First detection of the white dwarf cooling sequence of the galactic bulge

A. Calamida, K. C. Sahu, J. Anderson, S. Casertano, S. Cassisi, M. Salaris, T. Brown, J. Sokol, H. E. Bond, I. Ferraro, H. Ferguson, M. Livio, J. Valenti, R. Buonanno, W. Clarkson, A. Pietrinferni

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Abstract

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′× 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 2 yr, 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-1 (≈4 km s-1) at F606W ≈ 25.5 mag, and better than ≈0.5 mas yr-1 (≈20 km s-1) 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 day) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.

Original languageEnglish (US)
Article number164
JournalAstrophysical Journal
Volume790
Issue number2
DOIs
StatePublished - Aug 1 2014

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Calamida, A., Sahu, K. C., Anderson, J., Casertano, S., Cassisi, S., Salaris, M., Brown, T., Sokol, J., Bond, H. E., Ferraro, I., Ferguson, H., Livio, M., Valenti, J., Buonanno, R., Clarkson, W., & Pietrinferni, A. (2014). First detection of the white dwarf cooling sequence of the galactic bulge. Astrophysical Journal, 790(2), [164]. https://doi.org/10.1088/0004-637X/790/2/164