Metal Abundances and Kinematics of Bright Metal-poor Giants Selected from the LSE Survey: Implications for the Metal-weak Thick Disk

Abstract

We report medium-resolution (1–2 Å) spectroscopy and broadband ( UBV ) photometry for a sample of 39 bright stars (the majority of which are likely to be giants) selected as metal-deficient candidates from an objective-prism survey concentrating on Galactic latitudes below | b | = 30°, the Luminous Stars Extension (LSE) survey of Drilling & Bergeron. Although the primary purpose of the LSE survey was to select OB stars (hence the concentration on low latitudes), the small number of bright metal-deficient giant candidates noted during this survey provide interesting information on the metal-weak thick disk (MWTD) population. Metal abundance estimates are obtained from several different techniques and calibrations, including some that make use of the available photometry and spectroscopy and others that use only the spectroscopy; these methods produce abundance estimates that are consistent with one another and should be secure. All of the targets in our study have available high-quality proper motions from the Hipparcos or Tycho II catalogs, or both, that we combine with radial velocities from our spectroscopy to obtain full space motions for the entire sample. The rotational ( V ϕ ) velocities of the LSE giants indicate the presence of a rapidly rotating population, even at quite low metallicity. We consider the distribution of orbital eccentricity of the LSE giants as a function of [Fe/H] and conclude that the local fraction (i.e., within 1 kpc from the Sun) of metal-poor stars that might be associated with the MWTD is on the order of 30%–40% at abundances below [Fe/H] = -1.0. Contrary to recent analyses of previous (much larger) samples of nonkinematically selected metal-poor stars (assembled primarily from prism surveys that concentrated on latitudes above | b | = 30°), we find that this relatively high fraction of local metal-poor stars associated with the MWTD may extend to metallicities below [Fe/H] = -1.6, much lower than had been considered before. We identify a subsample of 11 LSE stars that are very likely to be members of the MWTD, based on their derived kinematics; the lowest metallicity among these stars is [Fe/H] = -2.35. Implications of these results for the origin of the MWTD and for the formation of the Galaxy are considered.