Navegando por Autor "Jeffers, S. V."
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Artigo Magnetic field and wind of kappa ceti: toward the planetary habitability of the young sun when life arose on earth(The American Astronomical Society, 2016-03-20) Nascimento Júnior, José Dias do; Vidotto, A. A.; Petit, P.; Folsom, C.; Castro, M.; Marsden, S. C.; Morin, J.; Mello, G. F. Porto de; Meibom, S.; Jeffers, S. V.; Guinan, E.; Ribas, I.We report magnetic field measurements for κ1 Cet, a proxy of the young Sun when life arose on Earth. We carry out an analysis of the magnetic properties determined from spectropolarimetric observations and reconstruct the large-scale surface magnetic field to derive the magnetic environment, stellar winds, and particle flux permeating the interplanetary medium around k1 Cet. Our results show a closer magnetosphere and mass-loss rate of M˙ = 9.7 ´ 10-13 M yr-1 , i.e., a factor of 50 times larger than the current solar wind mass-loss rate, resulting in a larger interaction via space weather disturbances between the stellar wind and a hypothetical young-Earth analogue, potentially affecting the planet’s habitability. Interaction of the wind from the young Sun with the planetary ancient magnetic field may have affected the young Earth and its life conditionsArtigo The connection between stellar activity cycles and magnetic field topology(Royal Astronomical Society, 2016-08-12) See, V.; Jardine, M.; Vidotto, A. A.; Donati, J. F.; Saikia, S. Boro; Bouvier, J.; Fares, R.; Folsom, C. P.; Gregory, S. G.; Hussain, G.; Jeffers, S. V.; Marsden, S. C.; Morin, J.; Moutou, C.; Nascimento Júnior, José Dias do; Petit, P.; Waite, I. A.Zeeman–Doppler imaging (ZDI) has successfully mapped the large-scale magnetic fields of stars over a large range of spectral types, rotation periods and ages. When observed over multiple epochs, some stars show polarity reversals in their global magnetic fields. On the Sun, polarity reversals are a feature of its activity cycle. In this paper, we examine the magnetic properties of stars with existing chromospherically determined cycle periods. Previous authors have suggested that cycle periods lie on multiple branches, either in the cycle period–Rossby number plane or the cycle period–rotation period plane.We find some evidence that stars along the active branch show significant average toroidal fields that exhibit large temporal variations while stars exclusively on the inactive branch remain dominantly poloidal throughout their entire cycle. This lends credence to the idea that different shear layers are in operation along each branch. There is also evidence that the short magnetic polarity switches observed on some stars are characteristic of the inactive branch while the longer chromospherically determined periods are characteristic of the active branch. This may explain the discrepancy between the magnetic and chromospheric cycle periods found on some stars. These results represent a first attempt at linking global magnetic field properties obtained from ZDI and activity cycles