The Impact of Accretion Disc Winds on the Optical Spectra of Cataclysmic Variables

Abstract

Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted absorption or P-Cygni profiles associated with ultraviolet (UV) resonance lines. These features imply the existence of powerful accretion disk winds in CVs. Here, we use our Monte Carlo ionization and radiative transfer code to investigate whether disk wind models that produce realistic UV line profiles are also likely to generate observationally significant recombination line and continuum emission in the optical waveband. We also test whether outflows may be responsible for the single-peaked emission line profiles often seen in high-state CVs and for the weakness of the Balmer absorption edge (relative to simple models of optically thick accretion disks). We find that a standard disk wind model that is successful in reproducing the UV spectra of CVs also leaves a noticeable imprint on the optical spectrum, particularly for systems viewed at high inclination. The strongest optical wind-formed recombination lines are Hα and He ii λ4686. We demonstrate that a higher-density outflow model produces all the expected H and He lines and produces a recombination continuum that can fill in the Balmer jump at high inclinations. This model displays reasonable verisimilitude with the optical spectrum of RW Trianguli. No single-peaked emission is seen, although we observe a narrowing of the double-peaked emission lines from the base of the wind. Finally, we show that even denser models can produce a single-peaked Hα line. On the basis of our results, we suggest that winds can modify, and perhaps even dominate, the line and continuum emission from CVs.

The origin and evolution of magnetic white dwarfs in close binary stars

“…We set η = 2.5/3. Additional mass loss from the system 68,69 was taken into account by using α = 0.75 in Equation 1. The donor magnetic field was assumed to be B 2 = 1 kG, which is consistent with recent measurements of field strengths of low-mass stars 70 .…”

A review of the disc instability model for dwarf novae, soft X-ray transients and related objects

“…Winds and outflows are observed in many if not all accreting systems, including cataclysmic variables (see Matthews et al, 2015, and references therein) and X-ray binaries (see Díaz Trigo and Boirin, 2016, for a review). These winds carry both mass and angular momentum, which should significantly affect the thermal/viscous instability.…”

“…These latter authors suggested that the problem might be with the assumptions on the angular momentum transport (see Sect. 5.1); it may be also that the impact of winds on accretion disc spectra is not well taken into account (Matthews et al, 2015), and in any case, one would like to couple a radiative transfer code to the MRI simulations, as the vertical distribution of energy release is important. The situation is even worse for cold disks, despite some progress (Idan et al, 2010).…”

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