2012A&A...537A...2M

There is an ongoing debate regarding the onset luminosity of dusty mass loss in population-II red giant stars. We present VLT/VISIR mid-infrared (MIR) 8.6µm imaging of 47 Tuc, the centre of attention of a number of space-based Spitzer observations and studies. The VISIR high-resolution (diffraction limited) observations allow excellent matching to existing optical Hubble Space Telescope catalogues. The optical-MIR coverage of the inner 1.15' core of the cluster provide the cleanest possible blending-free sampling of the upper 3mag of the giant branch. Our diagrams show no evidence of faint giants with MIR-excess. A combined near/mid-infrared diagram additionally confirms the near absence of dusty red giants. Dusty red giants and asymptotic giant stars are confined to the 47 Tuc long-period variable population. In particular, dusty red giants are limited to the upper one N_8.6µm magnitude below the giant branch tip. This particular luminosity level corresponds to ∼1000L_☉, which was suggested in previous determinations to mark the onset of dusty mass loss. Interestingly, starting from this luminosity level we detected a gradual deviation between the colours of red giants and the theoretical isochrones.