The VLT/VISIR mid-IR view of 47 Tucanae

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 1. 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.

An empirical mass-loss law for Population II giants from theSpitzer-IRAC survey of Galactic globular clusters

“…Similar arguments apply to the Momany et al (2012) photometric survey of 47 Tuc using the mid-IR imager VISIR at the VLT. While VISIR has an optimal, high spatial resolution, unfortunately its sensitivity is insufficient to probe small color excesses along the RGB in GCs.…”

“…A clear example of how large photometric uncertainties in both passbands can lead to misleading results is offered by the case of 47 Tuc. Indeed, our finding of dusty stars down to about the HB level Origlia et al (2007Origlia et al ( , 2010 has been questioned by McDonald et al (2011) and Momany et al (2012). However, an accurate comparison of the datasets has shown that their claims were mostly a consequence of an insufficient accuracy of their near-and mid-IR photometry.…”

“…Indeed, Fig. 3 and Table 2 of Momany et al (2012) show that about two magnitudes below the tip their photometric error rapidly increases from 0.1 up to 0.3 mag. Such errors are comparable with (at 1−2σ level) if not exceeding, the majority of color excesses measured by Origlia et al (2007).…”

A New Generation of Parsec-Colibri Stellar Isochrones Including the Tp-Agb Phase

“…The reason for this choice is that the mechanism for dust formation may act differently in other stars, and may even be inefficient in RGB stars (Gail & Sedlmayr 1998). Moreover, present-day claims of detection of circumstellar dust shells around RGB stars, as well as their interpretations in terms of mass loss, are still either controversial (see Boyer et al 2010;Momany et al 2012, and references therein) or depend on observations of very small samples of nearby stars (Groenewegen 2012b). …”

ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae

“…McDonald et al 2010). This combination has led to several estimates of their dust-production rates (Origlia et al 2007;Boyer et al 2010;Origlia et al 2010;McDonald et al 2011b,a;Momany et al 2012). High-resolution optical spectroscopy of moderately bright giants suggest relatively slow outflows from the stellar chromospheres of ∼10 km s −1 (McDonald & van Loon 2007).…”