The radial velocity dispersion of 67 RR Lyrae and BHB stars that are more than 4 kpc above the galactic plane at the North Galactic Pole is ≍110 km s-1 and shows no trend with Z (the height above the galactic plane). Nine stars with kpc show a smaller velocity dispersion (40±9 km s-1) as is to be expected if they mostly belong to a population with a flatter distribution. Both RR Lyrae stars and BHB stars show evidence of stream motion; the most significant is in fields RR 2 and RR 3, where 24 stars in the range 4.0≤Z≤11.0 kpc have a mean radial velocity of -59±16 km s-1. Three halo stars in field RR 2 appear to be part of a moving group with a common radial velocity of -90 km s-1 The streaming phenomenon therefore occurs over a range of spatial scales. The BHB and RR Lyrae stars in our sample both have a similar range of metallicity (-1.2≤[Fe/H]≤-2.2). Proper motions of BHB stars in fields SA 57 (NGP) and the anticenter field (RR 7) (both of which lie close to the meridional plane of the Galaxy) show that the stars that have Z<4 kpc as well as those with Z>4 kpc have a Galactic V motion that is <-200 km/s and which is characteristic of the halo. Thus the stars that have a flatter distribution are really halo stars and not members of the metal-weak thick disk.
The Astronomical Journal
- Pub Date:
- March 1996
- GALAXY: KINEMATICS AND DYNAMICS;
- SOLAR NEIGHBORHOOD;
- Accepted for publication in the March 1996 AJ. 15 pages, AASTeX V4.0 latex format (including figures), 2 eps figures, 2 separate AASTeX V4.0 latex tables