ence that POINTS can
address involving the
globular clusters (as well
as the open clusters, dis-
cussed next).
F. Open Clusters
Galactic and Magellanic
Cloud open clusters pro-
vide another rich source of
astrophysically interesting
problems. The old open
clusters are important for
determining galactic disk
evolution. Ground-based
observations are compli-
cated by the combined
effects of age, distance,
interstellar reddening, and
the lack of "standard can-
dles." The present rela-
tively crude radial velocity
data would be enormously
improved by accurate
proper motions. With
POINTS, the distances and
ages of old and young open clusters could be determined.
The latter would have a substantial
impact on the determination of galactic rotation and the structure of the spiral arms. In addition,
we should be able to determine the velocity dispersions and dissolution
timescales of associa-
tions; the open cluster in the Orion nebula is especially interesting. Another exciting possibility
is obtaining a fully three-dimensional picture of the nearby open clusters. For the
Pleiades (~130
pc), a 5
µ
as parallax error represents a 0.08 pc distance error, yet the cluster depth is ~4
pc.
Similarly, for a proper motion error of 5
µ
as/yr, the velocity error would be only 3 meters per
second, yet the internal velocity dispersion is on the order of 0.5
km/s. Young open clusters
serve as tracers for star formation, so that accurate distances would prove invaluable for mapping
the spiral arms, regions of sequential star formation,
[51]
and the history of star formation in the
Galaxy. Table 4 summarizes some of the many areas of star cluster science for which POINTS
d:\points\proposal\sz1\scifull.lwp
POINTS Science
16:29 Wednesday, January 03, 1996
page 14 of 20
[51]
Elmegreen, B.G., and Lada, C.J., 1977, Astrophys J 214, 725
[3]
level of accuracy required for significant scientific impact (current
accuracies
are ~1 mas and ~mas/yr)
[2]
distance range (top line of each science class)
[1]
approximate visual magnitude range for significant scientific impact
~10
µ
as/yr
Orion nebula
20
µ
as/yr
Internal dynamics
50
µ
as/yr
Galactic rotation
distances to 1%
Stellar luminosity calibrations
< 2-4 kpc
< 12-14
Young open clusters
10
µ
as/yr
< 5-10 kpc
16-17
Internal dynamics of old open clusters
20
µ
as/yr
< 10 kpc
13-15
Internal dynamics of globular clusters
100
µ
as/yr
< 10 kpc
14-16
Kinematics of old open clusters
40
µ
as/yr
2-50 kpc
14-17
Kinematics of globular clusters
parallax to < 5%
< 10 kpc
14-16
Ages & distances of old open clusters
distances to 1%
Globular cluster distance scale
distances to a few percent
Second parameter problem
distances to a few percent
Individual cluster age differences
distances to 1-2%
Universe-cluster age discrepancy
2-20 kpc
12-15
Ages & distances of globular clusters
D
[2]
/
[3]
m
V
[1]
Science Class / Problem
Star Cluster Science
Table 4
