Magnetic Fields in the Envelopes of Late-Type Stars: Circular Polarization of H2O Masers

1
Magnetic Fields in the Envelopes of Late-Type
StarsCircular 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

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Circumstellar Masers

• Onion model
• Dust at few AU
• Molecules until
• dissociation by UV
• Excitation varies
• SiO at few AU
• Water at up to few 100 AU
• OH at 500 2000 AU
• As Vexp increases
• from Tangential to
• radial amplification

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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
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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).

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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

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Polarization Analysis

• LTE method

• 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 BGauss /
?v km/s
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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, AA 375
L1
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Polarization Analysis

• Non-LTE method

• 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)

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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)

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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, AA
394, 589
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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)

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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 192962227 and
  K3-35