Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

By R. Gratton, A. Bragaglia, E. Carretta, Gisella Clementini, S. Desidera, F. Grundahl, S. L. I. A. D. Padova, Italy., I. Bologna, A. Univ. and 3 more. 2003

242 citations recorded by the source corpus.

New improved distances and absolute ages for the Galactic globular clusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the Main Sequence Fitting Method. We derived accurate estimates of reddening and metal abundance for these three clusters using a strictly dierential procedure, where the Johnson B V and Stromgren b y colours and UVES high resolution spectra of turn-o stars and early subgiants belonging to the clusters were compared to similar data for field subdwarfs with accurate parallaxes measured by Hipparcos. The use of a reddening free temperature indicator (the profile of H) allowed us to reduce the error bars in reddening determinations to about 0.005 mag, and in metal abundances to 0.04 dex, in the scales defined by the local subdwarfs. Error bars in distances are then reduced to about 0.07 mag for each cluster, yielding ages with typical random errors of about 1 Gyr. We find that NGC 6397 and NGC 6752 have ages of 13:9 1:1 and 13:8 1: 1G yr respectively, when standard isochrones without microscopic diusion are used, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by other techniques sensitive to relative ages. If we use models that include the eects of sedimentation due to microscopic diusion in agreement with our observations of NGC 6397, and take into account various sources of possible systematic errors with a statistical approach, we conclude that the age of the oldest globular clusters in the Galaxy is 13:4 0:8 0:6 Gyr, where the first error bar accounts for random eects, and the second one for systematic errors. This age estimate is fully compatible with the very recent results from WMAP, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z 2:5, in a standardCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z 1:3. On the other hand, our new age estimate once combined with values of H0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of M < 0:57, independently of type Ia SNe, and strongly supports the need for a dark energy. The new cluster distances lead to new estimates of the horizontal branch luminosity, that may be used to derive the zero point of the relation between the horizontal branch absolute magnitude and metallicity: we obtain MV (HB)= (0:22 0:05)((Fe=H)+ 1:5)+ (0:56 0:07). This zero point is 0.03 mag shorter than obtained by Carretta et al. (2000) and within the error bar it agrees with, but it is more precise than most of the previous individual determinations of the RR Lyrae absolute magnitude. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m M)0= 18:50 0:09.

Subfields: Astronomy and Astrophysics, Instrumentation

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In Astronomy and Astrophysics

1. Jay Anderson
2. G. Piotto
3. A. Milone
4. L. Bedin
5. R. Gratton
6. Aaron Dotter
7. F. D’Antona
8. E. Carretta

In Instrumentation

1. Jay Anderson
2. G. Piotto
3. F. D’Antona
4. L. Bedin
5. A. Milone
6. R. Gratton
7. E. Carretta
8. A. Bragaglia

Citations in the corpus

• Abundances of C, N, O in slightly evolved stars in the globular clusters NGC 6397, NGC 6752 and 47 Tuc (2004, methodology): “In all cases, the absolute magnitude scale is that defined by Gratton et al. (2003).”
• THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. IX. HORIZONTAL BRANCH MORPHOLOGY AND THE SECOND PARAMETER PHENOMENON (2009, unknown): “Other recent estimates include MV (HB) = (0.22 ± 0.05) ([Fe/H]+1.5)+(0.56 ± 0.07) (Gratton et al. 2003) and a further refinement of the slope to 0.214 ± 0.047 mag dex−1 (Gratton et al. 2004a).”
• A Deep Chandra Survey of the Globular Cluster 47 Tucanae: Catalog of Point Sources (2005, unknown): “In this paper we use a distance of 4.85 kpc (Gratton et al. 2003), a neutral gas column of NH ¼ 1:3 ; 1020 cm 2 [from E(B V ) of 0.024 and NH/E(B V ) ¼ 5:5 ; 1021 cm 2; Gratton et al. 2003; Predehl & Schmitt 1995; Cardelli et al. 1989], and a core radius of 2400 (Howell et al. 2000).”
• A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae (2005, methodology): “The distance is tightly constrained for some globular clusters (4% distance uncertainty to 47 Tuc, Gratton et al. 2003), making them an excellent target for such studies, as pioneered by Rutledge et al. (2002a); Gendre et al. (2003b); Heinke et al. (2003).”
• The evolution of binary fractions in globular clusters (2005, methodology): “For this cluster we adopted a core mass of105M⊙ and a total cluster mass of106M⊙ at present (Freire et al. 2001); we also taketrh = 109 yr (Harris 1996) and an age of11Gyr (Gratton et al. 2003).”
• MULTIPLE STELLAR POPULATIONS IN 47 Tucanae (2011, result): “The adopted distance modulus and reddening are those that provide the best fitting of the isochrones to the data in the mF606W versus mF606W − mF814W plane and are in agreement with those provided by Gratton et al. (2003) and Harris (1996, 2010 December update).”
• Is there supernova evidence for dark energy metamorphosis (2003, result): “…suggest t 0 = 12.5 ± 2.5 Gyr, at the 95 per cent confidence level (Krauss 2001; Krauss & Chaboyer 2001; Gnedin, Lahav & Rees 2001; Hansen et al. 2002; Gratton et al. 2003; De Marchi et al. 2004) and this age estimate is consistent with several other measurements including observations of eclipsing…”
• Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters (2009, methodology): “For the latter cluster, we also have a measure of the α–enhancement, [α/Fe]=+0.34±0.02 (Gratton et al. 2003), while Strömgren photometry provides [Fe/H]=–1.83 ±0.04 and [α/Fe]=0.3 (AnthonyTwarog & Twarog 2000).”
• Abundances in giant stars of the globular cluster NGC 6752 (2005, unknown): “While globular clusters constitute only 2% of the mass of the halo (Freeman & Bland-Hawthorn 2002), they are important targets to study because they are the oldest Galactic objects for which reliable ages have been obtained (Vandenberg et al. 1996; Gratton et al. 2003).”
• A catalogue of masses, structural parameters, and velocity dispersion profiles of 112 Milky Way globular clusters (2018, unknown): “…47 Tuc (d = 4.41 kpc) is now in much better agreement with other distance methods that generally find a cluster distance around d = 4.5 kpc (e.g. Gratton et al. 2003; Bono et al. 2008; Dotter et al. 2010; Woodley et al. 2012) than the kinematic distances derived by Baumgardt (2017) (d =…”
• Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo (2004, result): “These results agree well with both the relative ages from Rosenberg et al. (1999) (a smaller sample of35 clusters, fully contained as a subset of the Salaris & Weiss sample), and the recent analysis of Gratton et al. (2003).”
• THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. VII. RELATIVE AGES (2008, methodology): “We performed similar calculations using the NGC 0104 and NGC 6397 distance moduli listed in Gratton et al. (2003) of (m − M)V = 13.50 ± 0.08 and 12.58 ± 0.08 , respectively.”
• The Vertical Structure of Warm Ionised Gas in the Milky Way (2008, background): “For example, the 22 pulsars in the globular cluster 47 Tuc have an average DM of 24.4 pc cm−3 and are at a distance of≈ 5 kpc (Camilo et al. 2000; Gratton et al. 2003).”
• Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system (2004, background): “Again, the reddening agrees with independent empirical estimates (e.g. Gratton et al. 2003), whereas the distance is slightly longer than the MS-fitting estimates (Carretta et al. 2000; Gratton et al. 2003).”