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Astronomical Applications Department, U.S. Naval Observatory - Scifull (Page 9)

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Astronomical Applications Department, U.S. Naval Observatory - Scifull
Distance to the Galactic Center
The Sun's distance from the galactic center sets the galactic distance scale. Many different
approaches for determining this distance have been tried, with recent (1974-1993) results ranging
from 6.2-10.8 kpc. The best estimate appears to be ~8.0▒0.5 kpc.
The best primary indicator
consists of VLBI determinations of water maser proper motions near the center of the Galaxy.
Results are fairly unreliable due to large random motions of the masers and the
nonuniform dis-
tribution of masers around the exciting star. Hence, we look to secondary and indirect methods.
POINTS can contribute significantly to the determination of R
in several ways.
Accurate distances to the globular clusters would allow three estimates for R
. First, if we
assume the globular cluster system is symmetrically distributed about the galactic center, the den
sity peak of the cluster locations projected along the line joining the Sun and the galactic center
(GC) yields R
Accuracy currently depends on the horizontal branch absolute magnitude cali
bration as a function of metallicity, and upon the extinction correction. Neither of these sources
of error affects POINTS's direct measurements of the trigonometric parallaxes of the individual
clusters. Second, there is reason to believe that the density of globular clusters decreases in a
cone (opening angle ~15
) aligned on the galactic rotation axis.
Adjusting R
to maximize the
cone angle yields the GC distance. Accurate globular cluster distances are necessary to charac
terize this cone of avoidance. Third, the metallicity of globular clusters decreases with distance
from the GC. If the cluster distribution is azimuthally symmetric about the galactic rotation axis,
then R
can be determined by requiring that the metallicity be uncorrelated with galactocentric
azimuth. This method has the (ground-based) advantage that
metallicity determinations are rela-
tively insensitive to extinction corrections. Again, accurate cluster distances are needed.
RR Lyrae variables can be seen across the Galaxy through low-extinction windows (such as
Baade's Window or the one at
l = 0, b = -8
). Similar to the method for globular clusters, the
RR Lyrae distribution peak in such windows can be used to estimate R
Principal sources of
systematic error for this method include the unknown
metallicity dependence of the RR Lyrae
absolute magnitude calibration, uncertainties in the extinction correction, and the calibration of
the RR Lyrae PLC relation.
By measuring accurate distances, POINTS can calibrate the
POINTS Science
16:29 Wednesday, January 03, 1996
page 9 of 20
Reid 1993
e.g. Walker, A.R., and Terndrup, D.M. 1991, Astrophys J 378, 119;
Fernley, J.A., Jameson, R.F., Longmore, A.J., Watson, F.G., and Wesselink, T., 1987, MNRAS 226, 927, "The
Absolute Magnitude of RR Lyraes and the Distance to the galactic Centre"
Oort, J.H., and Plaut, L. 1975, Astron & Astrophys 41, 71
Wright, A.E., and Innanen, K.A., 1972, BAAS 4, 267;
Sasaki, T., and Ishizawa, T., 1978, Astron & Astrophys 69, 381
e.g. Racine, R., and Harris, W.E., 1989, Astron J 98, 1609, "Globular Clusters and the Distance to the galactic
For a review, see Reid, M.J., 1993, Ann. Rev. Astron. Astrophys. 31, 345, "The Distance to the Center of the Gal-

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