Broad Wings around H$\alpha$ and H$\beta$ in the Two S-Type Symbiotic Stars Z Andromedae and AG Draconis

TL;DR

This study investigates the broad wings around Hα and Hβ emission lines in two S-type symbiotic stars, using high-resolution spectra and Monte Carlo simulations to determine their origin, favoring Raman scattering as the primary cause.

Contribution

The paper provides high-resolution spectral analysis and Monte Carlo modeling to identify Raman scattering as the main mechanism behind broad Balmer wings in symbiotic stars.

Findings

Hα wings are broader than Hβ in both stars.

Raman scattering explains the width difference between Hα and Hβ.

Thomson scattering produces wings proportional to the square root of electron temperature.

Abstract

Symbiotic stars often exhibit broad wings around Balmer emission lines, whose origin is still controversial. We present the 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$\alpha$ and H$\beta$. When H$\alpha$ and H$\beta$ lines are overplotted in the Doppler space, it is noted that H$\alpha$ profiles are overall broader than H$\beta$ in these two objects. Adopting a Monte Carlo approach, we consider the formation of broad wings of H$\alpha$ and H$\beta$ through Raman scattering of far UV radiation around Ly$\beta$ and Ly$\gamma$ and Thomson scattering by free electrons. Raman scattering wings are simulated by choosing an H I region with a neutral hydrogen column density $N_{HI}$ and a covering factor $CF$. For Thomson wings, the ionized scattering region is assumed to cover fully the Balmer emission nebula and is characterized by the electron temperature $T_e$ and the electron column density $N_e$. Thomson wings of H$\alpha$ and H$\beta$ have the same width that is proportional to $T_e^{1/2}$. However, Raman wings of H$\alpha$ are overall three times wider than H$\beta$ counterparts, which is attributed to different cross section for Ly$\beta$ and Ly$\gamma$. Normalized to have the same peak values and presented in the Doppler factor space. H$\alpha$ wings of Z And and AG Dra are observed to be significantly wider than H$\beta$ counterpart, favoring the Raman scattering origin of broad Balmer wings.