The cluster ages experiment (CASE). IV. Analysis of the eclipsing binary V69 in the globular cluster 47 Tuc

Abstract

We use photometric and spectroscopic observations of the eclipsing binary V69-47 Tuc to derive the masses, radii, and luminosities of the component stars. Based on measured systemic velocity, distance, and proper motion, the system is a member of the globular cluster 47 Tuc. The system has an orbital period of 29.5d and the orbit is slightly eccentric with e = 0.056. We obtain Mp = 0.8762 ± 0.0048 M⊙, Rp = 1.3148 ± 0.0051 R, Lp = 1.94 ± 0.21 L for the primary and Ms = 0.8588 0.0060 M, Rs = 1.1616 ± 0.0062 R⊙, Ls = 1.53 ± 0.17 L⊙ for the secondary. These components of V69 are the first Population II stars with masses and radii derived directly and with an accuracy of better than 1%. We measure an apparent distance modulus of (m - M)V = 13.35 ± 0.08 to V69. We compare the absolute parameters of V69 with five sets of stellar evolution models and estimate the age of V69 using mass-luminosity-age, mass-radius-age, and turnoff mass-age relations. The masses, radii, and luminosities of the component stars are determined well enough that the measurement of ages is dominated by systematic differences between the evolutionary models, in particular, the adopted helium abundance. By comparing the observations to Dartmouth model isochrones we estimate the age of V69 to be 11.25 ± 0.21(random) 0.85(systematic) Gyr assuming [Fe/H] = -0.70, [α/Fe] = 0.4, and Y = 0.255. The determination of the distance to V69, and hence to 47 Tuc, can be further improved when infrared eclipse photometry is obtained for the variable.