The application of Monte Carlo methods to the synthesis of spectral line profiles from accretion disc winds

Abstract

A Monte Carlo line profile synthesis method is presented which allows the radiative transfer of resonantly scattered lines through a stellar wind to be solved exactly. The resulting code is designed to be used primarily in the context of non-spherical wind models, where its fully three-dimensional nature and elimination of assumptions made by codes based on approximate analytical solutions are of particular importance. A detailed description of the method is given as an Appendix. Applying our method to the UV resonance lines formed in the winds of cataclysmic variable stars, we have constructed a new kinematic disc wind model, in which the outflow emanates from the surface of the rotating accretion disc and has a biconical geometry. In this, it is assumed that specific angular momentum is conserved along streamlines. Within the parameter space of the model, a wide range of outflow geometries can be explored. An exploration is begun here in considering the extent to which a disc wind model can better explain phenomena that are difficult to accommodate within simpler, more approximate central wind models (with or without rotation). Our main findings are as follows. (i) Line profiles calculated from disc wind models are qualitatively consistent with observed UV line shapes. In particular, disc winds can give rise to low-inclination profiles which show maximum absorption near line centre even in the limit of constant outflow velocity. This behaviour can be reproduced by central wind models only on resorting to very slow wind acceleration. (ii) At high inclinations, disc winds can naturally give rise to the pure emission-line profiles that are seen in observations. Instead of the broadened, blueshifted absorption that tends to persist in profiles synthesized from central wind models, many high-inclination disc wind profiles show sharp reversals (`dips') cutting into a broader underlying wind emission line. If observed, these dips may be used as indicators for a biconical wind geometry, and their offset from line centre may allow the degree of wind collimation to be estimated. (iii) We confirm a result of earlier work that the introduction of a rotational velocity component into the outflow can broaden the line emission component, and reduce its core intensity at high inclination. The rotational component of motion only becomes important in shaping the line profile in highly collimated disc winds viewed at high inclination, in which it determines the apparent linewidth. (iv) Disc wind line profiles are sensitive to the overall wind geometry and the bias of mass loss towards the peak of the disc's radial energy distribution. There is also considerable sensitivity to the introduction of an outflow velocity gradient. In more collimated disc winds, gradual acceleration can cause high-inclination profiles to approach the double-peaked shape characteristic of rotating line-emitting media.