Differential Rotation and Magnetism across the HR Diagram

Differential rotation in very-low-mass stars: a clue to dynamo bistability?

Not scheduled

132:028 (Nordita)

Talk

Speaker

Julien Morin (Institut für Astrophysik, Georg-August-Universität Göttingen)

Description

M dwarfs are of prime interest for stellar dynamo theories. They indeed span a wide range of parameters, in terms of relative depth of the convection zone (the lowest-mass M dwarfs being fully convective) and rotation. The number of magnetic field measurements on M dwarfs has been rapidly growing in the past few years, trends are emerging and now need to be understood in the framework of dynamo theory. In this talk I will detail the analogy anelastic dynamo simulations by Gastine, Duarte & Wicht (2012) and observations of M-dwarf magnetism, focusing on field geometries derived from spectropolarimetric observations carried out with the instruments CFHT/ESPaDOnS and TBL/NARVAL (Morin et al. 2010 and references therein). In geodynamo models, the relative importance of inertia in the force balance is known to have a strong impact on the magnetic field geometry. This can be quantified by the so-called "local Rossby number", which has been found to be a rather universal quantity that allows to separate dipolar and multipolar dynamo models. I will discuss its relevance in setting the field geometry of M dwarfs and the transition towards a bistable regime (Gastine et al. 2013). In the dynamo models of Gastine, Duarte & Wicht (2012) a strong correlation exists between the type of magnetic field generated and differential rotation, I will detail an ongoing observing campaign aimed at finding evidence of this behavior among very-low-mass stars.