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Register a new userA sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations
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In this paper we present an analysis of the first high-resolution full Stokes vector spectropolarimetric observations of the active M dwarf AD Leo. Based on observations collected in 2016 with the ESPaDOnS instrument at CFHT, we derived the least-squares deconvolved Stokes profiles and detected linear polarisation signatures in spectral lines. At the same time, we discovered that the circular polarisation profiles corresponding to our data set are significantly weaker compared to all archival spectra of AD Leo, which exhibited approximately constant profiles over the timescale of at least 6 years until 2012. Magnetic maps obtained using Zeeman Doppler imaging confirm the sudden change in the surface magnetic field. Although the total magnetic field energy decreased by about 20% between 2012 and 2016, the field component responsible for the observed circular polarisation signatures corresponds to a stronger field occupying a smaller fraction of the stellar surface in the more recent map. These results represent the first evidence that active M dwarfs with dipole-dominated axisymmetric field topologies can undergo a long-term global magnetic variation.
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AU Mic is a young, very active M dwarf star with a debris disk and at least one transiting Neptune-size planet. Here we present detailed analysis of the magnetic field of AU Mic based on previously unpublished high-resolution optical and near-infrared spectropolarimetric observations. We report a systematic detection of circular and linear polarization signatures in the stellar photospheric lines. Tentative Zeeman Doppler imaging modeling of the former data suggests a non-axisymmetric global field with a surface-averaged strength of about 90 G. At the same time, linear polarization observations indicate the presence of a much stronger $approx$2 kG axisymmetric dipolar field, which contributes no circular polarization signal due to the equator-on orientation of AU Mic. A separate Zeeman broadening and intensification analysis allowed us to determine a mean field modulus of 2.3 and 2.1 kG from the Y- and K-band atomic lines respectively. These magnetic field measurements are essential for understanding environmental conditions within the AU Mic planetary system.
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It was recently claimed that the magnetic O-type star HD 54879 exhibits important radial velocity variability indicative of its presence in a spectroscopic binary. More remarkably, it was furthermore reported that the star underwent a short, sudden variation in spectral type and magnetic field. In this Letter we examine new Narval and ESPaDOnS data of this star in addition to the previously-published FORS2 data and conclude that both the reported velocity variations and the sudden spectral and magnetic changes are spurious.
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