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A magnetically collimated jet from an evolved star

EVN Symposium 2006, Torún. A magnetically collimated jet from an evolved star. Wouter H.T. Vlemmings (Jodrell Bank Observatory, U.K.) Phillip J. Diamond (JBO) H. Imai (Kagoshima University). Credit: NRAO/NSF. ‘Water Fountain’ sources. Show characteristics of evolved stars:

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A magnetically collimated jet from an evolved star

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  1. EVN Symposium 2006, Torún A magnetically collimated jet from an evolved star Wouter H.T. Vlemmings (Jodrell Bank Observatory, U.K.) Phillip J. Diamond (JBO) H. Imai (Kagoshima University) Credit: NRAO/NSF

  2. ‘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 <100 year • Only 5 objects known to date  evolved stars on their way to (Proto-)Planetary Nebula phase

  3. ‘Water Fountain’ sources (Likkel et al. 1992) (Boboltz & Marvel 2005)

  4. W43A • W43A is the archetypal ‘water fountain’ source • The H2O masers exist in a precessing jet • Outflow velocity: v=145 km/s • Inclination: 39° • 5° precession with P=55 yr (Imai et al. 2002)

  5. 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)

  6. 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)

  7. VLBA observation results VLBA observations at Dec 8 2004

  8. 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 T~500 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

  9. VLBA linear polarization results VLBA observations at Dec 8 2004

  10. 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 B=2-35 G. The magnetic field of W43A collimates the jet

  11. Magnetic fields in the envelopes of evolved stars Maser Magnetic Fields: • SiO at ~2 stellar radii • Typical magnetic field strength B~3.5 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 B~0.1-2 G (Vlemmings et al. 2002) • Supergiant VX Sgr shows dipole field (Vlemmings et al. 2005) • OH at ~250-10.000 AU • Field strengths of B~1-10 mG (e.g. Reid et al. 1982; Szymczak et al.) • Indication of alignment with circumstellar envelope (e.g. Etoka et al. 2004)

  12. 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

  13. 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

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