Dynamos of young stars & M dwarfs: guiding theory to advanced models with SPIRou observations

Student : Bonnie ROMANO ZAIRE

Advisors : Jean-Francois DONATI, Laurene JOUVE

Start : Octobre 2018

Group : PS2E

Magnetic fields in low-mass stars, hosting a convective envelope, are attributed to the so-called dynamo effect, involving the shearing and twisting of magnetic fields by turbulent convection and differential rotation.  In the Sun, dynamo processes taking place in the outer 30% of the solar radius are thought to generate a cyclic magnetic field causing sunspots, energetic eruptions and an outward flow of ionized particles (wind) which strongly interacts with the Earth magnetosphere.  Similar dynamo mechanisms also exist in stars other than the Sun but with properties that depend on the stellar parameters (e.g., rotation rate, mass, age). Numerical simulations provide an efficient way of exploring the complex interactions between turbulent convective flows and magnetic fields, leading to dynamo action. Whereas several studies based on multidimensional numerical simulations focused on magnetic fields of M dwarfs (e.g., Gastine et al 2012, A&A 546, 19;  Gastine et al 2013, A&A 549, L5; Yadav et al 2015 , ApJ 813, L31), only a few explored theoretical dynamos of PMS stars and more generally the impact of various types of internal structures on the dynamo processes operating in stellar convective zones (Schrinner et al 2012, ApJ 752, 121).  This PhD project is devoted to such theoretical studies, together with the modeling of SPIRou data for a sample of M dwarfs and low-mass PMS stars.

Zeeman-Doppler Imaging(ZDI) is a tomographic technique inspired from medical imaging, allowing to recover the magnetic fields that stars host at their surfaces, from sets of phase-resolved spectropolarimetric observations recorded as the stars rotate.  Results so far have shown that fields of  low-mass PMS and MS stars reflect mostly the internal structure of the host stars;  in particular, in both types of stars, a sharp transition seem to exist in the large-scale magnetic field properties between mainly-convective and mainly-radiative stars (Donati & Landstreet 2009, ARA&A 47, 333; Morin et al 2010, MNRAS 407, 2269; Gregory et al 2012, ApJ 755, 97; Donati et al 2013, MNRAS 436, 881).  Stellar rotation also plays a key role in low-mass MS stars with moderate rotation rates (Folsom et al 2016, MNRAS 457, 580).  With SPIRou coming on line, we can expect a wealth of new observations on a much larger sample of low-mass MS and PMS stars than previously accessible with existing instruments, allowing forthcoming ZDI studies to guide dynamo theories to more successful models applicable to both fully- and partly-convective stars.