THE ASTRONOMICAL JOURNAL    VOLUME 110, NUMBER 5, PAGE 2288       NOVEMBER 1995

          THE METALLICITIES AND KINEMATICS OF RR LYRAE VARIABLES. II.
               GALACTIC STRUCTURE AND FORMATION FROM LOCAL STARS

                               ANDREW C. LAYDEN

    Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
                       Electronic mail: alayden@noao.edu

                                   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.