First Detection of the Galactic Bulge WD Cooling Sequence

Monday, 05 May 2014 10 a.m. — 11 a.m. MST

AURA Lecture Hall

NOIRLab South Colloquia
ANNALISA CALAMIDA (Space Telescope Science Institute (AURA) and Gemini South Visiting Astronomer)

We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search) field  (~3'x3'), 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 clearly defined 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.