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Magnetic Fields in the Envelopes of Late-Type Stars: Circular Polarization of H 2 O Masers

Magnetic Fields in the Envelopes of Late-Type Stars: Circular Polarization of H 2 O Masers. Wouter Vlemmings, Cornell University Phil Diamond, Jodrell Bank Huib Jan van Langevelde, JIVE. Role of Magnetic Fields. Mass loss Alfv én waves can drive stellar winds and produce clumpy mass loss

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Magnetic Fields in the Envelopes of Late-Type Stars: Circular Polarization of H 2 O Masers

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  1. Magnetic Fields in the Envelopes of Late-Type Stars:Circular Polarization of H2O Masers Wouter Vlemmings, Cornell University Phil Diamond, Jodrell Bank Huib Jan van Langevelde, JIVE

  2. Role of Magnetic Fields • Mass loss • Alfvén waves can drive stellar winds and produce clumpy mass loss • Outflows • Shaped by magnetic fields • Magnetic pressure dominates the thermal/kinetic pressure for high magnetic fields • Planetary nebulae

  3. Previous Observations • SiO Masers: • Highly ordered Magnetic Fields • Field Strengths (Zeeman): • Supergiants: up to 100 G • Miras: 10-30 Gauss • But: non-Zeeman interpretation: • Fields factor 1000 less • OH Masers: • Some indication of alignment with CSE structure. • Field Strengths: • Both Supergiants and Miras show a few mG fields Kemball and Diamond, 1997, ApJ 481 L111

  4. H2O Masers • H2O maser 616 – 523 rotational transition. • 22.235 GHz • 6 Hyperfine transitions • Non-paramagnetic: • Factor 103 weaker than for radicals like OH. • Expected splitting 10-3 times typical maser line width (20 kHz).

  5. Observation Calibration • VLBA observations of 4 late type stars. • (S Per, U Her, VY CMa and NML Cyg) • Correlated twice: • All 4 polarizations, 0.1 km/s resolution. • RR and LL only, 0.027 km/s resolution. • Calibration: • First calibration on low spectral resolution • Apply solutions on high resolution data

  6. LTE method: Polarization Analysis • Calculate Zeeman splitting • For 3 dominant hyperfine lines • Create Synthetic Circular Polarization Spectrum • Proportional to derivative of total power, I’ • Determine AF-F’ in: PV  ( Vmax – Vmin ) / Imax = AF-F’· B[Gauss] / v [km/s]

  7. Magnetic Fields Results • Clear detections • Only few % • Rule out systematics: • Varying values and directions • B|| = 207 ±30 mG • But: • V spectrum narrower than thermal Zeeman • No linear polarization Vlemmings, Diamond, van Langevelde, 2001, A&A 375 L1

  8. Non-LTE method: Polarization Analysis • Calculate Equations of State • Linear maser geometry • Including interaction between: • 3 dominant Hyperfine lines • Their magnetic substates • Total of 99 non-linearly related equations • Solve for various thermal line widths of the maser medium • Directly fit the observations to the models • Partly explains narrowing • (2D or 3D could provide solution)

  9. Results • S Per: • H2O: 150 mG / 200 mG • OH: 1 mG (Masheder et al. 1999) • VY CMa: • H2O: 175 mG / 200 mG • SiO: 65 G (Barvainis et al.1987) • OH: 2 mG (Cohen et al. 1987) • NML Cyg: • H2O: 500 mG / 500 mG • OH: 2 mG (Cohen et al. 1987) • U Her: • H2O: 1.5 G / 2.5 G • OH: 1 mG (Palen & Fix 2000)

  10. Magnetic Fields in CSEs • Observations trace • Inner edge of the maser region • High density clumps • Favors Solar Type (r-2) magnetic fields • Surface field of 100 G (Miras) to 1 kG (Supergiants) • Magnetic pressure can drive outflows and help shape nebulae Vlemmings, Diamond, van Langevelde, 2002, A&A 394, 589

  11. Planetary Nebulae • Magnetic pressure in the H2O maser region: •  8  nH k T / B² (ratio of thermal and magnetic pressure) •   0.05; the magnetic pressure dominates by a factor of 20 for B  250 mG. • Asymmetric nebulae possibly due to: • magnetic shaping of the outflow (García-Segura, 1999) • wind interaction with a warped circumstellar disk (Icke, 2003) • warped disk may be caused by high magnetic fields (Lai, 1999)

  12. Conclusions • Zeeman interpretation is favored • No linear polarization • LTE models appear too simple • Coupled transfer models (non-LTE) promising • Constraints on saturation & beaming • Inferred magnetic fields fit nicely • Compared to OH & SiO values and solar type magnetic field • Indicate surface fields of  1 kG • Comparable to dynamo-produced fields (Blackman et al. 2001) • The one Mira star in sample appears to have a stronger field • Indicates H2O maser in thick shell, closer to the star • New VLBA observation will expand sample • observed: VX Sgr, R Cas, U Ori • Masers in P-PNe can provide clues on evolution of the magnetic fields • proposed observations on: IRAS 19296+2227 and K3-35

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