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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desh Raman Research Institute, Bangalore + Miller Goss, Eduardo Mendoza-Torres

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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

  • 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

  • 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


W49N: OH maser sources


The apparent sizes

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 sources


W49N: OH maser sources


Scattering in anisotropic medium

B

Image shape resulting from anisotropic diffraction

Image elongation orthogonal to that of the density irregularities


W49N: OH maser sources

B


W49N: OH maser sources

Gal. Plane

implied

PA~117 deg

107+/- 3 deg


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

  • Wolleben (2007) : North polar spur


W49N: OH maser sources: Zeeman pairs

  • Pairs selected by positional proximity (< 10 mas)‏


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


W49N: OH maser sources: Zeeman pairs


Circular Polarization & Scattering

  • 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

  • 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

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

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


Structure functions


Structure functions


Structure functionB_rms/B ~ 1


Summary:

  • 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.


W49N: OH maser sources


W49N: OH maser sources: Zeeman pairs


OH

energy

levels


  • Spatial Power Spectrum 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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