Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

TL;DR

This paper introduces a new method using cumulative Stokes V profiles to analyze stellar magnetic fields, providing a complementary approach to Zeeman Doppler imaging, especially useful for validation and cases with limited data.

Contribution

The paper develops a novel interpretation technique of circular polarisation signatures using cumulative Stokes V profiles, enhancing magnetic field topology analysis for rapidly rotating stars.

Findings

Applied to V410 Tau, the method confirms azimuthal magnetic dominance.

Applied to HD 37776, it reveals complex field regions, challenging previous quadrupolar assumptions.

Method provides qualitative magnetic topology insights from archival data.

Abstract

Information about stellar magnetic field topologies is obtained primarily from high-resolution circular polarisation (Stokes $V$) observations. Due to their generally complex morphologies, the stellar Stokes $V$ profiles are usually interpreted with elaborate inversion techniques such as Zeeman Doppler imaging (ZDI). Here we further develop a new method of interpretation of circular polarisation signatures in spectral lines using cumulative Stokes $V$ profiles (anti-derivative of Stokes $V$). This method is complimentary to ZDI and can be applied for validation of the inversion results or when the available observational data are insufficient for an inversion. Based on the rigorous treatment of polarised line formation in the weak-field regime, we show that, for rapidly rotating stars, the cumulative Stokes $V$ profiles contain information about the spatially resolved longitudinal magnetic field density. Rotational modulation of these profiles can be employed for a simple, qualitative characterisation of the stellar magnetic field topologies. We apply this diagnostic method to the archival observations of the weak-line T Tauri star V410 Tau and Bp He-strong star HD 37776. We show that the magnetic field in V410 Tau is dominated by an azimuthal component, in agreement with the ZDI map that we recover from the same data set. For HD 37776 the cumulative Stokes $V$ profile variation indicates the presence of multiple regions of positive and negative field polarity. This behaviour agrees with the ZDI results but contradicts the popular hypothesis that the magnetic field of this star is dominated by an axisymmetric quadrupolar component.