ASTROMETRIC AND ASTROPHYSICAL DISCONTINUITIES BETWEEN THE GALACTIC OLD DISK AND HALO STELLAR POPULATIONS

Intermediate band, RI, and DDO photometry of the weak-lined stars in the first three volumes (Dec. < - 16°) of the Michigan catalogs of spectral type are discussed on the basis of luminosity and heavy element abundance. The interface between the old disk ([Fe/H] > - 0.8 dex) and halo ([Fe/H] < - 1.2 dex) populations represents discontinuities in both the stellar motions and the stellar physics. The CN strengths of both evolved and unevolved halo stars decrease with decreasing temperature, in a mirror image of the increase with decreasing temperature for the disk objects. The result for the halo giants has been attributed to deep mixing in the stellar atmospheres but the similar result for unevolved dwarfs indicates a difference in formation rather than in evolutionary process of the two populations. The best available luminosity estimates lead to a division between the two populations in the lag velocity (V), behind galactic rotation, with halo stars showing V1 = V + 15 km/s < - 100 km/s.