2018ApJ...866..129C

Symbiotic stars often exhibit broad wings around Balmer emission lines, whose origin is still controversial. We present high-resolution spectra of the S type symbiotic stars Z Andromedae and AG Draconis obtained with the ESPaDOnS and the 3.6 m Canada-France-Hawaii Telescope to investigate the broad wings around Hα and Hβ. When Hα and Hβ lines are overplotted in the Doppler space, it is noted that Hα profiles are overall broader than Hβ in these two objects. Adopting a Monte Carlo approach, we consider the formation of broad wings of Hα and Hβ through Raman scattering of far-UV radiation around Lyβ and Lyγ and Thomson scattering by free electrons. Raman scattering wings are simulated by choosing an H I region with a neutral hydrogen column density N_H I_ and a covering factor CF. For Thomson wings, the ionized scattering region is assumed to fully cover the Balmer emission nebula and is characterized by the electron temperature T_e and the electron column density N_e. Thomson wings of Hα and Hβ have the same width that is proportional to T_e^1/2. However, Raman wings of Hα are overall three times wider than Hβ counterparts, which is attributed to different cross sections for Lyβ and Lyγ. Normalized to have the same peak values and presented in the Doppler factor space, Hα wings of Z And and AG Dra are observed to be significantly wider than their Hβ counterparts, favoring the Raman scattering origin for broad Balmer wings.