view Abstract Citations (116) References (99) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Galactic Spheroid: What is Population II? Wyse, Rosemary F. G. ; Gilmore, Gerard Abstract The term Population II stars when applied to spiral galaxies is generally assumed to refer to those stars that lie outside the thin exponential disk. Population II stars in the solar neighborhood were originally identified with the extreme subdwarfs. Spectroscopic and photometric studies of local subdwarfs show these stars to form a metal-poor system whose kinematics are dominated by pressure support. However, spectroscopic and photometric studies of external spiral galaxies show spheroid stars to form a metal-rich, flattened system whose kinematics are dominated by rotation, at least within a couple of effective radii of the spheroid (around 5 kpc). Accurate CCD photometric data for galactocentric distances beyond that of the solar neighborhood are now available for M31 (Mould and Kristian) and confirm the domination of a metal- rich spheroid over the metal-poor component until <~20 kpc from the center of that galaxy. This apparent contradiction between the properties of Population II in our Milky Way galaxy and that in external spiral galaxies is here resolved by identifying the vast majority of spatially defined 'Population II' stars in the solar neighborhood not with the metal-poor subdwarfs but with stars in the thick-disk component, which has recently been shown to dominate the stellar distribution within a few kiloparsecs of the Galactic plane. This proposal follows the spirit of the Vatican Symposium, although the thick disk has rather different defining characteristics from the components discussed there. Spatially defined Population II and metallicity-defined Population II are essentially distinct populations. Some implications of our proposal for disk galaxy formation and evolution are discussed. We show how detailed information about the element ratios present in spheroid stars can place constraints on the timescale on which the metal-poor system formed, and on how homogeneous the proto-Galaxy could have been at this time. Publication: The Astronomical Journal Pub Date: May 1988 DOI: 10.1086/114738 Bibcode: 1988AJ.....95.1404W Keywords: Galactic Evolution; Metallicity; Spiral Galaxies; Subdwarf Stars; Brightness Distribution; Charge Coupled Devices; Giant Stars; Globular Clusters; Milky Way Galaxy; Stellar Evolution; Astrophysics; STARS: STELLAR STATISTICS; GALAXIES: THE GALAXY full text sources ADS | data products SIMBAD (3)