The Red Giant Branch in the Tycho-2 Catalogue

TL;DR

This study presents a comprehensive sample of red giants within 500 pc, analyzing their distribution and kinematics to understand Galactic structure, stellar populations, and the Sun's position relative to the Galactic plane.

Contribution

It provides a large, high-purity catalog of red giants with photometric distances and kinematic analysis, challenging existing Galactic models and offering new insights into stellar populations and the Sun's offset.

Findings

The stellar density decreases with distance from the Galactic plane following a barometric law.

Most K and M giants are disk stars older than 3 Gyr, with some being very young or thick-disk stars.

C stars are likely young, massive giants associated with the Gould Belt.

Abstract

Based on multicolor photometry from the 2MASS and Tycho-2 catalogues, we have produced a sample of 38 368 branch red giants that has less than 1\% of admixtures and is complete within 500 pc of the Sun. The sample includes 30 671 K giants, 7544 M giants, 49 C giants, and 104 suspected supergiants or S stars. The photometric distances have been calculated for K, M, and C stars with an accuracy of 40\%. Tycho-2 proper motions and PCRV radial velocities are used to analyze the stellar kinematics. The decrease in the stellar distribution density with distance from the Galactic equator approximated by the barometric law, contrary to the Besancon model of the Galaxy, and the kinematic parameters calculated using the Ogorodnikov--Milne model characterize the overwhelming majority of the selected K and M giants as disk stars with ages of more than 3 Gyr. A small number of K and M giants are extremely young or, conversely, thick-disk ones. The latter show a nonuniform distribution in the phase space of coordinates and velocities, arguing against isothermality and full relaxation of the disk and for the theory of dynamical streams or superclusters. The spatial distribution and kinematics of the selected C stars force us to consider them as asymptotic branch giants with masses of more than 2 $M_{\odot}$ and ages of less than 2 Gyr probably associated with the Gould Belt. The offset of the Sun above the Galactic equator has been found from the distribution of stars to be $13\pm2$ pc, which coincides with the previously obtained value for the clump red giants.