Since K
z
is proportional to the derivative of the stellar space density distribution, the determina
-
tion of the local volume mass density depends on the square of the distance scale, and determina
-
tions of the surface mass density are linearly proportional to the distance scale.
[23]
Hence,
accurate determination of the galactic distance scale is crucial for understanding the local mass
structure of the Galaxy. Gilmore et al. (1989) conclude that systematic errors limit the local vol
-
ume mass density determinations (the Oort limit). The distribution of K giants perpendicular to
the disk is a major source of uncertainty. To resolve this, we would need ~20 parallax measure
-
ments of bright K giants (m
V
= 10), with individual accuracy 50
µ
as = 5% error at 1 kpc. This
falls well within POINTS capabilities. We would also need space velocities to rule out the exis
-
tence of separate kinematic subgroups: ~100 K giants to a level of several hundred
µ
as, again
well within the range of POINTS.
If we assume the Magellanic Clouds are bound to the Galaxy, measuring their proper motion
would provide another constraint on the total mass of the Galaxy, as well as a measure of the
dark matter component.
[24]
Proper motion measurements of the Magellanic Clouds would also
greatly improve galactic mass distribution determinations resulting from observations of the
Magellanic Stream.
[25]
The brighter LMC stars are m = 11, and the expected orbital motion is
large: ~15 mas/yr (assuming no dark matter).
[26]
Because POINTS measurements would be in a
reference frame tied to the quasars, the systematic errors that beset ground measurements of
LMC and SMC motions would be absent. Besides the LMC and SMC, at least seven other satel-
lite galaxies orbit the Milky Way within 300 kpc.
[27]
Determination of proper motions and dis-
tances to these objects would corroborate and extend the
Magellanic Cloud measurements.
D. Galactic Structure
Spiral Arm Structure
Are spiral arms due to traveling density waves or to propagating star formation? This is one of
the fundamental questions regarding galactic spiral arms. POINTS will be able to differentiate
between the two hypotheses by measuring the kinematics of stars near a spiral arm. Density
waves will affect stellar motions in a characteristic way; expected peculiar motions are on the
order ~20 km/s. Measurements of ~20-50 early-type stars (m
V
13) within ~200 pc of the
nearby Perseus arm (~2 kpc), to an accuracy of 2% = 10
µ
as in parallax and 1% = 21
µ
as/yr in
proper motion will suffice. Discrimination at this level is well within POINTS capabilities.
d:\points\proposal\sz1\scifull.lwp
POINTS Science
16:29 Wednesday, January 03, 1996
page 8 of 20
[27]
Zaritsky, D., Olszewski, E.W., Schommer, R.A., Peterson, R.C., and Aaronson, M., 1989, Astrophys J 345, 759
[26]
Lin, D.N.C., and Lynden-Bell, D., 1977, MNRAS 181, 59
[25]
Murai, T., and Fujimoto, M., 1980, PASJ 32, 581;
Lin, D.N.C., and Lynden-Bell, D., 1982, MNRAS 198, 707
[24]
Fich and Tremaine 1991
[23]
Gilmore et al. 1989
