| 1. |
- Bedding, Timothy R., et al.
(författare)
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A multi-site campaign to measure solar-like oscillations in Procyon. II. mode frequencies
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 713:2, s. 935-949
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Tidskriftsartikel (refereegranskat)abstract
- We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with 11 telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges, which we identify with even and odd values of the angular degree (l = 0 and 2, and l = 1 and 3, respectively). We interpret a strong, narrow peak at 446 mu Hz that lies close to the l = 1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of similar to 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29(-0.49)(+0.55) days, which is significantly shorter than the 2-4 days seen in the Sun.
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| 2. |
- Bonanno, Alfio, et al.
(författare)
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Analytic mean-field alpha(2)-dynamo with a force-free corona
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Tidskriftsartikel (refereegranskat)abstract
- Context. Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations, it is possible to find analytical solutions. Interior dynamo models often consider a current-free corona without taking into account the constraints imposed by the presence of currents in the corona. Aims. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an alpha(2) dynamo. We intend to estimate the change in geometry and dynamo excitation numbers with respect to the current-free case. Methods. We analytically solved the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence in a spherical domain surrounded by a linear force-free corona with the mean magnetic field B satisfying del x B = beta B. Results. The dynamo number is a decreasing function of beta. Moreover, if the current is parallel to the field (beta > 0), the dynamo number is smaller than in the force- free case. In contrast, for (beta < 0), the dynamo number is greater than in the force-free case. Conclusions. Currents in the corona need to be taken into account because they affect the condition for excitation of a dynamo.
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| 3. |
- Bonanno, Alfio, et al.
(författare)
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Breakdown of chiral symmetry during saturation of the Tayler instability
- 2012
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 86:1, s. 016313-
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Tidskriftsartikel (refereegranskat)abstract
- We study spontaneous breakdown of chiral symmetry during the nonlinear evolution of the Tayler instability. We start with an initial steady state of zero helicity. Within linearized perturbation calculations, helical perturbations of this initial state have the same growth rate for either sign of helicity. Direct numerical simulations (DNS) of the fully nonlinear equations, however, show that an infinitesimal excess of one sign of helicity in the initial perturbation gives rise to a saturated helical state. We further show that this symmetry breaking can be described by weakly nonlinear finite-amplitude equations with undetermined coefficients which can be deduced solely from symmetry consideration. By fitting solutions of the amplitude equations to data from DNS, we further determine the coefficients of the amplitude equations.
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| 4. |
- Bonanno, Alfio, et al.
(författare)
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On the possibility of helicity oscillations in the saturation of the Tayler instability
- 2017
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - : Wiley. - 0004-6337 .- 1521-3994. ; 338:5, s. 516-526
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Tidskriftsartikel (refereegranskat)abstract
- Recent numerical results of current-driven instabilities at low magnetic Prandtl number and high Hartmann number support the possibility of a saturation state characterized by helicity oscillations. We investigate the underlying mechanism by analyzing this possibility using a higher order Landau-Ginzburg effective Lagrangian for the weakly nonlinear amplitude dynamics, where the magnetic and velocity perturbations are linearly dependent. We find that, if the mirror symmetry between left- and right-handed modes is spontaneously broken, it is impossible to achieve an oscillating helical state. We argue that the result is likely to hold also for adding higher order terms and in the presence of an explicit symmetry breaking. We conclude that an oscillating saturating state for the Tayler instability is unlikely to depend on the interaction of chiral modes.
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