Kinematics and Velocity Ellipsoid of M Giants

Abstract

To study the kinematics of M giant stars (luminosity class III) use is made of 1,532 stars with proper motions taken from the Hipparcos catalog, of which 480 have radial velocities. Stars farther off than 700 pc were excluded because they induce a noticeable tilt in the distribution of the stars. Various solutions were performed, and the one calculated by the robust L1 norm was taken as the best. Kinematical parameters and the coefficients of the velocity ellipsoid are solved for simultaneously. The results obtained are reasonable: solar velocity of 24.20±0.70 km s ⁻¹; Oort's constant's, in units of km s ⁻¹ kpc ⁻¹, A = 16.86 ± 2:78 and B = -6.34 ± 2.56, implying a rotational velocity of 197.27±26:80 km s ⁻¹. The velocity ellipsoid is tilted considerably with respect to the Galactic plane in the y direction.