Three-Dimensional Numerical Simulations of Magnetized Winds of Solar-Like Stars

Abstract

By means of self-consistent three-dimensional (3D) magnetohydrodynamics (MHD) numerical simulations, we analyze magnetized solar-like stellar winds and their dependence on the plasma-β parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal 3D MHD equations. We adopt in our simulations a heating parameter described by γ, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from 1 to 20 G. We show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. For the maximum magnetic intensity simulated of 20 G and solar coronal base density, the wind velocity reaches values of ∼ 1000 km s −1 at r ∼ 20 r 0 and a maximum temperature of ∼ 6 × 10 6 K at r ∼ 6 r 0 . The increase of the field intensity generates a larger "dead zone" in the wind, i. e., the closed loops that inhibit matter to escape from latitudes lower than ∼ 45 o extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B 0 = 20 G, the system recover back to slower and cooler winds. For a fixed γ, we show that the key parameter in determining the wind velocity profile is the β-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-β parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We analyze the influence of γ in our results and we show that it is also important in determining the wind structure.

Nonradial and nonpolytropic astrophysical outflows

“…The lack of accretion disk prevents the formation of disk winds and the possibility of higher mass loss rates. The lower jet width may be caused either by a lower stellar magnetic activity (see Vidotto et al 2009Vidotto et al , 2010 or by a lack of magnetic connection into the disk. The magnetic braking time in this case would be on the order of 10 million years, which is indeed the time a T Tauri star is expected to spend in the weak-line stage.…”

Time-scales of close-in exoplanet radio emission variability

“…Petit et al 2008) and winds (e.g. Vidotto et al 2009) show that this is not the case. Indeed, the dynamic nature of the solar wind is one of the factors, among others, known to affect the intensity of radio activity at Jupiter and Saturn (Zarka 1998;Gurnett et al 2002;Crary et al 2005).…”

The effects of stellar winds on the magnetospheres and potential habitability of exoplanets

“…The most up to date models are fully three dimensional and self-consistent (Vidotto et al 2009(Vidotto et al , 2012 and use more physically motivated arguments to determine mass loss rates (Holzwarth & Jardine 2007;Cohen et al 2007;Cranmer & Saar 2011). However many assumptions are used and scaling relations are still present in some models.…”