The Cluster AgeS Experiment (CASE) – VIII. Age and distance of the Globular Cluster 47 Tuc from the analysis of two detached eclipsing binaries

Abstract

We use photometric and spectroscopic observations of the eclipsing binary E32 in the globular cluster 47 Tuc to derive the masses, radii, and luminosities of the component stars. The system has an orbital period of 40.9d, a markedly eccentric orbit with e = 0.24, and is shown to be a member of or a recent escaper from the cluster. We obtain M p = 0.862 ± 0.005M ⊙ , R p = 1.183 ± 0.003R ⊙ , L p = 1.65 ± 0.05L ⊙ for the primary and M s = 0.827 ± 0.005M ⊙ , R s = 1.004 ± 0.004R ⊙ , L s = 1.14 ± 0.04L ⊙ for the secondary. Based on these data and on an earlier analysis of the binary V69 in 47 Tuc we measure the distance to the cluster from the distance moduli of the component stars, and, independently, from a color -surface brightness calibration. We obtain 4.55±0.03 and 4.50±0.07 kpc, respectively -values compatible within 1 σ with recent estimates based on Gaia DR2 parallaxes. By comparing the M − R diagram of the two binaries and the color-magnitude diagram of 47 Tuc to Dartmouth model isochrones we estimate the age of the cluster to be 12.0 ± 0.5 Gyr, and the helium abundance of the cluster to be Y≈0.25.

The Villafranca catalog of Galactic OB groups

“…The ultimate answer to the possible presence of remaining systematic effects in the form of the distances in this paper being systematically higher or lower should come from a comparison with (yet inexistent) more accurate distances: only time will tell, as is often the case. However, we can still compare our results with the few high-accuracy geometric distances to some of the groups analyzed here and in previous papers: those of Thompson et al (2020) for 47 Tuc (Maíz Apellániz et al 2021e), Hensberge et al (2000) for Villafranca O-020, North et al (2007) for Villafranca O-024, and Smith (2006b) for η Car in Villafranca O-025. The excellent agreement with those values in combination with the analysis in Maíz Apellániz et al (2021e) leads us to conclude that our distances are accurate as systematic effects have been properly characterized.…”

FUV Observations of the Bright Star in the Globular Cluster 47 Tucanae

“…If the star's radius and distance are known, then the scale factor required to convert the model spectrum to the flux at earth is φ = 4π(R * /d) 2 (Kurucz 1979). Using Gaia EDR3 data, Maíz Apellániz et al ( 2021) derive a distance to 47 Tuc of 4.53 ± 0.06 kpc, in excellent agreement with the eclipsing-binary result of Thompson et al (2020). Adopting this distance and our scale factor, we derive a stellar radius R * /R of 9.63 ± 0.13.…”

Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. II. Application to the Colour-Magnitude Diagrams of the Globular Clusters 47 Tuc, NGC 6362, M5, M3, M55, and M92

“…Fortunately, member eclipsing binary stars provide important constraints on the chemical composition of 47 Tuc. Thompson et al (2020) have recently reduced (albeit only slightly) the uncertainties associated with the physical properties of V69, as compared with the findings of Brogaard et al (2017), and they have determined precise masses and radii for a second binary, E32, to within 0.55% and 0.40%, respectively. Their results for the two binaries are compared with the M-R relations from several isochrones in Figure 3.…”

“…2d-f, isochrones for a higher [Fe/H] value by 0.1 dex provide better fits to the cluster giants by virtue of being cooler and therefore redder; indeed, the region enclosed by the bluest and reddest isochrones contains (filled and open circles, respectively, and the associated error boxes). Their masses and radii were taken from the study by Thompson et al (2020). The adopted value of [O/Fe] is given explicitly; all other α-elements are assumed to have [m/Fe] = 0.4. most of the MS, TO, and giant-branch stars.…”

“…Comparison of the predicted relations between mass and radius from several isochrones that have the indicated ages and chemical abundances with the properties of the eclipsing binaries V69 and E32 in 47 Tuc (filled and open circles, respectively, and the associated error boxes). Their masses and radii were taken from the study byThompson et al (2020). The adopted value of [O/Fe] is given explicitly; all other α-elements are assumed to have [m/Fe] = 0.4.…”