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Register a new userThe Absolute Magnitude and Kinematics of RR Lyrae Stars via Statistical Parallax
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We present new statistical parallax solutions for the absolute magnitude and kinematics of RR Lyrae stars. New proper motion, radial velocity, and abundance data are used; the new data set is 50% larger, and of higher quality, than previously available data sets. Based on an a priori kinematic study, we separate the stars into halo and thick disk sub-populations. Statistical parallax solutions on these sub-samples yield M_V(RR) = +0.71 +/- 0.12 at <[Fe/H]> = -1.61 for the halo (162 stars), and M_V(RR) = +0.79 +/- 0.30 at <[Fe/H]> = -0.76 for the thick disk (51 stars). The solutions yield kinematic parameters (solar motion and velocity ellipsoid) in good agreement with estimates of the halo and thick disk kinematics derived from both RR Lyrae stars and other stellar tracers. Monte Carlo simulations indicate that the solutions are accurate, and that the errors may be smaller than the estimates above. The simulations reveal a small bias in the disk solutions, and appropriate corrections are derived. The large uncertainty in the disk M_V(RR) prevents ascertaining the slope of the M_V(RR)-[Fe/H] relation. We find that (1) the distance to the Galactic Center is 7.6 +/- 0.4 kpc; (2) the mean age of the 17 oldest Galactic globular clusters is 16.5 _{-1.9}^{+2.1} Gyr; and (3) the distance modulus of the LMC is 18.28 +/- 0.13 mag. Estimates of H_0 which are based on an LMC distance modulus of 18.50 (e.g., Cepheid studies) increase by 10% if they are recalibrated to match our LMC distance modulus.
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This paper presents results from photometric and statistical-parallax analysis of a sample of 850 field RR Lyrae (RRL) variables. The photometric and spectroscopic data for sample RRLs are obtained from (1) our new spectroscopic observations (for 448 RRLs) carried out with the Southern African Large Telescope (SALT); (2) our photometric observations using the 1.0-m telescope of the South African Astronomical Observatory (SAAO), and (3) literature. These are combined with accurate proper motion data from the second release of textit{Gaia} mission (DR2). This study primarily determines the velocity distribution of solar neighborhood RRLs, and it also calibrates the zero points of the RRLs visual V-band luminosity-metallicity (LZ or $M_V-$[text{Fe/H}]) relation and their period-luminosity-metallicity (PLZ) relations in the textit{WISE} $W_1-$ and textit{2MASS} $Ks-$band. The calibrated PLZ and LZ relations are used to estimate the Galactic Center distance and the distance modulus of the Large Magellanic Cloud (LMC), which are found to be 7.99$pm$0.49,kpc and 18.46$pm$0.09 ,mag, respectively. All our results are in excellent agreement with available literature based on statistical parallax analysis, but are considerably more accurate and precise. Moreover, the zero-points of our calibrated PLZ and LZ relations are quite consistent with current results found by other techniques and yield the LMC distance modulus that is within 0.04,mag of the current most precise estimate.
The present determination of the absolute magnitude $M_V(RR)$ of RR Lyrae stars is twofold, relying upon Hipparcos proper motions and trigonometric parallaxes separately. First, applying the statistical parallax method to the proper motions, we find $<M_V(RR)>=0.69pm0.10$ for 99 halo RR Lyraes with $<$[Fe/H]$>$ =--1.58. Second, applying the Lutz-Kelker correction to the RR Lyrae HIP95497 with the most accurately measured parallax, we obtain $M_V(RR)$=(0.58--0.68)$^{+0.28}_{-0.31}$ at [Fe/H]=--1.6. Furthermore, allowing full use of low accuracy and negative parallaxes as well for 125 RR Lyraes with -- 2.49$leq$[Fe/H]$leq$0.07, the maximum likelihood estimation yields the relation, $M_V(RR)$=(0.59$pm$0.37)+(0.20$pm$0.63)([Fe/H]+1.60), which formally agrees with the recent preferred relation. The same estimation yields again $<M_V(RR)>$ = $0.65pm0.33$ for the 99 halo RR Lyraes. Although the formal errors in the latter three parallax estimates are rather large, all of the four results suggest the fainter absolute magnitude, $M_V(RR)$$approx$0.6--0.7 at [Fe/H]=--1.6. The present results still provide the lower limit on the age of the universe which is inconsistent with a flat, matter-dominated universe and current estimates of the Hubble constant.
We present the first definitive measurement of the absolute magnitude of RR Lyrae c-type variable stars (RRc) determined purely from statistical parallax. We use a sample of 247 RRc selected from the All Sky Automated Survey (ASAS) for which high-quality light curves, photometry and proper motions are available. We obtain high-resolution echelle spectra for these objects to determine radial velocities and abundances as part of the Carnegie RR Lyrae Survey (CARRS). We find that M_(V,RRc) = 0.52 +/- 0.11 at a mean metallicity of [Fe/H] = -1.59. This is to be compared with previous estimates for RRab stars (M_(V,RRab) = 0.75 +/- 0.13 and the only direct measurement of an RRc absolute magnitude (RZ Cephei, M_(V, RRc) = 0.27 +/- 0.17). We find the bulk velocity of the halo to be (W_pi, W_theta, W_z) = (10.9,34.9,7.2) km/s in the radial, rotational and vertical directions with dispersions (sigma_(W_pi), sigma_(W_theta), sigma_(W_z)) = (154.7, 103.6, 93.8) km/s. For the disk, we find (W_pi, W_theta, W_z) = (8.5, 213.2, -22.1) km/s with dispersions (sigma_(W_pi), sigma_(W_theta), sigma_(W_z)) = (63.5, 49.6, 51.3) km/s. Finally, we suggest that UCAC2 proper motion errors may be overestimated by about 25%
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
Most of known RR Lyraes are type ab RR Lyraes (RRLab), and they are the excellent tool to map the Milky Way and its substructures. We find that 1148 RRLab stars determined by Drake et al.(2013) have been observed by spectroscopic surveys of SDSS and LAMOST. We derived radial velocity dispersion, circular velocity and mass profile from 860 halo tracers in our paper I. Here, we present the stellar densities and radial velocity distributions of thick disk and halo of the Milky Way. The 288 RRLab stars located in the thick disk have the mean metallicity of [Fe/H]$=-1.02$. Three thick disk tracers have the radial velocity lower than 215 km $rm s^{-1}$. With 860 halo tracers which have a mean metallicity of [Fe/H]$=-1.33$, we find a double power-law of $n(r) propto r^{-2.8}$ and $n(r) propto r^{-4.8}$ with a break distance of 21 kpc to express the halo stellar density profile. The radial velocity dispersion at 50 kpc is around 78 km $rm s^{-1}$.
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