Magnetic field and convection in Betelgeuse

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Transcript Magnetic field and convection in Betelgeuse

Magnetic field and convection in Betelgeuse
M. Aurière, J.-F. Donati, R. Konstantinova-Antova,
G. Perrin, P. Petit, T. Roudier
Roscoff, 2011 April 6
Outline
• Dynamo(s) in the Sun and cool stars
• The case of Betelgeuse
• Spectropolarimetric detection of stellar magnetic fields
• The cool supergiant Betelgeuse
• Systematic field measurements in supergiant stars
• Perspectives
The large-scale solar dynamo
Helical motions
Differential rotation
surface
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tachocline
poloidal
toroidal
Combination of both effects
(both linked to solar rotation)
toroidal
poloidal
Solar cycle
Parker 1955
Some open questions about the solar dynamo
• Toroidal field generation :
differential rotation (Parker 1955)
 tachocline alone ?
 convective zone as a whole ?
(Brown et al 2010, Petit et al. 2008)
 what about the subsurface shear layer ?
(Brandenburg 2005)
Poloidal field generation :
 cyclonic convection ? (Parker 1955)
 decay of active regions
+ meridional circ. ? (Dikpati et al. 2004)
Small-scale magnetism and solar dynamo
Origin of small-scale (intranetwork) magnetic elements :
• decay of active regions ? But: no or very limited variation over solar cycle
• small-scale dynamo (Meneguzzi & Pouquet 1989, Cattaneo 1999 etc) ?
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Lites et al. 2008 (Hinode observations)
Small-scale magnetism and solar dynamo
Origin of small-scale (intranetwork) magnetic elements :
• decay of active regions ? But: no or very limited variation over solar cycle
• small-scale dynamo ? (Meneguzzi & Pouquet 1989, Cattaneo 1999 etc)
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Vögler et al. 2007
Play with other stars to tune parameters
• How to make sure that small solar magnetic elements are not
residuals from active regions, generated by the large-scale dynamo ?
Observe a star without rotation (no global dynamo)
• How to resolve magnetic elements at the convective scale
on a distant star ?
Observe a star with huge convective cells
Betelgeuse : basic facts
Cool supergiant star
• Teff = 3600 K
• R = 600 - 800 Rsun , e.g. Perrin et al. 2004
(first stellar diameter ever measured,
Michelson & Pease 1921)
• M ~ 15 Msun
• Prot ~ 17 yr
(from space-resolved UV Doppler shifts)
HST/FOC
Convection in Betelgeuse
Giant convection cells
(a few tens of cells on visible hemisphere
vs ~ 106 cells on solar hemisphere)
• largest cells seen in nIR,
lifetime ~ years
• smaller cells in visible,
lifetime ~ weeks
(e.g. Schwarzshild 1975,
Chiavassa et al. 2010, 2011)
Magnetic fields in Betelgeuse ?
Prot ~ 17 yr
Ro = Prot/tconv >> 1
no solar dynamo expected
Convective dynamo simulations predict strong fields (500 G)
with small filling factors (Dorch 2004)
UV radius > optical radius
(hot material above photosphere, Gilliland et al. 1996)
… and :
Radio radius > optical radius
(cool material above photosphere, Lim et al. 1998)
Cool extended atmosphere coexists with hot extended atmosphere
Ayres et al. 2003 report strongly absorbed lines of highly ionized species
« Buried » coronal loops
Zeeman detection of stellar magnetic fields
J=1
J=0
Splitting of spectral lines in a magnetized atmosphere
(proportional to field strength, unsensitive to field orientation)
Zeeman 1896, Hale 1908 for the Sun, Babcock 1947 for a star
Zeeman detection of stellar magnetic fields
Zeeman splitting in a sunspot
Zeeman detection of stellar magnetic fields
J=1
J=0
Generally, B too weak to produce Zeeman splitting
… but still able to polarize light in spectral lines
Zeeman detection of stellar magnetic fields
J=1
J=0
(Zeeman 1896)
Light polarization controlled by strength and orientation of B
Extracting Zeeman signatures
• Generally, polarized Zeeman signatures signatures too weak to be detected
in individual lines
multi-line analysis (cross-correlation).
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Instrumental constraints
• Largest polarized Zeeman signatures in cool stars : V ~ 10-2Ic
• For low-activity stars (e.g. solar twins) : V ~ 10-5Ic
• Linear polarization (Q and U) ~ 10-2V ~ 10-7Ic for solar twins
•
•
optimize the instrumental throughput
(ESPaDOnS/NARVAL : about 15% including sky & detector)
use large reflectors
(ESPaDOnS/HARPSpol : 4m)
perform accurate polarimetric analysis
•
resolve spectral lines (R > 30,000)
•
TBL, Pic du Midi
NARVAL (2007)
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CFHT, Hawaii
ESPaDOnS (2004)
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La Silla, Chile
HARPS (2010)
The magnetic field of Betelgeuse
Field detection using 15,000 photospheric atomic lines
(note : thousands of molecular lines ignored)
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B ~ 1 Gauss
Aurière et al. 2010
The magnetic field of Betelgeuse
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Field variability < 1 month
• much faster than stellar rotation
• consistent with convective timescales (giant cells)
Likely similar to « Quiet Sun » magnetism
Aurière et al. 2010
Velocity fields
Asymmetric Zeeman signatures
generated by vertical gradients
of magnetic fields & velocities
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(Lopez Ariste 2002)
… seen also in solar intranetwork :
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Viticchié & Sanchez Almeida 2011
Are all cool supergiants magnetic ?
Grunhut et al. (2009) observed 30 late-type supergiants
with 30% magnetic detections (weak fields)
probably 100% of magnetic supergiants
(assuming 5x better S/N)
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What happens to the 5-10% of strongly magnetic,
main-sequence massive magnetic stars ?
organized, strongly magnetic evolved stars
(inclined dipole with ~500G field)
Aurière et al. 2008 for EK Eri
Magnetic field often ignored in proposed processes
creating highly structured wind
to be reconsidered ?
Kervella et al. 2009 (NACO observations)
Perspectives
• Look for periodicities in field variability
• Classical magnetic mapping prevented by long rot. period (17 yr)
use simultaneous interferometry and spectropolarimetry
use future ground-based solar facilities like ATST, EST.
(AO + spectropolarimetry)
• Combine optical spectropolarimetry and UV spectroscopy
UVMAG project (ask Coralie about that)