Deciphering the 3D structure of the old Galactic bulge from the OGLE RR
  Lyrae stars
  We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars
(type RRab) detected recently toward the Galactic bulge by the Optical
Gravitational Lensing Experiment (OGLE) survey. The data support our earlier
claim that these metal-poor stars trace closely the barred structure formed of
intermediate-age red clump giants. The distance to the Galactic center (GC)
inferred from the bulge RR Lyrae stars is R_0=8.27+/-0.01(stat)+/-0.40(sys)
kpc. We show that their spatial distribution has the shape of a triaxial
ellipsoid with an major axis located in the Galactic plane and inclined at an
angle of i=20+/-3 deg to the Sun-GC line of sight. The obtained scale-length
ratio of the major axis to the minor axis in the Galactic plane and to the axis
vertical to the plane is 1:0.49(2):0.39(2). We do not see the evidence for the
bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve
parameters, we derive metallicities of the RRab variables and show that there
is a very mild but statistically significant radial metallicity gradient. About
60% of the bulge RRab stars form two very close sequences on the
period-amplitude (or Bailey) diagram, which we interpret as two major old bulge
populations: A and B. Their metallicities likely differ. Population A is about
four times less abundant than the slightly more metal-poor population B. Most
of the remaining stars seem to represent other, even more metal-poor
populations of the bulge. The presence of multiple old populations indicates
that the Milky Way bulge was initially formed through mergers.
