EDP Sciences
Free access
Issue A&A
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 )

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 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

SIMBAD Objects



© ESO 2005

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.

Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
Frequency: 12 volumes per year
Published by: EDP Sciences

Mirror sites: CDS | EDP Sciences
  RSS feeds
© The European Southern Observatory (ESO)