A. Calamida Abstract

Detection of a Large Population of White Dwarfs in the Direction of the Galactic Bulge

A. Calamida (STScI) , K. Sahu, J. Anderson, S. Casertano

We performed time-series observations of 12 ACS/WFC3 fields in the Sagittarius low-reddening window in the Galactic bulge, with the aim of detecting isolated black holes through astrometric microlensing. I will present preliminary results based on the reduction of four ACS fields. The composite V,I-band photometric catalog includes about 1 million stars down to V = 31 mag, and it is the deepest optical color-magnitude-diagram ever obtained for the bulge. The deep and accurate photometry allowed us to identify a large population (~700) of candidate white dwarfs and ~40 extremely horizontal branch (EHB) stars, some of them showing ellipsoidal variability.

Proper motions have been measured for the SWEEPS ACS field, a deep observation of which was taken in 2004. The time baseline of eight years allowed us to achieve a proper motion accuracy of better than0.15 mas/yr at V = 26 mag in both coordinates. The analysis of the proper motions shows that the EHB stars belong to the bulge population, while the white dwarf cooling sequence is made up of a composite disk and bulge population.