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 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 )

Lites et al (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) ?

Vögler et al 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 = R sun, e.g. Perrin et al (first stellar diameter ever measured, Michelson & Pease 1921) M ~ 15 M sun P rot ~ 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 ~ 10 6 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 ? P rot ~ 17 yr Ro = P rot /t conv >> 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 ) … and : Radio radius > optical radius (cool material above photosphere, Lim et al ) Cool extended atmosphere coexists with hot extended atmosphere Ayres et al 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

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 ~ I c For low-activity stars (e.g. solar twins) : V ~ I c Linear polarization (Q and U) ~ V ~ I c 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, Hawaii ESPaDOnS (2004) TBL, Pic du Midi NARVAL (2007) La Silla, Chile HARPS (2010)

The magnetic field of Betelgeuse Aurière et al 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 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 signatures generated by vertical gradients of magnetic fields & velocities (Lopez Ariste 2002) … seen also in solar intranetwork :

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) 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 for EK Eri

Magnetic field often ignored in proposed processes creating highly structured wind to be reconsidered ? Kervella et al (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)