on the order of a Hubble time. Globular clusters represent important probes of the galactic halo
potential, their ages place a bound on the age of the universe, their member star absolute
nosities place constraints on stellar evolution models, and they are useful for tracing the develop
ment of the formation of the Milky Way.
The evidence that some metal-poor globulars appear to be older than some ages determined for
the universe poses an interesting problem. This can be resolved at the globular cluster end in
three ways. One of the principle uncertainties in the ages of the globular clusters is determining
the main sequence turn-off point which in turn requires accurate calibration of the absolute mag
nitude of the main sequence. For ages to be accurate to a billion years, cluster parallaxes must be
measured to better than 3%. POINTS could do this with a handful of observations for the 22
globular clusters within ~6 kpc of the Sun. A more extensive set of measurements such that
as increases the limiting distance to 30 kpc, giving access to ~120 clusters.
Aside from ques-
tions regarding crowded fields (see section III.C), this is within the capabilities of POINTS.
Another approach is improved absolute magnitude calibration of field
subdwarfs vs. color and
metallicity, for fitting globular cluster main sequences. A third and complementary avenue of
attack is calibration of absolute magnitudes of bright RR
Lyrae variables vs. metallicity and
period, fixing the globular cluster horizontal branch. POINTS could accomplish all three tasks
Determination of the proper motions of a representative sample of globular clusters would
directly yield the distribution of orbital angular momentum for the globular cluster system, pro
viding information about the galactic potential. It would also determine the orbits of individual
clusters, allowing studies of the correlations between metal abundance, perigalactic distance,
cluster radius, and orbital eccentricity, with strong consequences for theories of the formation of
Other problems involving the globular clusters include age differences among the individual
clusters, the so-called second parameter problem,
and cluster internal dynamics. POINTS
astrometry, in combination with radial velocity data, would provide information along all three
axes of the velocity dispersion tensor and unequivocally constrain
dynamical models. POINTS
will be able to make a significant impact in all of these areas. Because of their brightness, we
have an almost complete sample of clusters to observe. Because proper motions and positions
are with respect to the POINTS GRF, there is no need for nearby zero-proper-motion objects
(quasars). Hence, we can observe clusters near the galactic plane. Table 4 summarizes the sci
16:29 Wednesday, January 03, 1996
page 13 of 20
Zinn, R., 1993, in The Globular Cluster-Galaxy Connection, ASP Conf. Ser. 48, eds. G. Smith and J. Brodie, p.
Tello, C., 1994, Astron J 107, 1381, "Sampling of Globular Clusters and the Distance to the galactic Center. I.
Data Description and Analysis"