RR Lyrae stars: kinematics, orbits and z-distribution

Abstract

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 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 , and normalized maximum orbital z-distance . 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 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 Θ, 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] 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.