Measurements of stellar magnetic fields with the autocorrelation of spectra

TL;DR

This paper introduces a new autocorrelation-based method for measuring stellar magnetic fields from spectra, capable of detecting very small magnetic effects even at low spectral resolutions, thus expanding observational capabilities.

Contribution

The paper presents a novel autocorrelation technique for stellar magnetic field measurement that works effectively at low spectral resolutions and in faint or rapidly rotating stars, surpassing traditional Zeeman splitting methods.

Findings

Autocorrelation curves agree with Zeeman splitting measurements for known magnetic stars.

The method remains effective at lower spectral resolutions where Zeeman splitting is undetectable.

It enables magnetic field measurements in faint, rapidly rotating, or low-resolution spectra.

Abstract

We present a novel technique that uses the autocorrelation of the spectrum of a star to measure the line broadening caused by the modulus of its average surface magnetic field. The advantage of the autocorrelation comes from the fact that it can detect very small spectral line broadening effects because it averages over many spectral lines and therefore gives an average with a very high signal to noise ratio. We validate the technique with the spectra of known magnetic stars and obtain autocorrelation curves that are in full agreement with published magnetic curves obtained with Zeeman splitting. The autocorrelation also gives less noisy curves so that it can be used to obtain very accurate curves. We degrade the resolution of spectra of these magnetic stars to lower spectral resolutions where the Zeeman splitting is undetectable. At these resolutions, the autocorrelation still gives good quality curves, thereby showing that it can be used to measure magnetic fields in spectra where the Zeeman splitting is significantly smaller than the width of the spectral line. This would therefore allow observing magnetic fields in very faint Ap stars with low-resolution spectrographs, thereby greatly increasing the number of known magnetic stars. It also demonstrates that the autocorrelation can measure magnetic fields in rapidly rotating stars as well as weak magnetic fields that give a Zeeman splitting smaller than the intrinsic width of the spectral lines. Finally, it shows that the autocorrelation can be used to find unknown magnetic stars in low resolution spectroscopic surveys.