2017A&A...602A..71S

Context. Light curves (LCs) of some symbiotic stars show a different slope of the ascending and descending branch of their minimum profile. The origin of this asymmetry is not well understood.
Aims. We explain this effect in the ultraviolet LCs of the symbiotic binary SY Mus.
Methods. We model the continuum fluxes in the spectra obtained by the International Ultraviolet Explorer at ten wavelengths, from 1280 to 3080Å. We consider that the white dwarf radiation is attenuated by H⁰ atoms, H^– ions, and free electrons in the red giant wind. Variation in the nebular component is approximated by a sine wave along the orbit as suggested by spectral energy distribution models. The model includes asymmetric wind velocity distribution and the corresponding ionization structure of the binary.
Results. We determined distribution of the H⁰ and H⁺, as well as upper limits of H^– and H⁰ column densities in the neutral and ionized region at the selected wavelengths as functions of the orbital phase. Corresponding models of the LCs match well the observed continuum fluxes. In this way, we suggested the main UV continuum absorbing (scattering) processes in the circumbinary environment of S-type symbiotic stars.
Conclusions. The asymmetric profile of the ultraviolet LCs of SY Mus is caused by the asymmetric distribution of the circumstellar matter at the near-orbital-plane area.