THE APPLICATION OF MONTE-CARLO METHODS TO THE SYNTHESIS OF SPECTRAL-LINE PROFILES ARISING FROM ACCRETION DISC WINDS

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 importanc e. A detailed description of the method is given as an Appendix. Apply ing our method to the UV resonance lines formed in the winds of catacl ysmic variable stars, we have constructed a new kinematic disc wind mo del, in which the outflow emanates from the surface of the rotating ac cretion 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 b e explored. An exploration is begun here in considering the extent to which a disc wind model can better explain phemonena that are difficul t 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 consisten t with observed UV line shapes. In particular, disc winds can give ris e 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 slo w wind acceleration. (ii) At high inclinations, disc winds can natural ly give rise to the pure emission-line profiles that are seen in obser vations. Instead of the broadened, blueshifted absorption that tends t o persist in profiles synthesized from central wind models, many high- inclination disc wind profiles show sharp reversals ('dips') cutting i nto a broader underlying wind emission line. If observed, these dips m ay be used as indicators for a biconical wind geometry, and their offs et from line centre may allow the degree of wind collimation to be est imated.(iii) We confirm a result of earlier work that the introduction of a rotational velocity component into the outflow can broaden the l ine emission component, and reduce its core intensity at high inclinat ions. The rotational component of motion only becomes important in sha ping the line profile in highly collimated disc winds viewed at high i nclination, in which it determines the apparent linewidth. (iv) Disc w ind line profiles are sensitive to the overall wind geometry and the b ias of mass loss towards the peak of the disc's radial energy distribu tion. There is also considerable sensitivity to the introduction of an outflow velocity gradient. In more collimated disc winds, gradual acc eleration can cause high-inclination profiles to approach the double-p eaked shape characteristic of rotating line-emitting media.