Observing magnetic fields in star-forming regions Jim Cohen Jim Cohen The University of Manchester...
-
Upload
cecilia-crawford -
Category
Documents
-
view
223 -
download
0
Transcript of Observing magnetic fields in star-forming regions Jim Cohen Jim Cohen The University of Manchester...
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?