A barium central star binary in the Type-I diamond ring planetary nebula Abell 70

TL;DR

This paper reports the discovery of a binary central star in planetary nebula Abell 70, featuring a Barium star with implications for understanding s-process nucleosynthesis and binary evolution in PNe.

Contribution

It provides the first detailed characterization of a Barium star in a planetary nebula and discusses its significance for stellar nucleosynthesis and binary star evolution.

Findings

Central star is a binary with a G8IV-V secondary and a hot white dwarf.

Secondary shows enhanced Ba II and Sr II features, classifying it as a Barium star.

Nebula exhibits Type-I chemical abundances with helium and nitrogen enrichment.

Abstract

Abell 70 (PN G038.1-25.4, hereafter A 70) is a planetary nebula (PN) known for its diamond ring appearance due a superposition with a background galaxy. The previously unstudied central star is found to be a binary consisting of a G8IV-V secondary at optical wavelengths and a hot white dwarf (WD) at UV wavelengths. The secondary shows Ba II and Sr II features enhanced for its spectral type that, combined with the chromospheric Halpha emission and possible 20-30 km/s radial velocity amplitude, firmly classifies the binary as a Barium star. The proposed origin of Barium stars is intimately linked to PNe whereby wind accretion pollutes the companion with dredged-up material rich in carbon and s-process elements when the primary is experiencing thermal pulses on the Asymptotic Giant Branch (AGB). A 70 provides further evidence for this scenario together with the other very few examples of Barium central stars. The nebula is found to have Type-I chemical abundances with helium and nitrogen enrichment, which when combined with future abundance studies of the central star, will establish A 70 as a unique laboratory for studying s-process AGB nucleosynthesis. We also discuss guidelines to discover more binary central stars with cool secondaries in large orbits that are required to balance our knowledge of binarity in PNe against the currently better studied post common-envelope binary central stars.