Observing magnetic fields in star-forming regions

Observing magnetic fields in star-forming regions

Jim CohenJim Cohen Jim CohenJim Cohen

The University of ManchesterThe University of Manchester

Jodrell Bank ObservatoryJodrell Bank Observatory

The University of ManchesterThe University of Manchester

Jodrell Bank ObservatoryJodrell Bank Observatory

1717thth February 2004 February 20041717thth February 2004 February 2004

Zwolle Workshop

Introduction

Polarization Mechanisms

Zeeman Splitting

Maser Regions

Introduction

Polarization Mechanisms

Zeeman Splitting

Maser Regions

Outline of Talk

Bipolar Outflows Align with Polarization of Starlight

Cohen et al. 1984, MNRAS 210, 425-438

Magnetic pressure estimated from OH maser Zeeman splitting is significant in dynamics of bipolar outflow.

Virial equilibrium:

P s+ |W| = Ms + Mw + 2T

P s External pressure

W Gravitational energy

Ms Static B

Mw Alfven wave B

T Internal Kinetic Energy

Are Cloud Cores Collapsing?

Vallee & Bastien 2000, ApJ, 530, 806-816

Evolutionary Effects

What are the polarization signatures

of protostellar evolution?

There are many techniques available to estimate B but not usually in one and the same source.

Some measurements give B , some give B , some give B magnitude, some give the direction, some give the full vector B.

Polarized flux is often less than 1% so we are usually struggling for sensitivity.

Stokes parameters are additive. Therefore polarization structure that is unresolved either in frequency or spatially will lead us to underestimate the true degree of polarization.

General Remarks

Faraday Rotation

2 ne B cos dx

Can mask true direction of B

Pulsar DM 2 ne dx

B cos RM/DM

useful for large-scale Galactic B but not small scale studies of star-formation

Synchrotron

E B

Continuum Polarization

Aligned Dust Grains

Emission E B (FIR or submm)

Extinction E B (optical)

Scattering E B (optical, NIR)

Interstellar Polarization in Taurus Dark Clouds

Messinger, Whittet & Roberge 1997, ApJ 487, 314-319

Well organized on large scale, but only outer layers of dust clouds are probed.

Note wavelength dependent PA of two stars – dust properties change with grain size and location (depth) in cloud. Field direction twists inside cloud.

Lang et al. 1999, ApJ, 526, 727-743

Chuss et al. 2003,

ApJ, 599, 1116-1128

350m poln (Hertz on CSO)

overlaid on 20-cm continuum

Dense: B b (toroidal)

Rare: B b (poloidal)

Chuss et al. 2003,

ApJ, 599, 1116-1128

Collapse can produce toroidal B in mol cloud while leaving B poloidal outside.

Magnetic reconnection can produce the energy for the nonthermal filaments.

OR bipolar wind

Classical Zeeman Effect

An electron in a magnetic field B precesses at the Larmor frequency L = eB/2me .

Spectral lines are split into three polarized components at (angular) frequencies o ,

o + L and o - L

Blended: Bcos

Unblended: B

HI Zeeman

Weak splitting, sigma components dominate.

Stokes V = z Bcos dI/d where z is the splitting factor.

Measures line-of-sight component Bcos.

Instrumental issues limit usefulness to strong fields exceeding ~10G.

Sarma et al. 2000, ApJ 533, 271-280, VLA 35 x 20 arcsec

NGC6334 source E

Brogan & Troland 2001, ApJ 560, 821-840 VLA OH and HI

Bcos increases where Bsin (traced by 100m poln) decreases.

Either B is bending around the HII region or the dust properties are being changed by the HII region.

M17

Quantum Zeeman EffectQuantum Zeeman Effect

A magnetic dipole μ in a magnetic field B has a potential energy μ.B that is quantized:

μ.B = B g J μB / ħ

where μB = eħ/2me is the Bohr magneton. Lande factor g ~ 1 (paramagnetic) or ~ 10-3 (non-paramagnetic), but depends on total angular momentum F and is different for upper and lower states in general.

States split into 2F+1 substates with allowed transitions

Δm = +1 Δm = 0 Δm = -1 σ+ π σ -

Linear polarization is parallel to B for π components, perpendicular to B for σ components.

OH ZeemanOH Zeeman

Polarization and intensity depend on angle of B to line-of-sight

Splitting B provided hyperfine components don’t overlap. Otherwise see Elitzur (1996,8).

Complete Zeeman pattern can be complex.

Maser propagation/competive effects

Sarma et al. 2000, ApJ 533, 271-280, VLA 16 x 12 arcsec

OH Thermal Absorption NGC6334

OH Thermal Emission

Crutcher & Troland 2000, ApJ 537, L139-L142 Arecibo 2.8 x 3.2 arcmin

CN ZeemanCrutcher et al. 1999, ApJ 514, L121-L124 Pico Vateta

CN 1-0 at 113 GHz

Traces 105-106 cm-3

9 hyperfine components, well separated in velocity 4 strong Zeeman, 3 weak Zeeman effect, 2 useless

Different splitting factors reduce systematic errors

Simultaneous fitting to 4 strong (upper) and weak (lower) components

DR21(OH) 0.71mG OMC1n 0.36 mG

CN

Excited OH

OH Masers

H2O Masers

Magnetic Fields in Molecular Clouds

Crutcher 1999, ApJ 520, 706-713

B nH20.5

Ambipolar diffusion?

Or constant VAlfven

B(4)-1/2 0.7 km s-1

OH thermal emission and absorption generally traces the outer regions of molecular clouds but not the dense cores.

Crutcher et al. 2004 propose use of randomness in polarization vectors to estimate B (Chandrasekhar & Fermi 1953) based on MHD wave argument

Bsin n1/2 V -1

L1544 results in OH give smaller B than SCUBA polarimetry at 850 microns which penetrates core.

Could have angle = 16 to line of sight to be consistent.

We Need More Tracers of B

Prestellar Cores

Ward Thompson et al. 2000, ApJ 537, L135-L138

Crutcher et al. 2004, ApJ 600, 279-285

Bsin = 80G SCUBA 850 m 14 arcsec Bsin = 140 G

L183 L1544

MERLINMERLIN

Multi Element Radio Linked Interferometer Multi Element Radio Linked Interferometer NetworkNetwork

D = 218 km

0.170" 18 cm

0.042 " 4 cm

0.013" 1.4 cm

D

Orion-KLOrion-KL

OH masers trace a rotating and expanding molecular torus at the centre of the H2 outflow (Gasiprong 2000, PhD thesis).

13x1612-MHz, 430x1665-MHz, 3x1667-MHz masers

Magnetic Beaming in Magnetic Beaming in MasersMasers

Complete Zeeman patterns rarely observed.

σ-components grow fastest and can suppress π-comps (Gray & Field 1995).

100% circular polarization most common.

Zeeman shift has same effect as velocity shift. In a turbulent medium LHC and RHC trace different molecules in general.

σ -πσ +W75N

Vector B

OH maser polarization indicates 3-d magnetic field with suitable interpretation (need to identify -components)

Garcia-Baretto et al. 1988 ApJ 326, 954

W75N

W75N bipolar W75N bipolar outflowoutflow

Shepherd et al. 2003, ApJ 584, 882

•

0.6pc

Large-scale B-field parallel to outflow (submm poln).

OH MasersOH MasersHutawarakorn & Cohen 2002, MNRAS 330, 349

2000AU

0.010 pc1665 MHz

Kinematics show a rotating and expanding disc (torus) orthogonal to the outflow.

Strong linear poln up to 100%.

Vectors are either parallel to outflow or perpendicular.

OH Masers OH Masers continuedcontinued

Magnetic field reverses on opposite sides of disc (toroidal component).

Field lines twisted up in the rotating disc.

Uchida & Shibata (1985) model is supported.

1667 MHz and 1720 MHz

Twisted Magnetic FieldTwisted Magnetic Field

Uchida & Shibata 1985 hydrodynamical computation.

(a) large scale field contracts with disc

(b) disc twists field lines (c) close-up of core

PASJ 37, 515

Model of OH masers and polarizationModel of OH masers and polarizationSynthetic maser spectra generated using polarization-dependent model of propagation, with physical conditions taken from Uchida & Shibata (1985) model. Gray et al. 2003, MNRAS, 343, 1067-1080.

Masers originate at different depths in disc.

IRAS 20126+4104 Bipolar Bipolar Outflow

Cesaroni et al., in press Plateau de Bure

Edris et al., in preparation MERLIN

Vallee & Bastien 2000, ApJ 530, 806-816SCUBA

B outflow

Sarma et al. 2002, ApJ, 580, 928-937 VLA

H2O Maser Polarization

Hyperfines?

H2O Linear Polarization

Imai et al 2003, ApJ 595, 285-293 VLBA

Where Next?Where Next?

3-d magnetic field studies are sensitivity limited for now (key polarized flux is only a small % of total).

Potential to probe range of densities to 1010cm-3.

Major new IR/submm/mm facilities are coming and will overlap with masers at subarcsec resolution.

Some key questions:

• How to treat overlap of hyperfine components?

• Relation to galactic magnetic field?

• Magnetic field evolution, does B dominate?

• Maser lifetimes and source evolution?