The onset of photometric variability in red giant stars

The onset of variability in red giant stars is studied from the data collected by the 'Long-Term Photometry of Variables' project which operates at ESO (La Silla) since 1982. The differential nature of the observations combined with a multi-night reduction algorithm enables to study the variability of over 50 red giants with an accuracy of the order of 2 to 3 millimagnitudes on time scales ranging from days to years. All red giants with spectral types late G to early K (i.e. b - y similar to 0.6 - 0.8) are stable at the level sigma(y) < 6 millimag (where sigma(y) is the standard deviation of the Stromgren y magnitude). Red giants with later spectral types are all variable, and it is found that the minimum variability level sigma(y,min) increases with increasing b-y, thus defining a minimum-variability boundary. The new data obtained in this paper reveal that the time scale of the associated variability increases with increasing b - y (and thus amplitude), and that the stability of this time scale improves concomitantly. More precisely, irregular variations on time scales of 5 to 10 days (and sigma(y) similar to 7 millimag) characterize red giants with b-y similar to 1.0) whereas the variations (with sigma(y) similar to 100 millimag) become more regular with periods of the order of 50 d for red giants with b - y similar to 1.3. A radial-velocity jitter (of the order of 1.5 km s(-1) r.m.s.) is associated with this photometric variability, and suggests that stellar oscillations may be responsible for the observed variations.