Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

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.Comment: 11 pages, 9 figures; accepted for publication in A&

Remarkable non-dipolar magnetic field of the Bp star HD 137509

The southern magnetic Bp star HD 137509 exhibits complex rotational modulation of the longitudinal field and other magnetic observables. Interpretation of this magnetic variability in the framework of the low-order multipolar field models suggests a very strong quadrupolar component to dominate the surface field topology of HD 137509. I have examined the high-quality VLT/UVES spectra of HD 137509 and discovered resolved Zeeman split components in some of the spectral lines. The inferred mean surface field modulus, =29 kG, agrees with the multipolar model predictions. This confirms the presence of an extremely strong non-dipolar magnetic field in HD 137509 and establishes this star as the object with the second-largest field among magnetic chemically peculiar stars.Comment: accepted by A&A; 6 pages, 4 figure

Magnetic Doppler imaging of alpha^2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields

Strong organized magnetic fields have been studied in the upper main sequence chemically peculiar stars for more than half a century. However, only recently have observational methods and numerical techniques become sufficiently mature to allow us to record and interpret high-resolution four Stokes parameter spectra, leading to the first assumption-free magnetic field models of these stars. Here we present a detailed magnetic Doppler imaging analysis of the spectropolarimetric observations of the prototypical magnetic Ap star alpha^2 CVn. The surface abundance distributions of Fe and Cr and a full vector map of the stellar magnetic field are reconstructed in a self-consistent inversion using our state-of-the-art magnetic Doppler imaging code Invers10. We succeeded in reproducing most of the details of the available spectropolarimetric observations of alpha^2 CVn with a magnetic map which combines a global dipolar-like field topology with localized spots of higher field intensity. We demonstrate that these small-scale magnetic structures are inevitably required to fit the linear polarization spectra; however, their presence cannot be inferred from the Stokes I and V observations alone. Our magnetic Doppler imaging analysis of alpha^2 CVn and previous results for 53 Cam support the view that the upper main sequence stars can harbour fairly complex surface magnetic fields which resemble oblique dipoles only at the largest spatial scales. Spectra in all four Stokes parameters are absolutely essential to unveil and meaningfully characterize this field complexity in Ap stars. We therefore suggest that understanding magnetism of stars in other parts of the H-R diagram is similarly incomplete without investigation of their linear polarization spectra.Comment: 16 pages, 12 figures; Accepted for publication by Astronomy & Astrophysic

Observations of magnetic fields in hot stars

The presence of magnetic fields at the surfaces of many massive stars has been suspected for decades, to explain the observed properties and activity of OB stars. However, very few genuine high-mass stars had been identified as magnetic before the advent of a new generation of powerful spectropolarimeters that has resulted in a rapid burst of precise information about the magnetic properties of massive stars. During this talk, I will briefly review modern methods used to diagnose magnetic fields of higher-mass stars, and summarize our current understanding of the magnetic properties of OB stars.Comment: Proceedings of IAUS272: Active OB stars, review talk (11 pages

Magnetic field topology and chemical spot distributions of the Ap star HD119419

Analysis of high-resolution spectropolarimetric observations of early-type magnetic stars is currently the most advanced method of obtaining detailed information on their surface magnetic field topologies and horizontal spot distributions. In this study we analyse a new set of high-quality four Stokes parameter observations of the magnetic Ap star HD119419 - a member of the 14 Myr old Lower Cen-Cru association - for the purpose of studying the surface field topology and mapping the chemical abundance spots. We made use of the circular and linear polarisation data collected for HD119419 with the HARPSpol instrument at the ESO 3.6-m telescope. These observations were analysed with a multi-line magnetic diagnostic technique and modelled in detail with a Magnetic Doppler imaging code. We present a new set of mean longitudinal magnetic field measurements and derive a revised stellar rotational period. We also redetermine the basic stellar atmospheric parameters. Our four Stokes parameter magnetic inversions reveal a moderately complex surface field topology with a mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal dipolar component dominates the magnetic energy spectrum of the surface field in HD119419. However, significant contributions of the higher-order spherical harmonic components are also present. We show that the dipole plus quadrupole part of the reconstructed field geometry is incapable of reproducing the observed amplitudes and shapes of the Stokes Q and U profiles. The chemical abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with the magnetic field geometry, are characterised by large abundance gradients and a lack of clear correlation with the magnetic field structure. This full Stokes vector analysis of HD119419 extends the modern hot-star magnetic mapping investigations to an open cluster Ap star with a well-determined age.Comment: 15 pages, 11 figures; accepted for publication in A&

Pulsational line profile variation of the roAp star HR 3831

We report the first comprehensive investigation of the line profile variation caused by non-radial pulsation in a magnetic oscillating chemically peculiar star. Spectrum variation of the well-known roAp star HR 3831 is detected using very high-resolution high signal-to-noise spectroscopic time-series observations and are followed through the whole rotation cycle of the star. We confirm outstanding diversity of pulsational behaviour of different lines in the HR 3831 spectrum and attribute this phenomenon to an interplay between extreme vertical chemical inhomogeneity of the HR 3831 atmosphere and a running pulsation wave, propagating towards the upper photospheric layers with increasing amplitude. Rapid profile variation of the NdIII 6145 A line is characterized by measuring changes of its equivalent width and the first three moments. We demonstrate that rotational modulation of the radial velocity oscillations cannot be fully explained by an oblique axisymmetric dipole (ell=1, m=0) mode, implied by the classical oblique pulsator model of roAp stars. Pulsational variation of the higher moments reveal substantial contribution of the high-order (ell=3) spherical harmonics which appear due to distortion of pulsations in HR 3831 by the global magnetic field. We interpret observations with a novel numerical model of the pulsational variation and rotational modulation of the line profile moments in roAp stars. The comparison between observed and computed amplitudes and phases of the radial velocity and line width variation is used to establish parameters of the oblique pulsator model of HR 3831. Furthermore, definite detection of pulsational variation in lines of light and iron-peak elements enables the first 3-D mapping of pulsations in non-radially oscillating star.Comment: 21 pages, 11 figures; accepted by A&