Molecular Masers in Variable Stars

Abstract

When a star with a mass of one to a few solar masses enters the red giant stage of its evolution, the radius of its atmosphere reaches several astronomical units. Pulsational instability is typical for this stage. Most stars become Mira-type or semiregular variables with light cycles of a few hundred days. Red giants lose mass at a rate M = 10^–7–10^–5M_⊙ yr^–1. Extensive gas–dust circumstellar envelopes form. These envelopes contain various molecular species. Some of these molecules (OH, H₂O, SiO, HCN) manifest themselves in maser radio emission. Data on the H₂O maser variability and its connection with the stellar brightness variations are discussed. In the H₂O line circumstellar masers can be divided into [stable] (showing persistent emission — R Aql, U Her, S CrB, X Hya) and [transient] (appearing in the H₂O line once per 10–15 stellar light cycles — R Leo, R Cas, U Aur). Physical mechanisms of the maser variability are discussed. The most probable process explaining the observed visual–H₂O correlation is the influence of shock waves on the masing region. Usually it is assumed that shocks in Mira atmospheres are driven by stellar pulsations. Here an alternative explanation is proposed. If a star during its main sequence life possessed a planetary system, similar to the solar system, the planets will be embedded in a rather dense and hot medium. Effects of a planet revolving around a red giant at a short distance (inside its circumstellar envelope) are discussed. A shock produced by the supersonic motion of a planet can account for the correlated variability of the Hα line emission and H₂O maser. If the planetary orbit is highly eccentric, then the connected Hα–H₂O flare episodes may be explained by the periastron passage of the planet. New tasks for the upgraded ATCA are discussed.