The structure of the old Galactic Bulge revealed by Type II Cepheids

In the Galactic Bulge, metal-rich Red Clump stars, trace an X-shaped structure (Zoccali & Valenti 2016). However, the spatial distribution of old and more metal-poor Bulge stars, as traced by RR Lyrae (RRLs), is still under discussion (Dékany et al. 2013; Pietrukowicz et al. 2015; Kunder et al. 2016). Type II Cepheids (T2Cs) are old, low-mass post-Horizontal Branch stars. Like the RRLs, T2Cs trace old stellar populations, but they are 1-3 mag brighter. T2Cs are distance indicators that obey a Period-Luminosity relation which is more accurate in the NIR, and practically metallicity-independent (Di Criscienzo et al. 2007). We use the Ks-band VVV light curves of 894 T2Cs in the OGLE IV catalog (Soszynski et al. 2017) to inspect the structure of the Bulge. We find a distance of the Galactic Center of 8.46 kpc, which is larger than estimates obtained with similar tracers (RRLs, 8.30 kpc). The difference is due to the fact that we used a new reddening law (Alonso-Garcia et al. 2017) and a 3D, distance-dependent reddening map (Schultheis et al. 2014). We also find, as in Pietrukowicz et al. (2015), a tilted ellipsoid, with stars at positive Galactic longitude (l) closer than stars at negative l. We use VVV and Gaia proper motions to show that Bulge T2Cs have the properties an old, kinematically hot population, as expected. Finally, we find that the average difference between VVV and Gaia proper motions of T2Cs is similar to the proper motion of Sgr A*, in the Galactic Center.