Analysis of absorption lines in the high-resolution spectra of five hot post-AGB candidates

Abstract

From an analysis of absorption lines in the high-resolution spectra, we have derived the radial velocities, stellar parameters (Teff, log g, wind-strength parameter log Q, and projected rotational velocity), and abundances (C, N, O, and Si) of IRAS 17460-3114, IRAS 18131-3008, IRAS 19336-0400, LSE 45, and LSE 163. Abundances are found to be solar, except for a low Si abundance in IRAS 19336-0400 and a mild CNO pattern in LSE 163, which rotates at an unusual large rotational velocity for its spectral classification. Combining the stellar parameters information with the Gaia DR2 data, we are able to derive absolute magnitudes, radii, and luminosities and clarify the possible post-asymptotic giant branch (AGB) nature of the objects. IRAS 17460-3114 and IRAS 18131-3008 are found to be massive OB stars, whereas IRAS 19336-0400 is found to be a post-AGB star, already showing nebular lines in the spectrum. However, we could not confirm the nature of LSE 45 and LSE 163 as post-AGB stars, although their parameters are much more inconsistent with those of massive stars. In both cases, we find a discrepancy between the spectroscopic mass and that derived from the predictions of post-AGB evolutionary tracks. In addition, LSE 45 lacks nebular lines, which are present in IRAS 19336-0400 at a similar temperature. In the case of LSE 163, the rotational velocity (259 ± 15 kms−1⁠) would be extremely large for a star evolving to central stars of planetary nebulae. The combination of this rotational velocity, the high Galactic latitude, slightly large radial velocity, and mild CNO enhancement suggests a history of binary interaction.