The post-common-envelope binary central star of the planetary nebula ETHOS 1

TL;DR

This study characterizes the binary central star of planetary nebula ETHOS 1, confirming the alignment of the binary orbit with the nebula's symmetry axis and providing detailed insights into the star's properties and system geometry.

Contribution

It provides the first detailed modeling of ETHOS 1's binary system, confirming the orbital and nebular axis alignment in a post-common-envelope planetary nebula.

Findings

Binary consists of a pre-white-dwarf and an M-type companion.

Binary and nebular axes are aligned, with less than 0.00002% probability of chance alignment.

The companion's properties are consistent with low-mass star models.

Abstract

We present a detailed study of the binary central star of the planetary nebula ETHOS 1 (PN G068.1+11.0). Simultaneous modelling of light and radial velocity curves reveals the binary to comprise a hot and massive pre-white-dwarf with an M-type main-sequence companion. A good fit to the observations was found with a companion that follows expected mass-temperature-radius relationships for low-mass stars, indicating that despite being highly irradiated it is consistent with not being significantly hotter or larger than a typical star of the same mass. Previous modelling indicated that ETHOS 1 may comprise the first case where the orbital plane of the central binary does not lie perpendicular to the nebular symmetry axis, at odds with the expectation that the common envelope is ejected in the orbital plane. We find no evidence for such a discrepancy, deriving a binary inclination in agreement with that of the nebula as determined by spatio-kinematic modelling. This makes ETHOS 1 the ninth post-common-envelope planetary nebula in which the binary orbital and nebular symmetry axes have been shown to be aligned, with as yet no known counter-examples. The probability of finding such a correlation by chance is now less than 0.00002%.