The Observational Evidence

Abstract

The domain in the (U, V) plane of old-disk-population stars is defined by a sample of F- and G-type stars brighter than visual magnitude 6.5 for which luminosities are available from intermediate-band photometry. The majority of these old-disk stars avoid the domain of the young-disk population in the (U, V) plane except for (1) several objects with V near − 17 km s^–1 and (2) a half dozen possible members of a solar group. The sample of stars used to define the old-disk population is confined to distances less than 100 parsec from the Sun and those with V near − 17 km s^–1 may represent several generations of the objects formed in what is now the solar neighbourhood and which keep returning to this neighbourhood because they are isoperiodic (V near − 17 km s^–1) with the local standard of rest. The half dozen possible members of a solar group indicate an evolutionary age of some 5 × 10⁹ yr. In addition to some of the previously recognized stellar groups in the old-disk population (e.g. Wolf 630 and ζ Her), a new group with U = +40 to 50 km s^–1 and V = −36 km s^–1 is also discussed. The sample of red giants: (V_E≤5^m.0 and (R–I)₀ > + 0^m.42) contains eight probable members of the halo population of which five are members of the Arcturus group. Possible structure in the old-disk-population giant sequence is discussed on the basis of published narrow-band indices, and the luminosity functions of the old- and the young-disk-population red giants are compared.

Most of the material presented can be found in Eggen, O. J.: 1973, Publ. Astron. Soc. Pacific85, 542.