||The White Dwarf Cooling Sequence of the Galactic Bulge
||491, Fifty Years of Wide Field Studies in the Southern Hemisphere: Resolved Stellar Populations in the Galactic Bulge and the Magellanic Clouds
||Calamida, A.; Sahu, K.; Anderson, J.; Casertano, S.; Brown, T.; Cassisi, S.; Sokol, J.; Bond, H.; Ferguson, H.; Livio, M.; Salaris, M.; Valenti, J.
||We collected F606W- and F814W-band time series data of the Sagittarius
low-reddening window in the Galactic bulge with the Advanced Camera far
Surveys mounted on the Hubble Space Telescope. We sampled the region
approximately every two weeks over the period of one year, with the principal aim to
detect a potential hidden population of isolated black holes and neutron stars in
the Galactic disk through astrometric microlensing. We present preliminary
results here based on a photometric catalog including ≈3 ×105
stars down to F606W ≈ 31 mag. Proper motions were also measured,
with an accuracy better than ≈ 0.5 mas/yr at F606W ≈ 28 mag
in both coordinates. We were then able to separate disk and bulge stars
and obtain a clean bulge color-magnitude diagram. Together with a dozen
candidate extreme horizontal branch stars we were able to identify for the
first time a clearly defined white dwarf (WD) cooling sequence in the bulge.
The comparison between theory and observations shows that a substantial
fraction of the WDs (≈ 40%) is systematically redder than the
canonical cooling tracks for CO-core DA WDs. This evidence would suggest
the presence of a significant number of He-core WDs in the bulge, formed
in close binaries, as has been found in some Galactic globular and open
clusters. The presence of close binaries in the bulge population is further
supported by the detection of a candidate dwarf nova in outburst and a few
candidate cataclysmic variables in quiescence in the same field.