Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations

Details Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007a%26a...476..243s&link_type=abstract

Astronomy and Astrophysics, Volume 476, Issue 1, December II 2007, pp.243-253

Astronomy and Astrophysics

Astrophysics

18

Globular Clusters: Individual: 47 Tuc, Infrared: Stars, Stars: Distances, Stars: Luminosity Function, Mass Function, Stars: Population Ii

Scientific paper

Context: The Galactic globular cluster 47 Tucanae is central to studies of Galaxy formation, and a test-bed for theoretical models, distance determination and extragalactic age-dating techniques. Independent parallax-based distance determinations in the optical spectral range provide discrepant results; also, star counts along the Red Giant Branch from optical data have disclosed a worrying disagreement with theoretical predictions, that impacts not only the theory of red giant stars, but also the calibration of the age scale of extragalactic systems. Aims: Our new near-infrared data for 47 Tuc set constraints on its distance and test the reliability of theoretical red giant branch star counts, independently of previous conclusions from optical work. Methods: We have obtained deep near-infrared imaging of 47 Tuc using SOFI at the ESO New Technology Telescope. Colour-magnitude diagrams, isochrones and synthetic horizontal branch modelling have been used to determine the distance of 47 Tuc and constrain its age. We have also constructed a luminosity function of red giant stars, which has been compared with theoretical predictions of stellar evolution models. Results: We obtain a distance (m-M)0 = 13.18 ± 0.03 (random) ± 0.04 (systematic), for [Fe/H] = -0.7 ± 0.1 and E(B-V) = 0.04 ± 0.02. This supports the shorter end of the range of distances obtained from optical studies. The mean horizontal branch star mass is between 0.65 and 0.66 M&sun;, and its 1σ Gaussian dispersion is between 0.010 and 0.012 M&sun;. The cluster age can only be approximately estimated from the data, and is between ~10 and ~13 Gyr. The luminosity function of red giant branch (and early-asymptotic giant branch) stars does not show a statistically significant discrepancy with theory. The brightness of the red giant branch bump in the near-infrared is possibly fainter than the models, although the uncertainty on the spectroscopic metallicity and age prevents to reach a firm conclusion on this issue.
Based on data collected at the European Southern Observatory,
La Silla, Chile, Proposal 66.B-0247.

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