Alpha Enhancement and the Metallicity Distribution Function of Plaut's Window

TL;DR

This study analyzes the metallicity and alpha-element enhancement of giant stars in Plaut's Window, revealing a metallicity gradient along the bulge and similarities in alpha enhancement with Baade's Window, contributing to understanding bulge composition.

Contribution

It provides detailed high-resolution spectroscopic measurements of metallicity and alpha-element abundances in Plaut's Window, highlighting the absence of an alpha gradient and identifying a possible inner disk population.

Findings

Median metallicity is ~0.4 dex lower than Baade's Window.

Metallicity range spans -1.5 to +0.3, similar to other bulge fields.

Alpha enhancement is consistent with Baade's Window, indicating no gradient.

Abstract

We present Fe, Si, and Ca abundances for 61 giants in Plaut's Window (l=-1,b=-8.5) and Fe abundances for an additional 31 giants in a second, nearby field (l=0,b=-8) derived from high resolution (R~25,000) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. The median metallicity of red giant branch (RGB) stars in the Plaut field is ~0.4 dex lower than those in Baade's Window, and confirms the presence of an iron abundance gradient along the bulge minor axis. The full metallicity range of our (biased) RGB sample spans -1.5<[Fe/H]<+0.3, which is similar to that found in other bulge fields. We also derive a photometric metallicity distribution function for RGB stars in the (l=-1,b=-8.5) field and find very good agreement with the spectroscopic metallicity distribution. The radial velocity and dispersion data for the bulge RGB stars are in agreement with previous results of the BRAVA survey, and we find evidence for a decreasing velocity dispersion with increasing [Fe/H]. The [alpha/Fe] enhancement in Plaut field stars is nearly identical to that observed in Baade's window, and suggests that an [alpha/Fe] gradient does not exist between b=-4 and -8. Additionally, a subset of our sample (23 stars) appear to be foreground red clump stars that are very metal--rich, exhibit small metallicity and radial velocity dispersions, and are enhanced in alpha elements. While these stars likely belong to the Galactic inner disk population, they exhibit [alpha/Fe] ratios that are enhanced above the thin and thick disk.