Large-scale alpha^2-dynamo in low-mass stars and brown dwarfs

By G. Chabrier, M. Kueker. 2005

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We develop a model based on three dimensional mean-field magnetohydrodynamics computations for the generation of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous planets. The dynamo process is of α 2 type and thus differs from the shell-dynamo at work in more massive stars. The α 2 dynamo is found to become supercritical for Coriolis numbers Ω* ≥ 1, i.e. Rossby numbers Ro ≤ 10. It generates a large-scale, non-axisymmetric, steady field that is symmetric with respect to the equatorial plane. Saturation of the α 2 -generated field at the equipartition field strength yields strengths of several kiloGauss, in agreement with observations of active M dwarfs, and provides a qualitative explanation for the observed activity saturation in late M stars. For brown dwarfs with a conductive core, as occurs at the center of the most massive and oldest of these objects, we have also studied an α 2 Ω dynamo, i.e. the effect of differential rotation. In this case the field is predominantly toroidal, axisymmetric, and oscillatory, like the solar magnetic field. The topology of the field in the fully convective objects exhibits a high order multipole character that differs from the aligned dipole field generated by the an dynamo. The strong reduction of the dipolar component due to the non-axisymmetry of the field should considerably reduce the Alfven radius and thus the efficiency of magnetic braking, providing an appealing explanation for the decreasing angular momentum loss rate observed in low-mass stars and brown dwarfs. This may have also implications for cataclysmic variables below the period gap. In spite of this large-scale field, the decreasing conductivity in the dominantly neutral atmosphere of these objects may prevent the current generation necessary to support a chromosphere and thus activity. An observational signature of the present model would be (i) asymmetry of the chromospheric activity, contrary to the spatially uniform activity expected from small-scale turbulent dynamo and (ii) the absence of cycles in uniformly rotating (fully convective) low-mass objects.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b9990e2559e369b2c599e81d81a31e781772fe48

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Citations in the corpus

Simulations of Dynamo Action in Fully Convective Stars (2007, methodology): “Chabrier & Kuker (2006) performed analogous mean-field modeling for fully-convective low-mass stars, typically also assuming no differential rotation, and found large-scale nonaxisymmetric fields were generated by an α2 dynamo; these fields were steady and symmetric with respect to 3 the equatorial…”
Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, background): “Küker & Rüdiger (1999) and Chabrier & Küker (2006) performed mean-field modeling of dynamo action in fully convective stars and found purely non-axisymmetric α2 solutions, indicating that these objects can sustain large-scale magnetic fields.”
Observations of Cool-Star Magnetic Fields (2012, background): “Johns-Krull (2007) also provide evidence that fields in these young stars may be of fossil origin and not generated by a dynamo, although Chabrier and Küker (2006) estimate survival times for fossil fields in fully convective stars well below 1000 yr.”
The Volume-limited A-Star (VAST) survey – I. Companions and the unexpected X-ray detection of B6–A7 stars (2011, background): “…M3 (∼0.35 M ), the stars become fully convective (Chabrier & Baraffe 1997) and the high level of magnetic activity observed (e.g. Randich 2000) may be due to an α2-type dynamo-generation mechanism (Chabrier & Küker 2006), in which turbulent motions are able to generate large-scale magnetic fields.”
Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs (2008, background): “10 kG (Chabrier & Küker 2006; Browning 2007) and field divergence in the limiting case of a global dipole with B(R) ∝ R−3, we can determine that a source with local field strength ∼ 3 kG is confined to a height h ≤ 0.5 R above the stellar surface.”
WEATHER ON OTHER WORLDS. II. SURVEY RESULTS: SPOTS ARE UBIQUITOUS ON L AND T DWARFS (2014, background): “However, in general the neutral atmospheres of L and T dwarfs are considered too electrically resistive to support magnetic starspots (Mohanty et al. 2002; Gelino et al. 2002; Chabrier & Küker 2006).”
The Magnetic Fields of Classical T Tauri Stars (2007, background): “Matt & Pudritz (2005b) suggest that an accretion-powered stellar wind is responsible for spinning down CTTSs.”
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph (2007, background): “Given that fossil magnetic fields are presumably dissipated very quickly in largely convective stars (e.g., Chabrier & Küker 2006), the most natural assumption is that large-scale fields of cTTSs are generated through dynamo action.”