Broad Wings around Hα and Hβ in the Two S-type Symbiotic Stars Z Andromedae and AG Draconis

Abstract

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 H α and H β . When H α and H β lines are overplotted in the Doppler space, it is noted that H α profiles are overall broader than H β in these two objects. Adopting a Monte Carlo approach, we consider the formation of broad wings of H α and H β through Raman scattering of far-UV radiation around Ly β and Ly γ 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 H α and H β have the same width that is proportional to . However, Raman wings of H α are overall three times wider than H β counterparts, which is attributed to different cross sections for Ly β and Ly γ . Normalized to have the same peak values and presented in the Doppler factor space, H α wings of Z And and AG Dra are observed to be significantly wider than their H β counterparts, favoring the Raman scattering origin for broad Balmer wings.