  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 stellar
winds is still poorly understood. In the standard model, stellar pulsations
extend the atmosphere, allowing a wind to be driven through radiation pressure
on condensing dust particles. Here we investigate the onset of the wind, using
nearby RGB stars drawn from the Hipparcos catalogue. We find a sharp onset of
dust production when the star first reaches a pulsation period of 60 days. This
approximately co-incides with the point where the star transitions to the first
overtone pulsation mode. Models of the spectral energy distributions show
stellar mass-loss rate suddenly increases at this point, by a factor of ~10
over the existing (chromospherically driven) wind. The dust emission is
strongly correlated with both pulsation period and amplitude, indicating
stellar pulsation is the main trigger for the strong mass loss, and determines
the mass-loss rate. Dust emission does not strongly correlate with stellar
luminosity, indicating radiation pressure on dust has little effect on the
mass-loss rate. RGB stars do not normally appear to produce dust, whereas dust
production by asymptotic giant branch stars appears commonplace, and is
probably ubiquitous above the RGB-tip luminosity. We conclude that the strong
wind begins with a step change in mass-loss rate, and is triggered by stellar
pulsations. A second rapid mass-loss-rate enhancement is suggested when the
star transitions to the fundamental pulsation mode, at a period of ~300 days.
