Explaining the unusual Stokes V signatures of ultra-weak magnetic A stars
Abstract
Recently, extremely weak magnetic fields have been found in a small number of A-type stars. These magnetic fields are on the order of a few gauss or less, which is two to three orders of magnitude weaker than those found in magnetic Ap and Bp stars. The Stokes V (circularly polarized) line profiles of three out of five of these stars known (Sirius, beta UMa, and theta Leo) are highly unusual, consisting of a single positive lobe spanning most of the width of the line. While there is evidence that this signal is due to a magnetic field, the shape of the line profile cannot be explained by Zeeman splitting in a static atmosphere. Using synthetic spectra, we investigated introducing radial gradients in velocity and magnetic field strength, which can produce qualitatively similar model spectra. With some simple assumptions of geometry we produced disk integrated spectra that can match the observations. We propose this as a possible explanation for the unusual Stokes V signatures in these ultra-weak magnetic A stars.