Spectroscopic view on the outburst activity of the symbiotic binary AG Draconis

TL;DR

This study analyzes 14 years of spectral data from the symbiotic star AG Draconis, revealing how different outburst types affect emission lines and suggesting a common 'combination nova' mechanism for these phenomena.

Contribution

It provides a detailed spectroscopic analysis of AG Dra's outbursts, identifying distinct spectral signatures and proposing a unified model for outburst behavior based on similarities with Z And.

Findings

Cool and hot outbursts have distinct spectral signatures.

Raman O VI line nearly disappears during cool outbursts.

Emission lines vary with orbital motion and outburst type.

Abstract

Variations of the emission lines in the spectrum of the yellow symbiotic star AG Dra have been studied for over 14 years (1997 - 2011), using more than 500 spectra obtained on the 1.5-metre telescope at Tartu Observatory, Estonia. The time interval covered includes the major (cool) outburst of AG Dra that started in 2006. Main findings can be summarized as follows: (i) cool and hot outbursts of AG Dra can be distinguished from the variations of optical emission lines; (ii) the Raman scattered emission line of O VI at $\lambda\,6825$ almost disappeared during the cool outburst; (iii) lower excitation emission lines did not change significantly during the cool outburst, but they vary in hot outbursts and also follow orbital motion; (iv) similarity of variations in AG Dra to those in the prototypical symbiotic star Z And allows to suggest that a "combination nova" model proposed for the latter object might also be responsible for the outburst behaviour of AG Dra.