High-resolution spectroscopy of the high-velocity hot post-AGB star IRAS 18379–1707 (LS 5112)

Abstract

The high-resolution ($R\\sim48\\,000$) optical spectrum of the B-type supergiant LS 5112, identified as the optical counterpart of the post-AGB candidate IRAS 18379-1707, is analysed. We report the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9200 A for the first time. The absorption line spectrum has been analysed using non-LTE model atmosphere techniques to determine stellar atmospheric parameters and chemical composition. We estimate $T_\\text{eff}=18~000\\pm1000$ K, $\\log g=2.25\\pm0.08$, $\\xi_{\\rm t}=10\\pm4$ km/s and $v \\sin i=37\\pm6$ km/s, and the derived abundances indicate a metal-deficient ([M/H]$\\approx-0.6)$ post-AGB star. Chemical abundances of eight different elements were obtained. The estimates of the CNO abundances in IRAS 18379-1707 indicate that these elements are overabundant with [(C+N+O)/S]=+0.5$\\pm$0.2 suggesting that the products of helium burning have been brought to the surface as a result of third dredge-up on the AGB. From the absorption lines, we derived heliocentric radial velocity of $V_\\text{r}=-124.0\\pm0.4$ km/s. We have identified permitted emission lines of O I, N I, Na I, S II, Si II, C II, Mg II and Fe III. The nebula forbidden lines of [N I], [O I], [Fe II], [N II], [S II], [Ni II] and [Cr II] have also been identified. The Balmer lines H$\\alpha$, H$\\beta$ and H$\\gamma$ show P-Cygni behaviour clearly indicating post-AGB mass-loss process in the object with the wind velocity up to 170 km/s.