Detection of lithium in the spectrum of the symbiotic Mira star V407 Cygni

Abstract

We detected the resonance lithium doublet, ⁷LiI λ6708, in the spectra of the symbiotic Mira V407 Cyg (P≈763^d) obtained with a resolution R = λ/Δλ ≈ 18 500. The line equivalent width is ∼0.34 Å. The presence of lithium in the atmosphere of the Mira, which, judging by its period, appreciably ascended the asymptotic giant branch (AGB), can be explained by the penetration of its convective envelope into the hot-bottom-burning (HBB) hydrogen-shell source. At the same time, the spectrum of V407 Cyg does not reveal the ZrO λ5551, 6474 absorption bands, which are used to classify S-type stars from low-resolution spectra. We found only weak ZrO λ5718, 6412 bands, which are, however, invisible in low-resolution spectra. Thus, the Mira star V407 Cyg should be classified as being of the spectral type M. In general, the spectrum of V407 Cyg is similar to the spectrum of the red giant in the symbiotic star CH Cyg, but the latter exhibits no lithium lines. The switch-on of the HBB process without any significant enrichment of the atmosphere of an AGB star with s-process elements implies that the third dredge-up is not efficient for some of these stars.