The VLT/VISIR mid-IR view of 47 Tucanae

A further step in solving the puzzle of RGB mass loss^⋆,⋆⋆

¹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
e-mail: ymomany@eso.org; isaviane@eso.org; gmarconi@eso.org; asmette@eso.org; abayo@eso.org
² INAF, Oss. Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: yazan.almomany@oapd.inaf.it; leo.girardi@oapd.inaf.it; alessandro.bressan@oapd.inaf.it
³ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
e-mail: milone@iac.es
⁴ Department of Astrophysic, University of La Laguna, 38200 La Laguna, Tenerife, Canary Island, Spain
⁵ SISSA, via Bonomea, 265, 34136 Trieste, Italy

Received: 9 May 2011
Accepted: 10 October 2011

Abstract

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) 8.6   μm 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 $\mathrm{1}\stackrel{\mathrm{\prime }}{.}\mathrm{15}$ core of the cluster provide the cleanest possible blending-free sampling of the upper 3 mag 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_8.6   μm magnitude below the giant branch tip. This particular luminosity level corresponds to  ~1000 L_⊙, 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.