Molecular Masers in Variable Stars

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.

History of the Light Curves and Molecular Maser Emission of the Miras U Ori and R Leo

We have collected all the available data on light curves, OH, H2O and SiO maser observations for a sample of Mira-type variables. We consider in detail the data on two stars, U Ori and R Leo. There is a net correlation between optical and radio line variations for all the three molecular species in these stars. More pronounced maser flares seem to follow brighter-than-average visual maxima of the stars. We discuss also the drastic changes in the type of the OH maser radio emission which happened in these stars some years ago. Implications for the mechanisms of maser pumping and the evolutionary status of these stars (probably undergoing the helium flash) are discussed.

On the nature of protostellar H2O masers

Most sources of B2O maser radio emission at 1.35 cm, associated with star formation regions, show strong variability with, sometimes, rapid bursts of emission (see, e.g., Liljeström 1984, Rowland and Cohen 1986, and references therein). A preliminary conclusion on the possible cyclicity of H2O maser variability can be drawn (Lekht et al. 1982, 1983), with a quasiperiod of several years. The “quiet” state of a maser source, with moderate, slowly varying values of the line flux density, turns to the “active” phase with H2O line bursts (Lekht et al. 1983). The H2O maser generation region is probably located in a rotating gas-and-dust disc (torus) around a protostar (or young star). This is pointed to by VLBI observations showing in some sources maser features arranged in an ellipsoidal structure around a common centre (presumably, the protostellar object - see Downes et al. 1979), as well as by symmetrical character of E2O line profiles of many masers (Lekht et al. 1982). As an excitation mechanism for H2O, collisional pumping in two-temperature medium behind a shock front (with hot heavy particles and cold free electrons or vice versa) is widely accepted (Bolgova et al. 1982, Kylafis and Norman 1986).

Studies of variability of circumstellar H2O masers

From March 1980 to December 1983, the author took part in regular observations of variability of maser radio emission in the H2O line at 22 GHz. The observations were carried out at the 22-meter radio telescope of the P. Lebedev Physical Institute (USSR Academy of Sciences) in Pushchino (Moscow Region). The interval between consecutive observational sessions was usually 1.5–2 months. The observational program included 21 late-type variable stars (Miras and SRs): R Aql, RR Aql, RT Aql, SY Aql, U Aur, NV Aur, RX Boo, VY CMa, S CrB, KY Cyg, NML Cyg, U Her, W Hya, X Hya, R Leo, U Lyn, U Ori, UU Peg, VX Sgr, RS Vir, RT Vir. The results for eight stars ending June 1982 were published by Berulis et alt (1983). A comparison was made between the time dependences of the H2O line radio flux F and the curves of visual and near-infrared brightness of the stars. Miras (R Aql, R Leo, U Ori, U Aur), as a rule, have a rise in F connected with the visual maximum (phase 0), the maximum F occurring at phases 0.1–0.2 (see figure for an example). Not all visual maxima (only one out of each two or three) are accompanied by H2O flares. This Miras! behaviour was also noted earlier in the H2O line by Berulis et al. (1984), Gómez Balboa and Lépine (1986), as well as in the SiO maser line v=1, J=2−1 by Nyman and Olofsson (1986).