A magnetically collimated jet from an evolved star

1
A magnetically collimated jet from an evolved star
EVN Symposium 2006, Torún

• Wouter H.T. Vlemmings (Jodrell Bank Observatory,
  U.K.)
• Phillip J. Diamond (JBO)
• H. Imai (Kagoshima University)

Credit NRAO/NSF
2
Outline

• Water Fountain Sources
• W43A
• H2O maser polarization
• Results of the VLBA polarization observations of
  W43A
• Linear polarization
• Circular polarization
• Interpretation
• Magnetic field
• Physical properties of the maser region
• Magnetic fields around evolved stars
• Origin of the magnetic field
• Conclusions

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Water Fountain sources

• Show characteristics of evolved stars
• SiO, H2O and OH masers
• A-typical H2O maser spectrum with velocity spread
  well outside OH maser range (150 km/s)
• Often typical double peaked OH maser spectrum
  (20 km/s)
• Imaging reveals H2O masers at the red- and
  blue-shifted tip of bi-polar jet
• Dynamical age lt100 year
• Only 5 objects known to date
• ? evolved stars on their way to
• (Proto-)Planetary Nebula phase

4
Water Fountain sources
(Likkel et al. 1992)
(Boboltz Marvel 2005)
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W43A

• W43A is the archetypal water fountain source
• The H2O masers exist in a precessing jet
• Outflow velocity v145 km/s
• Inclination 39
• 5 precession with P55 yr

(Imai et al. 2002)
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W43A

• W43A is the archetypal water fountain source
• The H2O masers exist in a precessing jet
• OH masers in shell with off-set blue- and
  red-shifted peaks

(Imai et al. 2002)
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W43A

• W43A is the archetypal water fountain source
• The H2O masers exist in a precessing jet
• OH masers in shell with off-set blue- and
  red-shifted peaks
• SiO masers in a biconical outflow
• Additional continuum source at 1300 AU possibly
  related to the outflow

(Imai et al. 2005)
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H2O Maser polarization

• 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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Polarization Analysis

• Non-LTE method
• (Nedoluha Watson 1992)
• AF varies with maser saturation

PV ? ( Vmax Vmin ) / Imax AF BGauss / ?v
km/s

• Calculate Equations of State
• 3 dominant Hyperfine lines
• Their magnetic substates

• Direct fit of the observations to the models
• Yields magnetic field, emerging brightness
  temperature (saturation), intrinsic thermal width
• Unknown angle ? between line-of-sight and
  magnetic field
• Linear polarization can contstrain ?
• Direction of magnetic field perpendicular or
  parallel to polarization angle
• depends on ?

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VLBA observation results
VLBA observations at Dec 8 2004
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Intrinsic properties of the maser region

• From the H2O maser model fitting to the maser
  feature where circular polarization was detected
  we find
• Intrinsic thermal line width of the maser vth
  1.1 km/s
• This indicates a temperature in the masing
  region T500 K
• the masers are partially saturated
• H2O masers are typically excited in regions with
  hydrogen density nH2 108 - 1010 cm-3
• If the masers are shock excited, models indicate
  the pre-shock density to be 3?106 cm-3
• Unlikely at 1000 AU from the star
• Masers exist in swept up material
• High density enhances magnetic field by a factor
  between 50 and 250 assuming partial coupling

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VLBA linear polarization results
VLBA observations at Dec 8 2004
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Polarization interpretation

• The linear polarization vectors on the H2O
  masers in the tip of the W43Aprecessing jet are
  mostly perpendicular to the magnetic field
  direction. ? Toroidal magnetic field.
• The circular polarization fraction is PV 0.33
  0.09 .
• Using the H2O maser polarization models this
  indicates a magnetic field along the maser
  propagation direction of B 85 33 mG.
• The (de-projected) toroidal magnetic field
  component in the jet is estimated to be B? 200
  mG.
• The magnetic field is enhanced in the high
  density H2O masers which are excited in swept up
  material.
• ? Magnetic field around the jet in the lower
  density material at 1000 AU from W43A is B 0.5
  - 3 mG.
• Extrapolated to W43A (B? ? r-1) indicates a
  surface magnetic field of B2-35 G.
• The magnetic field of W43A collimates the jet

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Magnetic fields in the envelopes of evolved stars

• Maser Magnetic Fields
• SiO at 2 stellar radii
• Typical magnetic field strength B3.5 G (Herpin
  et al. 2006)
• up to several tens of Gauss
• Ordered (radial) magnetic field (Kemball
  Diamond 1997)
• H2O at 50-500 AU
• Magnetic fields of B0.1-2 G (Vlemmings et al.
  2002)
• Supergiant VX Sgr shows dipole field (Vlemmings
  et al. 2005)
• OH at 250-10.000 AU
• Field strengths of B1-10 mG
• (e.g. Reid et al. 1982 Szymczak et al.)
• Indication of alignment with circumstellar
  envelope (e.g. Etoka et al. 2004)

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Origin of the Magnetic Field

• Local magnetic fields ?
• Unable to explain large scale structure in SiO,
  H2O as well as OH maser observations
• ? large scale fields exist and collimated W43A
  jet
• Internal dynamo between stellar envelope and fast
  rotating core ?
• Extra source of rotation needed to counteract
  energy loss due to field drag ?
• Interaction with circumstellar disk ?
• Spin-up due to binary or heavy planet ?
• Possible source of the W43A jet precession though
  large sample of magnetic stars show no indication
  of companion
• Tight binary would likely disrupt maser action

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Conclusions

• VLBA polarization observations of the H2O masers
  at the tip of the jet of W43A indicate
• toroidal magnetic field
• magnetic field strength implies magnetic
  collimation
• (200 mG in the jet, 1 mG outside, 20 G at the
  stellar surface)
• field strength consistent with maser magnetic
  field measurements in large sample of evolved
  stars
• magnetic field origin unknown
• First direct detection of an
• astrophysical magnetically
• collimated jet

presented in Vlemmings, Diamond Imai, 2006,
Nature, 440, 58