Sub-giant stars as tracers of galactic chemical evolution

The low luminosities of dwarf stars and the modest sizes of most telescopes used for studies of galactic chemical evolution until recently clearly limit the volume of the Galaxy available for study. Although some stars with distant birth places are likely to visit the solar vicinity, a less model-dependent study of the galactic chemical evolution requires in situ study of the detailed properties of different stellar environments: fields within and outside the "solar circle", fields in the galactic halo and the bulge as well as globular cluster fields. The new class of very large telescopes (e.g. ESO's Very Large Telescope, VLT), with, high resolution spectrographs will considerably extend the volume available for such studies of dwarf stars. In order to further extend the range of such studies it is important also to use more luminous stars without loosing accuracy in the derived abundances. In a project managed by my PhD student Patrik Thorén, we are therefore making a systematic exploratory study of sub-giant stars of different metallicities in order to evaluate their specific usefulness for accurate galactic chemical evolution studies.

Except for being more luminous than dwarfs, sub-giant stars have the important advantage over most dwarf stars that their ages may be determined by means of isochrone fitting. This is necessary for the chronological aspects.

We have obtained high-resolution spectra with the CES spectrograph fed by the 1.4m CAT telescope of the European Southern Observatory and the SOFIN spectrograph of the 2.5m Nordic Optical Telescope, of 35 sub-giant field stars spanning metallicities, [Fe/H], from about -2.0 to +0.3. The analysis has been started and the spectra will enable abundance determinations for a wide range of chemical elements: CNO, "alpha" elements, iron-peak elements, s- and r-process elements. Lithium (which is thought to be affected by mixing and destruction processes in these stars) will also be searched for.

Latest update: June 2, 2000