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Low- and intermediate-mass stars eject much of their mass during the late,red giant branch (RGB) phase of evolution. The physics of their strong stellarwinds is still poorly understood. In the standard model, stellar pulsationsextend the atmosphere, allowing a wind to be driven through radiation pressureon condensing dust particles. Here we investigate the onset of the wind, usingnearby RGB stars drawn from the Hipparcos catalogue. We find a sharp onset ofdust production when the star first reaches a pulsation period of 60 days. Thisapproximately co-incides with the point where the star transitions to the firstovertone pulsation mode. Models of the spectral energy distributions showstellar mass-loss rate suddenly increases at this point, by a factor of ~10over the existing (chromospherically driven) wind. The dust emission isstrongly correlated with both pulsation period and amplitude, indicatingstellar pulsation is the main trigger for the strong mass loss, and determinesthe mass-loss rate. Dust emission does not strongly correlate with stellarluminosity, indicating radiation pressure on dust has little effect on themass-loss rate. RGB stars do not normally appear to produce dust, whereas dustproduction by asymptotic giant branch stars appears commonplace, and isprobably ubiquitous above the RGB-tip luminosity. We conclude that the strongwind begins with a step change in mass-loss rate, and is triggered by stellarpulsations. A second rapid mass-loss-rate enhancement is suggested when thestar transitions to the fundamental pulsation mode, at a period of ~300 days.

Pulsation-triggered mass loss from AGB stars: the 60-day critical period