full stokes (iquv) spectropolarimetry of agb and post agb

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

1IAU Symposium 305‐ Punta Leona, Costa Rica

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


Full Stokes Spectropolarimetry from Narval instrumentat Télescope Bernard Lyot, Pic du Midi Observatory, France

My collaborators :

IAU Symposium 305‐ Punta Leona, Costa Rica 3

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)



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)


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)


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


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)


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 ?


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)


1

2 3

1: U: ND ; Q : DD

2: U : DD ; Q : DD

3: U : DD ; Q : DD

LSD profiles : Thousand lines involved !


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


LSD profiles : thousand lines involved !


χ Cyg : Linear polarizationalso detected in individual lines !

[email protected] nm

[email protected] nm

Sto

kes

IS

toke

s U

Sto

kes

Qcoherent scattering


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 ?


(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 !

full stokes (iquv) spectropolarimetry of agb and post agb

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