uvby-β photometry of high-velocity and metal-poor star. VIII. Stars of very low metal abundance

Abstract

A catalogue of uvby-β photometry for 87 of the turn-off and subgiant stars from the HK survey of Beers et al. (1992) is given. Most of these stars have [Fe/H]≤ −2.5. These photometric data have been taken and reduced using the same techniques as in our previous two uvby-β catalogues. An error analysis has been made; typical mean observational errors for a star with V = 14. m 2 are 0.008, 0.007, 0.011, 0.010 and 0.012 in V , (b − y), m1, c1 and β, respectively. Our photometric data are also compared to the UBV photometry and spectral indices from the HK survey; satisfactory agreement is found indicating good quality for the two data sets. The stars are classified in reddening-free diagrams of the uvby-β system; several of the stars are found to have been mis-classified in the HK survey. Two metal-poor supergiants, three (possibly four) horizontal branch stars, one blue subluminous star and two stars with ambiguous photometry have been identified. Stellar parameters, such as E(b − y), MV , δMV , and Teff, plus error estimates for these parameters, are provided using our photometric calibrations. Although all stars have galactic latitudes |b| ≥ 30 • many of them are significantly reddened with E(b − y) ranging from 0.02 to 0.15. The average reddening towards the South Galactic Pole region, b < −60 • , is E(b − y) = 0.027 ± 0.004 corresponding to E(B − V) = 0.036 ± 0.005 in contrast to the low value of E(B − V) 0.01 derived from the Burstein & Heiles (1982) maps. The stars are analyzed in the (b − y)0, [Fe/H] diagram, and individual ages estimated using the isochrones of VandenBerg & Bell (1985) and of Bergbusch & VandenBerg (1992). These very metal-poor stars are found to be coeval, within 1−2 Gyr, with the halo and thick-disk stars studied previously, with a mean age in excess of 18 Gyr. Several blue "thick-disk" stars, which are perhaps analogous to the "blue metal-poor" stars of Preston et al. (1994), have been identified.

The age of the Milky Way halo stars from the Sloan Digital Sky Survey

“…There is a mean difference of 4 Gyr in the absolute age due to diffusion, as proposed by Salaris & Weiss (2001). The results obtained for ages from the nodiff-isochrones agree better with those of Unavane et al (1996) and Schuster et al (1996).…”

“…We could see that the results obtained using BaSTI models agreed better with those obtained from the nodiff-isochrones, while Y 2 results agreed better with those obtained from diff-isochrones. In this way, we could explain why the previous ages obtained with this method (Unavane et al 1996;Schuster et al 1996) are greater than our own. Given the current debate about the efficiency with which atomic diffusion acts in stellar interiors, our results show a strong argument against fully inhibited diffusion in metal-poor halo stars.…”

“…If gravitational settling is ignored in the isochrones employed (i.e. Bergbusch & Vandenberg 1992), the absolute ages obtained for the field stars can be up to 18 Gyr (Schuster et al 1996). Similarly, Unavane et al (1996) obtained ages of 15−16 Gyr for a stellar sample of Carney et al (1994) using Green et al (1987) isochrones.…”

uvby–βphotometry of solar twins

“…The uvby-β data presented here in Table 1 for the solar twins were taken using the H. L. Johnson 1.5 m telescope at the San Pedro Mártir Observatory, Baja California, México (hereafter SPM), and the same six-channel uvby-β photoelectric photometer as for the northern observations of Schuster & Nissen (1988, hereafter SN), for all the uvby-β observations of Schuster et al (1993, hereafter SPC), the northern data of very-metalpoor stars by Schuster et al (1996), the uvby-β data for verymetal-poor stars in Table 1 of Schuster et al (2004), and the uvby-β data for high-velocity and metal-poor stars in Table 1 of Schuster et al (2006). The new uvby-β values for solar twins included here in Table 1 were taken during three observing runs in November 2007 (8 nights), April 2008 (7 nights), and September 2008 (4 nights).…”

The Evolution of Old Stellar Populations in Our Galaxy

“…The age scatter in halo field stars is at least as great as that in the young halo cluster system (Schuster et al 1996), and perhaps greater -as shown by the apparently young, blue metal poor stars of Preston et al (1994) and the carbon stars of Totten & Irwin (1996), both of which are attributed to satellite accretion. Evidence for significant phase space complexity, manifested as "moving groups", streams, or other kinematic substructure of halo stars (Carney et al 1996, Majewski et al 1996a, summary in Majewski et al, 1996b) is consistent with a dynamically unrelaxed, accreted outer halo.…”