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
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Differential rotation Helical motions
Parker 1955
Solar cycle
poloidal toroidal toroidal poloidal
surface
tachocline
Combination of both effects(both linked to solar rotation)
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)
Lites et al. 2008 (Hinode observations)
Small-scale magnetism and solar dynamo
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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
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)
• 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
Play with other stars to tune parameters
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=0
J=1
Zeeman 1896, Hale 1908 for the Sun, Babcock 1947 for a star
Splitting of spectral lines in a magnetized atmosphere(proportional to field strength, unsensitive to field orientation)
Zeeman detection of stellar magnetic fields
Zeeman splitting in a sunspot
Generally, B too weak to produce Zeeman splitting… but still able to polarize light in spectral lines
J=0
J=1
Zeeman detection of stellar magnetic fields
J=0
J=1
(Zeeman 1896)
Light polarization controlled by strength and orientation of B
Zeeman detection of stellar magnetic fields
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• Generally, polarized Zeeman signatures signatures too weak to be detected in individual lines
multi-line analysis (cross-correlation).
Extracting Zeeman signatures
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)
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CFHT, HawaiiESPaDOnS (2004)
TBL, Pic du MidiNARVAL (2007)
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La Silla, ChileHARPS (2010)
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The magnetic field of Betelgeuse
Aurière et al. 2010
Field detection using 15,000 photospheric atomic lines(note : thousands of molecular lines ignored)
B ~ 1 Gauss
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The magnetic field of Betelgeuse
Aurière et al. 2010
Field variability < 1 month • much faster than stellar rotation• consistent with convective timescales (giant cells)
Likely similar to « Quiet Sun » magnetism
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Viticchié & Sanchez Almeida 2011
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Velocity fields
Asymmetric Zeeman signaturesgenerated by vertical gradientsof magnetic fields & velocities
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(Lopez Ariste 2002)
… seen also in solar intranetwork :
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Are all cool supergiants magnetic ?
Grunhut et al. (2009) observed 30 late-type supergiantswith 30% magnetic detections (weak fields) probably 100% of magnetic supergiants (assuming 5x better S/N)
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)