Magnetic instabilities in stellar interiors

Transcription

1 Magnetic instabilities in stellar interiors Laurène Jouve Institut de Recherche en Astrophysique et Planétologie Toulouse Journées 26/27 Novembre 2015

2 Stellar magnetism Our Sun

3 Stellar magnetism Its role in stellar evolution Disks around young stars where planets are formed Mass loss in massive stars

4 The tools: Observations: ESPADONS/NARVAL and next SPIROU Magnetic fields with wide range of intensity and complexity V374 Pegasi dwarf star (low-mass and cool) 30 light-years away from the Sun Morin, Donati et al. ( )

5 The tools: Models and numerical simulations 2D VS 3D:! 2D approach: " simplified version of the equations " faster/more efficient! 3D approach: " more complete version of the equations " more difficult to set up Local VS Global:! Local approach: " more detailed study of a specific physical process! Global approach: " geometrical effects are sometimes needed Bessolaz & Brun, 2011 Käpylä et al. 2009, 2010

6 Origin of magnetic fields The dynamo mechanism Dynamo mechanism: process through which motions of a conducting fluid can permanently regenerate and maintain a magnetic field against its ohmic dissipation It consists of the regeneration of both poloidal and toroidal fields Poloidal Toroidal Toroidal Poloidal Ω effect α effect Babcock-Leighton effect 6

7 Magnetic flux emergence The example of our Sun The dynamo mechanism is at the origin of the solar magetism Goal: to know in detail the process of magnetic flux emergence from the solar interior to the surface

8 Magnetic flux emergence The example of our Sun Creation of magnetic «fluxtubes» through buoyancy instabilities Evolution in the convective zone Link with the solar atmosphere and Sun/ Earth connections? Local 3D simulations Global 3D simulations Simplified 2D simulations

9 Creation of magnetic «fluxtubes» local simulations Local simulations of magnetic buoyancy instabilities (performed with Cambridge Code on Resolution: Number of cpus: 32 to 64 Computing time for 1 run: 4000h monoproc

10 Creation of magnetic «fluxtubes» local simulations Under certain circumstances (when the magnetic field is initially helical), large-scale coherent structures are formed. They resemble twisted magnetic fluxtubes Favier et al. (2012), Jouve et al No helicity: Destruction of magnetic layer Initial non-zero helicity: Creation of large-scale twisted magnetic fluxtubes

11 Evolution of magnetic «fluxtubes» global simulations We now solve the full set of MHD equations in a spherical shell. A hydrodynamical model is first computed: Radial velocity Rotation rate A magnetic flux tube is then introduced at the base of the domain. It has a density deficit and will thus rise: ρ in ρ out, % = exp ΔS (%. ' *' 1 B2 - & C v )& 8πP out (/ * 1 ) 0 1 γ

12 Evolution of magnetic «fluxtubes» global simulations Radial magnetic field at top Magnetic loop in a convective shell Simulation performed with ASH on Titane@CCRT Movie created with SDvision@CEA Resolution: to Number of cpus: 64 to 256 Computing time for 1 run: 5000h to 10000h monoproc Jouve et al. 2013

13 Evolution of magnetic «fluxtubes» global simulations Convective upflows and downflows control the rise velocity of the loop and modify the morphology of emerging regions. Vr+Br Vr+Emag

14 Evolution of magnetic «fluxtubes» global simulations Jouve et al # Faster rise at high latitudes, thus stronger fields: effect of rotation # Shift in longitude: effect of differential rotation # Tilt angle higher at high latitudes (in agreement with observations)

15 Intermediate-mass stars A peculiar type of magnetism X-ray luminosity of stars of various spectral classes Solar-type M/Ms<0.5 Massive and intermediatemass Dwarf 0.5<M/Ms<1.5 M/Ms>01.5 O B A F G K M

16 Intermediate-mass stars A peculiar type of magnetism Observations: Musicos + NARVAL Theory: Strong initial field: differential rotation suppressed strong large-scale stable field Weal initial field: differential rotation creates toroidal field instability weak small-scale field " Some stars possess a strong large-scale field " Some others possess a weak complex field " None possesses an intermediate field

17 Intermediate-mass stars Numerical approach Simulations: 2D simulations of differential rotation/magnetic field interaction Gaurat et al D simulations of magnetic field instability r Btor/Bpol Ω" " What kind of toroidal field created? " How does it depend on rotation? " Is the field unstable? " How does the surface field look like?

18 Jouve&et&al.&2015& Intermediate-mass stars 3D simulations Toroidal&field&created&by&differen7al&rota7on&becomes&unstable&& & & " &Strong&toroidal&field,& &&&an7symmetric,& &&&close&to&the&surface& & & " &Favored&modes:& &&&&m=4,&5&and&6& & Resolution: & " &Instability&around&& &&&the&regions&of&& &&&strong&toroidal&field& & Number of cpus: 20 to 320 Computing time for 1 run: 5000h monoproc

19 Intermediate-mass stars 3D simulations! &Stable&and&unstable&cases&dis7nguished&by&ra7o&growth'(me'of'the'instability/(me'scale'of'the'background'field'! &Surface&radial&field:&weak&smallQscale&VS&strong&largeQscale&! &Es7mate&of&threshold&field:&

20 Conclusions Goals: " Progress towards a better understanding of the stellar magnetism and key physical processes. " Prepare students and future researchers to the use of high-performance computing in stellar physics. Tools: " High-quality observations: ESPADONS/NARVAL and then SPIROU (2018) " Numerical simulations with various levels of complexity which necessitates state-of-the art codes, architectures and computing centers: eos@calmip, curie@tgcc,