Radiation-driven winds from luminous accretion discs

We study the two-dimensional, time-dependent hydrodynamics of radiation-driven winds from luminous accretion discs in which the radiation force is mediated primarily by spectral lines. We assume that the disc is flat, Keplerian, geometrically thin and optically thick, radiating as an ensemble of blackbodies according to the α-disc prescription. The effect of a radiant central star is included both in modifying the radial temperature profile of the disc and in providing a contribution to the driving radiation field. Angle-adaptive integration techniques are needed to achieve an accurate representation of the driving force 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 maximum value of the force multiplier associated with the line-driving force) exceeds the Eddington limit. If the dominant contribution to the total radiation field comes from the disc, then we find that the outflow is intrinsically unsteady and characterized by large-amplitude velocity and density fluctuations. Both infall and outflow can occur in different regions of the wind at the same time. The cause of this behaviour is the difference in the variation with height of the vertical components 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 steady. In either case, we find that the two-dimensional structure of the wind consists of a dense, slow outflow, typically confined to angles within ∼45° of the equatorial plane, that is bounded on the polar side by a high-velocity, less dense stream. The flow geometry is controlled largely by the geometry of the radiation field - a brighter disc/star produces a more polar/equatorial wind. Global properties such as the total mass loss rate and terminal velocity depend mainly on the system luminosity and are insensitive to geometry. The mass-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 Eddington 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 solutions agree qualitatively with the kinematics of outflows in cataclysmic 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 from active galactic nuclei and from high mass young stellar objects (YSOs).