Navegando por Autor "Bouvier, J."

Agora exibindo 1 - 3 de 3

Stellar magnetism: empirical trends with age and rotation
(Royal Astronomic Society, 2014-02-28) Vidotto, A. A; Gregory, S. G.; Jardine, M.; Donati, J. F.; Petit, P.; Morin, J.; Folsom, C. P.; Bouvier, J.; Cameron, A. C.; Hussain, G.; Marsden, S.; Waite, I. A.; Fares, R.; Jeffers, S.; Nascimento Júnior, José Dias do
We investigate how the observed large-scale surface magnetic fields of low-mass stars (∼0.1– 2M ), reconstructed through Zeeman–Doppler imaging, vary with age t, rotation and Xray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting premain sequence to main-sequence objects (1Myr t 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field is related to age as t−0.655 ± 0.045. This relation has a similar dependence to that identified by Skumanich, used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method (‘magnetochronology’). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star–disc interaction, rather than being driven by the dynamo
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
The energy budget of stellar magnetic fields
(Royal Astronomic Society, 2015-06-02) See, V.; Jardine, M.; Vidotto, A. A.; Donati, J. F.; Folsom, C. P.; Saikia, S. Boro; Bouvier, J.; Fares, R.; Gregory, S. G.; Hussain, G.; Jeffers, S.V.; Marsden, S. C.; Morin, J.; Moutou, C.; Nascimento Júnior, José Dias do; Petit, P.; Rosén, L.; Waite, A.
Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5M . We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5M having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotationactivityrelation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies

SIGAA

Navegando por Autor "Bouvier, J."