The VISIR@VLT Mid-IR view of 47Tuc: A further step in solving the puzzle of RGB mass loss

Abstract

There is an ongoing debate regarding the onset luminosity of dusty mass loss in population-II red giant stars. In this paper we present VISIR@VLT MIR 8.6 micron imaging of 47Tuc, 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 arcmin of the cluster provide the cleanest possible, blending-free, sampling of the upper 3 magnitudes of the giant branch. Our diagrams show no evidence of faint giants with MIR-excess. A combined NIR-MIR diagram further confirms the near absence of dusty red giants. Dusty red giants and asymptotic giant stars are confined to the 47Tuc long period variables population. In particular, dusty red giants are limited to the upper one 8.6 micron magnitude below the giant branch tip. This particular luminosity level corresponds to ~1000 solar luminosity, suggested in previous determinations to mark the onset of dusty mass-loss. Interestingly, at this luminosity level, we detect a small deviation between the colours of red giants and the theoretical isochrones.