Discovery of a very weak magnetic field on the Am star Alhena

TL;DR

This paper reports the first detection of a normal magnetic field in the Am star Alhena, using high-quality spectropolarimetric data, which could help understand magnetic phenomena in similar stars.

Contribution

It presents the discovery of a normal magnetic field in Alhena, contrasting with previously observed ultra-weak fields in other Am stars, offering new insights into stellar magnetism.

Findings

Detected a few Gauss magnetic field on Alhena

First normal Zeeman signature in an Am star

Potential clues to magnetic signature peculiarities in Am stars

Abstract

Alhena ($\gamma$ Gem) was observed in the frame of the BRITE (BRIght Target Explorer) spectropolarimetric survey, which gathers high resolution, high signal-to-noise, high sensitivity, spectropolarimetric observations of all stars brighter than V=4 to combine seismic and spectropolarimetric studies of bright stars. We present here the discovery of a very weak magnetic field \textbf{on} the Am star Alhena, thanks to very high signal-to-noise spectropolarimetric data obtained with Narval at T\'elescope Bernard Lyot (TBL). All previously studied Am stars show the presence of ultra-weak (sub-Gauss) fields with Zeeman signatures with an unexpected prominent positive lobe. However, Alhena presents a slightly stronger (but still very weak, only a few Gauss) field with normal Zeeman signatures with a positive and negative lobe, as found in stronger field (hundreds or thousands of Gauss) stars. It is the first detection of a normal magnetic signature in an Am star. Alhena is thus a very interesting object, which might provide the clue to understanding the peculiar shapes of the magnetic signatures of the other Am stars.