Long period variables in 47 Tuc: direct evidence for lost mass

摘要

We have identified 22 new variable red giants in 47 Tuc and determined periods for another 8 previously known variables. All red giants redder than V - I-c = 1.8 are variable at the limits of our detection threshold, which corresponds to delta V approximate to 0.1 mag. This colour limit corresponds to a luminosity log L/L-circle dot = 3.15 and it is considerably below the tip of the RGB at log L/L-circle dot = 3.35. Linear non- adiabatic models without mass loss on the giant branch can not reproduce the observed PL laws for the low amplitude pulsators. Models that have undergone mass loss do reproduce the observed PL relations and they show that mass loss of the order of 0.3 M-circle dot occurs along the RGB and AGB. The linear pulsation periods do not agree well with the observed periods of the large amplitude Mira variables, which pulsate in the fundamental mode. The solution to this problem appears to be that the nonlinear pulsation periods in these low mass stars are considerably shorter than the linear pulsation periods due to a rearrangement of stellar structure caused by the pulsation. Both observations and theory show that stars evolve up the RGB and first part of the AGB pulsating in low order overtone modes, then switch to fundamental mode at high luminosities.