1 / 13

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:

yvonne
Download Presentation

A magnetically collimated jet from an evolved star

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related