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Post Info TOPIC: EK Eridani
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RE: EK Eridani
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Title: A dominant magnetic dipole for the evolved Ap star candidate EK Eridani
Authors: M. Auriere, R. Konstantinova-Antova, P. Petit, T. Roudier, J.-F. Donati, C. Charbonnel, B. Dintrans, F. Lignieres, G.A. Wade, A. Morgenthaler, S. Tsvetkova

EK Eri is one of the most slowly rotating active giants known, and has been proposed to be the descendant of a strongly magnetic Ap star. We have performed a spectropolarimetric study of EK Eri over 4 photometric periods with the aim of inferring the topology of its magnetic field. We used the NARVAL spectropolarimeter at the Bernard Lyot telescope at the Pic du Midi Observatory, along with the least-squares deconvolution method, to extract high signal-to-noise ratio Stokes V profiles from a timeseries of 28 polarisation spectra. We have derived the surface-averaged longitudinal magnetic field Bl. We fit the Stokes V profiles with a model of the large-scale magnetic field and obtained Zeeman Doppler images of the surface magnetic strength and geometry. Bl variations of up to about 80 G are observed without any reversal of its sign, and which are in phase with photometric ephemeris. The activity indicators are shown to vary smoothly on a timescale compatible with the rotational period inferred from photometry (308.8 d.), however large deviations can occur from one rotation to another. The surface magnetic field variations of EK Eri appear to be dominated by a strong magnetic spot (of negative polarity) which is phased with the dark (cool) photometric spot. Our modelling shows that the large-scale magnetic field of EK Eri is strongly poloidal. For a rotational axis inclination of i = 60{\deg}, we obtain a model that is almost purely dipolar. In the dipolar model, the strong magnetic/photometric spot corresponds to the negative pole of the dipole, which could be the remnant of that of an Ap star progenitor of EK Eri. Our observations and modelling conceptually support this hypothesis, suggesting an explanation of the outstanding magnetic properties of EK Eri as the result of interaction between deep convection and the remnant of an Ap star magnetic dipole.

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Blobrana


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Posts: 131433
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Title: EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
Authors: M. Aurière (1), R. Konstantinova-Antova (2), P. Petit (1), C. Charbonnel (3,1), B. Dintrans (1), F. Lignières (1), T. Roudier (1), E. Alecian (4), J.F. Donati (1), J.D. Landstreet (5), G.A. Wade (4) ((1) Laboratoire d'Astrophysique de Toulouse- Tarbes, (2) Institute of Astronomy of Bulgarian Academy of Sciences, (3) University of Geneva, (4) Royal Military College of Canada, (5) The University of Western Ontario)

We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the CaII H and K lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modelling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.

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