocity space and has been particularly successful in resolving the location of emission components such as the red star (IP Peg; Harlaftis et al. 1994), the gas stream (OY Car in outburst; Harlaftis and Marsh 1996), the bright spot (GS2000+25; Harlaftis et al. 1996) and spiral waves in the outer accretion disc (Steeghs, Harlaftis and Horne 1997; Harlaftis et al. 1999). We built the Doppler images of the emission lines (see above references for the procedure) and, after subtracting the axisymmetric disc emission, we can zoom onto the Roche lobe of the red star (Fig. 3 from left to right, high-ionization to low-ionization lines, He II 4686, He I 4388, He I 4472, He I 4713, Mg II 4481).

Fig. 3. The irradiated Roche lobe of the red dwarf in IP Peg is imaged at various wavelengths. From left to right, the ionization potential decreases. These are the Doppler images zoomed on the red dwarf. The irradiated area may be moving to the polar regions with decreasing ionization potential.

© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999
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