Program: GS-2017A-Q-66

We propose to use the Doppler technique on time-resolved Gemini South/GMOS-S spectroscopy to infer the minimum mass of a likely compact object companion to a recently discovered ultra metal-poor star. Data already acquired suggests that the object may be a neutron star (and possibly a black hole), but the sparse radial velocity sampling currently available does not exclude the possibility that the companion is a white dwarf. If the compact object is confirmed to be a neutron star or black hole, it would be among the closest known massive compact objects to the Sun. The visible ultra metal-poor star is nearby and bright enough that Gaia will eventually measure the inclination of the system's orbit. It will therefore be possible to measure the mass of the compact object. The low metallicity of the visible star suggests that it and the compact object likely formed in the distant past, making the unseen companion the oldest compact object in the solar neighborhood.