High-Resolution Optical Spectroscopy of the Post-AGB Supergiant V340 Ser (${\mathbf{=}}$IRAS 17279$\mathbf{-}$1119)

Abstract

Evidence of wind variability and velocity stratification in the extended atmosphere has been found in the spectra of the supergiant V340 Ser (${=}$IRAS 17279$-$1119) taken at the 6-m BTA telescope with a resolution $R\geq 60\,000$. The H$\alpha$ line has a P Cyg profile whose absorption component ($V_{\odot}={+}34$ km s${}^{-1}$) is formed in the upper layers of the expanding atmosphere close the circumstellar medium. For four dates the mean velocity has been derived from the positions of 300–550 symmetric metal absorptions with an accuracy better than ${\pm}0.1$ km s${}^{-1}$: $V_{\odot}=59.30$, $60.09$, $58.46$, and $55.78$ km s${}^{-1}$. A lot of low-excitation metal lines have an inverse P Cyg profile. The mean positions of their emission components, $V_{\odot}=46.3\pm 0.4$ km s${}^{-1}$, differ systematically from the velocity inferred from symmetric absorptions, suggesting the presence of a velocity gradient in the supergiant’s extended atmosphere. The multicomponent profile of the Na I D lines contains the interstellar, $V_{\odot}={-}11.2$ km s${}^{-1}$, and circumstellar, $V_{\odot}={+}10$ km s${}^{-1}$, components and the component forming in the upper atmospheric layers, $V_{\odot}={+}34.0$ km s${}^{-1}$. The mean velocity from 20–30 diffuse interstellar bands (DIBs) identified in the spectra, $V_{\odot}\textrm{(DIBs)}={-}11.6\pm 0.2$ km s${}^{-1}$, agrees with the velocity from interstellar Na I and K I components. The equivalent width of the oxygen triplet $\textrm{W(O\,I 7774)}=1.25$ Å corresponds to an absolute magnitude of the star $Mv\approx{-}4.6^{m}$, which, given the total (interstellar $+$ circumstellar) extinction, leads to a distance to the star $d\approx 2.3$ kpc.