Full space velocities are computed for a sample of 130 nearby RR Lyrae variables using both ground-based and Hipparcos proper motions. In many cases proper motions for the same star from multiple sources have been averaged to produce approximately a factor of 2 improvement in the transverse space velocity errors. In most cases, this exceeds the accuracy attained using Hipparcos proper motions alone. The velocity ellipsoids computed for halo and thick-disk samples are in agreement with those reported in previous studies. A distinct sample of thin-disk RR Lyrae variables has not been isolated, but there is kinematic evidence for some thin-disk contamination in our thick-disk samples. Using kinematic and spatial parameters, a sample of 21 stars with [Fe/H] < -1.0 and disklike kinematics have been isolated. From their kinematics and spatial distribution we conclude that these stars represent a sample of RR Lyrae variables in the metal-weak tail of the thick disk that extends to [Fe/H] = -2.05. In the halo samples, the distribution of V velocities is not Gaussian, even when the metal-weak thick-disk stars are removed. Possibly related, a plot of U and W velocities as a function of V velocity for the kinematically unbiased halo sample shows some curious structure. The cause of these kinematic anomalies is not clear. In addition, systematic changes to the distance scale within the range of currently accepted values of M_v(RR) are shown to significantly change the calculated halo kinematics. Fainter values of M_v(RR), such as those obtained by statistical parallax (~0.60 to 0.70 at [Fe/H] = -1.9), result in local halo kinematics similar to those reported in independent studies of halo kinematics, while brighter values of M_v(RR), such as those obtained through recent analysis of Hipparcos subdwarf parallaxes (~0.30 to 0.40 at [Fe/H] = -1.9), result in a halo with retrograde rotation and significantly enlarged velocity dispersions.
The Astronomical Journal
- Pub Date:
- October 1998
- GALAXIES: STRUCTURE;
- STARS: VARIABLES: RR LYRAE VARIABLE;
- STARS: KINEMATICS;
- 40 pages (including tables), 7 PostScript figures, accepted for publication in the Astronomical Journal