Observations abundance analysis and age determinations

High-resolution (R ~ 65,000), high-quality (S/N > 250) echelle spectra were obtained for 60 F and G dwarfs by T.B. in January, April, and August 2006 with the MIKE spectrograph (Bernstein et al. 2003) on the Magellan/Clay 6.5-m telescope at the Las Campanas Observatory in Chile. A Toomre diagram for the sample can be seen in Figure 7.1(a).

For the abundance analysis we used the Uppsala MARCS stellar model atmospheres (Gustafsson etal. 1975; Edvardsson etal. 1993; Asplund etal. 1997). These models are one-dimensional, plane-parallel, and calculated under the assumption of local thermodynamic equilibrium (LTE). Their chemical compositions have been scaled with metallicity relative to the standard Solar abundances as given in Asplund et al. (2005), using enhanced abundances for the a-elements at subsolar metallic-ities. To determine effective temperatures we use excitation equilibrium of Fe i and to estimate the microturbulence parameter we require all Fe i lines to yield the same abundance independently of line strength. To estimate the surface gravities we utilize the fact that for all our stars we have accurate Hipparcos parallaxes (ESA 1997). Final abundances were first normalized on a line-by-line basis with our Solar values as reference and then averaged for each element.

Toomre Diagram

Figure 7.1. (a) A Toomre diagram. (b) Stellar ages versus [Fe/H]. Data for Hercules-stream stars are marked by gray stars, data for thin- and thick-disk stars from Bensby et al. (2003,2005) by open and filled circles, respectively. Error bars are explained in the text.

Figure 7.1. (a) A Toomre diagram. (b) Stellar ages versus [Fe/H]. Data for Hercules-stream stars are marked by gray stars, data for thin- and thick-disk stars from Bensby et al. (2003,2005) by open and filled circles, respectively. Error bars are explained in the text.

Stellar ages were determined from the Yonsei-Yale (Y2) a-enhanced isochrones (Kim et al. 2002; Demarque et al. 2004) in the Teff-MV plane. Lower and upper limits on the ages were estimated by taking the error bars arising from an assumed uncertainty of ±70 K (Bensby et al. 2003) in Teff and the uncertainty in MV due to the error in the parallax.

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