Structure and kinematics of Type II Cepheids in the Galactic bulge based
  on near-infrared VVV data
  Type II Cepheids (T2Cs) are radially pulsating variables that trace old
stellar populations and provide distance estimates through their
period-luminosity (PL) relation. We trace the structure of old stars in the
Bulge using new distance estimates and kinematic properties of T2Cs. We present
new NIR photometry of T2Cs in the bulge from the VVV survey. We provide the
largest sample (894 stars) of T2Cs with JHKs observations that have accurate
periods from the OGLE catalog. Our analysis makes use of the Ks-band
time-series to estimate mean magnitudes and distances by means of the PL
relation. To constrain the kinematic properties of our targets, we complement
our analysis with proper motions (PMs) based on both VVV and Gaia DR2. We
derive an empirical Ks-band PL relation that depends on Galactic longitude and
latitude:
Ks0=(10.66+-0.02)-(2.21+-0.03)*(logP-1.2)-(0.020+-0.003)*l+(0.050+-0.008)*|b|
mag; individual extinction corrections are based on a 3D reddening map. Our
targets display a centrally concentrated distribution, with solid evidence of
ellipsoidal symmetry--similar to the RR Lyrae (RRL) ellipsoid--and a few halo
outliers up to >100 kpc. We obtain a distance from the Galactic center of
R0=8.46+-0.03(stat.)+-0.11(syst.) kpc. We also find evidence that the bulge
T2Cs belong to a kinematically hot population, as the tangential velocity
components (sigma vl*=104.2+-3.0 km/s and sigma vb=96.8+-5.5 km/s) agree within
1.2sigma. Moreover, the difference between absolute and relative PM is in good
agreement with the PM of Sgr A* from VLBA measures. We conclude that bulge T2Cs
display an ellipsoidal distribution and have kinematics similar to RRLs, which
are other tracers of the old, low-mass stellar population. T2Cs also provide an
estimate of R0 that agrees excellently well with the literature, taking account
of the reddening law.
