# Wind asymmetry imprint in the UV light curves of the symbiotic binary SY Mus

## TL;DR

This study models the ultraviolet light curves of the symbiotic binary SY Mus, revealing that the asymmetry in the light curves is caused by an asymmetric distribution of circumstellar matter around the binary system.

## Contribution

It introduces a detailed model of UV flux attenuation considering asymmetric wind velocity and ionization structure, explaining the observed light curve asymmetry in SY Mus.

## Key findings

- Asymmetric distribution of circumstellar matter causes UV light curve asymmetry.
- Model matches observed UV fluxes well, confirming the proposed absorption processes.
- Identifies key hydrogen and electron distributions affecting UV attenuation.

## Abstract

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 understood well. 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 10 wavelengths, from 1280 to 3080 \AA. We consider that the white dwarf radiation is attenuated by $H^0$ 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 $H^0$ and $H^+$, as well as upper limits of $H^-$ and $H^0$ 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.