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You are here Research > Research Groups > Cool Stars & Exoplanets > Simulations

Simulations

(page under construction and not complete)

Movies and Images

I use magnetohydrodynamics (MHD) numerical simulations as a tool in my research of stellar winds of cool magnetised stars. In this page, I provide some images and movies that I have created in the past. Feel free to use them in your presentation, I just ask you to properly credit them.

M dwarf winds: effects on exoplanets (Vidotto et al 2011, 2013, 2014)

The magnetic field of M dwarf stars, currently the main targets in searches for terrestrial planets, is very different from the solar one (in topology and intensity).

M Dwarf Star Download Image

Therefore, the magnetised environment surrounding a planet orbiting in the habitable zone (HZ) of M-dwarf stars can differ substantially to the one encountered around the Earth.

Likewise, the extreme magnetic pressure of M dwarfs can compress magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind (see figure on the right).

Credit: NASA/Goddard/Aaron Kaase Download Image

We calculated the minimum degree of planetary magnetospheric compression caused by the intense stellar magnetic fields for a sample of 15 M dwarf stars whose magnetic fields have been observationally reconstructed (Donati+2008, Morin+2008,2010). Hypothetical Earth-like planets with similar terrestrial magnetisation orbiting at the inner (outer) edge of the HZ of these stars would present magnetospheres that extend at most up to 5.0 planetary radii rp (9.7 rp).

At present, it is unknown what would be the minimum degree of planetary magnetospheric compression (and for how long it is allowed to last) before it starts affecting the potential for formation and development of life in a planet.

More on this work can be found here.

For a few of the stars mentioned above, we also performed stellar wind simulations, using the realistic (observed) geometry of their magnetic fields (more here). Because of their complex field topologies, the winds of these stars are not smooth. This implies that their Alfven surfaces (the loci where stellar wind velocity equals the Alfven velocity) have odd shapes (see the case of DT Vir, on the right).

Planetary magnetospheric sizes are roughly set by pressure equilibrium between the planet’s magnetic field and the stellar wind total pressure ptot (i.e., the sum of thermal, magnetic and ram pressures). Because ptot is modulated essentially by the stellar magnetic field for a close-in planet, the more non-axisymmetric topology of the stellar magnetic field produces more asymmetric distributions of ptot. The figure below shows the distribution of the stellar wind total pressure at a spherical surface of radius ~19 stellar radii (~0.05 au). As an exoplanet orbits around its host star, it probes regions of different ptot. Consequently, its magnetospheric size becomes smaller (larger) when the external ptot is larger (smaller).

More on this work can be found here.

τ Boo (Vidotto et al. 2012)

τ Boo is an intriguing planet-host star that is believed to undergo magnetic cycles similar to the Sun, but with a duration that is about one order of magnitude smaller than that of the solar cycle. With the use of observationally derived surface magnetic field maps (see here), we simulated the magnetic stellar wind of τ Boo using BATS-R-US.

The movie above shows the magnetic field lines that τ Boo is expected to have if no wind existed: this magnetic field configuration is in the lowest state of energy (potential field). Download Image
However, because the stellar wind interacts with the magnetic field lines, it stresses the magnetic field, changing its configuration. Such a magnetic field configuration is seen on the animation shown above. Download Image

You can read more about this work here.

Interactions between exoplanets and
the stellar winds of young stars (Vidotto et al. 2009, 2010)

The topology of stellar magnetic field is important not only for the investigation of magnetospheric accretion, but also responsible for shaping the large-scale structure of stellar winds, which are crucial for regulating the rotation evolution of stars. We modelled the stellar winds of young stars by means of 3D MHD simulations, adopting simplified topologies of the stellar magnetic field.

Initial topology Download Image
Final wind solution: the wind is modulated with the same period of the star Download Image


Because stellar winds of young stars are believed to have enhanced mass-loss rates compared to those of cool MS stars, the interaction of winds with newborn exoplanets might affect the early evolution of exoplanetary systems.

A hypothetical hot-Jupiter orbiting in the equatorial plane of a young star would interact with a stellar wind with varying conditions (e.g., velocities) along its orbit (illustrated by a dashed circle). Download Image
As a consequence, the sizes of planetary magnetospheres will vary as a function of the stellar phase of rotation. The hypothetical planet is assumed to be orbiting at 0.05AU and to have an intrinsic dipolar field of 50G. Download Image


This interaction can also give rise to observable signatures which could be used as a way to detect young planets, while simultaneously probing the presence of their still elusive magnetic fields.

Predicted planetary radio emission generated in the interaction between a hot-Jupiter and the stellar wind. The shaded area lies between maximum and minimum powers that can be released in the interaction, according to the rotational phase of the star. The red curve assumes an aligned stellar dipole. For comparison, Jupiter radio emission is ≈1010.5W. Download Image

Torques from the stellar wind can remove orbital angular momentum of the planet, causing planetary migration.

Timescales for planetary migration caused by torques from the stellar wind. Although τw are long (≫ timescale for disc dissipation), complex stellar fields are more efficient in causing planetary migration than aligned fields. Download Image

We showed that asymmetric field topologies can lead to an enhancement of the stellar wind power, resulting not only in an enhancement of angular momentum losses, but also intensifying and rotationally modulating the wind interactions with exoplanets.

More about this work can be found here and here.