The H$\alpha$ and H$\beta$ Raman-scattering line profiles of the symbiotic star AG Peg

TL;DR

This study analyzes the Hα and Hβ line profiles of the symbiotic star AG Peg, modeling the Raman scattering process and the star's bipolar shell structure to understand the ionized and neutral zones around the white dwarf.

Contribution

It introduces a Monte Carlo simulation approach to fit Raman scattering wings and proposes a bipolar conical shell model for the star's circumstellar environment.

Findings

The neutral scattering zone has a column density of 3-5 x 10^{19} cm^{-2}.

The ionized region is radiation-bounded with specific density and temperature.

The star's structure may be a bipolar shell formed by mass inflow and stellar winds.

Abstract

The H$\alpha$ and H$\beta$ line profiles of the symbiotic star AG Peg observed on 1998 September (phase $\phi$ = 10.24), display the top narrow double Gaussian and bottom broad (FWHM of 200 -- 400 kms$^{-1}$) components. The photo-ionization model indicated that the ionized zone, responsible for the hydrogen Balmer and Lyman lines, is radiation-bounded, with a hydrogen gas number density of $n_{\rm H} \sim 10^{9.85}$ cm$^{-3}$ and a gas temperature of T$_e$ = 12\,000 -- 15\,000~K. We carried out Monte Carlo simulations to fit the Raman scattering broad wings, assuming that the hydrogen Ly$\beta$ and Ly$\gamma$ lines emitted within the radiation-bounded HII zone around a white dwarf have the same double Gaussian line profile shape as the hydrogen Balmer lines. The simulation shows that the scattering HI zones are attached to (or located just outside of) the inner HII shells. The best fit to the observed broad HI line profiles indicates that the column density of the scattering neutral zone is $N_{\rm H} \simeq $ 3 -- 5 $\times$ 10$^{19}$ cm$^{-2}$. We examined whether the geometrical structure responsible for the observed H$\alpha$ and H$\beta$ line profiles is a bipolar conical shell structure, consisting of the radiation-bounded ionized zone and the outer material bounded neutral zone. The expanding bipolar structure might be two opposite regions of the common envelope or the outer shell of the Roche lobe around the hot white dwarf, formed through the mass inflows from the giant star and pushed out by the fast winds from the hot white dwarf.