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Astronomical Applications Department, U.S. Naval Observatory - Scifull (Page 6)

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Astronomical Applications Department, U.S. Naval Observatory - Scifull
would be able to accurately determine A and B, and hence
and R
, by observing solar neigh-
borhood stars. It could also measure directly the distances and proper motions of bright objects
throughout the Galaxy to determine the rotation curve
The local escape speed may be used to estimate the local mass density. If we assume all nearby
stars are bound to the Galaxy,
then the escape speed must be greater than the largest observed
velocity. Current estimates of the cutoff velocity range from 450 to 650 km/sec.
The most
serious uncertainty in this method is velocity error, resulting from proper motion and distance
uncertainties. (These stars are > 300 pc distant, making ground-based parallaxes inadequate.)
POINTS would measure proper motions of and distances to a representative selection (~1000) of
solar neighborhood stars with greatly improved accuracy. This will sharpen the cutoff velocity
measurement and help discriminate between the various theoretical mass models, including the
distribution of the dark matter component.
Another way to probe the galactic potential is to observe the distances and proper motions of
halo stars and of the globular clusters and derive their galactic orbits. More than half (84 of 149)
of the globular clusters in our Galaxy lie in the range 2-10
kpc from the Sun.
At 10 kpc,
POINTS could make distance determinations accurate to better than 10% from a few observa
tions, better than 1% from an extensive set; and transverse velocities could be determined to
0.2 km/sec. Hence, the orbits of many globular clusters could be determined. This has
implications for the formation history and the mass distribution of the Galaxy and would help
constrain the various local and global models of the cluster system dynamics. The unknown
value of the velocity dispersion anisotropy parameter
, where
is the mean
h 1
square tangential velocity and
is the mean square radial velocity, plagues attempts to con
strain the mass distribution from globular cluster kinematics.
could be determined if accurate
distances were available. Distance errors act to erroneously
isotropize the dispersion tensor, and
observations of tangential velocities do not even exist for the more distant clusters.
POINTS Science
16:29 Wednesday, January 03, 1996
page 6 of 20
Fich and Tremaine 1991
e.g. Lang, K.R., 1992, Astrophysical Data: Planets and Stars, Springer-Verlag, New York;
Ruprecht, J., Balazs, B., and White, R. E. 1981, "Catalogue of Star Clusters and Associations. II. Globular Clus
The density of dark matter alone in the disk appears to exceed the density of visible matter by ~5-50%
(Oort, J.
1932, Bull. Astron. Inst. Neth. 6, 249; Bahcall 1984a,b,c), though this is still a matter of dispute (see Carr, B.,
1994, Ann Rev Astron Astrophys 32, 531, "Baryonic Dark Matter" for a summary).
Leonard and Tremaine 1990
Within limits, this appears to be a safe assumption. Two contaminating populations are Local Group interlopers
(Dawson, P.C., and de Robertis, M.M., 1988, in The Mass of the Galaxy, ed. M. Fich, p. 21, Toronto: Can. Inst.
Theoret. Astrophys.) and escaping stars (Leonard, P.J.T., and Duncan, M.J., 1990, Astron J 99, 608; and Hills,
J.G., 1988, Nature 331, 687). However, a spatially uniform population of interlopers of the required density
would be inconsistent with local star counts (Leonard,
P.J.T., and Tremaine, S., 1990, Astrophys J 353, 486). For
escapers, in order for even one to be within a survey radius of, say, 200
pc, ~10
such stars must have been ejected
over the lifetime of the Galaxy. Such an ejection rate is improbable
(Fich and Tremaine 1991). Thus, the assump-
tion that nearby stars are bound to the Galaxy is relatively sound.

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