A&A 476, 243–253 (2007) Astronomy DOI: 10.1051/0004-6361:20078445 & c ESO 2007 Astrophysics Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations M. Salaris1,E.V.Held2, S. Ortolani3, M. Gullieuszik2,andY.Momany2 1 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf Birkenhead CH41, 1LD, UK e-mail: [email protected] 2 Osservatorio Astronomico di Padova, INAF, vicolo dell’Osservatorio 5, 35122 Padova, Italy e-mail: [enrico.held;marco.gullieuszik;yazan.almomany]@oapd.inaf.it 3 Dipartimento di Astronomia, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy e-mail: [email protected] Received 8 August 2007 / Accepted 3 October 2007 ABSTRACT 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 theo- retical 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 ± 0.1andE(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, and its 1σ Gaussian dispersion is between 0.010 and 0.012 M. The cluster age can only be approxi- mately 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. Key words. globular clusters: individual: 47 Tuc – infrared: stars – stars: distances – stars: luminosity function, mass function – stars: population II 1. Introduction therein) employed to calibrate the Galactic and extragalactic dis- tance scale. The metal-rich globular cluster 47 Tucanae (NGC 104) has played and continues to play a fundamental role in both Galactic The integrated properties of 47 Tuc play an important role and extragalactic studies. First of all, 47 Tuc belongs to the for testing methods to estimate the mean age and metallicity “thick disk” population of Galactic globular clusters (GCs), and of extragalactic systems. To derive reliable information about the comparison of its age with that of the more metal-poor stellar age and metallicity from the integrated light of unre- solved galaxies one must overcome the age-metallicity degen- “halo” GCs and the oldest “thin disk” open clusters provides ff clues about the timescale for the formation of the Galactic stel- eracy, which a ects both integrated colours and absorption- lar populations (Salaris & Weiss 1998; Liu & Chaboyer 2000; line strengths (Worthey 1994). Age-dating techniques based on VandenBerg 2000; Salaris et al. 2004b). It also provides the zero the Balmer lines (Jones & Worthey 1995) have shown great point for the age determination of “bulge” GCs, since their ages promise in breaking this degeneracy, but they need to be tested are most reliably determined from the differential comparison of on Galactic GCs for which independent age estimates based their colour−magnitude diagrams (CMDs) with that of 47 Tuc on their resolved stellar populations are possible. Gibson et al. (Ortolani et al. 1995). Moreover, 47 Tuc has been used as test- (1999) applied the Hγ-Fe4668 line indices diagram to the determi- bed to compare different distance determination methods (such nation of the age of 47 Tuc from its integrated spectrum, obtain- ing an age well in excess of 20 Gyr, much larger than CMD ages, as white dwarf-fitting, main sequence-fitting, red clump method; − see, e.g., Zoccali et al. 2001; Percival et al. 2002, and references currently estimated in the range 11 13 Gyr. Vazdekis et al. (2001) and Schiavon et al. (2002) have in- Based on data collected at the European Southern Observatory, vestigated this issue in detail. In their analysis of the prob- La Silla, Chile, Proposal 66.B–0247. lem, Schiavon et al. (2002) compared the observed differential Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20078445 244 M. Salaris et al.: Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations luminosity function (star counts as a function of magnitude – LF) Table 1. The journal of observations of 47 Tuc. of the cluster’s stars, with theoretical counterparts from differ- ent authors; this comparison disclosed a worrying discrepancy Field RA(J2000) Dec(J2000) Filter Nima DIT × NDIT (s) along the upper red giant branch (RGB) that, according to the deep 00:24:09.5 −72:02:59 J 310× 6 authors, could be one of the main causes of the spectroscopic- H 35× 12 × CMD age problem. The predicted number of RGB stars above Ks 3512 the horizontal branch appears to be about a factor of 2 lower than shallow 00:24:08.7 −72:03:53 J 31.18× 10 × observed. This discrepancy found by Schiavon et al. (2002) adds H 31.1810 K 31.18× 10 to the discrepancy between predicted and observed values of the s quantity Rbump (ratio between star counts across the RGB bump and fainter RGB stars) introduced by Bono et al. (2001). The σ exposures were obtained about 2 off the cluster centre to observed value of Rbump for 47 Tuc is more than 2 larger than ff the theoretical predictions, whereas for almost all other clusters avoid the most crowded regions, while the o set was somewhat in the sample of Bono et al. (2001) no significant disagreement smaller for the shallow exposures. A typical observing sequence in each filter consists of 3 images of 47 Tuc interspersed with is found. Given that the luminosity function of RGB stars tests ff the composition stratification above the outward moving thin 3 frames on o set sky positions. The center of our 47 Tuc point- H-shell (Renzini & Fusi Pecci 1988; Cassisi et al. 2002), dif- ings is given in Table 1, together with the number of images in ferences between predicted and observed RGB luminosity func- each filter (3 dithered images were obtained in each of the JHKs / filters), and on-target total exposure times given as the product of tions may be caused by additional physics (e.g. rotation and or × additional element transport mechanisms) not included in the DIT NDIT (the number of integrations co-added before read- model computations. out). In this paper we present the deepest to date near-infrared Observations of 4 standard stars from Persson et al. (1998) were obtained on the same night as the 47 Tuc data for calibra- (near-IR) CMD of 47 Tuc, in the 2MASS JHKs system; its well populated RGB allows us to reassess the extent of the dis- tion purposes. agreement between observed and predicted star counts along the cluster RGB. Apart from minimising the effect of extinc- 2.2. Pre-reduction tion, the advantage of using near-IR filters is that they bracket the spectral region of maximum flux density for RGB stars, plus Our pre-reduction, photometry, and calibration procedures are the bolometric corrections are essentially unaffected by the star similar to those used by Momany et al. (2003) in a deep near- chemical composition. We also provide a new estimate of the infrared study of the globular cluster NGC 6528, using a sim- cluster distance and mean mass loss along the RGB, by fitting ilar data set and the same observing strategy. We only briefly synthetic horizontal branch (HB) models to the observed coun- comment here on the reduction process, and refer the reader to terpart. Constraints on the cluster age from the Turn Off region that study for details. In short, for each set of 47 Tuc images, of the CMD will also be discussed. The whole theoretical anal- a median background frame was created from the 3 sky frames ysis has been performed employing the recent and widely used scaled to a common level, and subtracted from the individual BaSTI library of stellar models and isochrones by Pietrinferni science images. The background-subtracted images were flat- et al. (2004, 2006). fielded and cleaned using master flat-fields, filter-dependent il- The paper is structured as follows. Section 2 describes the lumination corrections, and bad pixel masks available from the observations and data reduction procedures, while the observed ESO SOFI webpages. CMD and luminosity function are presented in Sect. 3. Section 4 compares the CMD with theoretical isochrones and presents a 2.3. Stellar photometry determination of the cluster distance and age.