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 V<12.5 mag, 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 has 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, 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 +-0.1 kpc for the disk component and 4.6 +-0.3 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. 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. Keywords: astrometry -- Stars: kinematics -- Stars: variables -- Stars: RR-Lyrae -- Galaxy: Halo -- Galaxy: structure

arXiv comment10 pages with 10 figures, A and A accepted Online-Data are available under http://www.astro.uni-bonn.de/~gmaintz

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• The Kinematic Properties of BHB and RR Lyrae stars towards the Anticentre and the North Galactic Pole: The Transition between the Inner and the Outer Halo
• Evidence for Galactic disc RR~Lyrae stars in the Solar neighbourhood
• Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics
• The QUEST RR Lyrae Survey: III. The Low Galactic Latitude Catalogue
• RR Lyrae From Binary Evolution: Abundant, Young and Metal-Rich
• Local RR Lyrae Stars: native and alien
• Proper identification of RR Lyrae Stars brighter than 12.5 mag
• The Population of Pulsating Variable Stars in the Sextans Dwarf Spheroidal Galaxy
• Mid-infrared Period-Luminosity Relations of RR Lyrae Stars Derived from the WISE Preliminary Data Release
• Do the nearby BHB stars belong to the Thick Disk or the Halo?
• Phase mixing due to the Galactic potential: steps in the position and velocity distributions of popped star clusters
• A cautionary tale of interpreting O-C diagrams: period instability in a classical RR Lyr Star Z CVn mimicking as a distant companion
• Astrophysics in 2006
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