Kinematics of Tycho-2 Red Giant Clump Stars

TL;DR

This study analyzes the kinematics of Tycho-2 Red Giant Clump stars using the Ogorodnikov-Milne model, revealing Galactic rotation, contraction effects, and disk warp characteristics through proper motions and radial velocities.

Contribution

It provides new measurements of Galactic kinematic parameters and demonstrates the contraction and warp effects in RGC stars using a large stellar sample.

Findings

Oort constants A=15.9 and B=-12.0 km/s/kpc

Contraction velocity of -3.5 km/s near the Galactic plane

Galactic X-axis rotation of -2.5 km/s/kpc

Abstract

Based on the Ogorodnikov-Milne model, we analyze the proper motions of 95 633 red giant clump (RGC) stars from the Tycho-2 Catalogue. The following Oort constants have been found: A = 15.9+-0.2 km/s/kpc and B = -12.0+-0.2 km/s/kpc. Using 3632 RGC stars with known proper motions, radial velocities, and photometric distances, we show that, apart from the star centroid velocity components relative to the Sun, only the model parameters that describe the stellar motions in the XY plane differ significantly from zero. We have studied the contraction (a negative K-effect) of the system of RGC stars as a function of their heliocentric distance and elevation above the Galactic plane. For a sample of distant (500--1000 pc) RGC stars located near the Galactic plane (|Z|<200 pc) with an average distance of d=0.7 kpc, the contraction velocity is shown to be Kd= -3.5+-0.9 km/s; a noticeable vertex deviation, lxy = 9.1+-0.5 degrees, is also observed for them. For stars located well above the Galactic plane (|Z|>=200 pc), these effects are less pronounced, Kd = -1.7+-0.5 km/s and lxy = 4.9+-0.6 degrees. Using RGC stars, we have found a rotation around the Galactic X axis directed toward the Galactic center with an angular velocity of -2.5+-0.3 km/s/kpc, which we associate with the warp of the Galactic stellar-gaseous disk.