Detection of an extremely strong magnetic field in the double-degenerate binary merger product HD 144941

TL;DR

This paper reports the first detection of an extremely strong magnetic field in the white dwarf merger product HD 144941, providing evidence that stellar mergers can generate fossil magnetic fields, with implications for stellar evolution.

Contribution

It presents the first high-resolution spectropolarimetric detection of a strong magnetic field in HD 144941, a likely white dwarf merger product, and discusses its unique properties and implications.

Findings

Detected a line-of-sight magnetic field of ~8 kG.

Observed Zeeman splitting indicating a magnetic modulus of ~17 kG.

Identified weak Hα emission consistent with a Centrifugal Magnetosphere.

Abstract

HD 144941 is an extreme He (EHe) star, a rare class of subdwarf OB star formed from the merger of two white dwarf (WD) stars. Uniquely amongst EHe stars, its light curve has been reported to be modulated entirely by rotation, suggesting the presence of a magnetic field. Here we report the first high-resolution spectropolarimetric observations of HD 144941, in which we detect an extremely strong magnetic field both in circular polarization (with a line-of-sight magnetic field averaged over the stellar disk $\langle B_z \rangle \sim -8$ kG) and in Zeeman splitting of spectral lines (yielding a magnetic modulus of $\langle B \rangle \sim 17$ kG). We also report for the first time weak H$\alpha$ emission consistent with an origin an a Centrifugal Magnetosphere (CM). HD 144941's atmospheric parameters could be consistent with either a subdwarf or a main sequence (MS) star, and its surface abundances are neither similar to other EHe stars nor to He-strong magnetic stars. However, its H$\alpha$ emission properties can only be reproduced if its mass is around 1 M$_\odot$, indicating that it must be a post-MS object. Since there is no indication of binarity, it is unlikely to be a stripped star, and was therefore most likely produced in a WD merger. HD 144941 is therefore further evidence that mergers are a viable pathway for the generation of fossil magnetic fields.