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: (1) (1) Polarization by dust grains (2) (2) The Zeeman effect in spectral lines

6. 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 NGC 1333 IRAS 4A is first textbook example of hourglass magnetic field

7. Maser polarization 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 Leads to `Zeeman pairs’ of masers (same position, different velocity) Selection rule Δ m F =0,±1 Δ m F =0 gives σ transitions (linear) Δ m F =±1 gives π transitions (circular)

9. (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 Stokes Parameters Degree of linear polarization Polarization angle χ The Stokes parameters are related to the amplitudes of the components of the electric field E x and E y perpendicular to the direction of propagation of the wave

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

11. Vlemmings W.H.T. Harvey-Smith L., Cohen R.J. (2006) MNRAS, 371, L26 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)

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