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

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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) ?



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 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


J=0

J=1

(Zeeman 1896)

Light polarization controlled by strength and orientation of B


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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)



CFHT, HawaiiESPaDOnS (2004)

TBL, Pic du MidiNARVAL (2007)





La Silla, ChileHARPS (2010)



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





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



Viticchié & Sanchez Almeida 2011



Velocity fields

Asymmetric Zeeman signaturesgenerated by vertical gradientsof magnetic fields & velocities



(Lopez Ariste 2002)

… seen also in solar intranetwork :



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)

Magnetic field and convection in Betelgeuse

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