CDROM/AJ/V110/P2288 Local RR Lyrae Stars (Layden 1995) ================================================================================ The Metallicities and Kinematics of RR Lyrae Variables. II. Galactic Structure and Formation from Local Stars Andrew C. Layden <1995, AJ, 110, 2288> =1995AJ....110.2288L ================================================================================ Abstract: We investigate the kinematics and spatial distribution of a sample of 302 nearby RR Lyrae variable stars with the goal of learning more about the formation history and present-day structure of the galaxy. We find that the kinematics change abruptly at [Fe/H] ~ -1.0. Above this value, the stars exhibit kinematic properties like those of other tracers of the galaxy's thick disk, while below [Fe/H] = -1.3, halo kinematics are observed. The stars with -1.3 <= [Fe/H] <= -1.0 are a mixture of these populations, though the fraction of thick disk RR Lyrae stars with [Fe/H] <= -1.0 is significantly smaller than found among red giant stars by other authors; we discuss several possibilities for this discrepancy. The RR Lyraes with [Fe/H] > -0.5 exhibit somewhat cooler kinematics and a flatter spatial distribution than those of the thick disk, and may include some members of the old thin disk population. We find the kinematics of the halo RR Lyraes to be uncorrelated with abundance over the range -2.2 <= [Fe/H] <= -1.0, and interpret this as evidence that the motions and chemical enrichment of gas in the early galaxy was characterized by a high degree of randomization, consistent with fragment accretion pictures of galaxy formation. We argue that the small number of thick disk stars with [Fe/H] <= -1.0 cannot be called upon to balance the net kinematics of a retrograde-rotating halo to produce the slow prograde rotations seen in this and other local samples; however, a hotter, slowly rotating, somewhat flattened second halo component may be consistent with kinematics both of local samples and of samples far from the galactic plane. We find the local density of halo RR Lyraes to be 11-15 kpc^-3, with a firm lower limit of 8 +/- 3 kpc^-3. This is larger than that predicted from counts of faint RR Lyraes far from the plane; either the flattening of the local halo is larger than c/a ~ 0.7, or a two- component halo exists in which one component is quite flattened. We derive an exponential scale height of 0.7 (+0.5)(-0.3) kpc and a midplane density of 10 +/- 4 kpc^-3 for the disk RR Lyraes with [Fe/H] > -1.0. File Summary: -------------------------------------------------------------------------------- File Name Lrecl Records Explanations -------------------------------------------------------------------------------- table2.dat 74 154 Proper motions and space velocities of 154 RR Lyrae variables -------------------------------------------------------------------------------- Byte-by-byte Description of files: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 A8 --- Star Star name 9-16 F8.2 arcsec/100yr pmRA *Proper motion in right ascension 17-22 F6.2 arcsec/100yr e_pmRA Error in pmRA 23-29 F7.2 arcsec/100yr pmDE Proper motion in declination 30-35 F6.2 arcsec/100yr e_pmDE Error in pmDE 36 1X --- --- Blank 37 A1 --- Note *[a-d] Note flag 38-42 I5 km/s Vrho *Rho component of space velocity 43-46 I4 km/s e_Vrho Error in Vrho 47-52 I6 km/s Vphi *Phi component of space velocity 53-56 I4 km/s e_Vphi Error in Vphi 57-62 I6 km/s Vz *z component of space velocity 63-66 I4 km/s e_Vz Error in Vz 67-74 F8.2 dex [Fe/H] Metal abundance -------------------------------------------------------------------------------- Notes for file: table2.dat -------------------------------------------------------------------------------- pmRA: Proper motion in right ascension times the cosine of the declination, in arcsec/century Note: a = proper motion from Hemenway (1975) b = proper motion from Clube (1968) c = proper motion from van Herk (1965) d = [Fe/H] taken from the literature Vrho, Vphi, Vz: Space velocities were computed via the formulation of Johnson & Soderblom (1987), which includes the treatment of errors. Many of the stars are sufficiently far from the Sun that the UVW coordinate system becomes misaligned with the principal galactocentric axes. Thus, the authors have rotated the UVW velocities of each star, and their errors, into the cylindrical frame, rho-phi-z. In this system, rho increases outward from the axis of galactic rotation, parallel to the galactic plane, phi is in the direction of galactic rotation, and z is toward the north galactic pole. -------------------------------------------------------------------------------- ================================================================================ (End) Lee Brotzman [ADS] 03-Nov-1995