Stellar halo tracers include K giants, blue horizontal branch stars, carbon stars,
[18]
and RR Lyrae
variables.
[19]
The halo stars have a large vertical velocity dispersion, about 90
km/s at 6 kpc.
This seems to drop off at larger distances: ~60
km/s at 25 kpc from the plane. POINTS could
measure all three components of the halo tracer velocity dispersions, as well as determine the
bulge rotation rate.
[20]
Thus, we could make significant progress in discriminating between two
possible causes of the drop-off: 1) a change in the orbital structure of the Galaxy, with implica
-
tions for the separability of the potential and a third integral of the motion; and 2) a true decrease
in the mass density of the halo. This second possibility would imply both a truncation and a sub
-
stantial flattening of the dark matter distribution. Measurements of the proper motions, to an
accuracy of 5 km/s = 40
µ
as, and distances, to a parallax accuracy of 5 % = 2
µ
as, of, say, ~50 K
giants farther than 10 kpc from the disk would suffice to make this distinction. Such stars are
readily observable: m
V
16. In addition, we could make use of parallaxes to calibrate nearer
stars and determine spectroscopic parallaxes for the farther stars, which would then reduce the
needed parallax accuracy to 10
µ
as. Additionally, an understanding of the orbital structure of the
halo stars will provide clues to the formation of the Galaxy and thereby indirectly improve our
understanding of its mass structure.
The volume density of the disk has recently been estimated by
Bahcall
[21]
and by Kuijken and
Gilmore.
[22]
These two groups differ in their estimation of the dark matter component of the disk.
A substantial amount of disk dark matter implies a
dissipational character, which would rule out
most non-baryonic candidates and many baryonic ones. The local volume density can be written
in the form
(4)
q
= -
1
4oG
¹K
z
¹z
+
2 ( A
2
-
B
2
)
where A and B are the Oort constants and K
z
is the vertical component of the galactic gravita
-
tional field. The disk surface density is then
(5)
S(z)
=
°
-
z
z
q(z) dz
=
1
2oG K
z
-
1
oG
( A
2
-
B
2
) z
d:\points\proposal\sz1\scifull.lwp
POINTS Science
16:29 Wednesday, January 03, 1996
page 7 of 20
[22]
MNRAS 239, Aug. 1, 1989, p. 571-603, 605-649, 651-664, "The mass distribution in the galactic disc. I - A tech
-
nique to determine the integral surface mass density of the disc near the Sun. II - Determination of the surface
mass density of the galactic disc near the Sun. III - The local volume mass density"
[21]
Bahcall, J.N., 1984 Astrophys J 276, 156; 276, 169; 287, 926
[20]
Minniti, D., White, S.D.M., Olszewski, E.W., and Hill, J.M., 1992, Astrophys J Lett 393, L47, "Rotation of the
galactic Bulge"
[19]
Gilmore, G., Wyse, R.F.G., and Kuijken, K., 1989, Ann Rev Astron Astrophys 27, 555, "Kinematics, Chemistry,
and Structure of the Galaxy";
Fich and Tremaine 1991
[18]
Bothun, G., Elias, J.H., and MacAlpine, G., 1991, Astron J 101, 2220, "Carbon Stars at High galactic Latitude";
Jura, M., Joyce, R.R., and Kleinmann, S.G., 1989, Astrophys J 336, 924, "High-Luminosity Carbon Stars in the
galactic Anticenter"
