Slavek M. Rucinski

THE CLUSTER AGES EXPERIMENT (CASE). IV. ANALYSIS OF THE ECLIPSING BINARY V69 IN THE GLOBULAR CLUSTER 47 Tuc

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.

THE CLUSTER AGES EXPERIMENT (CASE). V. ANALYSIS OF THREE ECLIPSING BINARIES IN THE GLOBULAR CLUSTER M4

We use photometric and spectroscopic observations of the eclipsing binaries V65, V66, and V69 in the field of the globular cluster M4 to derive masses, radii, and luminosities of their components. The orbital periods of these systems are 2.29, 8.11, and 48.19xa0days, respectively. The measured masses of the primary and secondary components (Mp and Ms ) are 0.8035xa0± 0.0086 and 0.6050xa0± 0.0044 M ☉ for V65, 0.7842xa0± 0.0045 and 0.7443xa0± 0.0042 M ☉ for V66, and 0.7665xa0± 0.0053 and 0.7278xa0± 0/0048 M ☉ for V69. The measured radii (Rp and Rs ) are 1.147xa0± 0.010 and 0.6110xa0± 0.0092 R ☉ for V66, 0.9347xa0± 0.0048 and 0.8298xa0± 0.0053 R ☉ for V66, and 0.8655xa0± 0.0097 and 0.8074xa0± 0.0080 R ☉ for V69. The orbits of V65 and V66 are circular, whereas that of V69 has an eccentricity of 0.38. Based on systemic velocities and relative proper motions, we show that all three systems are members of the cluster. We find that the distance to M4 is 1.82xa0± 0.04xa0kpc—in good agreement with recent estimates based on entirely different methods. We compare the absolute parameters of V66 and V69 with two sets of theoretical isochrones in mass-radius and mass-luminosity diagrams, and for assumed [Fe/H] = –1.20, [α/Fe] = 0.4, and Y = 0.25 we find the most probable age of M4 to be between 11.2 and 11.3xa0Gyr. Color-magnitude diagram (CMD) fitting with the same parameters yields an age close to, or slightly in excess of, 12xa0Gyr. However, considering the sources of uncertainty involved in CMD fitting, these two methods of age determination are not discrepant. Age and distance determinations can be further improved when infrared eclipse photometry is obtained.

THE CLUSTER AGES EXPERIMENT (CASE). VII. ANALYSIS OF TWO ECLIPSING BINARIES IN THE GLOBULAR CLUSTER NGC 6362

We use photometric and spectroscopic observations of the detached eclipsing binaries V40 and V41 in the globular cluster NGC 6362 to derive masses, radii, and luminosities of the component stars. The orbital periods of these systems are 5.30 and 17.89 days, respectively. The measured masses of the primary and secondary components (Mp, Ms) are (0.8337 ± 0.0063, 0.7947 ± 0.0048) M⊙ for V40 and (0.8215 ± 0.0058, 0.7280 ± 0.0047) M⊙ for V41. The measured radii (Rp, Rs) are (1.3253 ± 0.0075, 0.997 ± 0.013) R⊙ for V40 and (1.0739 ± 0.0048, 0.7307 ± 0.0046) R⊙ for V41. Based on the derived luminosities, we find that the distance modulus of the cluster is 14.74 ± 0.04 mag—in good agreement with 14.72 mag obtained from color–magnitude diagram (CMD) fitting. We compare the absolute parameters of component stars with theoretical isochrones in mass–radius and mass–luminosity diagrams. For assumed abundances [Fe/H] = −1.07, [α/Fe] = 0.4, and Y = 0.25 we find the most probable age of V40 to be 11.7 ± 0.2 Gyr, compatible with the age of the cluster derived from CMD fitting (12.5 ± 0.5 Gyr). V41 seems to be markedly younger than V40. If independently confirmed, this result will suggest that V41 belongs to the younger of the two stellar populations recently discovered in NGC 6362. The orbits of both systems are eccentric. Given the orbital period and age of V40, its orbit should have been tidally circularized some ~7 Gyr ago. The observed eccentricity is most likely the result of a relatively recent close stellar encounter.

Eclipsing binaries in globular clusters as age and distance indicators

We show that DEBs can serve as excellent age and distance indicators for globular clusters. A short review of past surveys for eclipsing binaries in globular clusters is followed by presentation of the current status of the CASE project. A few selected systems from the CASE sample are discussed in some detail.

Division V: Variable Stars

Division V, Variable Stars, consists of Commission 27, also called Variable Stars and Commission 42, Close Binaries. Thus the former deals with stars whose variations are intrinsic, whereas in the latter the variations are caused by the interactions between the components in the binary. It is evident that the definition of the Division is predominantly observational, and there may be cases where the assignment of an object to one of the two commissions might be in doubt (a recent somewhat related example was the first detection of an extra-solar planet, in 54 Pegasi, where intrinsic variability of the star in the form of high-order g modes was also initially suspected).

COMMISSION 42: CLOSE BINARIES

During the commission business session, the past President presented the new Organizing Committee which was selected by the OC through a e-mail vote conducted during the months before the Rio de Janeiro General Assembly. The new OC will consist of Ignasi Ribas (President), Mercedes Richards (Vice President), and Slavek Rucinski (Past President) with the members: David Bradstreet, Petr Harmanec, Janusz Kaluzny, Joanna Mikolajewska, Ulisse Munari, Panos Niarchos, Katalin Olah, Theo Pribulla, Colin Scarfe and Guillermo Torres.