Metallicity and Kinematic distributions of Red Horizontal-Branch Stars from the SDSS Survey

TL;DR

This study analyzes the metallicity and kinematic properties of red horizontal-branch stars from SDSS to understand their origins and sub-populations within the Milky Way's thick disk and halo.

Contribution

It identifies distinct metallicity and kinematic sub-populations of RHB stars, revealing detailed structure of the thick disk and halo components.

Findings

Thick disk stars peak at [Fe/H] ~ -0.6 with Vrot ~ 170 km/s.

Halo stars split into two sub-populations with different rotational properties.

Inner and outer halo components are distinguished by their spatial and kinematic distributions.

Abstract

On the basis of a recently derived color-metallicity relation and stellar parameters from the Sloan Digtal Sky Survey Data Release 7 spectroscopic survey, a large sample of red horizontal branch candidates have been selected to serve as standard candles. The metallicity and kinematic distributions of these stars indicate that they mainly originate from the thick-disk and the halo populations. The typical thick disk is characterized by the first group peaking at [Fe/H] ~ -0.6, Vrot ~ 170 km/s with a vertical scale height around |Z| ~ 1.2 kpc, while stars with [Fe/H] < -0.9 are dominated by the halo population. Two sub-populations of the halo are suggested by the RHB stars peaking at [Fe/H] ~-1.3: one component with Vrot > 0 km/s (Halo I) shows a sign of metallicity gradient in the [Fe/H] versus |Z| diagram, while the other with Vrot < 0 km/s (Halo II) does not. The Halo I mainly clumps at the inner halo with R < 10 kpc and the Halo II comes both from the inner halo with R < 10 kpc and the outer halo with R > 10 kpc based on the star distribution in the R versus |Z| diagram.