Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy

TL;DR

This study provides high-quality spectroscopic data confirming the existence of a metal-weak thick disk in the Milky Way, refining abundance measurements and supporting its role in galactic formation models.

Contribution

It introduces a new, accurate calibration of stellar parameters for weak-metal stars, confirming the metal-weak thick disk's presence and characteristics.

Findings

Confirmed the presence of a metal-weak thick disk population.

Improved abundance measurement accuracy over previous photometric estimates.

Supported the existence of the MWTD through orbital and chemical analysis.

Abstract

A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R~3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the SEGUE Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and [Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities -1.8 < [Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy vs. angular momentum diagram. A comparison is made with recent results for a similar-size sample of RAVE stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.