RR Lyrae variables provide one of the important consistency checks on the galactic distance
scale. Yet there is conflict between the distances inferred from
Cepheid absolute magnitudes,
RR Lyrae absolute magnitudes,
[7]
and the dependence of RR Lyrae magnitudes on metallicity.
RR Lyrae absolute magnitudes as a function of metallicity are critical to determining the ages of
globular clusters, which in turn constrain the Hubble constant.
POINTS can measure trigonometric distances to better than 1% to all 28 of the calibrating galac
-
tic Cepheids
[8]
(m < 12) and to many additional ones. This represents an order of magnitude
increase in accuracy and an enlargement of the calibration class. POINTS can also test for
metal-
licity effects, both in Cepheids and in RR Lyrae variables. Other distance indicators for which
POINTS can make direct distance determinations include O stars
[9]
and planetary nebulae.
[10]
There is currently no high-accuracy global reference frame to which
HST can tie its observations.
However, since HST can observe Cepheids and RR Lyrae variables in all Local Group galaxies,
it can provide a direct link to the local galactic distance scale determined by POINTS. Thus,
POINTS will greatly improve the most crucial bottom rung of the distance ladder that provides
our link to knowledge of the size and age scales of the Universe.
C. Mass of the Galaxy
The total mass of the Galaxy, and the mass distributions within the several components that make
up our Galaxy, are ill-determined. Four important parameters are R
0
, the distance to the center of
the Galaxy,
0
, the rotation speed of the local standard of rest around the center of the Galaxy,
and the Oort constants, A (a measure of the local shear) and B (the local
vorticity), both quanti-
ties being mainly due to differential galactic rotation. The
Oort constants may be determined
from measurements of radial velocity, transverse velocity, and distance for nearby stars. We
have
(1)
v
r
=
A $ r $ sin 2k
l
=
A $ cos 2k
+
B
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16:29 Wednesday, January 03, 1996
page 4 of 20
[10]
Jacoby, G.H., Ciardullo, R., and Ford, H.C., 1988, in The Extragalactic Distance Scale, Proceedings of the ASP
100th Anniversary Symposium, ASP, p. 42, "Planetary Nebulae as Distance Indicators";
Jacoby, G.H., Branch, D., Ciardullo, R., Davies, R.L., Harris, W.E., Pierce, M.J., Pritchet, C.J., Tonry, J.L., and
Welch, D.L., 1992, PASP 104, 599, "A Critical Review of Selected Techniques for Measuring
Extragalactic Dis-
tances";
Hui, X., Ford, H.C., Ciardullo, R., and Jacoby, G.H., 1993, Astrophys J 414, 463, "The Planetary Nebula System
and Dynamics of NGC 5128. I. Planetary Nebulae as Standard Candles";
Jacoby, 1989, Astrophys J 339, 39;
Kaler, J.B., 1985, Ann Rev Astron Astrophys 23, 89, "Planetary Nebulae and their Central Stars"
[9]
Garmany, C.D., and Stencel, R.E., 1992, Astron & Astrophys Supp 94, 211, "galactic OB Associations in the
Norhtern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg"
[8]
Feast and Walker, 1987
[7]
currently uncertain at a level ~0.2 mag (Barnes, T.G., and Hawley, S.L., 1987, Astrophys J Lett 307, L9)
