Observing magnetic fields in star-forming regions

1. Observing magnetic fields in star-forming regions Jim Cohen 17th February 2004 The University of Manchester Jodrell Bank Observatory Zwolle Workshop

2. Outline of Talk Introduction Polarization Mechanisms Zeeman Splitting Maser Regions

3. Bipolar Outflows Align with Polarization of Starlight Cohen et al. 1984, MNRAS 210, 425-438 Magnetic pressure estimated from OH maser Zeeman splitting is significant in dynamics of bipolar outflow.

4. Are Cloud Cores Collapsing? Virial equilibrium: P s+ |W| = Ms + Mw + 2T P sExternal pressure W Gravitational energy MsStatic B MwAlfven wave B T Internal Kinetic Energy

5. Evolutionary Effects What are the polarization signatures of protostellar evolution? Evolutionary Effects Vallee & Bastien 2000, ApJ, 530, 806-816

6. General Remarks There are many techniques available to estimate B but not usually in one and the same source. Some measurements give B , some give B, some give B magnitude, some give the direction, some give the full vector B. Polarized flux is often less than 1% so we are usually struggling for sensitivity. Stokes parameters are additive. Therefore polarization structure that is unresolved either in frequency or spatially will lead us to underestimate the true degree of polarization.

7. Faraday Rotation   2neB cos dx Can mask true direction of B Pulsar DM  2nedx B cos  RM/DM useful for large-scale Galactic B but not small scale studies of star-formation

8. Continuum Polarization Synchrotron E B Aligned Dust Grains Emission E B (FIR or submm) Extinction EB (optical) Scattering E B (optical, NIR)

9. Interstellar Polarization in Taurus Dark Clouds Messinger, Whittet & Roberge 1997, ApJ 487, 314-319 Well organized on large scale, but only outer layers of dust clouds are probed. Note wavelength dependent PA of two stars – dust properties change with grain size and location (depth) in cloud. Field direction twists inside cloud.

10. Lang et al. 1999, ApJ, 526, 727-743

11. Chuss et al. 2003, ApJ, 599, 1116-1128 350m poln (Hertz on CSO) overlaid on 20-cm continuum Dense: B  b (toroidal) Rare: B b (poloidal)

12. Chuss et al. 2003, ApJ, 599, 1116-1128 Collapse can produce toroidal B in mol cloud while leaving B poloidal outside. Magnetic reconnection can produce the energy for the nonthermal filaments. OR bipolar wind

13. Classical Zeeman Effect An electron in a magnetic field B precesses at the Larmor frequency L = eB/2me . Spectral lines are split into three polarized components at (angular) frequencieso , o + Land o - L Blended: Bcos Unblended: B

14. HI Zeeman NGC6334 source E Weak splitting, sigma components dominate. Stokes V = z BcosdI/d where z is the splitting factor. Measures line-of-sight component Bcos. Instrumental issues limit usefulness to strong fields exceeding ~10G. Sarma et al. 2000, ApJ 533, 271-280, VLA 35 x 20 arcsec

15. M17 Brogan & Troland 2001, ApJ 560, 821-840 VLA OH and HI Bcos increases where Bsin (traced by 100m poln) decreases. Either B is bending around the HII region or the dust properties are being changed by the HII region.

16. Quantum Zeeman Effect A magnetic dipole μ in a magnetic field B has a potential energy μ.B that is quantized: μ.B = B g J μB / ħ where μB = eħ/2me is the Bohr magneton. Lande factor g ~ 1 (paramagnetic) or ~ 10-3 (non-paramagnetic), but depends on total angular momentum F and is different for upper and lower states in general. States split into 2F+1 substates with allowed transitions Δm = +1 Δm = 0 Δm = -1 σ+π σ - Linear polarization is parallel to B forπ components, perpendicular to B for σ components.

17. OH Zeeman Polarization and intensity depend on angle of B to line-of-sight Splitting  B provided hyperfine components don’t overlap. Otherwise see Elitzur (1996,8). Complete Zeeman pattern can be complex. Maser propagation/competive effects

18. OH Thermal Absorption NGC6334 Sarma et al. 2000, ApJ 533, 271-280, VLA 16 x 12 arcsec

19. OH Thermal Emission Crutcher & Troland 2000, ApJ 537, L139-L142 Arecibo 2.8 x 3.2 arcmin

20. CN Zeeman Crutcher et al. 1999, ApJ 514, L121-L124 Pico Vateta DR21(OH) 0.71mG OMC1n 0.36 mG CN 1-0 at 113 GHz Traces 105-106 cm-3 9 hyperfine components, well separated in velocity 4 strong Zeeman, 3 weak Zeeman effect, 2 useless Different splitting factors reduce systematic errors Simultaneous fitting to 4 strong (upper) and weak (lower) components

21. Magnetic Fields in Molecular Clouds H2O Masers Crutcher 1999, ApJ 520, 706-713 OH Masers Excited OH CN B nH20.5 Ambipolar diffusion? Or constant VAlfven B(4)-1/2  0.7 km s-1

22. We Need More Tracers of B OH thermal emission and absorption generally traces the outer regions of molecular clouds but not the dense cores. Crutcher et al. 2004 propose use of randomness in polarization vectors to estimate B (Chandrasekhar & Fermi 1953) based on MHD wave argument Bsin n1/2V -1 L1544 results in OH give smaller B than SCUBA polarimetry at 850 microns which penetrates core. Could have angle  = 16 to line of sight to be consistent.

23. Prestellar Cores Ward Thompson et al. 2000, ApJ 537, L135-L138 Crutcher et al. 2004, ApJ 600, 279-285 L183 L1544 Bsin = 80G SCUBA 850 m 14 arcsec Bsin = 140 G

24. MERLIN Multi Element Radio Linked Interferometer Network D = 218 km 0.170" 18 cm 0.042 " 4 cm 0.013" 1.4 cm

25. Orion-KL 13x1612-MHz, 430x1665-MHz, 3x1667-MHz masers OH masers trace a rotating and expanding molecular torus at the centre of the H2 outflow (Gasiprong 2000, PhD thesis).

26. Magnetic Beaming in Masers W75N σ + π σ - Complete Zeeman patterns rarely observed. σ-components grow fastest and can suppress π-comps (Gray & Field 1995). 100% circular polarization most common. Zeeman shift has same effect as velocity shift. In a turbulent medium LHC and RHC trace different molecules in general.

27. W75N Vector B OH maser polarization indicates 3-d magnetic field with suitable interpretation (need to identify -components) Garcia-Baretto et al. 1988 ApJ 326, 954

28. W75N bipolar outflow 0.6pc Shepherd et al. 2003, ApJ 584, 882 Large-scale B-field parallel to outflow (submm poln).

29. 2000AU 0.010 pc OH Masers 1665 MHz Hutawarakorn & Cohen 2002, MNRAS 330, 349 Kinematics show a rotating and expanding disc (torus) orthogonal to the outflow. Strong linear poln up to 100%. Vectors are either parallel to outflow or perpendicular.

30. 1667 MHz and 1720 MHz OH Masers continued Magnetic field reverses on opposite sides of disc (toroidal component). Field lines twisted up in the rotating disc. Uchida & Shibata (1985) model is supported.

31. Twisted Magnetic Field • Uchida & Shibata 1985 hydrodynamical computation. • large scale field contracts with disc • disc twists field lines (c) close-up of core PASJ 37, 515

32. Model of OH masers and polarization Synthetic maser spectra generated using polarization-dependent model of propagation, with physical conditions taken from Uchida & Shibata (1985) model. Gray et al. 2003, MNRAS, 343, 1067-1080. Masers originate at different depths in disc.

33. IRAS 20126+4104 BipolarOutflow Cesaroni et al., in press Plateau de Bure Edris et al., in preparation MERLIN Vallee & Bastien 2000, ApJ 530, 806-816 SCUBA B  outflow

34. H2O Maser Polarization Sarma et al. 2002, ApJ, 580, 928-937 VLA Hyperfines?

35. H2O Linear Polarization Imai et al 2003, ApJ 595, 285-293 VLBA

36. Where Next? • 3-d magnetic field studies are sensitivity limited for now (key polarized flux is only a small % of total). • Potential to probe range of densities to 1010cm-3. • Major new IR/submm/mm facilities are coming and will overlap with masers at subarcsec resolution. • Some key questions: • How to treat overlap of hyperfine components? • Relation to galactic magnetic field? • Magnetic field evolution, does B dominate? • Maser lifetimes and source evolution?