OH maser sources in W49N: probing differential anisotropic scattering & local magnetic fields
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OH maser sources in W49N: probing differential anisotropic scattering & local magnetic fields with Zeeman pairs. desh Raman Research Institute, Bangalore + Miller Goss, Eduardo Mendoza-Torres (also R. Ramachandran ‏ & Sarah Streb) ‏. desh Raman Research Institute, Bangalore

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OH maser sources in W49N: probing differential anisotropic scattering & local magnetic fields with Zeeman pairs

desh

Raman Research Institute, Bangalore

+ Miller Goss,

Eduardo Mendoza-Torres

(also R. Ramachandran‏ & Sarah Streb)‏

desh

Raman Research Institute, Bangalore

+ Miller Goss,

Eduardo Mendoza-Torres

(also R. Ramachandran‏,

desh

Raman Research Institute, Bangalore

+ Miller Goss,

Eduardo Mendoza-Torres

(also R. Ramachandran‏,


W49N scattering & local magnetic fields

  • a well-known, extensively studied star-forming region

  • distance 11.4 kpc; low Galactic latitude‏

  • Interstellar scattering: severe (comparable to the Vela case)‏

  • Short time-scale variability in W3OH (Ramachandran et al 2006); assessing & removing contamination from interstellar scintillations: W49N serves as a reference source (for effect of scattering)

  • Studied earlier by Desai, Gwinn & Diamond (1994): found anisotropic scattering


Oh maser sources in w49n
OH maser sources in W49N scattering & local magnetic fields

  • 12-hour synthesis observations with VLBA

  • high angular-resolution images at 1612, 1665 & 1667 MHz‏

  • region span: ~0.5 pc (at the distance of ~11 kpc)‏

  • beam size: ~20 mas x ~15 mas (Outer antennas excluded)‏

  • 240 spectral channels: resolution: 0.1 km/s; span: 22 km/s

  • 205 spots: elliptical shape, location, velocity, etc. estimated‏

  • A few dozen Zeeman pairs


W49n oh maser sources
W49N: OH maser sources scattering & local magnetic fields


W49n oh maser sources1
W49N: OH maser sources scattering & local magnetic fields


W49n oh maser sources2
W49N: OH maser sources scattering & local magnetic fields


W49n oh maser sources3

The apparent sizes scattering & local magnetic fields

smaller by a factor

of >= 2 compared to

those reported by

Desai et al (1994): OH,

but consistent with

Gwinn(1994): H2O

W49N: OH maser sources

ar=3

ar=1


W49n oh maser sources4
W49N: OH maser sources scattering & local magnetic fields


W49n oh maser sources5
W49N: OH maser sources scattering & local magnetic fields


Scattering in anisotropic medium
Scattering in anisotropic medium scattering & local magnetic fields

B

Image shape resulting from anisotropic diffraction

Image elongation orthogonal to that of the density irregularities


W49n oh maser sources6
W49N: OH maser sources scattering & local magnetic fields

B


W49n oh maser sources7
W49N: OH maser sources scattering & local magnetic fields

Gal. Plane

implied

PA~117 deg

107+/- 3 deg


Magnetic field induced anisotropy in electron density irregularities
Magnetic-field induced anisotropy in electron-density irregularities

  • Desai et al (1994): elongation in Gal plane

  • Their limited sample showed PA variation

  • Some random spread in PA is not unexpected

  • Our data show similar overall correspondence, BUT a significant mean deviation (~10deg) is evident from the PA suggested by the anisotropy induced by magnetic filed in the Galactic plane

  • Is the field direction deviation related to NPS ?


W49 irregularities

  • Wolleben (2007) : North polar spur


W49n oh maser sources zeeman pairs
W49N: OH maser sources irregularities : Zeeman pairs

  • Pairs selected by positional proximity (< 10 mas)‏


W49n oh maser sources zeeman pairs1
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs2
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs3
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs4
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs5
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs6
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs7
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs8
W49N: OH maser sources irregularities : Zeeman pairs


W49n oh maser sources zeeman pairs9
W49N: OH maser sources irregularities : Zeeman pairs


Circular polarization scattering
Circular Polarization & Scattering irregularities

  • Magneto-ionic medium would, in principle, render different refractive indices for the two hands of circular polarization

  • Diffractive scintillations and scatter-image shapes should therefore differ for LHC, RHC due to LOS component of B

  • Hence scattering-dominated images of even a randomly polarized source might show circular polarization in unmatched parts of the images (when Faraday rotation is significant)‏

  • Macquart & Melrose (2000) indeed consider this possibility, but estimate the effect to be too small (10^-8) to be observable!

  • Contrary to that expectation, the scatter-broadened images of some of the W49N OH maser sources seem to significantly differ in L&R polarizations !!


Significant diffrences in image pa
Significant diffrences in image PA irregularities

  • Observed differences in some cases: range between 6 to 30 deg.s (and are significant:~ 7-sigma)‏

  • Difference in the line-velocities, and hence in frequencies, is too small to account for the differential scattering

  • Position differences are also within a few mas


Simulation of differential diffractive effects for the two circular polarizations
Simulation of differential diffractive effects for the two circular polarizations

Column-density distribution

of free electrons

following a power-law

(Kolmogorov; -11/3)‏

spatial-spectrum, with

mild anisotropy.

Simulated spatial extent:

a few Fresnel scales

A uniform magnetic field

would cause a uniform scaling

between phase patterns due to

the medium for the two

circular polarizations.


Simulated images for the two hands of circular polarization
Simulated images for the two hands of circular polarization circular polarizations

Apparent average

elongations and sizes,

i.e. Shapes, of the

two images differ.

Of course, a more

detailed simulations,

with thick screen,

need to be viewed.


Structure functions
Structure functions circular polarizations


Structure functions1
Structure functions circular polarizations


Structure functions2
Structure functions circular polarizations


Structure functions3
Structure functions circular polarizations


Structure function b rms b 1
Structure function circular polarizationsB_rms/B ~ 1


Summary
Summary circular polarizations:

  • The OH maser sources in W49N do show anisotropic scattering, but the apparent scatter broadening is much less that reported earlier.

  • The PAs of the source images deviate significantly from the value expected if scattering density irregularities were to be “stretched” due to magnetic field strictly aligned parallel to the Galactic plane.

  • Differential scattering during propagation of the two circular polarization (due to Faraday rotation) is detected, and providing an interesting probe of the intervening magneto-ionic medium


Thank you
Thank you. circular polarizations


W49n oh maser sources8
W49N: OH maser sources circular polarizations


W49n oh maser sources zeeman pairs10
W49N: OH maser sources circular polarizations: Zeeman pairs


OH circular polarizations

energy

levels


  • Spatial Power Spectrum circular polarizations from different media?

  • How different are SPS slopes and what is this telling us?

  • HI emission: ~ 2.5-3.5 (=<1 deg scale)‏

  • HI absorption: 2.8

  • Optical, IR (Gibson): 2.8

  • IR: 3.5, @0.3pc --> 2.7

  • H2O masers 3.7

  • DM, SM: 3.7

  • DM (Terzan 5): ~3.7

  • - And what does Kolmogorov

  • spectrum actually mean?

  • - Does it imply turbulence

  • or other processes?

  • - How can we observationally

  • probe turbulent dissipation scales?


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