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Forming Field Structures around Cores (  Discs)

courtesy Jonathan Ferreira. Forming Field Structures around Cores (  Discs). Jane Greaves University of St Andrews, Scottish Universities Physics Alliance. magnetised dust. torques align grain minor axis with B-field p linear from net emission parallel to major axis.

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Forming Field Structures around Cores (  Discs)

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  1. courtesy Jonathan Ferreira Forming Field Structures around Cores ( Discs) Jane Greaves University of St Andrews, Scottish Universities Physics Alliance

  2. magnetised dust • torques align grain minor axis with B-field • plinear from net emission parallel to major axis details & figures from Lazarian (2007), JQSRT 106, 225

  3. (reality) • likely very approximate, for real grains growing in the cold mid-plane of discs! possible grain structure (Helen Fraser)

  4. molecules in magnetic fields • Goldreich-Kylafis effect • if Zeeman splitting << Doppler shifts • if the projection M of J changes by 1, photon can be  to B-field > anisotropy • resulting plincan be II or  to Bplane-of-sky ; need to know v-field to determine +1 0 -1 1 J M 0 0 B numerical results for CO J=1-0 in 1-D geometry (Deguchi & Watson 1984)

  5. polarimetry challenges • ‘photon starved’ if e.g. plinear ~ 1% • p-vectors are projection of net Bplane-of-sky • hence degenerate solutions of field structure • GK data can separate regions by vline-of-sight … very useful for (unresolved) rotating discs! B F B ? ? v Bdust 

  6. inversion problem means interpretation may involve range of plausible models • example: are B-fields helical in and around filamentary molecular clouds? Matthews, Wilson & Fiege 2001

  7. different assumptions about physics can also affect results • e.g. field strengths inferred from the Chandrasekhar-Fermi method • linear B distorted by turbulence? • or field shaped by other effects? Nutter et al. (2004)

  8. surveys = knowledge! • wide fields, many objects is the key to understanding very diverse polarisation results • well resolved cores & discs also essential… but need to understand the larger scale magnetised environment in which these formed • mm imaging-polarimeters exist for dust & lines POL-2 (PI Pierre Bastien) for SCUBA-2 camera at 450, 850 microns; HARP 345 GHz band camera & ROVER polarimeter

  9. line polarimetry is very photon starved… bandwidths ~104 times less than continuum! • e.g. ROVER (PI Greaves) at JCMT in 1.3, 0.8 mm bands; latter with 4x4 pixel HARP camera, 2’ FOV • verified by XPOL cross-checks at IRAM 30m, courtesy Helmut Wiesemeyer, Clemens Thum… I plinear pcircular angle R Leo, SiO 2-1 < ROVER XPOL >

  10. dust polarisation surveys • Hertz atlas • Jessie Dotson et al., 2010: 9 years of pcontinuum maps at 350 microns • 56 Galactic sources, mainly star-forming regions • SCUpol atlas • Brenda Matthews et al., 2009: 7 years of pcontinuum maps at 850 microns • 15 cores/globules, 11 YSOs, 48 star-forming regions… … examples shown next (E-vectors shown; rotate 90 for B)

  11. Matthews & Wilson 2002 – organised field, but depolarisation around cores

  12. Matthews et al. 2002 – filament wrapped by a helical field?

  13. Ward-Thompson et al. 2000 – starless core + YSO (field is  to disc)

  14. isolated few-MSun pre-star-forming cores at ~1 kpc

  15. VeLLO Class 0 protostars: ~0.15 LSun (N.E.) & binary of ~ 0.08, <0.03 Lsun (S.W.)

  16. field patterns • patterns around individual cores are not simple to interpret… but overall: • organised fields seen in pre-star-forming material • examples along filaments, through flattened cores • more complex patterns around star-formation sites • reduced p% - due to unresolved structure, or to less efficient grain alignment? • need to know what initial fields are like, on larger scales, as parent clouds form • e.g. material swept up by SNR & ISM bubbles…

  17. initial fields • B-field in Cas A (SNR) dust ring appears radial • NB, difficult experiment, as foreground clouds also seen (CO contours) • dust polarisation (see e.g. uncontaminated north side) shows high percentages… • to do with newly formed dust, or ordered field? Dunne et al. 2009: B vectors, I scale, CO contours

  18. find AFGL 333, part of W4 shell, is very similar! • net radial field & high p% • field seems to ‘wrap around’ most prominent cores Greaves & Holland 2002 (JCMT Newsletter); B-vectors

  19. fields around or in discs • two examples with SCUpol showed net field directions aligned with disc major axes • interpreted as toroidal fields, on scales << beamsize Tamura, Hough, Greaves et al. 1999: 850 microns

  20. fields within discs • however… polarimetry at other wavelengths and with interferometry  no plinear! Hughes et al. (2013): no p-vectors on 100’s AU scales and at similar wavelength to Tamura et al. (1999) Krejny et al. (2009): “Plot in Stokes space of Tamura et al. (1999) 850 μm polarization (dark gray) together with 350 μm (light gray). Circles signify 1σ and 2σ errorbars. The vertically hatched region denotes the locus of points consistent with polarization orientated orthogonally to the plane of the disk.”

  21. organised field – where?? • are we seeing polarised surrounding material, rather than the disc? • penvelope or poutflow could dominate, if very organised fields, even if dust flux contribution is less than disc example: CO line polarisation seen in bright outflow in Orion; Greaves et al. (2001)

  22. Tau/Aur is a diffuse star-forming region, but see cloud structures at 850 microns … from the SCUBA atlas, Di Francesco et al. 2008

  23. discs & outflows • SCUpol observation logs show 6 T Tauri stars were observed, only 2 published • 1997-8; data acquisition/reduction not then optimised! • comparison of disc orientations to net B-field (high-resolution disc images from: Guilloteau / Isella / Kitamura / Qi) PA(disc) PA(Bnet) PA • DG Tau 119 115  8 few • GM Aur 57 55  10 few • DG Tau B 26 116  7 90 • T Tau ~4 93  6 ~90 • HL Tau 140 145-170 <30 • TW Hya 135 (p < 2%) ? ( face-on)

  24. analogous situations? • these results suggest that there are magnetic field contributions from disc, outflow, envelope…? • interestingly ! … similar results for several evolved stars with outflows and possibly discs • NB fast flows mean line polarisation is also feasible • common physics in action? Sabin, Zijlstra & Greaves 2002; B-vectors for CRL 2688 (Egg Nebula)

  25. the future • POL-2 science commissioning is well underway! • much larger FOV, multiple regions… • will be able to study field structures from filament-to-disc scales  POL-2 commissioning image, courtesy PI Pierre Bastien

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