No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars

TL;DR

This study used high-resolution spectropolarimetry to search for magnetic fields in Am and HgMn stars, finding no detectable fields and confirming a magnetic dichotomy among A-type stars.

Contribution

It provides the first deep spectropolarimetric survey of Am and HgMn stars, establishing the absence of large-scale magnetic fields in these stars and reinforcing the magnetic dichotomy concept.

Findings

No magnetic fields detected in 15 Am and HgMn stars.

Confirmed at least an order of magnitude difference in magnetic field strength between Ap/Bp and other A-type stars.

Supports the idea that only Ap/Bp stars have detectable large-scale magnetic fields.

Abstract

We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars. Using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 Am stars and 3 HgMn stars. Using Least Squares Deconvolution (LSD), no magnetic field is detected in any of the 15 observed stars. Uncertaintiies as low as 0.3 G (respectively 1 G) have been reached for surface-averaged longitudinal magnetic field measurements for Am (respectively HgMn) stars. Associated with the results obtained previously for Ap/Bp stars, our study confirms the existence of a magnetic dichotomy among A-type stars. Our data demonstrate that there is at least one order of magnitude difference in field strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected stars (Am and HgMn stars). This result confirms that the spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring detectable large-scale surface magnetic fields.