Full Stokes (IQUV) spectropolarimetry of AGB and post‐AGB stars :
probing surface magnetism and atmospheric dynamics
Agnès Lèbre, University of Montpellier, FranceAgnes.Lebre@univ‐montp2.fr
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Eric Josselin, Benjamin Tessore, University of Montpellier & LUPM/CNRS, FranceMichel Aurière, Philippe Mathias, Pascal Petit, Univ. of Toulouse & IRAP/CNRS, France
Denis Gillet, Observatoire Haute Provence, FranceNicolas Fabas, Kiepenheuer‐Institute, Freiburg, Germany
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Full Stokes Spectropolarimetry from Narval instrumentat Télescope Bernard Lyot, Pic du Midi Observatory, France
My collaborators :
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Outlines :
‐ Cool and evolved stars : AGB and Post‐AGB
‐ Surface Magnetic Fields : Circular Polarisation (Stokes V)
‐RGB & early‐AGB magnetic fields
‐TP‐AGB magnetic fields (Mira stars)
‐Post‐AGB stars (RV Tauri stars) / PN magnetism
- RSG magnetic fields (e.g. : Betelgeuse) ‐> see Poster B14 ( E. Josselin)
Active giants (global dynamo)Descendant of Ap stars (magneto‐convection)
Amplification by shock waves ?
Turbulent dynamo
‐ Atmospheric dynamics : Linear Polarisation (Stokes Q and U)in the S‐type Mira star : χ Cyg ; in the RV Tauri star : R Sct
AGB :Cool (Teff <4 000K) and evolved radially pulsating stars= Mira stars
At the tip of the AGBHigh mass loss rate
Post‐AGB/proto NP :4 000 K < Teff < 6 000 Kradially pulsating stars= RV Tauri stars
Evolution of lowand intermediatemass stars
IRC 10216
Egg Nebula
Bug Nebula
IRAS 13208‐6020Ring Nebula
During the transition from AGB to PN :
Severe change of the morphology of the circumstellar envelope of an AGB(departure from spherical symmetry)
Binarity ? Magnetic fields ?
and
Observational evidences of magnetic fieldsaround PNe and AGB /post‐AGB(see works of V. Vlemmings ; L. Sabin)
HST images
(From Kwok,1987)
(From , Amiri Ph.D. 2011)
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AGB stars : He‐ and H‐shell burning phase
Teff 4000‐2500 K ; log g 0 − 2
Convection :Large‐scale convective motions in an extendedatmosphere, with a few granules /giant cells covering the surface (Schwarzschild 1975)
Radiative hydrodynamic simulations (Chiavassa+ 2010)
Cool and Evolved Stars
Freytag & Hofner 2008 3D simulation of the atmosphere of an AGB
Pulsations :In Mira stars (AGB) : stellar pulsations are expected to (periodically) generate radiative shocks waves => convection‐pulsation
And also in the pulsating RV Tauri stars (Post‐AGB )
Mass‐loss : In Miras levitation due to pulsations + radiation pressure on dust (Höfner, 2011)
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Magnetic fields in Mira stars (Thermal ‐Pulsing AGB with 2‐4 M_sun) :
Circumstellar magnetic field through CSE from Masers SiO & CN lines Geometry of the field : B ~ 1/r ... (Herpin et al. 2006 & 2009 ; Vlemmings et al., 2011)
Hydrogen Balmer lines in emission
=> shock wave (atmospheric dynamics)
o Ceti and R Leo (M‐type Mira stars) :
‐ photospheric field (~ a few G) expected from theoretical works (Thirumalai et al. 2013)=> not detected with Narval … so far ?
χ Cyg (S‐type Mira star) :
1st Mira star with a positive detection of a weak photospheric magnetic field.Detected through circular polarization – with Narval (Lèbre et al. 2014)
Connexion surface magnetic field ‐ atmospheric dynamics / shock waves
Associated linear polarization @ max. light (Fabas et al. 2011)
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First detection of a surface magnetic field on a Mira starNarval observations of χ Cyg around its 2012 maximum light
Definite Detectionχ2=1.81 , fap=5.2 10‐10
a (magnetic) Zeeman effect origin
Stokes I profile : typical line doubling of metallic lines due to a shock wave in the atmosphere.Stokes V signal : associated to the blue component of the I profile
(Lèbre et al. 2014)
Surface field estimation : 2‐3 G
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Magnetic fields in Post‐AGB stars (including RV Tauri stars) :
Detection of large scale magnetic fields in the circumstellar environment mainly from radioastronomy(Sabin et al. 2013 ; Vlemmings et al. 2011)
RV Tauri stars : U Mon, R Sct (K0‐type) :1st positive detections of a photospheric magnetic field through circular polarization – with ESPaDOnS and Narval (Sabin et al. 2014, MNRAS in press)
arXiv:1410.6224
Planetary Nebulae :Detection of large scale magnetic fields in the nebulaeCentral star : null or inconclusive detections (Jordan+2012 ; Leone+2014, Steffen+2014)
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1 – 21-23 July 2014 2 – 01-03 September 20143 – 10-12 September 2014
Spectropolarimetric monitoring of pulsating variables : R Sct
Impact of atmospheric shock waves ?
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1
2 3
1: DD Bl = 0.6 +/- 0.72 G
2: MD Bl = -0.23 +/- 0.72 G
3: ND Bl = -1.62 +/- 0.83 G
(Sabin et al., 2014 MNRAS 446, 1988)
R SctCircular polarization (Stokes V)
R SctLinear polarization (Stokes U & Q)
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1
2 3
1: U: ND ; Q : DD
2: U : DD ; Q : DD
3: U : DD ; Q : DD
LSD profiles : Thousand lines involved !
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R Sct : Linear polarization detected in individual lines !
[email protected] nm R Sct – 01 Sept. 2014 [email protected] nm
Linear polarization (Stokes U & Q) around the maximum light of 2007
Stokes Q (1 sequence) : Definite Detection (χ2=3.01)
Stokes U (1 sequence) :Definite Detection(χ2=4.57)
Stokes V (1 sequence) : No Detection (χ2=1.16) Noise level ~ 0.5 10‐4
→ The shock favours a direc on, inducing a net linear polariza on.In agreement with Fabas et al., 2011
→ Departure from spherical symmetry at the photospheric level.also seen from interferometric data (Ragland et al., 2006)
The Mira star, Χ Cyg :
Q/U ~1
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LSD profiles : thousand lines involved !
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χ Cyg : Linear polarizationalso detected in individual lines !
Sto
kes
IS
toke
s U
Sto
kes
Qcoherent scattering
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Linear polarization in χ Cyg (Mira star) and R Sct (RV Tauri star)
Scattering
Polarization of the continuum : assuming pure Rayleigh scattering, pL ~ 2% in the blue (variable with λ)
Polarization in the atomic lines :‐ Partial depolarization in lines due to collisions (indirect measurement of NLTE effect ?)
‐ In the case of the Sun (2nd Solar spectrum) => coherent scattering
Hanle effect : coherent scattering + weak magnetic field=> A measurement of weak surface magnetic fields in stars ?
Linear polarization in the lines (individual / global) :‐ Detected in AGB (Miras), in Post‐AGB (RV Tauri) + time variable ‐ Also detected in Red Supergiants‐ But not detected in RGB / early‐AGB
In AGB and Post‐AGB : Toward a second stellar spectrum ?
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(Konstantinova‐Antova et al. 2014, UAI 302)
Magnetic RGB with Bl < 1 Gauss (Pollux)
« 2nd magnetic strip » :
Tip RGB / AGB
‐ low surface rotation ‐ convection=> Local dynamo ?
Exploration of unbiased sample :
40 Red Giants (magV < 4)Incl. M giants
Observations/detection of 24 giantswith activity signatures+ 6 bright giants
The most active magnetic giants are concentrated in a
« Magnetic Strip » :
1rst Dredge‐up and Core Helium burning phases.
Evolutionnary models including rotation (Charbonnel et al., in prep.)
(Aurière et al., 2014, A&A, in press)arXiv:1411.6230
RGB & AGB surface magnetic fields
~ 50 % of RGB with a magnetic field above the Gauss levelMagnetic field and activity is more common than expected !