The search for magnetic fields in mercury-manganese stars

TL;DR

This study used high-sensitivity spectropolarimetry to search for magnetic fields in mercury-manganese stars and found no evidence of large-scale magnetic fields, suggesting alternative mechanisms for surface inhomogeneities.

Contribution

It provides the most sensitive search to date for magnetic fields in HgMn stars, establishing stringent upper limits and confirming the absence of large-scale magnetic fields.

Findings

No magnetic fields detected above 3 sigma in 47 stars.

Upper limits of 2-10 Gauss for simple global fields.

Chemical spots likely caused by non-magnetic mechanisms.

Abstract

We performed a highly sensitive search for magnetic fields on a large set of HgMn stars. With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines resulting in significantly greater sensitivity to the presence of magnetic fields, including very weak fields. For the 47 individual objects studied, including 6 components of SB2 systems, we do not detect any magnetic fields at greater than the 3 sigma level. The lack of detection in the circular polarization profiles indicates that if strong fields are present on these stars, they must have complex surface topologies. For simple global fields, our detection limits imply upper limits to the fields present of 2-10 Gauss in the best cases. We conclude that HgMn stars lack large-scale magnetic fields, typical for spotted magnetic Ap stars, sufficient to form and sustain the chemical spots observed on HgMn stars. Our study confirms that in addition to magnetically altered atomic diffusion, there exists another differentiation mechanism operating in the atmospheres of late-B main sequence stars which can compositional inhomogeneities on their surfaces.