Quantitative spectral analysis of the sdB star HD 188112: A helium-core white dwarf progenitor

Context. HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass too low to ignite core helium burning and is therefore considered a pre-extremely low-mass (ELM) white dwarf (WD). ELM WDs (M ≲ 0.3 M_⊙) are He-core objects produced by the evolution of compact binary systems.
Aims: We present in this paper a detailed abundance analysis of HD 188112 based on high-resolution Hubble Space Telescope (HST) near- and far-ultraviolet spectroscopy. We also constrain the mass of the star's companion.
Methods: We use hybrid non-LTE model atmospheres to fit the observed spectral lines, and to derive the abundances of more than a dozen elements and the rotational broadening of metallic lines.
Results: We confirm the previous binary system parameters by combining radial velocities measured in our UV spectra with the previously published values. The system has a period of 0.60658584 days and a WD companion with M ≥ 0.70 M_⊙. By assuming a tidally locked rotation combined with the projected rotational velocity (v sin I = 7.9 ± 0.3 km s^-1), we constrain the companion mass to be between 0.9 and 1.3 M_⊙. We further discuss the future evolution of the system as a potential progenitor of an underluminous type Ia supernova. We measure abundances for Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Ni, and Zn, and for the trans-iron elements Ga, Sn, and Pb. In addition, we derive upper limits for the C, N, O elements and find HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE effects on the line strength of some ionic species such as Si II and Ni II. The metallic abundances indicate that the star is metal-poor, with an abundance pattern most likely produced by diffusion effects.