A disc wind origin for the optical spectra of dwarf novae in outburst

Abstract

Many high-state cataclysmic variables (CVs) exhibit blue-shifted absorption features in their ultraviolet (UV) spectra -- a smoking-gun signature of outflows. However, the impact of these outflows on {\em optical} spectra remains much more uncertain. During its recent outburst, the eclipsing dwarf nova V455 And displayed strong optical emission lines whose cores were narrower than expected from a Keplerian disc. Here, we explore whether disc + wind models developed for matching UV observations of CVs can also account for these optical spectra. Importantly, V455~And was extremely bright at outburst maximum: the accretion rate implied by fitting the optical continuum with a standard disc model is ${\dot{M}}_{\mathrm{acc}}\simeq 10^{-7}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. Allowing for continuum reprocessing in the outflow helps to relax this constraint. A disk wind can also broadly reproduce the optical emission lines, but only if the wind is (i) highly mass-loaded, with a mass-loss rate reaching ${\dot{M}}_{\mathrm{wind}}\simeq 0.4{\dot{M}}_{\mathrm{acc}}$, and/or (ii) clumpy, with a volume filling factor $f_V\simeq 0.1$. The same models can describe the spectral evolution across the outburst, simply by lowering ${\dot{M}}_{\mathrm{acc}}$ and ${\dot{M}}_{\mathrm{wind}}$. Extending these models to lower inclinations and into the UV produces spectra consistent with those observed in face-on high-state CVs. We also find, for the first time in simulations of this type, P-Cygni-like absorption features in the Balmer series, as have been observed in both CVs and X-ray binaries. Overall, dense disc winds provide a promising framework for explaining multiple observational signatures seen in high-state CVs, but theoretical challenges persist.

13 pages, 9 figures, 3 tables. Accepted for publication in MNRAS