Radio spectroscopy of late-type variable stars

Abstract

The results of the monitoring of a sample of late-type variable stars (Mira Ceti-type and semiregulars) are reported. Since 1980 a sample of 60 stars has been observed in the maser line of the H2O molecule at a wavelength of 1.35 cm. The observations are performed on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory. Since 1994 optical spectra of some of these stars have also been monitored on the 1.25-meter telescope of the Crimean Laboratory of the Sternberg Astronomical Institute. Variations of the circumstellar H2O masers correlate with the visual light curves with a time lag of 0.30.4 P (P is the star’s period). Flares of the H emission noted in R Leo, U Aur, R Cas, and R LMi were followed 1.52 years later by the corresponding ares of the H2O masers. These phenomena can be interpreted as a consequence of propagation of a shock wave driven by stellar pulsation. However, radio continuum observations suggest that such shocks (if any) may be rather weak, with a velocity that does not exceed 1520 km/s. Alternatively, the shock can be produced by the motion of a low-mass companion (a planet or a brown dwarf) in the inner layers of the circumstellar envelope. The eects of such a companion on the spectrum and light curve of the primary star are discussed. It is shown that a cone shock (similar to that produced in the Earth’s atmosphere by the motion of a large meteoritic body) can produce sucien t Balmer line emission without signican t accompanying radio continuum. The companion’s motion may also account for a signican t portion of the primary’s light variations without invoking large-amplitude stellar pulsations, which so far have been accepted as the main mechanism of Mira-type variability in red giants.