magnetic fields in supernova remnants kashi & urumqi, 2005 sept. 7 th -14 th
Post on 09-Jan-2016
28 Views
Preview:
DESCRIPTION
TRANSCRIPT
Magnetic Fields inSupernova Remnants
Kashi & Urumqi, 2005 Sept. 7th-14th
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.)
Evolution of SNRs(based on Woltjer 1972)
log
Rad
ius
log Time
R t R t2/5 R t2/7 R t1/4
FreeExpansion
Adiabatic Radiation RadiationSedov internal pressure momentum
merg
ing into
the inte
rste
llar
mediu
m
Magnetic Field and Evolution of SNRs
Magnetic pressure number
RH = magnetic pressure = B0
2/8 476
B02(mGs)
.
dynamic
pressure 1/20vs
2 n0(cm-
3)vs2(100km/s)
100
10
1
0.1
0.01
RH
10Gs 100Gs 1mGs 10mGs
B0
10-8 dyne cm-2
10-7 dyne cm-2
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)
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)
(rad) = 0(rad) + RM(rad/m2)(m)2 +n
G127.1+0.5=11cm E-Vectors = 6cm
Ambiguity of Rotation Measure HB9 100-m-RT
+
(rad) = 0.2+114 (m)2
6cm
11cm 21cm
Ambiguity of Rotation Measure HB9 100-m-RT
+
(rad) = 0.2+114 (m)2
6cm
11cm 21cm
S1476cm
Urumqi25m-RT
TP + B-Field + Pulsar ( )
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
Young Shells
Tycho 10.55 GHzTP +B-Field 100-m-RT
Fine structure at 15 arcsec scale (0.2 pc) VLA 5 GHz (Wood et al., 1992)
Tycho’s SNR
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
Magnetic Field Direction in SNRs
(Whiteoak & Gardner 1968)
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
Evolved Shells
CTB1 10.55 GHzTP+B-Field 100-m-RT
The Orientation of bilateral SNRs and the Galactic Magnetic Field
G127.1+0.5 HC30 G93.3+6.9
Magnetic Field Direction in SNRs
(Whiteoak & Gardner 1968)
Magnetic Field Direction in G179.0+2.5
= 6cmTP + E-Vectors
Old SNR with radial B-Field!!
Filled-center SNRs (Tau A)
VLA 21cm/6cm, (Bietenholz & Kronberg 1990)
100-m-RT 32GHz,(Reich 2002)
G21.5-0.9
Nobeyama Array 22.3 GHz 100-m-RT 32 GHz, (Reich et al. 1998)
Depolarization
Polarization degree:
P(%) = 3+3 sin B02 / (B0
2 + Br2), (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
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)
RHTycho 0.1
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)
W44 (Claussen et al. 1997)
0.28±0.09mG
W51C (Brogan et al. 2000)
1.5±0.05mG 1.9±0.10mG
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
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.
Thank You
On Xinwei day the new star fadedaway (China, 14th century B.C.)
top related