On the post-common-envelope central star of the planetary nebula NGC 2346

TL;DR

This study confirms the 16-day orbital period of the central star in planetary nebula NGC 2346, revises its surface gravity, and identifies the secondary as the most massive known post-common-envelope companion, providing new insights into binary evolution.

Contribution

The paper conclusively determines the orbital period as 16 days and revises the surface gravity, revealing the secondary as the most massive post-common-envelope star known.

Findings

Confirmed 16-day orbital period using high-resolution spectroscopy.

Revised surface gravity indicating a sub-giant status.

Identified the secondary as the most massive post-common-envelope star.

Abstract

The common-envelope phase is one of the most poorly understood phases of (binary) stellar evolution, in spite of its importance in the formation of a wide-range of astrophysical phenomena ranging from cataclysmic variables to cosmologically important supernova type Ia, and even recently discovered gravitational wave producing black hole mergers. The central star of the planetary nebula NGC~2346 has long been held as one of the longest period post-common-envelope systems known with a published period of approximately 16 days, however the data presented were also consistent with much shorter periods of around 1 day (a more typical period among the known sample of post-common-envelope binary central stars). Here, using the modern high-stability, high-resolution spectrograph HERMES, we conclusively show the period to, indeed, be 16 days while also revising the surface gravity to a value typical of a sub-giant (rather than main-sequence) resulting in an intrinsic luminosity consistent with the recently published GAIA parallax distance. Intriguingly, the implied mass for the secondary ($\gtrsim$3.5 M$_\odot$) makes it, to our knowledge, the most massive post-common-envelope secondary known, whilst also indicating that the primary may be a post-RGB star.