Spectral analysis of the barium central star of the planetary nebula Hen 2-39

Abstract

Barium stars are peculiar red giants characterized by an overabundance of s-process elements along with an enrichment in carbon. These stars are discovered in binaries with white dwarf companions. The more recently formed of these stars are still surrounded by a planetary nebula. Precise abundance determinations of the various s-process elements, especially, of the lightest, short-lived radionuclide technetium will establish constraints for the formation of s-process elements in asymptotic giant branch stars as well as mass transfer through, for example, stellar wind, Roche-lobe overflow, and common-envelope evolution. We performed a detailed spectral analysis of the K-type subgiant central star of the planetary nebula Hen 2-39 based on high-resolution optical spectra obtained with the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope using LTE model atmospheres. We confirm the effective temperature of $T_{\mathrm{eff}}=4350\pm 150$ K for the central star of the planetary nebula Hen 2-39. It has a photospheric carbon enrichment of $[C/H]=0.36\pm 0.08$ and a barium overabundance of $[Ba/Fe]=1.8\pm 0.5$. We find a deficiency for most of the iron-group elements (calcium to iron) and establish an upper abundance limit for technetium ($\log {\epsilon }_{\mathrm{Tc}}<2.5$). The quality of the available optical spectra is not sufficient to measure abundances of all s-process elements accurately. Despite large uncertainties on the abundances as well as on the model yields, the derived abundances are most consistent with a progenitor mass in the range 1.75-3.00 $M_{\odot }$ and a metallicity of $[Fe/H]=-0.3\pm 1.0$. This result leads to the conclusion that the formation of such systems requires a relatively large mass transfer that is most easily obtained via wind-Roche lobe overflow.

arXiv comment26 pages, 18 figures

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