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
Outline of Talk
• possible roles of magnetic fields
• important parameters
• observational techniques
• observational result exemplars
• conclusions
• implications for study of CMB polarization
• the future
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
Field Morphology
Shu, The Physical Universe (1982)
Strong B, magnetic support implies:• non-tangled (smooth) field lines • hourglass morphology
Mass-to-Flux Ratio: M/
( / )
( / )observed
critical
M
M
• Uniform disk Nakano & Nakamura (1978)
• Observing M/
• definition
/ 3C observed • Geometry correction
1
2critical
M
G
Ciolek & Mouschovias (1994)
mass/flux ratio gravitational collapse / magnetic support
2( )observed
observed
N HM
B
sub
critical
critica
l
sup
erc
ritic
al
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)
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
L1544 Starless Core
n(H2) 5 105 cm-3, N(H2) 4 1022, 13, Bpos 140 G, c 0.8 Crutcher et al. (2004)
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
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
NGC1333 IRAS4 (BIMA 230 GHz)Girart et al. (1999)
Bpos > 1 mG
NGC1333 IRAS4 (SMA 345 GHz)
Rao, Girart and Marrone
DR21(OH)
Blos = 0.4, 0.7 mG
Lai et al. (2003) Crutcher et al. (1999)
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
# o
f p
ositi
ons
21 – 10
DR21(OH)
Cortes, Crutcher, & Watson (2005)
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
The Orion Molecular Cloud
NGC 2024 (Orion B) Magnetic Field Maps
Crutcher et al. (1999)
NGC 2024 (Orion B)
Lai, Crutcher, et al. (2001)
NGC 2024 SCUBA Dust Polarization
Matthews et al. (2002)
Orion Molecular Cloud
Girart et al. 2004
Orion Molecular Cloud
Girart et al. 2004
Orion Molecular Cloud
Rao et al. 1998Houde et al. 2004
W3OH
CN Zeeman, Blos =1.1 mG
Turner & Welch 1984 Falgarone, Crutcher, & Troland 2005
W3OH
Gusten et al. 1994
8-11 mG
n(H2) 6 106, N(H2) 5 1023, Blos 3.1 mG, c
0.5
Mass to Magnetic Flux Ratios mass/flux ratio () gravitational collapse /magnetic support
H I clouds,subcritical!
Field Strength vs. Density
B
Weak B = 2/3
Strong B 0.4
0.47 ± 0.08
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
Dust Polarization and the CBM
Arce, et al 1998
Molecular Cirrus
Desert, Bazell, & Boulanger 1988
Stark 1995
Some Telescopes Used for Study of B
Coming Telescope for Study of B