CDROM/APJS/V96/P175 Kinematics of Metal-Poor Stars (Beers & SommerLarsen 1995)
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Kinematics of Metal-Poor Stars in the Galaxy
Timothy C. Beers & Jesper Sommer-Larsen
<1995, ApJS, 96, 175>
=1995ApJS...96..175B
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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 "minimalassumptions" 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 thickdisk 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.
Keywords: stars: kinematics -- stars: Population II
File Summary:
------------------------------------------------------------------------------File Name
Lrecl
Records
Explanations
------------------------------------------------------------------------------table2.dat
68
1936
Positions, radial velocities, distances,
and
abundances
------------------------------------------------------------------------------Byte-by-byte Description of file: table2.dat
------------------------------------------------------------------------------Bytes Format Units
Label
Explanations
------------------------------------------------------------------------------1-12
A12
--Star
Star name
13-15
I3
h
RAh
Right ascension (1950.0)
16-18
I3
min
RAm
R.A.
19-23
F5.1
s
RAs
R.A.
24
1X
----Blank
25
A1
--DEDeclination sign
26-27
I2
deg
DEd
Declination (1950.0)
28-32
F5.1
arcmin DEm
Dec.
33-38
F6.2
mag
V
[]? V magnitude
39
1X
----Blank
40-42
A3
--Type
*Object classification
43-47
I5
km/s
RV
Best available heliocentric radial
velocity
48-53
I6
pc
Dist
Distance from Sun
54-59
F6.2
dex
Fe_H
Abundance estimate, parameterized as
[Fe/H]
60-62
I3
--r_RV
*Source code for RV
63-65
I3
--r_Dist *Source code for Dist
66-68
I3
--r_Fe_H *Source code for Fe_H
------------------------------------------------------------------------------Notes for file: table2.dat
------------------------------------------------------------------------------Type:
Object classification codes:
A = Apparent main-sequence A-type star
AGB = aymptotic giant branch star
D = main-sequence dwarf star
FHB = field horizontal-branch star
G = giant star
SG = subgiant star
TO = main-sequence turnoff star
V = RR Lyraes and other variable stars
r_RV, r_Dist, r_Fe_H:
Source codes are listed in Table 1 of Beers and Sommer-Larsen (1995).
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(End)
Lee Brotzman [ADS]
08-May-95