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, 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: Θ < 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 ± 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 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] < 1.0 have a wide symmetric distribution about Θ = 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.

The Leo Elliptical NGC 3379: A Metal‐Poor Halo Emerges

“…The spatial distribution for metalpoor stars and globular clusters follows a three-dimensional profile $ r Àa , where a ' 3:0Y3:5 ( Harris 2001;Kinman et al 1994;Sluis & Arnold 1998;Maintz & de Boer 2005) and continues outward well past r > 50 kpc, similar to what we are beginning to find specifically for M31 and NGC 3379. In a few other nearby disk galaxies that are edge-on, evidence for low-metallicity, very low-density halos is also now beginning to emerge (e.g., Seth et al 2007;Tikhonov et al 2006) with the same method of resolved stellar photometry.…”

MID-INFRARED PERIOD-LUMINOSITY RELATIONS OF RR LYRAE STARS DERIVED FROM THEWISEPRELIMINARY DATA RELEASE

“…Finally, we compute the prior mean of the distance modulus of the ith RRL as μ 0,i = m * V ,i − M V ,i , with the uncertainty in this quantity propagated assuming that the errors on m * V ,i 5 Note that z i will depend on the value of distance determined; however, we simply use the coordinate information provided by Maintz & de Boer (2005) when available or transform the sky coordinates using the Fernley et al (1998) distance results when necessary. Our results are not sensitive to the precise value of z i .…”

BV I c photometry of RR Lyrae stars

“…For most of the RR Lyraes, the scatter of the data points of the light curves (Figs. [1][2][3][4][5][6] suggests that observational uncertainties are close to 0.01 m .…”

“…Being relatively old stars (with ages over 10 9 years), they are perfect objects for studying the structure and kinematics of the Galaxy's old subsystems: its halo and thick disk. Since the most complete recent studies in this field [1,2] are based on properties of only a few hundred stars, the existing data were obviously insufficient for detailed investigations of the thick disk and halo.…”