Magnetic Fields in Molecular Clouds Richard M. Crutcher University of Illinois

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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 Jason Kirk, University of
Illinois Doug Roberts, Northwestern
University Josep 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

• Strong B, magnetic support implies
• non-tangled (smooth) field lines
• hourglass morphology

Shu, The Physical Universe (1982)
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Mass-to-Flux Ratio M/?
mass/flux ratio ? gravitational collapse /
magnetic support

• Uniform disk Nakano Nakamura (1978)

critical?
subcritical?
?

• Observing M/?

supercritical?

• ? definition

• Geometry correction

Ciolek Mouschovias (1994)
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Scaling of B with ? B ? ??
B ? ?0
B ? ?1
Spherical collapse (weak magnetic fields)
Magnetic support, ambipolar diffusion

• flux freezing M ? ?
• mass conservation

B ? ?0.4
Ciolek Mouschovias (1994)
Mestel (1966)
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Observational Techniques
1. Zeeman effect
V ? dI/d? Blos Q,U ? dI/d?2 Bpos
2. Polarization of dust emission ? linear
polarization ? B ? morphology of Bpos ?
indirectly (Chandrasekhar Fermi) ?
Bpos ? 0.5(4??)1/2 ?Vlos /??
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
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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
n(H2) ? 1 ? 104, N(H2) ? 9 ? 1021, Blos 11 µG,
?c ? 1.1
n(H2) ? 5 ? 105 cm-3, N(H2) ? 4 ? 1022, ?? ? 13?,
Bpos ? 140 ?G, ?c ? 0.8
Crutcher et
al. (2004)
Crutcher Troland (2000)
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L183 L1498 Starless Cores
L183
L1498
Crutcher et al. (2004)
Kirk Crutcher (2005)
n(H2) ? 3 ? 105, N(H2) ? 3 ? 1022, ?? ? 13?, Bpos
? 80 µG, ?c ? 0.9
?? ? 40?
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NGC1333 IRAS4 (BIMA 230 GHz)
Girart et al. (1999)
Bpos gt 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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DR21(OH)

• Linearly Polarized J2-1 and J1-0 Lines
• J2-1 polarization is perpendicular to dust
  polarizaton and therefore parallel to the
  magnetic field
• J1-0 polarization is
• orthogonal to J2-1
• polarization!
• requires two sources of anisotropic CO excitation
• anisotropic velocity gradient (and ?), and photon
  trapping
• IR from compact dust cores

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of positions
4
2
50 70 90 110
?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. 1998
Houde 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
8-11 mG
n(H2) ? 6 ? 106, N(H2) ? 5 ? 1023, Blos ? 3.1 mG,
?c ? 0.5
Gusten et al. 1994
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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

• B ? ?0, n lt 103
• ? molecular clouds form
• by accumulation along B
• Magnetic fields usually not tangled
• ? B dominates turbulence
• Hourglass B morphology on cores
• ? magnetic support
• M/? critical in molecular cores
• ? magnetic support
• 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