THE INTRICATE OPTICAL-LINE BEHAVIOR OF THE NOVA-LIKE SYSTEM V795 HERCULIS

The optical emission-line properties of the nova-like cataclysmic vari able V795 Her are studied using high-resolution Isaac Newton Telescope [Intermediate Dispersion Spectrograph (IDS) and Faint-Object Spectrog raph (FOS)] data secured during 1994 June. The observed Balmer and He emission lines, and He I lambda 4471 absorption, modulate on the 2.6-h orbital period of this system. The detailed nature of these changes i s complex, however, with the incidence of separate low- and high-veloc ity fluctuations. Low-intensity 'S'-wave features are apparent in the blue and red outer wings of the Balmer lines, extending to similar to +/- 1600 km s(-1). These phased changes are seen as two synchronized c omponents with net blueshift and redshift. The Balmer profiles are als o affected by absorption which dominates during the first part of the cycle. We propose an empirical interpretation of this behaviour in ter ms of a single broad emission feature which follows a high-velocity 'S ' wave, plus a superimposed, sympathetically phased, absorption compon ent. The associated physical interpretation is that of a gas stream (s een in absorption) that overflows the outer accretion disc and re-impa cts on the inner disc regions. The 'double-peaked' inner cores of the Balmer lines have relatively fixed central velocities, and do not reve al an obvious 'S'-wave variation. This inner-core emission likely aris es from material located in the outer regions of the accretion disc. A n similar to 10 per cent variation is present in the continuum flux of V795 Her, restricted to less than 25 per cent of the system orbit. We find some correspondence between the overall optical emission changes and those found by us in the UV resonance lines of V795 Her in a sepa rate study based on Hubble Space Telescope data. We discuss our observ ational results in terms of magnetic white dwarf and disc-overflow gas stream scenarios. Constraints are presented that argue against recent ly proposed models for V795 Her which invoke channelled accretion colu mns flowing on to a synchronously rotating white dwarf. Disc-overflow model simulations are presented which quantitatively account for sever al of the observed dynamical properties of V795 Her. There remain some discrepancies, however, which call for a more detailed physical model .