The central star of the planetary nebula PB 8: a Wolf-Rayet-type wind of an unusual WN/WC chemical composition

TL;DR

This paper analyzes the central star of planetary nebula PB 8, revealing a unique hydrogen-deficient, non-carbon-rich composition, and proposes a new spectral classification [WN/WC] for such stars based on spectral analysis and evolutionary considerations.

Contribution

It introduces a new spectral type [WN/WC] for CSPNe based on the unique chemical composition of PB 8, which differs from typical [WC] stars and resembles WN/WC transition types.

Findings

PB 8 has a hydrogen-deficient, non-carbon-rich surface composition.

The star's chemical profile suggests a new [WN/WC] spectral classification.

PB 8's properties challenge existing post-AGB evolutionary models.

Abstract

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB 8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs greatly from the 30-50% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB 8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB 8 as its first member. The central star of PB 8 has a relatively low temperature of T=52kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB 8. We discuss various scenarios that might explain the origin of this unique object.