Stellar loci IV. Red giant stars

TL;DR

This paper presents metallicity-dependent stellar color loci for red giant stars from SDSS data, enabling reliable photometric metallicity estimates and a new method to distinguish red giants from main-sequence stars, aiding Galactic halo studies.

Contribution

It introduces the first metallicity-dependent color loci for red giants and a novel photometric technique to identify and analyze metal-poor red giants in the SDSS survey.

Findings

Red giant loci are narrow with minimal intrinsic width.

Photometric metallicities can be estimated with 0.2-0.25 dex accuracy.

New method achieves ~70% completeness and ~80% efficiency in selecting metal-poor red giants.

Abstract

In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 -- 1.2 mag in color g-i, -0.3 -- -2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u-g, g-r, r-i, and i-z colors simultaneously to an accuracy of 0.2 -- 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ~ 70 per cent and efficiency of ~ 80 per cent in selecting metal-poor red giant stars of [Fe/H] $\le$ -1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.