Dynamo in subsurface convective layers of hot massive stars

IAU General Assembly, Beijing 2012

Transcript

1. Dynamo in subsurface convective layers of hot massive stars Matteo Cantiello Kavli Institute for Theoretical Physics (UCSB) ( J.Braithwaite, A.Brandenburg, F. del Sordo, P. Käpylä, N. Langer)
2. Which stars is this talk about? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
3. Which stars is this talk about? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
4. Which stars is this talk about? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
5. Massive stars  Energy / Momentum in ISM  Stellar Winds, SNe  Nucleosynthesis  Remnants: NS and BHs  Magnetars, Pulsars, Long GRBs... Importance of magnetic fields and final angular momentum budget Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
6. Convection in hot stars Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
7. Stellar structure Low Mass stars Massive stars Radiative core Convective envelope Convective core Radiative envelope e.g 1MSun e.g 20 MSun Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
8. Stellar structure Low Mass stars Massive stars Radiative core Convective envelope Convective core Radiative envelope e.g 1MSun e.g 20 MSun Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
9. Opacity Inside a massive star Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
10. Opacity Inside a massive star Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
11. Near-surface convection (e.g. Maeder et al. ‘08 , Cantiello et al. ’08) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
12. Near-surface convection (e.g. Maeder et al. ‘08 , Cantiello et al. ’08) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
13. The physical mechanism (e.g. Lighthill ‘52, Stein ‘67, Edmunds ’78, Goldreich & Kumar ’90, de Jager et al. ’91) Cantiello et al. 2009 Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
14. Magnetic fields in hot, massive stars Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
15. B fields in massive stars (direct evidence) About 30 magnetic OB stars found (e.g. Donati, Hubrig, Neiner, Petit) Detection through Zeeman spectral signature Bias toward strong, large scale fields Origin unclear. Likely Fossil (Wade et al. 2010) Similar to Ap/Bp stars ~ 5-10% ? Important evolutionary consequences (e.g. ud-doula & Owocki 02, Meynet et al. 2010) Tau Sco Credits: Jardine & Donati Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
16. OB stars show puzzling surface phenomena (e.g. DACs, LPV, Wind Clumping, Solar- Like Oscillations, Red Noise, Photometric variability, X-ray emission...) Some of these phenomena are ubiquitous. Therefore can not be explained by large scale fields! (e.g. Schnerr+08) Small scale / small amplitude fields? (e.g. Cranmer & Owocki 96, Fullerton+96, Kaper+97, Henrichs+05) (DACs: Kaper, Henrichs et al. 1999) B fields in massive stars (indirect evidence) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
17. OB stars show puzzling surface phenomena (e.g. DACs, LPV, Wind Clumping, Solar- Like Oscillations, Red Noise, Photometric variability, X-ray emission...) Some of these phenomena are ubiquitous. Therefore can not be explained by large scale fields! (e.g. Schnerr+08) Small scale / small amplitude fields? (e.g. Cranmer & Owocki 96, Fullerton+96, Kaper+97, Henrichs+05) (DACs: Kaper, Henrichs et al. 1999) B fields in massive stars (indirect evidence) Where these fields could come from? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
18. Dynamo Action in FeCZ Cantiello et al. 2009,2010; Cantiello & Braithwaite 2011 Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
19. Dynamo Action in turbulent convection Käpylä et al. 2008,2010; Hughes & Proctor 2008 Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
20. Dynamo Action in FeCZ (Cantiello, Braithwaite, Brandenburg et al. 2010) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012 Pencil Code (Brandenburg & Dobler 2002) Setup: piecewise polytropic (stable- unstable-stable) Cartesian grid 128 x 128 x 256 Fcon/Frad ~ 0.3 Re ~ 80 Shown is B/Beq
21. Dynamo Action in FeCZ (Cantiello, Braithwaite, Brandenburg et al. 2010) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
22. Dynamo Action in FeCZ (Cantiello, Braithwaite, Brandenburg et al. 2010) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012 Subsurface convection Rotation + Shear
23. Equipartition B Field in FeCZ 4.8 4.6 4.4 4.2 4.0 logT eff 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Log L 0 0 500 500 500 1000 1000 1000 1500 1500 2000 B (G) 0 500 1000 1500 2000 2500 GAL 4.8 4.6 4.4 4.2 4.0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Log L 120 MSun 35 MSun 20 MSun 10 MSun 7 MSun Cantiello & Braithwaite ’11 Here we assume dynamo action! Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
24. Surface B Field from FeCZ 4.8 4.6 4.4 4.2 4.0 logT eff 3 4 5 6 Log L 5 5 10 10 20 20 40 40 80 160 320 B (G) 0 5 10 20 40 80 160 > 320 GAL 4.8 4.6 4.4 4.2 4.0 3 4 5 6 Log L 35 MSun 20 MSun 10 MSun 7 MSun 120 MSun Cantiello & Braithwaite ’11 Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
25. Magnetic Spot in the Sun Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
26. For fields of ~ 100 G emerging at the surface this leads to a temperature increase of ~ 300 K. A hot, bright spot How they would look like? Cantiello & Braithwaite (2011) A very simple model: 1) Assume hydrostatic equilibrium 2) Assume thermal equilibrium 3) Assume beta >> 1 e.g. Parker (1955) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
27. Spot in a convective star Spot in a radiative star (?) How they would look like? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
28. High precision photometry: CoRoT CoRoT: HD 46149 (Degroote et al. 2010) CNES / David Ducros Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
29. CoRoT: Spots in a O8V star? HD 46149 (Degroote et al. 2010) Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
30. HD51756 (Papics et al. 2011) CoRoT: Spots in a B0.5IV star? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
31. HD51756 (Papics et al. 2011) CoRoT: Spots in a B0.5IV star? Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
32. X-ray emission • OB stars emit X rays.These stars follow the relation Lx/Lbol ~ 10-7 (e.g. Pallavicini+81) • The Lx/Lbol relation only applies for stars earlier than B0-B1. It breaks down at lower L (e.g.: Cohen+97) 4.8 4.6 4.4 4.2 4.0 logT eff 3 4 5 6 Log L 5 5 10 10 20 20 40 40 80 160 320 B (G) 0 5 10 20 40 80 160 > 320 GAL 4.8 4.6 4.4 4.2 4.0 3 4 5 6 Log L Log(Lx/Lbol) > -7 -7 > Log(Lx/Lbol) > -8 Log(Lx/Lbol) < -8 35 MSun 20 MSun 10 MSun 7 MSun 120 MSun Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
33. Take home message(s)  The envelope of early type stars is not fully radiative Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
34. Take home message(s)  The envelope of early type stars is not fully radiative  Dynamo action might be at work in sub-surface convective layers of massive Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
35. Take home message(s)  The envelope of early type stars is not fully radiative  Dynamo action might be at work in sub-surface convective layers of massive  Massive stars might have bright magnetic spots at their surface Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
36. Take home message(s)  The envelope of early type stars is not fully radiative  Dynamo action might be at work in sub-surface convective layers of massive  Massive stars might have bright magnetic spots at their surface  Implications for photometric variability and X-ray emission Matteo Cantiello Dynamo in subsurface convection in Massive Stars IAUS 294 – Beijing 2012
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