Selected topics on Cosmic Magnetic fields Huirong Yan KIAA-PKU hryan.
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Transcript of Selected topics on Cosmic Magnetic fields Huirong Yan KIAA-PKU hryan.
Astrophysical magnetic fields are ubiquitous
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Cluster of galaxies
Accretion disk
galaxy QSO
Solar wind
Also in Molecular clouds, outflows, supernovae remnants, planetary nebulae…
There is no universal magnetic diagnostics in diffuse medium
Zeeman splitting (BZeeman splitting (B ||||):):
Dense and cold clouds, with molecules or even denser with masersDense and cold clouds, with molecules or even denser with masers
Faraday rotation (BFaraday rotation (B ||||):):
Uncertainty with the electron density distribution along losUncertainty with the electron density distribution along los
Light polarization by dust (BLight polarization by dust (B⊥⊥):):
••Star light polarizationStar light polarization
••Grain IR emissionGrain IR emission
Most common ways of magnetic field studies:
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Magnetic realignment
Goldreich - Kylafis effect (upper state)polarization in radio and FIR emission due to anisotropic optical depth
applicable to molecular clouds
Ground state realignment
unique mechanism to induce polarization in absorption lines
due to anisotropic radiation
ranging from submilimeter, IR, optical to UV
general diffuse medium7
Toy model:s
Atomic alignment is differential occupation of the sublevels of the ground (or metastable) state.
Atomic species on the ground state can be aligned by anisotropic radiation
radiation
Classical AnalogyClassical Analogy
−−
Thermal systemRadiatively pumped system
Induced 1 transition followed by isotropic emission.
• anisotropic radiation (unpolarized light is sufficient)
• there are at least 3 sublevels on the ground state.
Alignment by unpolarized light requires >2 sublevels
Incoming light transmitted lightmedium
History: Laboratory studies on masers (experimental) (Brossel et al. 1952; Hawkins 1955; Kastler 1957)
Suggested by Varshalovich (1971) for B studies in diffuse medium
Magnetic field induces precession and realigns atoms
M=2M=2
M=1M=1
M=0M=0
M=-1M=-1
M=-2M=-2
zz
BB J(F)J(F)
θθrr
• Long lived ground state means
sensitivity to weak B.
• Alignment is either parallel or
perpendicular to B due to fast
magnetic precession.
BB
Magnetic field induces precession and realigns atoms
M=2M=2
M=1M=1
M=0M=0
M=-1M=-1
M=-2M=-2
zz
BB J(F)J(F)
θθrr
• Long lived ground state means sensitivity to weak B.
• Alignment is either parallel or perpendicular to B due
to fast magnetic precession.
BB
Atomic realignment is sensitive to weak magnetic fields
νL–Larmor frequency
A-Einstein coefficient, νΔJ– EΔJ /h
ννL L (s(s-1-1))
ΔΔv(cm/s)v(cm/s) ΒΒGG
HanleHanleeffecteffect
LowerLowerlevellevelHanleHanleeffecteffect
Level interferences Level interferences occuroccur
AtomicAtomicrealignmerealignmentnt Zeeman Zeeman
effecteffect
ννΔΔJJ⋅⋅
ΑΑ⋅⋅
⋅⋅
ττRR-1-1
For a wide range of field (>~10For a wide range of field (>~10-15-15G, ~<1G) in G, ~<1G) in diffuse medium, atomic alignment happens.diffuse medium, atomic alignment happens.
ντπτννΒμπτνπ
Realignment depends on the angle between the B and light
Alignment is determined by θr. The polarization degree depends on θ and θr . For the degenerate case, θr and θ coincide.
r
r
Polarization of absorption is either || or ⊥ to the magnetic field
Polarization changes direction at Van Vleck angle θr =54.7o,180o- 54.7o.
Yan & Lazarian (2006)Yan & Lazarian (2006)
Our results: many observed absorption lines have appreciable polarization
Ion C I C II Si I Si II O I S I
Wavl(A)
1329-1561
1336 1695-2529
1265 1302 1807
Pmax 18% 15% 20% 7% 29% 22%
Ion S II Ti II Cr I Cr II NI S III
Wavl(A)
1250 3385 4254-4290
2741-2767
1200 1012-1202
Pmax 12% 7% 5% 21% 5.5% 24.5%
Calculated Examples:Calculated Examples:
Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I … Many more lines:Many more lines:
18
Magnetic field changes optical depth and line intensity!
πτπτμννντντ
One has to take into account the effect of magnetic field when analyzing spectrum.
Effect of nuclear spin
2. Reduce alignment for atoms with fine structures.
Enables alignment for Alkalii atoms.
Similar structureSimilar structure
θθ θθ
θθ rr θθ rr
polarization of NI absorptionpolarization of NI absorption
Yan & Lazarian (2007)Yan & Lazarian (2007)
polarization of SII absorptionpolarization of SII absorption
NI and SII have same electron configurations (4SNI and SII have same electron configurations (4S3/23/2 4P 4P3/2,3/23/2,3/211). The ). The
difference in their polarizations shows the effect of hyperfine structure.difference in their polarizations shows the effect of hyperfine structure.
Our results: polarization of resonant lines scattered from aligned atoms
ion HI(Bal)
Na I K I N V O I P V S II Al II Ti II
Wavl 3646-6365
5892
7667
1243 5555-7254
1118
1254-1259
8843
3073
Pmax 25% 21% 20% 22% 2.3% 27% 31% 20% 7.3%
Many more lines:Many more lines:
N I, N II, N III, P III, Al I, Al III, Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I …
Calculated Examples:Calculated Examples:
Using several lines, it’s possible to get 3D B
3D information: from degree of polarization 3D information: from degree of polarization PP((22
00((θθrr),),θθ). ). Two lines are enough ((
P/P/ττ
θθrrθθ
ττ τ
τ
SII
SII
-1.82-1.82
-11%-11%
Optical depth
20(θr) ---Normalized
density matrix coefficient from atomic physics
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3rd possibilty: Forbidden (submm, IR) transitions within the aligned ground state
Schematics of UV pumpingof OI 63.2μm emissionSchematics of UV pumpingof OI 63.2μm emissionSchematics of UV pumpingof OI 63.2μm emission
Schematics of UVpumpingof CI 610μm emission μττντ 3Po
610μm3P1
3P0
3P2
3P2
[CI] Emission
θττμμντθττμννΔντπτπν
Forbidden (submm, IR) transitions within the aligned ground state are
polarizedCalculated Examples:Calculated Examples:
Many more lines:Many more lines:
Ex. I: alignment allows studies of magnetic field in a comet wake
Resonance scattering of solar light by sodium tail from comet.
MHD simulations of comet’s wake.
P: -8%~14% for Na D2P: -8%~14% for Na D2
Time variation of turbulent magnetic field can be studied.
Ex. II: Alignment allows tracing heliosphere B with comet
Satellite based model of Heliospheric B (Opher et al. 2008)
Earth
Comet orbit
Cost effective way of study of B in heliosphere!
Sun
synthetic measurements of comet B
∗
Ex. III: Polarization from circumstellar envelope
O I ( ) emissionQuickTime™ and aTIFF (Uncompressed) decompressor
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Polarization vectors are spherically symmetric without magnetic realignment and theclassical expression applies Spherical symmetry is broken
With alignment
Ex. V: Atomic alignment influences the FIR line intensity
ouudistortion to CMB
This influences Distortion of CMB and our estimates for early universe metalicity.
ΔT/T
cmbτ
Schematics of UV pumpingof OI 63.2μm emission
3S1
63.2μm
3P0
3P13P2
1306A 1302A
1304A
Ex. VI: alignment allows study of magnetic field in the epoch of Reionization
OI 63.2 μm transmission
UV excitation rate/ CMB excitation rate (Yan & Lazarian 2008)
From Cantalupo et al. (2008)
Schematics of reionization
63.2μm
3S1
3P0
3P13P2
1306A 1302A
1304A
Schematics of UV pumpingof OI 63.2μm emission
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CSO
Steward Observatory
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Sounding rocket flight is funded.(PI Nordsieck)
Observations for IBEX. (w. Paul Smith)
Studies of [C I] polarization are intended.(w.Houde)
Ongoing projects open wide avenues for the technique
Observations for SNRs. (w. F. Snick)
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Paramagnetic alignment and its modifications fail observational testing
• Grains get aligned with rotation axis parallel to B (grain 1)
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B
Davis-Greenstein mechanism 1
2
This is textbook solution
ττθνν
Δντπννννττντντππτν“ττν”νππττθνμντ
ΔνντνντνΔν
μτ
τ
ΒΔνντττθνΔΔΑ
τντνπτν
“τ”
ν
Draine & Weingrartner 97
L. Spitzer: Alignment torques are not universal. How can a universal alignment exist?
Angle between B and light direction
a1
k
e2
e3
e1Θ
Lazarian & Hoang 07
νπτνΘνΘμττΘνπνν
a1 is grain max inertia axis
k is light direction
μ
ΑντμΑπθντττντντττθΑνμντ
μπντπνΘνΘτθ
Θ
Θ
Θ
is angle between magnetic field and radiation
is angle between a and magnetic field
k
B
a
Βμτ
No wobbling: “wrong alignment”
“wrong” alignment for a narrow range of angles
μππν“ν”νμντ
Suppression of “wrong alignment” by wobbling
νν
ντπττνπμτ
• RAT alignment explains the existing data.• RAT testing constrains grain composition.• RAT alignment is present beyond ISM
νμμνμνττνττττντπντνΑνμντπττ
B
Davis-Greenstein mechanism
1
2
Ανμντ
• Atomic alignment is sensitive to smaller scale fluctuations of magnetic field.
• Combining the information from both, we
can get 3D information of magnetic field.
• Provide independent test to grain alignment
theory.
Comparison of atomic and grain alignment:
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Get magnetic strength from Chandrasekar-Fermi (CF) method
• Assuming equipartition between kinetic energy and perturbed magnetic energy
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μμττντ
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How good is the assumption of equipartition?
sub-Alfvenic turbulence
super-Alfvenic turbulence, turbulence dynamo
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Summary of interstellar medium polarimetry
Magnetic field in ubiquitous in astrophysics and need to be diagnosed from multiple scales with the synergy of various polarimetry techniques.
Ground state alignment is a unique mechanism to induce
polarization in absorption lines and detect ISM magnetic field.
It also influence emission polarimetry and forbidden line
polarimetry
Multiple wavebands from submillimeter, IR, optical to UV can be
used to detect 3D small scale magnetic field.
Combined with grain alignment, CF techniques, it can be used to
study both spatial and temporal variations of magnetic field.
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