Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
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)
2 2
≈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
3 3
Geometry makes the difference
2-4 December 2013 International Francqui Symposium, Bruxelles
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).
4 4
Content
2-4 December 2013 International Francqui Symposium, Bruxelles
1. How to measure magnetic properties of stars?
2. The stellar magnetic zoo
3. Summary
4. (A bigger –galactic- picture?) … for next time
5 5
Field strength and orientation
2-4 December 2013 International Francqui Symposium, Bruxelles
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
6 6
Poor men‘s version: residual line broadening due to Zeeman effect
2-4 December 2013 International Francqui Symposium, Bruxelles
• 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
7 7
Not-so-poor version: add time-series information
2-4 December 2013 International Francqui Symposium, Bruxelles
Strassmeier et al. 2011, A&A
8 8
The rich men‘s version: Stokes V
2-4 December 2013 International Francqui Symposium, Bruxelles
SOLIS - Synoptic map - CR 2007 Stokes V
Carroll, Strassmeier & Ilyin 2010, IAU S273 The Sun as a star
9 9
A „little“ problem
2-4 December 2013 International Francqui Symposium, Bruxelles
Stokes V
V/Ic
This is about what we expect !
V=11.2mag 3.6m CFHT Espadons R=60,000 600sec
10 10
Signal enhancement techniques
2-4 December 2013 International Francqui Symposium, Bruxelles
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)
11 11
Reconstruction from SVD profile
2-4 December 2013 International Francqui Symposium, Bruxelles
Original Spectrum
First Eigenspectrum ~ LSD Profile
SVD reconstruction SVD
Subspace averaging + clever line list
12 12
WFA & the effective longitudinal magnetic field Beff
2-4 December 2013 International Francqui Symposium, Bruxelles
local WFA
However, we can observe only disk-integrated Stokes I and Stokes V profiles:
= the „center-of-gravity“ (COG) method (Semel 1967)
13 13
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
14 14
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
15 15
The non-linear ZDI problem is solved simultaneously in V & I
International Francqui Symposium, Bruxelles
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
16 16
V410Tau: a very young (pms) Sun
International Francqui Symposium, Bruxelles 2-4 December 2013
Carroll, Strassmeier, Rice, Künstler 2012, A&A 548, A95 K4, P(rot)=1.87d, B ± 2kG
17 17
From principles to reality
International Francqui Symposium, Bruxelles
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
LBT
PEPSI/LBT
20 20
2. The stellar magnetic zoo
International Francqui Symposium, Bruxelles
Berdyugina 2009, IAU S259
2-4 December 2013
21 21
The M supergiant Betelgeuse
International Francqui Symposium, Bruxelles
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
22 22
Arcturus, K1.5III
International Francqui Symposium, Bruxelles 2-4 December 2013
• 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
23
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)
International Francqui Symposium, Bruxelles 2-4 December 2013
24
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’).
International Francqui Symposium, Bruxelles 2-4 December 2013
25 25
The M dwarf – B dwarf surprise
International Francqui Symposium, Bruxelles 2-4 December 2013
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
28 28
HD179949: planet-star interaction?
International Francqui Symposium, Bruxelles 2-4 December 2013
• 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
29 29
White dwarf
International Francqui Symposium, Bruxelles 2-4 December 2013
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.
International Francqui Symposium, Bruxelles 2-4 December 2013
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“?
International Francqui Symposium, Bruxelles 2-4 December 2013