EDP Sciences
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Volume 442, Number 1, October IV 2005
Page(s) 229 - 237
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20053231

A&A 442, 229-237 (2005)
DOI: 10.1051/0004-6361:20053231

RR Lyrae stars: kinematics, orbits and z-distribution

G. Maintz and K. S. de Boer

Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: [gmaintz;deboer]@astro.uni-bonn.de

(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: $\Theta < 100$ 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 $1.3\pm0.1$ kpc for the disk component and $4.6\pm0.3$ 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 $\Theta$, 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 $\Theta=0$, 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

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