Transcript Methanol maser polarization in W3(OH)

Methanol maser
polarization in W3(OH)
Lisa Harvey-Smith
Collaborators: Vlemmings, Cohen, Soria-Ruiz
Joint Institute for VLBI in Europe
Overview
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Formation of massive stars
Support against cloud collapse
-Turbulence
-Magnetic support
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Maser polarization theory
-Zeeman splitting
-Stokes parameters
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Observations
- Results for methanol polarization in W3(OH)
- Latest results from OH
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Future work using the EVN
Formation of massive stars
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Massive stars form in clusters within dense clumps in GMCs
Local cloud collapse often triggered by a shock, causing a
density enhancement
free-fall gravitational collapse
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GMCs are not collapsing globally: support mechanism
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Supporting processes-
(1) Thermal support (vastly insufficient in GMCs)
(2) Turbulent support (CO linewidth, should dissipate easily)
(3) Magnetic fields (flux freezing)
Turbulent support of clouds
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Turbulence should quickly dissipate by damping
in a molecular cloud
Possible mechanisms of turbulence regeneration:
MHD waves
Dynamical
feedback from
other ‘events’
(external shocks,
outflows etc.)
Magnetic fields more influential than
turbulence in cloud support
Ambipolar diffusion
Neutral particles coupled to ions by collisions.
Ions coupled to B field
cloud supported by
magnetic field
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Weakly ionized gas: Neutrals only weakly
coupled to B field. Neutrals can slip through B
field support structure and trigger clump
collapse
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This is called ambipolar diffusion- drags
magnetic field into distinctive ‘hourglass
shape’
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Acts in proto-stellar accretion disks, molecular
clouds and star-forming cores
We can look for this effect by observing:
(1)
Polarization by dust grains
(2)
The Zeeman effect in spectral lines
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Observations of magnetic support
NGC 1333 IRAS 4A is first textbook example of hourglass magnetic field
Polarization of dust
emission was recorded
using the Smithsonian
Sub-mm Array
 Green & red regions
mark the locations of
two protostars.
 Magnetic field is
warped by collapsing
material
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Girart J.M., Rao R. & Marrone D.P. (2006) Science, 313(5788), 812
Maser polarization
So what can we learn about the starformation process by observing
masers?
 Bright spectral lines – can undergo
Zeeman splitting (yielding magnetic field
strength)
 Full polarization observations allow
measurements of the direction and
strength of linear polarization by
comparing Stokes-IQU maps
Zeeman splitting
ΔmF=0 gives σ transitions (linear)
Selection rule ΔmF=0,±1
ΔmF=±1 gives π transitions (circular)
Leads to `Zeeman pairs’ of masers (same position, different velocity)
Stokes Parameters
The Stokes parameters are related to the amplitudes of the
components of the electric field Ex and Ey perpendicular to the
direction of propagation of the wave
Degree of linear polarization
Polarization angle
χ
Q2  U 2
L 
I
1
U 
  tan1  
2
Q
(1) Make maps of Stokes I,Q & U and calculate polarization vectors
(strength and direction) using AIPS.
(2) Make maps of LCP and RCP to search for Zeeman
pairs
W3(OH)
Harvey-Smith L. & Cohen R.J. (2006) MNRAS, 371, 1550
Methanol
polarization maps
 First maps of methanol
polarization published this
month
 Methanol polarization
vectors lie perpendicular
to the large-scale maser
filament
 Infer that magnetic field
is parallel to filament
(seeing polarization of σ
components)
 The magnetic field is in
the ‘broadline region’
aligned at 90° to largescale field
 No Zeeman splitting was
observed (methanol
molecule is diamagnetic)
Vlemmings W.H.T. Harvey-Smith L., Cohen R.J. (2006) MNRAS, 371, L26
Methanol polarization summary
 From non-detection: upper limit to magnetic field strength = 22
mG
 Consistent with 2-11 mG (1.6-GHz OH), 15 mG (6.0-GHz OH)
and 10 mG (13.4-GHz OH)
 The linear polarization was between 1-8%
 Consistent with methanol 12.2-GHz
 Faraday rotation affects higher frequencies less that lower
frequencies
 Methanol polarization is a better measure of B than OH
polarization
 Magnetic field is oriented parallel to the N-S extended maser
filament in W3(OH)
 Large spread of polarization vectors in broadline region – need
theoretical basis for this (disc, outflow, shock?)
Comparison with 6.0GHz OH masers
 Polarization of OH
molecule is much stronger
(molecule is paramagnetic)
 Many components are
polarized- confusion by
blending of adjacent
features
Work in progress:
OH 6031-MHz, 6035-MHz
in W3(OH)
Methanol 6.7-GHz
polarization in other
sources e.g. DR21(OH)
Harvey-Smith et al. in prep
Future: EVN/VLBA