The boundary layer of VW Hydri in quiescence

• Published in: Monthly notices of the Royal Astronomical Society Vol. 361; no. 3; pp. 809 - 812
• Main Authors: Godon, P., Sion, E. M.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 11.08.2005; Blackwell Science; Oxford University Press
• Subjects: accretion; accretion discs; Accretion disks; accretion, accretion discs; Astrophysics; Boundary layer; cataclysmic variables; Luminosity; novae; novae, cataclysmic variables; Stars & galaxies; stars: individual: VW Hyi; Ultraviolet radiation; X-rays; Viscosity; Accretion; White dwarf stars; Ultraviolet spectra; Luminosity; Rotation speed; Dwarf novae; stars: individual: VW Hyi; Accretion disks; accretion, accretion discs; FUSE satellite; Quiescence; novae, cataclysmic variables; Multifrequency observation; Cataclysmic variable stars; Ultraviolet observation; Boundary layers
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2005.09209.x

In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the ‘accretion belt’. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer (FUSE) observations of VW Hyi in quiescence, which confirm the presence of a second component (the ‘accretion belt’) in the far-UV spectrum. The temperature (≈ 50 000 K) and rotational velocity (≈ 3000 km s −1) of this second component are entirely consistent with the optically thick region (τ ≈ 1) located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to LBL= (0.53 ± 0.25) Ldisc, while the theory (Kluzniak) predicts, for the rotation rate of VW Hyi's white dwarf, LBL≈ (0.76 ± 0.03)Ldisc. About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Córdova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of ν ≈ 1013–1014 cm2 s −1, depending on the white dwarf mass and the size of the boundary layer.