RADIATION-DRIVEN WINDS FROM LUMINOUS ACCRETION DISCS

We study the two-dimensional, time-dependent hydrodynamics of radiatio n-driven winds from luminous accretion discs in which the radiation fo rce is mediated primarily by spectral lines. We assume that the disc i s flat, Keplerian, geometrically thin and optically thick, radiating a s an ensemble of blackbodies according to the alpha-disc prescription. The effect of a radiant central star is included both in modifying th e radial temperature profile of the disc and in providing a contributi on to the driving radiation field. Angle-adaptive integration techniqu es are needed to achieve an accurate representation of the driving for ce near the surface of the disc. Our hydrodynamic calculations use non -uniform grids to resolve both the subsonic acceleration zone near the disc and the large-scale global structure of the supersonic wind. We find that line-driven disc winds are produced only when the effective luminosity of the disc (i.e. the luminosity of the disc times the maxi mum value of the force multiplier associated with the line-driving for ce) exceeds the Eddington limit. If the dominant contribution to the t otal radiation field comes from the disc, then we find that the outflo w is intrinsically unsteady and characterized by large-amplitude veloc ity and density fluctuations. Both infall and outflow can occur in dif ferent regions of the wind at the same time. The cause of this behavio ur is the difference in the variation with height of the vertical comp onents of gravity and radiation force: the former increases while the latter is nearly constant. On the other hand, if the total luminosity of the system is dominated by the central star, then the outflow is st eady. In either case, we find that the two-dimensional structure of th e wind consists of a dense, slow outflow, typically confined to angles within similar to 45 degrees of the equatorial plane, that is bounded on the polar side by a high-velocity, less dense stream. The flow geo metry is controlled largely by the geometry of the radiation held - a brighter disc/star produces a more polar/equatorial wind. Global prope rties such as the total mass loss rate and terminal velocity depend ma inly on the system luminosity and are insensitive to geometry. The mas s-loss rate is a strong function of the effective Eddington luminosity ; at values of less than 1 there is virtually no wind at all, whereas above 1 the mass-loss rate in the wind scales with the effective Eddin gton luminosity as a power law with index 1.5. Matter is fed into the fast wind from within a few stellar radii of the central star. Our sol utions agree qualitatively with the kinematics of outflows in cataclys mic variable (CV) systems inferred from spectroscopic observations. We predict that low luminosity systems may display unsteady behaviour in wind-formed spectral lines. Our study also has application to winds f rom active galactic nuclei and from high mass young stellar objects (Y SOs).