Hoyle-Lyttleton Accretion onto Accretion Disks

We investigate Hoyle-Lyttleton accretion for the case where the central source is a luminous accretion disk. %In classical Hoyle-Lyttleton accretion onto a ``spherical'' source, accretion takes place in an axially symmetric manner around a so-called accretion axis. The accretion rate of the classical Hoyle-Lyttleton accretion onto a non-luminous object and $\Gamma$ the luminosity of the central object normalized by the Eddington luminosity. %If the central object is a compact star with a luminous accretion disk, the radiation field becomes ``non-spherical''. %Although the gravitional field remains spherical. In such a case the axial symmetry around the accretion axis breaks down; the accretion radius $R_{acc}$ generally depends on an inclination angle $i$ between the accretion axis and the symmetry axis of the disk and the azimuthal angle $\phi$ around the accretion axis. %That is, the cross section of accretion changes its shape. Hence, the accretion rate $\dot{M}$, which is obtained by integrating $R_{acc}$ around $\phi$, depends on $i$. % as well as $M$, $\Gamma$, and $v_\infty$. %In the case of an edge-on accretion ($i=90^{\circ}$), The accretion rate is larger than that of the spherical case and approximately expressed as $\dot{M} \sim \dot{M}_{HL} (1-\Gamma)$ for $\Gamma \leq 0.65$ and $\dot{M} \sim \dot{M}_{HL} (2-\Gamma)^2/5$ for $\Gamma \geq 0.65$. %Once the accretion disk forms and the anisotropic radiation fields are produced around the central object,the accretion plane will be maintained automatically (the direction of jets associated with the disk is also maintained). %Thus, the anisotropic radiation field of accretion disks drastically changes the accretion nature, that gives a clue to the formation of accretion disks around an isolated black hole.Comment: 5 figure

Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Intermediate resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23 -- 27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of He\textsc{II} at 4686 \AA, which were absent on July 23, emerged on July 24 together with emission lines of C\textsc{III} / N\textsc{III} Bowen blend. Analyses of the He\textsc{II} emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.Comment: 5 pages, 8 figures, accepted for publication in PAS