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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. Formation of massive stars Support against cloud collapse -Turbulence -Magnetic support Maser polarization theory -Zeeman splitting

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Methanol maser polarization in W3(OH)

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  1. Methanol maser polarization in W3(OH) Lisa Harvey-Smith Collaborators: Vlemmings, Cohen, Soria-Ruiz Joint Institute for VLBI in Europe

  2. Overview • Formation of massive stars • Support against cloud collapse -Turbulence -Magnetic support • Maser polarization theory -Zeeman splitting -Stokes parameters • Observations • Results for methanol polarization in W3(OH) • Latest results from OH • Future work using the EVN

  3. Formation of massive stars 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 GMCs are not collapsing globally: support mechanism Supporting processes- (1) Thermal support (vastly insufficient in GMCs) (2) Turbulent support (CO linewidth, should dissipate easily) (3) Magnetic fields (flux freezing)

  4. Turbulent support of clouds • 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

  5. Ambipolar diffusion • Neutral particles coupled to ions by collisions. Ions coupled to B field cloud supported by magnetic field • Weakly ionized gas: Neutrals only weakly coupled to B field. Neutrals can slip through B field support structure and trigger clump collapse • This is called ambipolar diffusion- drags magnetic field into distinctive ‘hourglass shape’ • Acts in proto-stellar accretion disks, molecular clouds and star-forming cores We can look for this effect by observing: • Polarization by dust grains • The Zeeman effectin spectral lines

  6. NGC 1333 IRAS 4A is first textbook example of hourglass magnetic field Observations of magnetic support 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 Girart J.M., Rao R. & Marrone D.P. (2006) Science, 313(5788), 812

  7. Maserpolarization So what can we learn about the star-formation 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

  8. Zeeman splitting ΔmF=0 gives σ transitions (linear) ΔmF=±1 gives πtransitions (circular) Selection rule ΔmF=0,±1 Leads to `Zeeman pairs’ of masers (same position, different velocity)

  9. 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

  10. W3(OH) Harvey-Smith L. & Cohen R.J. (2006) MNRAS, 371, 1550

  11. 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 large-scale field • No Zeeman splitting was observed (methanol molecule is diamagnetic) Vlemmings W.H.T. Harvey-Smith L., Cohen R.J. (2006) MNRAS, 371, L26

  12. 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?)

  13. Comparison with 6.0-GHz 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

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