A&A 442, 229-237 (2005)
RR Lyrae stars: kinematics, orbits and z-distributionG. Maintz and K. S. de Boer
Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
(Received 12 April 2005 / Accepted 8 July 2005 )
RR Lyrae stars in the Milky Way are good tracers to study the kinematic behaviour and spatial distribution of older stellar populations. A recently established well documented sample of 217 RR Lyr stars with V<12.5 mag, for which accurate distances and radial velocities as well as proper motions from the Hipparcos and Tycho-2 catalogues are available, has been used to reinvestigate these structural parameters. The kinematic parameters allowed to calculate the orbits of the stars. Nearly 1/3 of the stars of our sample have orbits staying near the Milky Way plane. Of the 217 stars, 163 have halo-like orbits fulfilling one of the following criteria: km s-1, orbit eccentricity >0.4, and normalized maximum orbital z-distance >0.45. Of these stars roughly half have retrograde orbits. The z-distance probability distribution of this sample shows scale heights of kpc for the disk component and kpc for the halo component. With our orbit statistics method we found a (vertical) spatial distribution which, out to z=20 kpc, is similar to that found with other methods. This distribution is also compatible with the ones found for blue (HBA and sdB) halo stars. The circular velocity , the orbit eccentricity, orbit z-extent and [Fe/H] are employed to look for possible correlations. If any, it is that the metal poor stars with [Fe/H] <1.0 have a wide symmetric distribution about , thus for this subsample on average a motion independent of disk rotation. We conclude that the Milky Way possesses a halo component of old and metal poor stars with a scale height of 4-5 kpc having random orbits. The presence in our sample of a few metal poor stars (thus part of the halo population) with thin disk-like orbits is statistically not surprising. The midplane density ratio of halo to disk stars is found to be 0.16, a value very dependent on proper sample statistics.
Key words: astrometry -- stars: kinematics -- stars: variables: RR Lyr -- Galaxy: halo -- Galaxy: structure
© ESO 2005