Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur

TL;DR

This study investigates the chemical composition, line profile variability, and magnetic field of the unique eclipsing binary AR Aur, revealing a HgMn primary with surface abundance patches and no detectable magnetic field.

Contribution

It provides the most precise chemical abundances for AR Aur's primary and sets stringent upper limits on its magnetic field, challenging previous assumptions about HgMn stars.

Findings

Primary is a HgMn star with surface abundance patches.

No magnetic field detected, upper limit of 100 G.

Secondary is a weak Am star, possibly pre-main sequence.

Abstract

AR Aur is the only eclipsing binary known to contain a HgMn star, making it an ideal case for a detailed study of the HgMn phenomenon. HgMn stars are a poorly understood class of chemically peculiar stars, which have traditionally been thought not to possess significant magnetic fields. However, the recent discovery of line profile variability in some HgMn stars, apparently attributable to surface abundance patches, has brought this belief into question. In this paper we investigate the chemical abundances, line profile variability, and magnetic field of the primary and secondary of the AR Aur system, using a series of high resolution spectropolarimetric observations. We find the primary is indeed a HgMn star, and present the most precise abundances yet determined for this star. We find the secondary is a weak Am star, and is possibly still on the pre-main sequence. Line profile variability was observed in a range of lines in the primary, and is attributed to inhomogeneous surface distributions of some elements. No magnetic field was detected in any observation of either stars, with an upper limit on the longitudinal magnetic field in both stars of 100 G. Modeling of the phase-resolve longitudinal field measurements leads to a 3 sigma upper limit on any dipole surface magnetic field of about 400 G.