Spectroscopic Survey of Faint Planetary-Nebula Nuclei. VIII. The Dwarf Barium Central Star of Kohoutek 1-9

TL;DR

This study presents optical spectroscopy and imaging of the faint planetary nebula Kohoutek 1-9, revealing its central star as a rare dwarf barium star, and discusses its binary evolution and morphology.

Contribution

First identification of a dwarf barium star as the central star of a planetary nebula, with detailed spectroscopic and imaging analysis.

Findings

Central star is a G-type dwarf with s-process element enhancements.

The nebula has a thin-ring morphology similar to other binary PNNi.

The star is likely part of a wide binary system with a history of mass transfer.

Abstract

In the course of our ongoing survey of faint planetary-nebula nuclei (PNNi), we obtained optical spectroscopy of the central star of the little-studied PN Kohoutek~1-9 (K 1-9). Its spectrum is found to be that of a G-type dwarf with strong absorption features of carbon molecules and s-process elements such as Sr and Ba--a dwarf barium star. K 1-9 thus joins a very small group of PNe with barium-star nuclei. Their likely progenitors are wide binaries in which the primary star reached the thermally pulsing asymptotic-giant-branch (AGB) phase, dredged up C and s-process elements from its interior, and transferred enriched material to the companion through a dense stellar wind. The remnant core is now a hot, optically inconspicuous (pre-)white dwarf, responsible for ionizing the AGB ejecta, and leaving the optical spectrum dominated by the cool barium star. We present deep narrow-band images of K 1-9, obtained by accumulating long exposure times using amateur telescopes. The PN shows a thin-ring morphology, remarkably similar to the "wedding-ring" shapes seen around other members of this class of binary PNNi. The thin ring probably represents material preferentially ejected into the orbital plane of the binary; we note that the PNN is slightly off-center within the ring, as has been predicted theoretically. We suggest several follow-up studies, including precision photometry to search for periodic variations due to starspots on the rotating barium star, and high-resolution spectroscopy to determine atmospheric parameters of the star, chemical abundances, and its rotation velocity.