Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

Abstract

Information about stellar magnetic field topologies is obtained primarily from high-resolution circular polarisation (Stokes V) observations. Because of 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 for interpreting 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 to validate 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.

Confirmation of xi^1 CMa's ultra-slow rotation: magnetic polarity reversal and a dramatic change in magnetospheric UV emission lines

“…This stands in contrast with other magnetic B-type stars such as 𝜏 Sco (HD 149438, B0.2 V) and HD 37776 (B2 V), for which the higher-order moments of the field are dominant (see e.g. Thompson & Landstreet 1985;Donati et al 2006;Donati & Landstreet 2009;Kochukhov et al 2011;Kochukhov 2015;Kochukhov & Wade 2016).…”

Magnetic fields of intermediate-mass T Tauri stars

“…stars from Alecian et al (2013) -using similar symbols as the IMTTS but with faded colors; as well as other IMTTS and LMTTS -using blue symbols (Donati et al 2007(Donati et al , 2008(Donati et al , 2010b(Donati et al , 2011b(Donati et al ,c, 2013(Donati et al , 2015Hussain et al 2009;Kochukhov 2015;Hill et al 2017;Lavail et al 2017;Yu & Donati 2017). In the latter case, all stars are magnetic, and the light-blue (blue) color indicates a lack (or not) of information on the B values.…”

Magnetic field topology and chemical abundance distributions of the young, rapidly rotating, chemically peculiar star HR 5624

“…The cumulative Stokes V (CSV) profile diagnostic technique was suggested by Gayley & Owocki (2014) and further developed by Kochukhov (2015). In this method the observed circular polarisation profiles of individual spectral lines or multi-line LSD profiles are converted to a spatially resolved measure of the longitudinal magnetic field density B z , which essentially represents a Doppler-resolved equivalent of the widely used mean line-ofsight (longitudinal) magnetic field B z .…”

“…The observed CSV profiles were obtained by a weighted average of the blue-to-red and red-to-blue integrals over the Stokes V profiles, normalised by the corresponding residual Stokes I profiles (see Kochukhov 2015 for details). The resulting dynamic CSV spectra are presented in the panels a) and b) of Fig.…”