High-resolution spectroscopy of the variable hot post-AGB star LS 4331 (IRAS 17381−1616)

An analysis of the high-resolution (\(R\sim 48000\)) optical spectrum of hot (B1Ibe) post-AGB star LS 4331 (IRAS 17381−1616) is presented. The detailed identification of the observed absorption and emission features in the wavelength range 3700–9200 Å is carried out for the first time. The atmospheric parameters and chemical composition of the star are derived from the non-LTE analysis of absorption lines. We estimated \(T_\textrm{eff}=20900\pm 500\) K, \(\log g=2.57\,\pm \,0.08\), \(V_r=-51.7\,\pm \,0.8\) km s\(^{-1}\), \(\xi _\textrm{t}=24\,\pm \,4\) km s\(^{-1}\)  and \(v \sin i=30\,\pm \,5\) km s\(^{-1}\). An abundance analysis for C, N, O, Mg, Al, S, and Si reveals that the N and O abundance is close to solar while metal underabundances relative to the solar value (i.e., \([\mathrm{Mg/H}] = -1.04\) dex, \([\mathrm{Al/H}] = -1.20\) dex, \([\mathrm{Si/H}] = -0.46\) dex) are found. LS 4331 is a high galactic latitude metal-poor and carbon-deficient hot post-AGB star. The underabundance of carbon (\([\mathrm{C/H}]=-0.64\) dex) is similar to that found in other hot post-AGB stars and indicates that the star’s AGB phase of evolution was terminated before the third dredge-up. Plasma diagnostics are derived from the nebular emission lines. The presence of nebular emission lines in the spectrum of LS 4331 indicates that the photoionization of the circumstellar envelope has already started. The nebular parameters and expansion velocity of the nebula are derived. Using the Gaia DR3 distance, the absolute luminosity of the star is derived, and the star’s position on the post-AGB evolutionary tracks suggests that its initial main sequence mass is about 1.2 \(M_{\odot }\). It is also reported that fast irregular brightness variations with an amplitude of up to 0.3 mag in the V band have been found in the star, typical of hot post-AGB objects.