Magnetic Fields in Supernova Remnants Kashi & Urumqi, 2005 Sept. 7 th -14 th.
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Transcript of Magnetic Fields in Supernova Remnants Kashi & Urumqi, 2005 Sept. 7 th -14 th.
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Magnetic Fields inSupernova Remnants
Kashi & Urumqi, 2005 Sept. 7th-14th
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SNRs,some historical
Comments • Synchrotron emission predicted by Alvén , Herlofson, Kiepenheuer
• First detected as optical emission from the Crab nebula 1953
• Optical linear polarization discovered (Dombrovsky 1954)
• Radio polarization from the Crab detected, (Mayer et al. 1957)
On Jisi day, the 7th day of the month, a big new star appeared near the Ho star (China, 14th century B.C.)
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Evolution of SNRs(based on Woltjer 1972)
log
Radi
us
log Time
R t R t2/5 R t2/7 R t1/4
FreeExpansion
Adiabatic Radiation RadiationSedov internal pressure momentum
mer
ging
into
the
inte
rste
llar m
ediu
m
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Magnetic Field and Evolution of SNRs
Magnetic pressure number
RH = magnetic pressure = B02/8 476 B02(mGs)
.
dynamic pressure 1/20vs2 n0(cm-
3)vs2(100km/s)
100 10 1 0.10.01
RH
10Gs 100Gs 1mGs 10mGs
B0
10-8 dyne cm-2
10-7 dyne cm-2
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Magnetic Field and Heat Conduction
The evaporation of clouds depends on heat conduction dQ/dt = K gradT.
For a typical cloud QK > 10⁸, the low magnetic heat conduction reduces the evaporation significantly. The cloud may survive, a star may be born .
QK = Kthermal 105 T(K)3 B(G)2
Kgyro n(cm-3)
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Observation
of Magnetic
Fields
Faraday rotation angle:
rot(rad) = RM(rad/m2) (m)2
Rotation measure: RM(rad/m2) = 8.1105 N(cm-3) B‖(G) dz(pc)
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(rad) = 0(rad) + RM(rad/m2)(m)2 +n
G127.1+0.5=11cm E-Vectors = 6cm
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Ambiguity of Rotation Measure HB9 100-m-RT
+
(rad) = 0.2+114 (m)2
6cm11cm 21cm
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Ambiguity of Rotation Measure HB9 100-m-RT
+
(rad) = 0.2+114 (m)2
6cm11cm 21cm
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S1476cm
Urumqi25m-RT
TP + B-Field + Pulsar ( )
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Types of SNRs• Young shells, historical SNRs:
Tycho, SN1006, Kepler • Old shells, evolved SNRs:
G127.1+0.5, G116.9+0.2, many others
• Filled centered SNRs, Pulsar powered: Crab nebular, 3C58, ….
• Combined SNRs
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Young Shells
Tycho 10.55 GHzTP +B-Field 100-m-RT
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Fine structure at 15 arcsec scale (0.2 pc) VLA 5 GHz (Wood et al., 1992)
Tycho’s SNR
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Young Shells• Predominantly radial field• Small scale variations (sub-pc scales)• Polarized fraction (PI/TP) 4 to 15% with
local enhancements. A large fraction of random magnetic field exists (Reynolds & Gilmore 1993)
• Radial field caused by external field directed towards observer (Whiteoak & Gardner, 1968)
• Rayleigh-Taylor instabilities between shock and ejecta, streching of magnetic field
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Magnetic Field Direction in SNRs
(Whiteoak & Gardner 1968)
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Young Shells• Predominantly radial field• Small scale variations (sub-pc scales)• Polarized fraction (PI/TP) 4 to 15% with
local enhancements. A large fraction of random magnetic field exists (Reynolds & Gilmore 1993)
• Radial field caused by external field directed towards observer (Whiteoak & Gardner, 1968)
• Rayleigh-Taylor instabilities between shock and ejecta, streching of magnetic field
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Evolved Shells
CTB1 10.55 GHzTP+B-Field 100-m-RT
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The Orientation of bilateral SNRs and the Galactic Magnetic Field
G127.1+0.5 HC30 G93.3+6.9
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Magnetic Field Direction in SNRs
(Whiteoak & Gardner 1968)
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Magnetic Field Direction in G179.0+2.5
= 6cmTP + E-Vectors
Old SNR with radial B-Field!!
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Filled-center SNRs (Tau A)
VLA 21cm/6cm, (Bietenholz & Kronberg 1990)
100-m-RT 32GHz,(Reich 2002)
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G21.5-0.9
Nobeyama Array 22.3 GHz 100-m-RT 32 GHz, (Reich et al. 1998)
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Depolarization Polarization degree: P(%) = 3+3 sin B0
2 / (B02 + Br
2), (Burn 1966)
3+3
=2r 2.83r
R=1
r 8.1 105 n B║ r (rad)
n(cm-3)B(Gs)r(pc)
Variation of total power rVariation of pol. Int. Sedov equations + strong
shock n0, B0, E0, tage, Vshock r
I
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Magnetic Field Strength Assumption: Minimum total energy of electrons, protons and
magnetic field. For =-2 (flux density spectral index = -0.5), and heavy particle energy 100 times electron energy,
lower frequency cut 107Hz, upper cut 1011Hz:
= relative radiating volumeR = radius (arcmin)d = distance (kpc)S1GHz = flux density (Jy)B = magnetic induction (µGs)
Tycho ~ 0.2 mGG127.1+0.5 ~ 12G
Bmin = 199 -2/7 R-6/7 d-2/7 S1GHz
2/7
(Pacholczyk 1970)RH
Tycho 0.1
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Magnetic Field Strength:the OH Line at 1720 MHz
• OH first detected (Weinreb et al. 1963)
• Maser theory (Litvak et al. 1966)• Collision pumping (Elizur 1976)• OH about 100 AU behind shock front
(Hollenbach & McKee 1989), (Neufeld & Dalgarno 1989)
• Zeeman splitting 1.31 kHz/mG (Heiles et al. 1993), (Frail et al. 1994, W28)
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W44 (Claussen et al. 1997)
0.28±0.09mG
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W51C (Brogan et al. 2000)
1.5±0.05mG 1.9±0.10mG
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OH 1720 Zeeman Data• 10 sources observed• Magnetic fields between 0.1 and a few
mG• W44:
• W51C Magnetic pressure 10-7 dyne cm-2
Dynamic pressure: 1/20Vs2 2 10-7
dyne cm-2
Magnetic pressure: B2/8 3 10-9 dyne cm-2
Thermal pressure: nkT 6-8 10-9 dyne cm-2
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Conclusions What can we learn from magnetic field
observation?• Interaction of SNRs with the Galactic
magnetic field• SNR parameters• In general, the dynamics of SNRs is
not affected by the magnetic field• In SNRs postshock regions with strong
cooling the magnetic field may have increased influence on the dynamics.
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Thank You
On Xinwei day the new star fadedaway (China, 14th century B.C.)