Discovery of a helium-core white dwarf progenitor

We discovered that HD 188112, a bright (V = 10(m).2), nearby ( 80 pc) B-type star, is a unique subluminous B (sdB) star. SdB stars are usually identified with models of core helium burning Extreme Horizontal-Branch (EHB) stars of half a solar mass. A spectral analysis of the hydrogen and helium lines resulted in T-eff = 21 500 +/- 500 K, log g = 5.66 +/- 0.05 placing the star below the EHB. HD 188112 was found to be radial velocity ( RV) variable and the RV curve has been measured to be perfectly sinusoidal with a period of 0.606585 days and a semi-amplitude of 188.3 km s(-1) indicating that it is a close binary system. From the atmospheric parameters and the Hipparcos parallax we conclude that the sdB star is of low mass (0.24 M-circle dot). The mass of the sdB is too low to sustain core helium burning and it is now evolving into a helium core white dwarf. A lower limit to the mass of its unseen companion of 0.73 M-circle dot is derived from the mass function. Because the companion does not contribute to the spectral energy distribution from the UV to the infrared it cannot be a main sequence star but must be a white dwarf (WD), a neutron star (NS) or a black hole. The system would qualify as pre-supernova Ia candidate (sdB+WD) if its total mass is above the Chandrasekhar limit (1.4 M-circle dot), or as post-supernova (sdB+NS) if the companion mass is above that limit, requiring the inclination angle to be lower than 51degrees or 48degrees, respectively.