Post-common-envelope Wolf-Rayet central stars of planetary nebulae

TL;DR

This paper reports on a radial velocity survey of Wolf-Rayet central stars in planetary nebulae, aiming to understand their binary nature and formation, revealing most have wider orbits than typical post-CE binaries.

Contribution

It provides initial results from a survey searching for binary companions of [WR] CSPNe, highlighting the potential for wider orbit binaries and challenging existing formation scenarios.

Findings

Most post-CE [WR] CSPNe likely have wider orbit companions.

Binary mergers are not the only pathway to [WR] star formation.

The survey suggests a diversity of binary configurations among [WR] CSPNe.

Abstract

Nearly 50 post-common-envelope (post-CE) close binary central stars of planetary nebulae (CSPNe) are now known. Most contain either main sequence or white dwarf (WD) companions that orbit the WD primary in around 0.1-1.0 days. Only PN~G222.8-04.2 and NGC~5189 have post-CE CSPNe with a Wolf-Rayet star primary (denoted [WR]), the low-mass analogues of massive Wolf-Rayet stars. It is not well understood how H-deficient [WR] CSPNe form, even though they are relatively common, appearing in over 100 PNe. The discovery and characterisation of post-CE [WR] CSPNe is essential to determine whether proposed binary formation scenarios are feasible to explain this enigmatic class of stars. The existence of post-CE [WR] binaries alone suggests binary mergers are not necessarily a pathway to form [WR] stars. Here we give an overview of the initial results of a radial velocity monitoring programme of [WR] CSPNe to search for new binaries. We discuss the motivation for the survey and the associated strong selection effects. The mass functions determined for PN~G222.8-04.2 and NGC~5189, together with literature photometric variability data of other [WR] CSPNe, suggest that of the post-CE [WR] CSPNe yet to be found, most will have WD or subdwarf O/B-type companions in wider orbits than typical post-CE CSPNe (several days or months c.f. less than a day).