2012MNRAS.421..679S

The first high-resolution (R ∼ 50 000) optical spectrum of the B-type star, LS III +52°24, identified as the optical counterpart of the hot post-asymptotic giant branch (post-AGB) candidate IRAS 22023+5249 (I22023) is presented. We report the detailed identifications of the observed absorption and emission features in the full wavelength range (4290-9015 Å) as well as the atmospheric parameters and photospheric abundances (under the local thermodynamic equilibrium approximation) for the first time. The nebular parameters (T_e, N_e) are also derived. We estimate T_eff= 24 000 K, log g= 3.0 and ξ_t= 7 km/s, and the derived abundances indicate a slightly metal-deficient evolved star with C/O < 1. The observed P-Cygni profiles of hydrogen and helium clearly indicate ongoing post-AGB mass-loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionization has just started. The observed spectral features, large heliocentric radial velocity, atmospheric parameters and chemical composition indicate that I22023 is an evolved post-AGB star belonging to the old disc population. The derived nebular parameters (T_e= 7000 K, N_e= 1.2 {x} 10⁴/cm3) also suggest that I22023 may be evolving into a compact, young low-excitation planetary nebula. Our optical spectroscopic analysis together with the recent Spitzer detection of double-dust chemistry (the simultaneous presence of carbonaceous molecules and amorphous silicates) in I22023 and other B-type post-AGB candidates may point to a binary system with a dusty disc as the stellar origin common to the hot post-AGB stars with O-rich central stars.