A study of B6‐9 and A giants

To study the kinematics of the B6‐9 and A giant stars (luminosity class III), proper motions taken from van Leeuwen's new reduction of the Hipparcos catalogue are used. 36 per cent of the B6‐9 stars appear to pertain to the Gould belt, and were excluded from the study. No A star, however, seems to be associated with the Gould belt. 597 B6‐9 giants, of which 147 have radial velocities, and 692 A giants, of which 144 have radial velocities, were used in the final study. Semi‐definite programming solves, using least squares, for the kinematical parameters such as the Oort constants and simultaneously for the coefficients of the velocity ellipsoid, reduced with the robust L 1 criterion. The condition that the solution for the solar velocity calculated both from the kinematical parameters and from the velocity ellipsoid calculation should be the same is enforced. The results obtained seem reasonable. For the B6‐9 giants: solar velocity of 16.45 ± 0.40 km s −1 ; Oort's constants, in units of km s −1 kpc −1 , A = 11.77 ± 1.66 and B =−9.05 ± 1.38 , implying a rotational velocity of 176.95 ± 27.07 km s −1 if we take the distance to the Galactic Centre as 8.5 ± 1.1 kpc ; velocity dispersions, in units of km s −1 , of σ x = 39.25 ± 3.29, σ y = 19.83 ± 1.16, σ z = 14.07 ± 0.85 with a vertex deviation of . For the A giants, the corresponding values are as follows: solar velocity 13.95 ± 0.58 km s −1 ; Oort's constants of A = 11.48 ± 5.45 and B =−8.29 ± 4.23 , implying a rotational velocity of 168.06 ± 26.90 km s −1 ; velocity dispersions of σ x = 26.95 ± 4.26, σ y = 23.08 ± 2.14, σ z = 16.46 ± 0.55 with a vertex deviation of . The generally higher mean errors for the A giants are explained by the higher dispersion in the residuals.