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