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Metallicity of the double Red Clump in the Milky Way Bulge

Zofia Budzik, Matylda Łukaszewicz, Radosław Poleski

TL;DR

The paper reanalyzes bulge metallicities to test whether the Milky Way's double red clump reflects multiple stellar populations or a geometric X-shaped structure. It uses infrared photometry and spectroscopic [Fe/H] from a prior study, adopting a Bayesian framework that assigns stars probabilistically to RC1, RC2, or RGB, and explicitly accounts for the RGB bump. The main result is a metallicity difference Δ[Fe/H] = $0.23^{+0.19}_{-0.24}$ dex (RGBB included) or $0.28^{+0.17}_{-0.18}$ dex (RGBB excluded), with large uncertainties preventing a definitive confirmation of multiple populations. Overall, the probabilistic approach reduces bias from arbitrary RC separations and suggests only a small, statistically inconclusive metallicity difference, leaving room for either a gradient or a minor population split rather than two distinct RC populations.

Abstract

We present a reanalysis of the metallicity of the double red clump (RC) stars from the Milky Way bulge. Two leading explanations for the existence of the double RC concern possible differences in chemical composition (multiple populations of stars) or distance (X-shaped bulge). We aim to verify the chemical composition hypothesis by determining the mean metallicity of each RC. We use infrared photometric data and metallicities from a previous study (that are based on the Michigan/Magellan Fiber System spectra). In contrast with previous studies, we assign stars to RCs or the red giant branch using a Bayesian approach. Our resulting difference between mean metallicity of the two RCs equals: $0.23^{+0.19}_{-0.24}$ or $0.28^{+0.17}_{-0.18}~\mathrm{dex}$, if the red giant branch bump is included or not, respectively. Because of the high statistical error, the result does not confirm the multiple populations hypothesis.

Metallicity of the double Red Clump in the Milky Way Bulge

TL;DR

The paper reanalyzes bulge metallicities to test whether the Milky Way's double red clump reflects multiple stellar populations or a geometric X-shaped structure. It uses infrared photometry and spectroscopic [Fe/H] from a prior study, adopting a Bayesian framework that assigns stars probabilistically to RC1, RC2, or RGB, and explicitly accounts for the RGB bump. The main result is a metallicity difference Δ[Fe/H] = dex (RGBB included) or dex (RGBB excluded), with large uncertainties preventing a definitive confirmation of multiple populations. Overall, the probabilistic approach reduces bias from arbitrary RC separations and suggests only a small, statistically inconclusive metallicity difference, leaving room for either a gradient or a minor population split rather than two distinct RC populations.

Abstract

We present a reanalysis of the metallicity of the double red clump (RC) stars from the Milky Way bulge. Two leading explanations for the existence of the double RC concern possible differences in chemical composition (multiple populations of stars) or distance (X-shaped bulge). We aim to verify the chemical composition hypothesis by determining the mean metallicity of each RC. We use infrared photometric data and metallicities from a previous study (that are based on the Michigan/Magellan Fiber System spectra). In contrast with previous studies, we assign stars to RCs or the red giant branch using a Bayesian approach. Our resulting difference between mean metallicity of the two RCs equals: or , if the red giant branch bump is included or not, respectively. Because of the high statistical error, the result does not confirm the multiple populations hypothesis.

Paper Structure

This paper contains 6 sections, 5 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Data
  3. Bright and Faint Red Clump
  4. Mixture model
  5. RGBB
  6. Effective number of stars

Figures (1)

  • Figure 1: CMD, the distribution of $K$-band brightness and PDFs of the two RCs. All plots include 10,406 stars after the imposed colour and the $K$ magnitude restrictions. Warmer tones on the CMD indicate more densely populated regions. The black and red lines are the best fits of the mixture model with or without RGBB.