Three windows towards the Galactic bulge,
located at (l, b) =
(1.6,-2.8), (5.2,-3.5), and (4.2,-4.8), were
identified from deep astrometric plates. Deep BVRI
CCD photometry of these fields reaching V = 18.5
was obtained at the ESO Dutch telescope. The
color-color and color magnitude diagrams of these
fields allow us to select different bulge tracers,
including blue horizontal branch stars, red clump
giants, and RGB tip stars, as well as disk
main-sequence stars. A first catalogue of some
2000 red clump giants was constructed, and
measurement of their proper motions in under way.
We are deriving absolute proper motions in the
Hipparcos system using a two step procedure.
First, plates from the Cordoba archive of the
Astrographic Catalogue (epochs 1903-1913) are used
in combination with recent plates from theESO GPO
telescope to tie a set of intermediate magnitude
field stars in the Hipparcos system of positions
and proper motions. Second, deeper intermediate
epoch (1958) and final epoch CCD images (epoch
1996.5) are used to determine the absolute proper
motions of the fainter bulge stars. The average
uncertainty in the absolute proper motion of a
single giant, based on a preliminary sample of few
tens of stars, is 0”.006
yr-1. This relatively small
error, in combination with the sample size, would
allow usto study the shape of the velocity
ellipsoid for the different Galactic components
present in the inner regions as function of
Galactocentric distance, and also to measure an
accurate distance to the Galactic center and the
mass of the bulge. In addition, we identified
about 20 faint objects which are potential members
of the Sgr dwarf, based on their location in the
color-magnitude diagrams, and are also measuring
their proper motions.
Other windows with measured proper motions
are located along the Galactic minor axis,
including Baade’s window at
l, b = (1, −4)
(Spaenhauer, A., Jones, B. F., &: Whitford, A.
E., 1992, A3, 103, 297),
and the Plaut field at
l, b = (0, - 8 )
(Mendez, R. et al., 1997, in 4th ESO/CTIO Workshop
on “The Galactic Center”, in press). Our proper
motions are complementary, probing the kinematics
off the Galactic minor axis, where the signature
of rotation should be evident. The fields chosen
for this study overlap fields that the MACHO
project is following since 1993. Their variable
star database would provide with excellent tracers
of different populations. The 3-D motions of these
different Galactic components would be measured
for the first time. These include RR Lyrae tracers
of the inner metal-poor halo, Miras, LPVs and
delta Scuti stars tracers of the metal-rich bulge,
and eclipsing binaries tracers of the Galactic
disk (Minniti, D., et al., 1996, in IAP Coll. on
“Variable Stars and the Astrophysical Returns of
Microlensing Surveys”, ed. R. Ferlet, p. 257;
Alcock, C, et al., 1997,
astro-ph/9707311).
On the Possibility of Determining the Distance to the Galactic Center from the Geometry of Spiral Arm Segments
