Magnetic field topology of the unique chemically peculiar star CU Virginis

The nearby M dwarf binary GJ65 AB, also known as BL Cet and UV Cet, is a unique benchmark for investigation of dynamo-driven activity of low-mass stars. Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as studies of flares, photometric variability, X-ray and radio emission. Here we present a direct analysis of large-scale and local surface magnetic fields in both components. Interpreting high-resolution circular polarisation spectra (sensitive to a large-scale field geometry) we uncovered a remarkable difference of the global stellar field topologies. Despite nearly identical masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like global field with an average strength of 1.3 kG while the primary has a much weaker, more complex and non-axisymmetric 0.3 kG field. On the other hand, an analysis of the differential Zeeman intensification (sensitive to the total magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and 6.7 kG for GJ65 A and B, respectively, although there is an evidence that the field strength distribution in GJ65 B is shifted towards a higher field strength compared to GJ65 A. Based on these complementary magnetic field diagnostic results we suggest that the dissimilar radio and X-ray variability of GJ65 A and B is linked to their different global magnetic field topologies. However, this difference appears to be restricted to the upper atmospheric layers but does not encompass the bulk of the stars and has no influence on the fundamental stellar properties.

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“…Our ZDI analysis of GJ65 was carried out with the code described by Kochukhov et al (2014) and Rosén et al (2016).…”

Section: Global Magnetic Field Topologymentioning

confidence: 99%

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Kochukhov

Lavail

2017

ApJL

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“…The latter is feasible for early-type stars (see e.g. Kochukhov et al 2014), but much more demanding for late-type stars when imaging both the surface magnetic field and temperature (or brightness). Furthermore, none of the stars in this study showed any strong bumps in the Stokes I, reducing the error of the approximation.…”

Section: Zeeman-doppler Imagingmentioning

confidence: 99%

“…For this purpose we applied the ZeemanDoppler imaging (ZDI) technique using Stokes IV spectropolarimetry with a code developed by O. Kochukhov (Kochukhov et al 2014).…”

Section: Introductionmentioning

confidence: 99%

Zeeman-Doppler imaging of active young solar-type stars

Hackman

Lehtinen

Rosén

et al. 2016

A&A

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Context. By studying young magnetically active late-type stars, i.e. analogues to the young Sun, we can draw conclusions on the evolution of the solar dynamo. Aims. We determine the topology of the surface magnetic field and study the relation between the magnetic field and cool photospheric spots in three young late-type stars. Methods. High-resolution spectropolarimetry of the targets was obtained with the HARPSpol instrument mounted at the ESO 3.6 m telescope. The signal-to-noise ratios of the Stokes IV measurements were boosted by combining the signal from a large number of spectroscopic absorption lines through the least squares deconvolution technique. Surface brightness and magnetic field maps were calculated using the Zeeman-Doppler imaging technique.Results. All three targets show clear signs of magnetic fields and cool spots. Only one of the targets, V1358 Ori, shows evidence of the dominance of non-axisymmetric modes. In two of the targets, the poloidal field is significantly stronger than the toroidal one, indicative of an α 2 -type dynamo, in which convective turbulence effects dominate over the weak differential rotation. In two of the cases there is a slight anti-correlation between the cool spots and the strength of the radial magnetic field. However, even in these cases the correlation is much weaker than in the case of sunspots. Conclusions. The weak correlation between the measured radial magnetic field and cool spots may indicate a more complex magnetic field structure in the spots or spot groups involving mixed magnetic polarities. Comparison with a previously published magnetic field map shows that on one of the stars, HD 29615, the underlying magnetic field changed its polarity between 2009 and 2013.

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“…P lm ≡ P lm (cos θ) is the associated Legendre polynomial of degree l and order m. α lm , β lm , γ lm are the coefficients that provide the best fit to the spectropolarimetric data and are such that Equations (1) to (3) Kochukhov et al 2014) define Y lm (θ, ϕ) = c lm P lm (cos θ)e imϕ , where c lm is a normalisation constant…”

Section: The Mathematical Description Used By Zdimentioning

confidence: 99%

The magnetic field vector of the Sun-as-a-star

Vidotto

2016

Mon. Not. R. Astron. Soc.

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Direct comparison between stellar and solar magnetic maps are hampered by their dramatic differences in resolution. Here, we present a method to filter out the smallscale component of vector fields, in such a way that comparison between solar and stellar (large-scale) magnetic field vector maps can be directly made. Our approach extends the technique widely used to decompose the radial component of the solar magnetic field to the azimuthal and meridional components as well. For that, we self-consistently decompose the three-components of the vector field using spherical harmonics of different l degrees. By retaining the low l degrees in the decomposition, we are able to calculate the large-scale magnetic field vector. Using a synoptic map of the solar vector field at Carrington Rotation CR2109, we derive the solar magnetic field vector at a similar resolution level as that from stellar magnetic images. We demonstrate that the large-scale field of the Sun is not purely radial, as often assumed -at CR2109, 83% of the magnetic energy is in the radial component, while 10% is in the azimuthal and 7% is in the meridional components. By separating the vector field into poloidal and toroidal components, we show that the solar magnetic energy at CR2109 is mainly (> 90%) poloidal. Our description is entirely consistent with the description adopted in several stellar studies. Our formalism can also be used to confront synoptic maps synthesised in numerical simulations of dynamo and magnetic flux transport studies to those derived from stellar observations.

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Magnetic field topology of the unique chemically peculiar star CU Virginis

Cited by 105 publications

References 49 publications

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Zeeman-Doppler imaging of active young solar-type stars

The magnetic field vector of the Sun-as-a-star

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