magnetic fields in molecular clouds richard m. crutcher university of illinois
DESCRIPTION
Magnetic Fields in Molecular Clouds Richard M. Crutcher University of Illinois Collaborators:Tom Troland, University of Kentucky Edith Falgarone, Ecole Normale Superieure Shih-Ping Lai, University of Maryland Ramprasad Rao, SubMillimeter Array Paulo Cortes, University of Illinois - PowerPoint PPT PresentationTRANSCRIPT
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Magnetic Fields in
Molecular Clouds
Richard M. Crutcher University of Illinois
Collaborators: Tom Troland, University of KentuckyEdith Falgarone, Ecole Normale SuperieureShih-Ping Lai, University of MarylandRamprasad Rao, SubMillimeter ArrayPaulo Cortes, University of IllinoisJason Kirk, University of IllinoisDoug Roberts, Northwestern UniversityJosep Girart, University of Barcelona
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Outline of Talk
• possible roles of magnetic fields
• important parameters
• observational techniques
• observational result exemplars
• conclusions
• implications for study of CMB polarization
• the future
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Possible Roles of Magnetic Fields
• formation of molecular clouds
• fragmentation to form cores
• support against collapse
• transport of angular momentum from central regions of cores, enabling star formation
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Field Morphology
Shu, The Physical Universe (1982)
Strong B, magnetic support implies:• non-tangled (smooth) field lines • hourglass morphology
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Mass-to-Flux Ratio: M/
( / )( / )
observed
critical
MM
• Uniform disk Nakano & Nakamura (1978)
• Observing M/
• definition
/ 3C observed • Geometry correction
12critical
MG
Ciolek & Mouschovias (1994)
mass/flux ratio gravitational collapse / magnetic support
2( )observed
observed
N HMB
subcritical
critical
supe
rcrit
ical
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Scaling of B with : B
• flux freezing: M
• mass conservation:
3 2R R BBR
3 1/
2 / 3
3M R R
B
Spherical collapse (weak magnetic fields)
B 0
Ciolek & Mouschovias (1994)
Magnetic support, ambipolar diffusion
B 1
B 0.4
Mestel (1966)
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2. Polarization of dust emission linear polarization B morphology of Bpos
indirectly (Chandrasekhar & Fermi):
Bpos 0.5(4)1/2 Vlos /
Observational Techniques
1. Zeeman effect
3. Goldreich-Kylafis effect anisotropic radiation field non-LTE magnetic sublevels linear polarization or B morphology of Bpos
Chandrasekhar-Fermi may be applied to estimate Bpos
V [dI/d] Blos
Q,U [dI/d]2 Bpos
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L1544 Starless Core
n(H2) 5 105 cm-3, N(H2) 4 1022, 13, Bpos 140 G, c 0.8 Crutcher et al. (2004)
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L1544 Starless Core
Crutcher & Troland (2000)
n(H2) 5 105 cm-3, N(H2) 4 1022, 13, Bpos 140 G, c 0.8 Crutcher et al. (2004)
n(H2) 1 104, N(H2) 9 1021,Blos = 11 µG, c 1.1
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L183 & L1498 Starless Cores
n(H2) 3 105, N(H2) 3 1022, 13, Bpos 80 µG, c 0.9
Crutcher et al. (2004) Kirk & Crutcher (2005)
L183 L1498
40
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NGC1333 IRAS4 (BIMA 230 GHz)Girart et al. (1999)
Bpos > 1 mG
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NGC1333 IRAS4 (SMA 345 GHz)
Rao, Girart and Marrone
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DR21(OH)
Blos = 0.4, 0.7 mG
Lai et al. (2003) Crutcher et al. (1999)
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Linearly Polarized J=2-1 and J=1-0 Lines
• J=2-1 polarization is perpendicular to dust polarizaton and therefore parallel to the magnetic field
• J=1-0 polarization is orthogonal to J=2-1 polarization!
• requires two sources of anisotropic CO excitation– anisotropic velocity gradient (and ), and photon trapping– IR from compact dust cores
DR21(OH)
2
4
6
50 70 90 110
# of
po
sitio
ns
21 – 10
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DR21(OH)
Cortes, Crutcher, & Watson (2005)
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DR21(OH)
1. CO polarization: n(H2) ~ 102, Bpos 0.01 mG
2. Dust polarization & CN Zeeman: n(H2) ~ 106, N(H2) 3 1023
Bpos Blos 0.7 mG, c 1.1
Combining 1 and 2, B 0.45
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The Orion Molecular Cloud
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NGC 2024 (Orion B) Magnetic Field Maps
Crutcher et al. (1999)
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NGC 2024 (Orion B)
Lai, Crutcher, et al. (2001)
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NGC 2024 SCUBA Dust Polarization
Matthews et al. (2002)
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Orion Molecular Cloud
Girart et al. 2004
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Orion Molecular Cloud
Girart et al. 2004
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Orion Molecular Cloud
Rao et al. 1998Houde et al. 2004
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W3OH
CN Zeeman, Blos =1.1 mG
Turner & Welch 1984 Falgarone, Crutcher, & Troland 2005
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W3OH
Gusten et al. 1994
8-11 mG
n(H2) 6 106, N(H2) 5 1023, Blos 3.1 mG, c 0.5
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Mass to Magnetic Flux Ratios mass/flux ratio () gravitational collapse /magnetic support
H I clouds,subcritical!
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Field Strength vs. Density
B
Weak B = 2/3
Strong B 0.4
0.47 ± 0.08
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Conclusions for Molecular Cores 1. B 0, n < 103
molecular clouds form by accumulation along B
2. Magnetic fields usually not tangled B dominates turbulence
3. Hourglass B morphology on cores magnetic support
4. M/ ~ critical in molecular cores magnetic support
5. B , 0.4-0.5 2/3 magnetic support
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Dust Polarization and the CBM
Arce, et al 1998
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Molecular Cirrus
Desert, Bazell, & Boulanger 1988
Stark 1995
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Some Telescopes Used for Study of B
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Coming Telescope for Study of B