THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 96:175-221, 1995 January

                 KINEMATICS OF METAL-POOR STARS IN THE GALAXY

                               TIMOTHY C. BEERS
        Department of Physics and Astronomy, Michigan State University,
                            East Lansing, MI 48824
                        E-mail: beers@msupa.pa.msu.edu

                                      AND

                             JESPER SOMMER-LARSEN
 Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen 0, Denmark
                        E-mail: jslarsen@nbivax.nbi.dk


                                   ABSTRACT

        We discuss the kinematic properties of a sample of 1936 Galactic
     stars, selected without kinematic bias, and with abundances [Fe/H] <=
     -0.6.  The stars selected for this study all have measured radial
     velocities, and the majority have abundances determined from
     spectroscopic and narrow-/intermediate-band photometric techniques.
     In contrast to previous examinations of the kinematics of the
     metal-poor stars in the Galaxy, our sample contains large numbers of
     stars that are located at distances in excess of 1 kpc from the
     Galactic plane.  Thus, a much clearer picture of the nature of the
     metal-deficient populations in the Galaxy can now be drawn.
        Our present data can be well described in terms of a two-component
     kinematic model consisting of a thick disk, rotating at roughly 200
     km/s (independent of metal abundance), and an essentially nonrotating
     halo.  The kinematics of these two components suggest a very broad
     overlap in metallicity; the thick disk is shown to possess an
     extremely metal-weak tail, extending to abundances even lower than
     previously reported, down to at least [Fe/H] ~ -2.0.  A "minimal-
     assumptions" maximum-likelihood model is used to show that below
     [Fe/H] = -1.5 roughly 30% of stars in the solar neighborhood can be
     kinematically associated with the thick disk.  Over the metallicity
     interval -1.6 <= [Fe/H] <= -1.0, the thick-disk proportion rises to
     60%.  This fraction is only slightly smaller than contribution of
     thick-disk stars derived by Morrison, Flynn, and Freeman in the same
     metallicity interval (80%).  Our confirmation that significant numbers
     of stars with thick-disk-like kinematics exist in the solar
     neighborhood at arbitrarily low metal abundance suggests that previous
     disagreements about the correlation of population rotation velocities
     and metal abundance (e.g. Sandage & Fouts vs. Norris) may be due
     primarily to the selection criteria employed, and the resulting
     different contribution of metal-weak thick-disk stars to the
     respective data sets.  The non-Gaussian nature of the velocity
     distribution of extremely metal-poor stars ([Fe/H] <= -1.5) in the
     directions of the Galactic poles reported by previous workers can also
     be understood as a consequence of the overlap between a cold
     metal-weak thick-disk population and a hot halo population.
        A maximum-likelihood technique has been developed in order to
     estimate the velocity ellipsoids of the thick-disk and halo components
     of the Galaxy.  From the 349 stars in our sample with -1.0 <= [Fe/H]
     <= -0.6 and |z| <= 1 kpc, the velocity ellipsoid of the thick disk is
     (sigma_U, sigma_V, sigma_W) = (63 +/- 7, 42 +/- 4, 38 +/- 4) km/s.
     These values are in remarkably good accord with the predicted
     thick-disk velocity ellipsoid obtained by Quinn, Hernquist, and
     Fullagar from simulations of a satellite-merger formation scenario.
     Based on this velocity ellipsoid, a radial scale length for thick-
     disk stars of h_R = 4.7 +/- 0.5 kpc is obtained, larger than reported
     by Morrison, and similar to the value obtained for the old-disk
     population.  However, the apparent equality of sigma_V and sigma_W is
     evidence that the thick disk is kinematically distinct from the
     old-disk population, where sigma_V:sigma_W ~ 2^{1/2}:1.  We find a
     substantially smaller asymmetric-drift velocity gradient for presumed
     thick-disk stars (delta Vrot/delta |z| = -13 +/- 6 km/s/kpc) than
     reported by Majewski (delta Vrot/delta |z| = -21 +/- 1 km/s/kpc).
        From 887 stars in our sample with [Fe/H] <= -1.5 the local velocity
     ellipsoid of the halo is (sigma_r, sigma_phi, sigma_theta) = (153 +/-
     10, 93 +/- 18, 107 +/- 7) km/s, that is, strongly radially peaked, as
     indicated by previous studies.  We find little difference in the
     velocity ellipsoids of this sample when it is split into two roughly
     equal pieces with -2.2 <= [Fe/H] <= -1.5 and [Fe/H] <= -2.2, which
     indicates a lack of radial metallicity gradient in the halo, as found
     from studies of the Galactic globular cluster system.  The velocity
     ellipsoid obtained from the small number of stars in our sample with
     Galactocentric distances r > 10 kpc (N = 61) is (sigma_r, sigma_phi,
     sigma_theta) = (115 +/- 18, 138 +/- 78, 110 +/- 24) km/s, much less
     radially elongated than found for the local sample.

     Subject headings: stars: kinematics -- stars: Population II