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Description
From a sample of nearly 2000 GK giants a group of young disk stars with well determined space motions has been selected. The zero point of the luminosity calibrations, both from the ultraviolet flux (modified Stromgren system) and that in the region of 4200 to 4900A (DDO system), show a discontinuity of about a half magnitude at the border of the young disk and old disk domains. The population separation is based on the space velocity components, which are also an age discriminants with the population interface near 2x10^9yr, based on models with convective overshoot at the core. This age corresponds to giant masses near 1.7M_{sun}_, near the critical mass separating the young stars that do not burn helium in degenerate cores from older stars that do. Ten percent of both populations show CN anomalies in that the derived value of P[Fe/H] from CN(C_m_) and from Fe(M_1_) differ by more than 0.1dex and the weak and strong CN stars occur equally in the old disk but the weak CN stars predominate in the young disk. Peculiar stars, where flux distortions affect the luminosity calibrations, are of the CH+ (Ba II) and CH- (weak G band) variety and represent less than 1% of the stars in both populations. The young disk giants are restricted to ages greater than about 10^9^yr, because younger stars are bright giants or supergiants (luminosity class II or I), and younger than about 2x10^9yr, because the old disk-young disk boundary occurs near 1.7M_{sun}_. The distribution of heavy element abundances, P[Fe/H], for young disk giants is both more limited in range (+/-0.4dex) and is skewed toward higher abundances, compared with the nearly normal distribution for old disk giants. The distribution of (U,V) velocity vectors gives (U,V,W) and their dispersions =(+17.6+/-18.4, -14.8+/-8.4, -6.9+/-13.0) and (+3.6+/-38.4, -20.7+/-27.5, -6.7+/-17.3) km/s for young and old disk giants, respectively. For a description of the DDO and RI Eggen photometric system, see e.g. <GCPD/12> and <GCPD/17> , respectively.
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