J/ApJS/96/175 Kinematics of Metal-Poor Stars. I. (Beers+ 1995)
Kinematics of Metal-Poor Stars in the Galaxy
Beers T.C., Sommer-Larsen J.
<Astrophys. J. Suppl. Ser. 96, 175 (1995)>
=1995ApJS...96..175B 1995ApJS...96..175B
ADC_Keywords: Stars, metal-deficient; Positional data; Radial velocities;
Abundances
Keywords: stars: kinematics -- stars: Population II
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 neighbourhood
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
neighbourhood 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 sigmaV:sigmaW ∼ 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.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 68 1936 Positions, radial velocities, distances, and
abundances
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See also:
J/AJ/119/2866 : Kinematics of metal-poor stars. II. (Beers+, 2000)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
-------------------------------------------------------------------------------
1-12 A12 --- Star Star name
14-15 I2 h RAh Right ascension (1950.0)
17-18 I2 min RAm R.A.
20-23 F4.1 s RAs R.A.
25 A1 --- DE- Declination sign
26-27 I2 deg DEd Declination (1950.0)
29-32 F4.1 arcmin DEm Dec.
34-38 F5.2 mag Vmag []? V magnitude
40-42 A3 --- Type *Object classification
44-47 I4 km/s RV Best available heliocentric radial velocity
48-53 I6 pc Dist Distance from Sun
54-59 F6.2 Sun [Fe/H] Abundance estimate
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
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Note on Type:
Object classification codes:
A = Apparent main-sequence A-type star
AGB = asymptotic 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
Note on r_RV, r_Dist, r_[Fe/H]:
Source codes are listed in Table 1 of Beers and Sommer-Larsen (1995).
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Origin: AAS CD-ROM series, Volume 4, 1995 Lee Brotzman [ADS] 08-May-95
(End) [CDS] 12-Jul-1995