absolute magnitude scale as a function of metallicity, sidestep the extinction correction problem,
and calibrate the RR Lyrae PLC relation.
Other bright stars, such as Mira variables, are visible through interstellar windows.
[34]
These
luminous giants currently have an uncertain absolute magnitude calibration, due mainly to the
complex molecular opacity sources in their cool atmospheres. Additionally, there is uncertainty
regarding the differences between the solar neighborhood (Pop. I)
Miras and the galactic center
(Pop. II) Miras. POINTS, again through direct distance determinations, can provide the neces
-
sary absolute magnitude calibrations for both populations, in addition to another determination of
R
0
.
Indirect methods for determining R
0
can be useful. One is to adopt a galactic rotation model and
combine it with distances of tracers that participate in that rotation. [Combin
ing radial velocities
with the kinematic model for the Galaxy to derive kinematic distances, one can solve for R
0
such
that the kinematic and luminosity distances agree. Another method is to use Oort's A parameter
(i.e., the local velocity shear) to obtain a galactic distance scale: given radial velocities and dis
-
tances for solar neighborhood stars, one can solve for R
0
so that the observed velocity shear
matches that of a model.] Finally, one may use radial velocities and distances of
Cepheid vari-
ables in a galactic rotation model to determine the galactic distance scale.
[35]
Most of these indi-
rect methods are sensitive to local deviations from circular motion and to extinction corrections,
and some are sensitive to the circular rotation speed of the Sun.
Galactic Rotation Curve Beyond the Solar Circle
The rotation curve inside the solar circle is well-determined, mainly from H I observations.
[36]
Such is not the case beyond the Sun's orbit. By measuring the distances and proper motions of
disk tracers (OB stars) outside the solar circle, POINTS can conclusively determine the rotation
curve for the outer Galaxy to ~15 kpc. The necessary observations fall well within the capabili
-
ties of POINTS: m
V
12, distance accuracy 10% = 10
µ
as, and proper motion accuracy 10 km/s
= 200
µ
as/yr.
Reddening
POINTS can contribute substantially to the determination of the structure of our Galaxy. Essen
-
tial to setting the size scale of the Galaxy, in addition to determining absolute
luminosities of
massive stars in various evolutionary stages, is the relationship between reddening and
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POINTS Science
16:29 Wednesday, January 03, 1996
page 10 of 20
[36]
Fich and Tremaine 1991; Fich et al. 1989
[35]
Caldwell, J.A.R., and Coulson, I.M., 1987, Astron J 93, 1090, "Milky Way Rotation and the Distance to the
Galactic Center from Cepheid Variables"
[34]
Glass, I.S., and Feast, M.W. 1982, MNRAS 198 199, "Infrared Photometry of Mira Variables in the Baade Win-
dows and the Distance to the galactic Centre"
