White dwarf radii and boundary-layer constraints in three dwarf novae

• Published in: Monthly notices of the Royal Astronomical Society Vol. 242; no. 4; pp. 606 - 615
• Main Authors: WOOD, J. H, HORNE, K
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford University Press 01-08-1990; Blackwell Science
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Novae, dwarf novae, recurrent novae, and other cataclysmic (eruptive) variables; Stars; Variable and peculiar stars (including novae); Mass; Model matching; Dwarf nova; Eclipse; Digital simulation; Close binary; Light curve; White dwarf; Accretion disk; Diameter; Boundary layer
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/242.4.606

The structure of the boundary layer between the accretion disc and white dwarf in three quiescent dwarf novae is explored with high signal-to-noise eclipse light curves obtained by phase folding 12–20 eclipses. Models of the eclipse shapes of various white dwarf/boundary layer configurations that might be at the centres of the accretion discs are calculated and compared with observations of the eclipses in Z Cha, OY Car and HT Cas. Possible models for the central objects are found to be a white dwarf with or without its lower hemisphere occulted by the disc, or a white dwarf with an optically thick boundary layer significantly extended in latitude up and down its sides. The most likely of these models for each system is an unocculted white dwarf with no boundary layer contributing significantly to the optical flux, or a white dwarf totally covered by an optically thick boundary layer. A spherical white dwarf with no boundary layer is fitted to the observed eclipses. The best-fit values of the eclipse width and the phase of mid-eclipse are consistent with the values we found previously by direct measurement of the light curves, as is the white dwarf radius for HT Cas. However, the fitted radii of the white dwarfs are at least 10 per cent smaller, for Z Cha and OY Car, than the radii derived from earlier direct measurements of eclipse phases. This systematic effect introduces comparable uncertainties in the mass determinations. The fitted radii depend on the limb darkening, but, only if the central object produces an ingress and egress more gradual at the beginning and end than that of a white dwarf with maximum limb darkening, could the fitted radii be consistent with the measured radii for Z Cha and OY Car. If the outer layers of the white dwarf were significantly flattened because of rapid rotation, a smaller radius would be required to fit the light curves. The optical fluxes produced in the boundary layers of Z Cha and OY Car are very low compared to those expected if the boundary layers are optically thick and the mass transfer rate on to the white dwarf is the same as that into the outer disc. This could indicate that material piles up in the outer discs, or that the white dwarfs are rotating rapidly. If the boundary layers are optically thin with ${\tau}_{\text{v}}\sim $ constant, then with a kinetic temperature of ≥ 33 500 K for Z Cha and ≥ 48 000 K for OY Car, the accretion rate on to the white dwarf can match those into the bright spot without requiring material to pile up in the disc or the white dwarf to rotate rapidly.