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Paper: The White Dwarf Cooling Sequence of the Galactic Bulge
Volume: 491, Fifty Years of Wide Field Studies in the Southern Hemisphere: Resolved Stellar Populations in the Galactic Bulge and the Magellanic Clouds
Page: 160
Authors: Calamida, A.; Sahu, K.; Anderson, J.; Casertano, S.; Brown, T.; Cassisi, S.; Sokol, J.; Bond, H.; Ferguson, H.; Livio, M.; Salaris, M.; Valenti, J.
Abstract: 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.
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