Ages of white dwarf-red subdwarf systems

Abstract

We provide the fi rst age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the VJRKCIKCJH bands and spectroscopy covering from 6,000°A to 9,000°A (our spectral coverage)for the two new systems, as well as for a comparison white dwarfmain sequence red dwarf system, GJ 283 AB. Using model grids, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and total lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 gigayears (Gyr).