IR-array photometry of Galactic globular clusters - II. JK photometry of 47 TUC

Details IR-array photometry of Galactic globular clusters - II. JK photometry of 47 TUC IR-array photometry of Galactic globular clusters - II. JK photometry of 47 TUC

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.276..739m&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 276, Issue 3, pp. 739-752.

Astronomy and Astrophysics

Astronomy

53

Techniques: Photometric, Stars: Evolution, Hertzsprung-Russell (Hr) Diagram, Globular Clusters: Individual: 47 Tuc, Infrared: Stars

Scientific paper

We present near-infrared J and K photometry obtained with IRAC-2 at the European Southern Observatory for a sample of 3512 giant (red giant and asymptotic giant branch) and horizontal branch stars in the 4x4 arcmin^2 central region of the Galactic globular cluster 47 Tuc (NGC 104). Optical V data for 2345 stars in common from Bica, Ortolani & Barbuy are also used to study very populous and complete colour-magnitude diagrams down to K<~15 (~2 mag fainter than the HB). The main results are as follows. (1) The brightest non-variable star (K=6.75, V-K=5.89, J-K=0.98) yields a statistically significant estimate of the RGB tip and a good match with the He-flash luminosity predicted by the Straniero & Chieffi theoretical evolutionary models, at M_bol=3.74 for metallicity [Fe/H]=-0.7. Such a metallicity value, independently deduced from the `photometric' CMD indicators, is in full agreement with previous `spectroscopic' determinations and yields an excellent fit (to within ~50K) between the observed RGB ridge line and the same theoretical tracks at the level of the so-called RGB bump. The RGB bump, clearly detected at K=12.05+/-0.05, is, however, ~0.4 mag fainter than predicted by current models, as previously found by Fusi Pecci et al. All of the brightest stars brighter than the predicted RGB tip are long-period variables and presumably belong to the asymptotic giant branch. (2) At variance with previous claims, no statistically significant radial gradient is detected either in colour or in stellar population within the central area (r<1 arcmin). The cluster centre of gravity differs by about 5 arcsec from the centre of K luminosity, and by a similar quantity from the centre of V luminosity, and the values here determined are in agreement with those presented by Calzetti et al. (3) The population ratios of the various branches are consistent with the predictions of the `standard and canonical' models and, via the R-method, yield an estimate of the primordial He abundance of Y=0.22+/-0.02. (4) The heliocentric distance estimated using various standard candles and adopting K_HB=11.95+/-0.05, V_HB=14.10+/-0.10, E(B-V)=0.04 and [Fe/H]=-0.7 turns out to be 4.6 kpc.

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