Observing magnetic fields in starforming regions

1
Observing magnetic fields in star-forming regions
Jim Cohen
17th February 2004
The University of Manchester Jodrell Bank
Observatory
Zwolle Workshop
2
Outline of Talk
Introduction Polarization Mechanisms Zeeman
Splitting Maser Regions
3
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.
4
Are Cloud Cores Collapsing?
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
5
Evolutionary Effects
What are the polarization signatures of
protostellar evolution?
Evolutionary Effects
Vallee Bastien 2000, ApJ, 530, 806-816
6
General Remarks
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.
7
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

8
Continuum Polarization
Synchrotron E ? B
Aligned Dust Grains
Emission E ? B (FIR or submm) Extinction E ?? B
(optical) Scattering E ? B (optical, NIR)
9
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.
10
Lang et al. 1999, ApJ, 526, 727-743
11
Chuss et al. 2003, ApJ, 599, 1116-1128
350?m poln (Hertz on CSO) overlaid on 20-cm
continuum Dense B ? b (toroidal) Rare B ?? b
(poloidal)
12
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
13
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
14
HI Zeeman
NGC6334 source E
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 10?G.
Sarma et al. 2000, ApJ 533, 271-280, VLA 35 x 20
arcsec
15
M17
Brogan Troland 2001, ApJ 560, 821-840 VLA OH
and HI
Bcos? increases where Bsin? (traced by 100?m
poln) decreases. Either B is bending around the
HII region or the dust properties are being
changed by the HII region.
16
Quantum 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 / h
where µB eh/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 2F1
substates with allowed transitions ?m 1
?m 0 ?m -1 s p s
- Linear polarization is parallel to B for p
components, perpendicular to B for s components.
17
OH Zeeman
Polarization and intensity depend on angle of B
to line-of-sight Splitting ? B provided
hyperfine components dont overlap. Otherwise
see Elitzur (1996,8). Complete Zeeman pattern can
be complex.
Maser propagation/competive effects
18
OH Thermal Absorption
NGC6334
Sarma et al. 2000, ApJ 533, 271-280, VLA 16 x 12
arcsec
19
OH Thermal Emission
Crutcher Troland 2000, ApJ 537, L139-L142
Arecibo 2.8 x 3.2 arcmin
20
CN Zeeman
Crutcher et al. 1999, ApJ 514, L121-L124 Pico
Vateta
DR21(OH) 0.71mG
OMC1n 0.36 mG
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
21
Magnetic Fields in Molecular Clouds
H2O Masers
Crutcher 1999, ApJ 520, 706-713
OH Masers
Excited OH
CN
B ? nH20.5 Ambipolar diffusion? Or constant
VAlfven B(4??)-1/2 ? 0.7 km s-1
22
We Need More Tracers of B
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.
23
Prestellar Cores
Ward Thompson et al. 2000, ApJ 537,
L135-L138 Crutcher et al. 2004, ApJ 600, 279-285
L183
L1544
Bsin? 80?G SCUBA 850 ?m 14 arcsec Bsin?
140 ?G
24
MERLIN
Multi Element Radio Linked Interferometer Network
D 218 km 0.170" 18 cm 0.042 " 4 cm 0.013"
1.4 cm
25
Orion-KL
13x1612-MHz, 430x1665-MHz, 3x1667-MHz masers
OH masers trace a rotating and expanding
molecular torus at the centre of the H2 outflow
(Gasiprong 2000, PhD thesis).
26
Magnetic Beaming in Masers
s -
p
s
W75N
Complete Zeeman patterns rarely
observed. s-components grow fastest and can
suppress p-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.
27
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
28
W75N bipolar outflow
0.6pc

Shepherd et al. 2003, ApJ 584, 882
Large-scale B-field parallel to outflow (submm
poln).
29
2000AU 0.010 pc
OH Masers
1665 MHz
Hutawarakorn Cohen 2002, MNRAS 330, 349
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.

30
1667 MHz and 1720 MHz
OH Masers continued
Magnetic field reverses on opposite sides of disc
(toroidal component). Field lines twisted up in
the rotating disc. Uchida Shibata (1985)
model is supported.
31
Twisted Magnetic Field

• Uchida Shibata 1985 hydrodynamical computation.
• large scale field contracts with disc
• disc twists field lines

(c) close-up of core
PASJ 37, 515
32
Model of OH masers and polarization
Synthetic 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.
33
IRAS 201264104 Bipolar Outflow
Cesaroni et al., in press Plateau de Bure Edris
et al., in preparation MERLIN
Vallee Bastien 2000, ApJ 530, 806-816 SCUBA
B ? outflow
34
H2O Maser Polarization
Sarma et al. 2002, ApJ, 580, 928-937 VLA
Hyperfines?
35
H2O Linear Polarization
Imai et al 2003, ApJ 595, 285-293 VLBA
36
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?