Magnetic fields of stars: from cool to hot stars · ζ Oph; a run-away Oe star • ζ Oph is one of...

2-4 December 2013 International Francqui Symposium, Bruxelles 1 1

Magnetic fields of stars: from cool to hot stars

Klaus G. Strassmeier Leibniz-Institute for Astrophysics Potsdam (AIP)

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≈27dex of field strengths in the universe

Galaxy intracluster fields ~nG-pG Milky Way average field ~few µG ULIRGs ~few mG Earth magnetic field ~1 G Solar global poloidal field ~10 G OBA stars ~10-30 G FGK m-s stars ~0.1-1 kG M dwarfs ~0-3 kG T Tauri stars ~1-3 kG Sunspots and faculae ~2-3 kG Ap/Bp star global field ~1-30 kG (Magnetic) white dwarfs ~MG Neutron stars ~GG-TG Magnetars ~PG (1015 G)

TRACE

EF Eri; Beuermann et al. 2007, A&A 463

2-4 December 2013 International Francqui Symposium, Bruxelles

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Geometry makes the difference


For the Sun: Gravitational energy (GM2/R) = 1041 J Rotational kinetic energy (Iω2) = 1035 J Magnetic energy in convection zone (B2/2μ0) = 1028 J (propto energy density of 140 erg/cm3; Trujillo Bueno et al. 2004, Nature 430, 326)

But: from top to bottom, there is an increasing geometric influence! … from pure isotropic (i.e. gravity), to axisymmetric (i.e. rotation), to non-axisymmetric (i.e. magnetic field).

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Content


1. How to measure magnetic properties of stars?

2. The stellar magnetic zoo

3. Summary

4. (A bigger –galactic- picture?) … for next time

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Field strength and orientation


BglB2131067.4 λλ −×=∆

gl , quantum Lande factor, measuring the line’s magnetic sensitivity

Saar et al., ApJ

Donati et al., MNRAS

Note. 1G ∝ 1 m/s

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Poor men‘s version: residual line broadening due to Zeeman effect


• Example HD123351, K0IV in close binary

• Differential analysis (FeI6173-FeI6240) • B.f.T inversion per line

profile • Time series of 273 echelle

spectra from 1.2m STELLA

Many previous apps to M dwarfs, WTTS… by Saar et al., Johns-Krull & Valenti et al., Ruedi et al., and others!

Vsini=1.2 km/s; Bf=542±72G

Strassmeier et al. 2011, A&A

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Not-so-poor version: add time-series information


Strassmeier et al. 2011, A&A

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The rich men‘s version: Stokes V


SOLIS - Synoptic map - CR 2007 Stokes V

Carroll, Strassmeier & Ilyin 2010, IAU S273 The Sun as a star

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A „little“ problem


Stokes V

V/Ic

This is about what we expect !

V=11.2mag 3.6m CFHT Espadons R=60,000 600sec

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Signal enhancement techniques


1. Construction of the observational covariance matrix CV 2. Decomposition of S and N eigenvalues 3. Projection of each line profile onto above eigenprofiles

and reconstruction with QS

4. Enhance QS from QN by subspace averaging of only S. 5. Introduce a „weighting factor“ Z = line list 6. Line list is constructed from a model star with

parameters of the target and a preset radial field. Full radiative transfer gives a large number of synthetic polarized spectra.

LSD (Donati et al. 1997), iLSD (Kochukhov et al. 2010) PCA (Carroll et al. 2007, Martinez Gonzalez et al. 2008) CA (Semel et al. 2009, Ramirez et al. 2010) ZCD (Sennhauser & Berdyugina 2010) SVD (Carroll et al. 2012)

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Reconstruction from SVD profile


Original Spectrum

First Eigenspectrum ~ LSD Profile

SVD reconstruction SVD

Subspace averaging + clever line list

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WFA & the effective longitudinal magnetic field Beff


local WFA

However, we can observe only disk-integrated Stokes I and Stokes V profiles:

= the „center-of-gravity“ (COG) method (Semel 1967)

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OMP + multi-res wavelet dictionaries: a new technique to extract Beff

International Francqui Symposium, Bruxelles 2-4 December 2013

Center-of-gravity method (COG) versus Orthogonal Matching Pursuit (OMP)

Carroll & Strassmeier 2014, A&A = σNoise where S/N = 1/σNoise

Beff Signal = Beff Noise Beff Signal < Beff Noise

OMP error: <Beff > max 0.7 G

COG error: <Beff > ≈ 2 - 100 G

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The real rich men‘s version: ZDI

2-4 December 2013

International Francqui Symposium, Bruxelles

SOLIS - Synoptic map - CR 2007 Sun with vsini=50 km/s

Stokes V

Carroll, Strassmeier & Ilyin 2010, IAU S273

2-4 December 2013

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The non-linear ZDI problem is solved simultaneously in V & I


Model

Reg. Optimization (Inverse Process)

Stokes V

Stokes I Stokes Q

Stokes U

Polarized Radiative Transfer (Forward Process)

Ω = χ² + λS

2-4 December 2013

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V410Tau: a very young (pms) Sun


Carroll, Strassmeier, Rice, Künstler 2012, A&A 548, A95 K4, P(rot)=1.87d, B ± 2kG

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From principles to reality


Current (hi-res echelle) spectropolarimeter workhorses: Espadons/3.6mCFHT; Narval/2mTBL; HARPS-P/3.6mESO; SoFINe/2.5mNOT Next steps: PEPSI/11.8mLBT, Spirou/3.6mCFHT (NIR) … a.o. Future: HIRES-P/39mESO E-ELT (400-2400nm)

2-4 December 2013

PEPSI/LBT

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2. The stellar magnetic zoo


Berdyugina 2009, IAU S259

2-4 December 2013

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The M supergiant Betelgeuse


Auriere et al. 2010, A&A 516, L2

2-4 December 2013

• Stokes V + LSD • Signal amplitude 3x10-5 • Surface averaged longitudinal B≈1G • Variable 0.5-1.5G on 1 month • Due to convection-cell dynamo?

Freytag et al. 2002, AN 323, 213

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Arcturus, K1.5III


• Three measurements w/ Espadons@CFHT: Blos 0.65±0.26G; 0.43±0.16G; Null

Sennhauser & Berdyugina 2011, A&A 529, A100

S/N>1000:1

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First detection of a magnetic field in an A supergiant

A mean longitudinal field of order of a few hundred Gauss was detected in two out of three FORS2 observations. It is one of the rare cases where a field is detected in an early A-type supergiant.

Hubrig et al. 2012, A&A 546, L6,

Magnetic field detection in the bright A0-type supergiant HD 92207 in NGC3324 (≈5 Myrs)


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ζ Oph; a run-away Oe star

• ζ Oph is one of the brightest massive stars in the northern hemisphere.

• Spectropolarimetric observations of ζ Oph with VLT/FORS 1.

• Using all available spectral absorption lines, a mean longitudinal magnetic field of 141 ± 45 G was detected, confirming the magnetic nature of this star.

Hubrig, S.; Oskinova, L. M.; Schöller, M. 2011, AN 332, 146 Combined Spitzer IRAC image of the bow shock around the star (36’x31’).


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The M dwarf – B dwarf surprise


V474 Peg (M4V) Donati et al. 2006a τ Sco (B0V) Donati et al. 2006b

• M4 star is old but P=0.44d and M=0.28Ms • B0 star is young but P=41d and M=15Ms • Yet, B0 star resembles solar corona while M4 star resembles pure dipole

Star formation: star-disk locking by magnetic fields

Surface Doppler image obtained with UVES/VLT showing hot spots due to magnetospheric accretion that fit the Doppler image and predict a polar field of 3 kG.

MN Lupi: M2 wTTS in Lupus star forming region

Strassmeier. et al. 2005, A&A 440

Towards observing solar analogs

18 Sco G2V, P=22.7 d Mean B ≈ 4 G 34% in dipole 56% in quadrupole Petit et al. (2008), MNRAS 388, 80

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HD179949: planet-star interaction?


• F8V with hot Jupiter • P(rot)=7.6d • P(orbit)=3.1d • HARPS-P in Stokes V • ZDI from I&V • B(max)≈10G • B relates to planet?

Carroll et al. 2014, A&A, in prep

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White dwarf


Beuermann et al. 2007, A&A 463, 647

• CV EF Eri wd+M2V • Deconvolution of spectra • Stokes V w/ FORS1/VLT • Use of H and He lines • B(dipole)≈44 MG • Offset dipole with multipole expansion of up to lmax = 5 • B(phot)≈13 MG

Summary

• 27dex observed magnetic field strengths in the universe • Geometry makes the difference for global plasma

interaction • Even if MHD approach, Lorentz force remains neglected • Stellar magnetic field measurements mostly rely on

Zeeman effect at optical and IR wavelengths • ≈1G∝1m/s @ δP/P≈10-4…-5 → requires signal-

enhancement techniques • Stokes I and V polarization are most common:

Zeeman broadening → |Bf | ; Stokes V/I → Blos & f • „Rich men‘s“ version: ZDI → gives B and its geometry • Galactic-, CMF- & n-stars measurements mostly based on

Farady rotation at radio waves rather than Zeeman eff.


Summary

• HRD: effective B ranges from 10-3 G to 1015 G • RGB stars with <1 kG, AGB stars with <<10 G. Why? • M supergiant Beteigeuze 1 G! → If so, Eddy dynamo? • Runaway O-star ζ Oph: 145 G. Where from? • Pre-main sequence stars very similar to enhanced Sun • And, yes, magnetic fields reside in cool starspots! • An orbiting planet can trigger or enhance magnetic

activity of the host star • White dwarfs can have high-order B components, not just

classical dipole. EF Eri → B(phot)≈13MG instead of 44 MG • Is there a „galactic magnetic web“? • Does B influence the dynamical and chemical evolution

of stars beyond the „magnetic brake“?


Magnetic fields of stars: from cool to hot stars · ζ Oph; a run-away Oe star • ζ Oph is one of...

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