Abstract

First detection of the White-Dwarf Cooling Sequence of the Galactic Bulge

Annalisa Calamida (Space Telescope Science Institute)

K. C. Sahu, J. Anderson, S. Casertano (STScI), S. Cassisi (OACTe - INAF), M. Salaris (Astrophysics Research Institute, Liverpool John Moores University), et al.

We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar Planet Search field (~ 3' x 3'), together with three more Advanced Camera for Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and F814W filters, approximately every two weeks for two years, with the principal aim of detecting a hidden population of isolated black holes and neutron stars through astrometric microlensing. Proper motions were measured with an accuracy of ~ 0.1 mas/yr (~ 4 km/s) at F606W ~ 25.5 mag, and better than ~ 0.5 mas/yr (~ 20 km/s) at F606W ~ 28 mag, in both axes.

Proper-motion measurements allowed us to separate disk and bulge stars and obtain a clean bulge color-magnitude diagram. We then identified for the first time a white dwarf (WD) cooling sequence in the Galactic bulge, together with a dozen candidate extreme horizontal branch stars. The comparison between theory and observations shows that a substantial fraction of the WDs (~ 30%) are systematically redder than the cooling tracks for CO-core H-rich and He-rich envelope WDs. This evidence would suggest the presence of a significant number of low-mass WDs and WD - main sequence binaries in the bulge. This hypothesis is further supported by the finding of two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.

Mode of presentation: oral