uvby-β photometry of high-velocity and metal-poor stars^*,**

X. Stars of very low metal abundance: Observations, reddenings, metallicities, classifications, distances, and relative ages

¹ Observatorio Astronómico Nacional, UNAM, Apartado Postal 877, Ensenada, B.C., CP 22800, México e-mail: [schuster;rmm;gabi]@astrosen.unam.mx
² Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No. 1, Tonantzintla, Puebla, CP 72840, México, e-mail: schuster@inaoep.mx
³ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA e-mail: beers@pa.msu.edu
⁴ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark e-mail: pen@phys.au.dk

Received: 10 November 2003
Accepted: 3 March 2004

Abstract

uvby(-β) photometry has been obtained for an additional 411 very metal-poor stars selected from the HK survey, and used to derive basic parameters such as interstellar reddenings, metallicities, photometric classifications, distances, and relative ages. Interstellar reddenings adopted from the Schlegel et al. ([CITE]) maps agree well with those from the intrinsic-color calibration of Schuster & Nissen ([CITE]). [Fe/H] values are obtained from the CaII K line index of the HK survey combined with the uvby and UBV photometry. The diagram is seen to be very useful for classifying these very metal-poor field stars into categories similar to those derived from globular cluster color-magnitude diagrams; the HK survey has detected metal-poor candidates extending from the red-giant to the blue-horizontal branch, and from the horizontal branch to subluminous stars. Distances derived from UBV photometry agree reasonably well with those from uvby, considering the paucity of good calibrating stars and the extrapolations required for the most metal-poor stars. These very metal-poor stars are compared to M 92 in the diagram, and evidence is seen for field stars 1-3 Gyrs younger than this globular cluster; uncertainties in the [Fe/H] scale for M 92 would only tend to increase this age difference. Significant reddening uncertainties for M 92 are unlikely but might decrease this difference. The significance of these younger very metal-poor stars is discussed in the context of Galactic evolution, mentioning such possibilities as hierarchical star-formation/mass-infall of very metal-poor material and/or accretion events whereby this material has been acquired from other (dwarf) galaxies with different formation and chemical-enrichment histories.