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

Abstract

The high-resolution ($R\sim 48000$) 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 $T_{\text{eff}}=18000\pm 1000$ K, $\log g=2.25\pm 0.08$, ${\xi }_{\mathrm{t}}=10\pm 4$ km/s and $v\sin i=37\pm 6$ 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 $V_{\text{r}}=-124.0\pm 0.4$ 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.

Accepted by MNRAS, 18 pages, 13 figures

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