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

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 B z ∼ −8 kG) and in Zeeman splitting of spectral lines (yielding a magnetic modulus of B ∼ 17 kG). We also report for the first time weak Hα 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 Hα emission properties can only be reproduced if its mass is around 1 M ⊙ , 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.

HD144941: The most extreme helium-strong star

“…Using the rigidly rotating magnetosphere model of Townsend & Owocki (2005), they propose this as strong evidence of the star having a mass around 1 M , which they derive from adopting the radius determined from the Gaia EDR3 parallax and the low surface gravity value of Pandey & Lambert (2017) that we ruled out here. Interestingly, Shultz et al (2021) note that their model could be reconciled with the massive star nature of HD144941 in important aspects if v sin i were lower than what they derived (11.2±0.7 km s −1 ), requiring about 7 km s −1 (c.f. our value in Table 2, though unfortunately with a large uncertainty).…”

“…A parallel work on HD144941, Shultz et al (2021) find very similar magnetic properties for the star from high-resolution spectropolarimetry and spectroscopy. Moreover, a detection of weak Hα emission is reported for the first time, suggesting the existence of a centrifugal magnetosphere (Petit et al 2013).…”