Water vapour masers in long-period variable stars IV. Mira variables RCas, oCet and RLeo

Abstract

Between 1987 and 2023 we carried out single-dish monitoring campaigns of water-maser emission at 22 GHz in the circumstellar envelopes (CSEs) of four stars: RCas, oCet, RLeo and ChiCyg. The variability in integrated flux in RCas and oCet followed that in the optical with the same period, but with a lag of about one-third in phase. RLeo was too often below our sensitivity threshold for us to determine a radio period. No maser was detected in ChiCyg. The variability in the masers in RCas has a distinctive pattern that confirms the existence of a zone in the CSE with favourable conditions for maser excitation. The total flux, modulated by the pulsations of the star, gradually increases to a maximum, which is followed by a similar decrease. This takes about 20 years. The pattern is repeated after an interval of quiescence of several years. During its decline, the variation in the flux resembles a damped harmonic oscillator. There are two dominant emission components that move almost tangentially on either side of the star with respect to the observer. The redshifted one is consistent with an origin in a single cloud that lived for at least about eight years, falling back towards the star. The blue component, moving in the CSE hemisphere nearest to us, has no drift in the line-of-sight velocity and appears to originate in a time series of short-living clouds with line-of-sight velocities within ca. 1 km/s of each other. No systematic drifts are found in RLeo and oCet. A few bursts of emission were detected in RCas and RLeo that lasted about a year and caused an increase in the flux density by 1-2 orders of magnitude. The velocity range of the maser emission is < 10 km/s and is clearly correlated with the bolometric luminosity of the stars.