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OH maser sources in W49N: probing differential anisotropic scattering with Zeeman pairs. desh Raman Research Institute, Bangalore + Miller Goss, Eduardo Mendoza-Torres, Tanmoy Laskar, & R. Ramachandran . W49N. a star-forming region
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OH maser sources in W49N: probing differential anisotropic scattering with Zeeman pairs desh Raman Research Institute, Bangalore + Miller Goss, Eduardo Mendoza-Torres, Tanmoy Laskar, & R. Ramachandran
W49N • a star-forming region • distance 11.4 kpc; low Galactic latitude • Interstellar scattering: severe (comparable to the Vela case) • Short time-scale variability in W3OH (Tanmoy's talk); assessing 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
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
Scattering in anisotropic medium B Image shape resulting from anisotropic diffraction Image elongation orthogonal to that of the density irregularities
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)
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.
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
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?
