uvby-β photometry of high-velocity and metal-poor stars

XI. Ages of halo and old disk stars

¹ Observatorio Astronómico Nacional, UNAM, Apartado Postal 877, Ensenada, BC, CP 22800, Mexico e-mail: schuster@astrosen.unam.mx
² CAAUL, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisbon, Portugal e-mail: andre@oal.ul.pt
³ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No. 1, Tonantzintla, Puebla, CP 72840, Mexico e-mail: amarquez@inaoep.mx
⁴ Institute of Astronomy, UNAM, México, DF, CP 04510, Mexico e-mail: laura@astroscu.unam.mx
⁵ Universidad de las Americas, San Pedro Cholula, Puebla, CP 72820, Mexico

Received: 8 July 2005
Accepted: 13 September 2005

Abstract

New uvby–β data are provided for 442 high-velocity and metal-poor stars; 90 of these stars have been observed previously by us, and 352 are new. When combined with our previous two photometric catalogues, the data base is now made up of 1533 high-velocity and metal-poor stars, all with uvby–β photometry and complete kinematic data, such as proper motions and radial velocities taken from the literature. Hipparcos, plus a new photometric calibration for M_v also based on the Hipparcos parallaxes, provide distances for nearly all of these stars; our previous photometric calibrations give values for and [Fe/H]. The [Fe/H], V(rot) diagram allows us to separate these stars into different Galactic stellar population groups, such as old-thin-disk, thick-disk, and halo. The X histogram, where X is our stellar-population discriminator combining V(rot) and [Fe/H], and contour plots for the [Fe/H], V(rot) diagram both indicate two probable components to the thick disk. These population groups and Galactic components are studied in the , M_v diagram, compared to the isochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), to derive stellar ages. The two thick-disk groups have the mean characteristics: ([Fe/H], V(rot), Age,  dex, 120 km s^-1, 12.5 Gyr, 62.0 km s^-1), and (-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups, Fe/H, show a mean age of (mean error) Gyr, giving a mean difference from the WMAP results for the age of the Universe of  Gyr. These results for the ages and components of the thick disk and for the age of the Galactic halo field stars are discussed in terms of various models and ideas for the formation of galaxies and their stellar populations.