Asteroseismology Reveals Strong Internal Magnetic Fields in Red Giant Stars

KASC8 / TASC1 Meeting, Aarhus (Denmark)

Transcript

1. Asteroseismology Reveals Strong Internal Magnetic Fields in Red Giant Stars J.Fuller (Caltech, KITP) & M.Cantiello (KITP) Collaborators: D. Stello, R. A. Garcia, L. Bildsten, T. Bedding, D. Huber, V. Silva Aguirre
2. evanescent zone g-mode cavity p-mode cavity νmax p-mode cavity (envelope) g-mode cavity (core) evanescent zone Courtesy: Dennis Stello Mixed Modes
3. evanescent zone g-mode cavity p-mode cavity νmax p-mode cavity (envelope) g-mode cavity (core) evanescent zone Courtesy: Dennis Stello Mixed Modes 2
4. Mosser et al. 2012 Puzzle: Suppressed Dipolar Modes Stars evolve this direction Stars with Suppressed Dipoles
5. Mosser et al. 2012 Puzzle: Suppressed Dipolar Modes Stars evolve this direction Stars with Suppressed Dipoles Important fact: l=0 normal, l=1 suppressed!
6. Stars evolve this direction Stello, MC, JF et al. (Submitted)
7. Mode Visibility
8. Core Mode Visibility Not to Scale!
9. Core Mode Visibility Not to Scale! In normal red giants with mixed modes, wave energy that tunnels into the core eventually tunnels back out to produce the observed oscillation mode
10. Excitation Damping Convective Envelope p-modes B Alfven Waves Magneto Gravity Waves Evanescent Region Core Not to Scale! At equilibrium, damping and excitation rate balance each other (Dupret et al. 2009)
11. Excitation Damping Convective Envelope p-modes B Alfven Waves Magneto Gravity Waves Evanescent Region Core Not to Scale! For suppressed mixed modes let’s assume that all the energy leaking into the g-mode cavity never makes it back to the envelope At equilibrium, damping and excitation rate balance each other (Dupret et al. 2009)
12. Excitation Damping Convective Envelope p-modes B Alfven Waves Magneto Gravity Waves Evanescent Region Core Not to Scale! Transmission through the evanescent region Envelope crossing time At equilibrium, damping and excitation rate balance each other (Dupret et al. 2009)
13. Convective Envelope p-modes B Alfven Waves Magneto Gravity Waves Evanescent Region Core One can write the ratio of visibility of suppressed/normal modes as: Envelope crossing Inverse of the envelope damping rate (typical value for RGB ~ 10d) Not to Scale! norm e.g. Corsaro et al. 2015 r2 r1
14. 0 100 200 300 400 500 nmax(µHz) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 V2 Normal Stars Suppressed Stars KIC 8561221 KIC 9073950 Stars evolve this direction
15. 0 100 200 300 400 500 nmax(µHz) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 V2 Normal Stars Suppressed Stars KIC 8561221 KIC 9073950 Fuller + Cantiello et al. (submitted to Science) Stars evolve this direction
16. 0 100 200 300 400 500 nmax(µHz) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 V2 Normal Stars Suppressed Stars KIC 8561221 KIC 9073950 Fuller + Cantiello et al. (submitted to Science) Stars evolve this direction Conclusion: Suppressed dipoles explained by a mechanism efficiently trapping wave energy in the core
17. Magnetic Fields
18. In the presence of strong B-fields, magnetic tension forces can become comparable to buoyancy Critical Field Strength Lorentz Force ~ Buoyancy Force
19. Critical field strength for magnetic domination:
20. Critical field strength for magnetic domination: Corresponding critical frequency: Fuller + Cantiello et al. (submitted to Science)
22. Magnetic Greenhouse Effect Fuller + Cantiello et al. (submitted to Science) Dipolar waves “scattered” to high harmonic degrees l Magnetic fields break spherical symmetry in the core High l waves trapped in the core Reese et al. 2004, Rincon & Rieutord 2003, Lee 2007,2010, Mathis & De Brye 2010,2012
23. Magnetic Greenhouse Effect Fuller + Cantiello et al. (submitted to Science) Dipolar waves “scattered” to high harmonic degrees l Magnetic fields break spherical symmetry in the core High l waves trapped in the core Reese et al. 2004, Rincon & Rieutord 2003, Lee 2007,2010, Mathis & De Brye 2010,2012 Typical Critical B-field ~ 105 G
24. Courtesy: Kyle Augustson  Convective core dynamos on the MS: Beq~105 G  Magnetic field topology is complex  Flux conservation can easily lead to B~106-107 G on the RG  Stable magnetic configurations of interlocked poloidal+toroidal fields exist in radiative regions Prendergast 1956, Kamchatnov 1982, Mestel 1984, Braithwaite & Nordlund 2006, Duez et al. 2010 Brun et al. 2005 2Msun
25.  New Asteroseismic technique allowing to measure / put limits on stellar internal magnetic fields for the first time! Droopy Fuller + Cantiello et al. (submitted to Science) (Garcia+ 2014)  This technique can be applied to thousand of red giants
26. Thousand of Giant Stars...
27. Stello, MC, JF et al. (Submitted)
28. Stello, MC, JF et al. (Submitted) MS: Radiative Cores MS: Convective Cores
29. Conclusions:  In red giants, strong core magnetic fields create a magnetic greenhouse effect that traps oscillation mode energy within the core  The effect is created by spherical symmetry-breaking caused by magnetic fields  Suppressed dipole modes reveal the presence of strong internal magnetic fields  Fields of roughly 105 G sufficient for suppression
30. Tnx!
31.  New Asteroseismic technique allowing to measure / put limits on stellar internal magnetic fields for the first time!  KIC 8561221 has BH-shell ~ 2x107 G  This technique can be applied to thousand of red giants Stars evolve this direction Droopy Fuller + Cantiello et al. (submitted to Science)
32.  Sun-like stars do not posses strong internal magnetic fields during the MS  ~ 60% of intermediate mass stars have powerful convective core dynamos during the MS  Convective Core dynamo generated fields are likely to assemble into stable equilibrium configurations Stello, MC, JF et al. (Submitted) Convective Core Dynamo
33. Fuller, Cantiello et al. (Sub.) Stars evolve this direction Stello, MC, JF et al. (Submitted)
34.  This technique doesn’t require long observational times. K2 (e.g. Clusters), TESS
35. Stars evolve this direction Stello, MC, JF et al. (Submitted)
36. Role of Joule Heating <<1
37. adapted from Cantiello et al. 2014
38. Fuller + Cantiello et al. (submitted to Science)
39. Magneto-gravity Waves: Alfven Waves Fuller + Cantiello et al. (submitted to Science)
40. Core
41. Core
42. Asteroseismology in the year 2015 Space-Based Asteroseismology has opened a window into the interiors of Red Giants Made it possible to distinguish between H- and He-burning Red Giants (Bedding+ 2011, Mosser+ 14) Rotational Splitting of Mixed Modes Allowed to Measure degree of Differential Rotation (Beck+ 2012, Mosser+ 2012) Can we use red giants asteroseismology to probe other fundamental internal properties of stars?
43. M < 2 MSun: Ignite He in a degenerate core M < 1.1 MSun: Core masses below S-C limit. In HE and TE throughout H-shell burning. No H.gap. Long- lived phase (~Gyrs) [subgiant- branch] M = 1.1-1.5 MSun: same as M< 1.1 MSun, but small convective cores during core H-burning: ‘hook’ in the evolution M = 1.5-2.0 MSun: Do exhibit a small H.gap as they reach the S-C limit (mass limit above which an inert core can not remain in TE) before their cores become degenerate Huber et al. 2011 Stochastically Excited Oscillations
44. Fuller + Cantiello et al. (submitted to Science)
45. Garcia et al. 2014 Droopy
46. Garcia et al. 2014
47. Garcia et al. 2014