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The near-circumstellar environment of TX Cam

The near-circumstellar environment of TX Cam. Athol Kemball (NRAO) , Phil Diamond (JBO) and Yiannis Gonidakis (JBO) National Radio Astronomy Observatory P.O. Box 0, Socorro, NM 87801, USA akemball@nrao.edu Jodrell Bank Observatory Jodrell Bank, Univ. Manchester, UK

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The near-circumstellar environment of TX Cam

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  1. The near-circumstellar environment of TX Cam Athol Kemball (NRAO), Phil Diamond (JBO) and Yiannis Gonidakis (JBO) National Radio Astronomy Observatory P.O. Box 0, Socorro, NM 87801, USA akemball@nrao.edu Jodrell Bank Observatory Jodrell Bank, Univ. Manchester, UK pdiamond@jb.man.ac.uk, yiannis@jb.man.ac.uk

  2. The NCSE of late-type, evolved stars • Near-circumstellar environment: • dominated by the mass-loss process • permeated by shocks from stellar pulsation • local temperature and density gradients • circumstellar magnetic fields • complex kinematics and dynamics (Reid & Menten1997)

  3. What does synoptic VLBA monitoring of SiO masers add to NCSE models ? • SiO masers are unique astrophysical probes of the near-circumstellar environment: • Located in the extended atmosphere close to the stellar surface • Compact spatial structure and high brightness temperature • Significant linear and circular polarization • In concert with a theory of maser polarization propagation: • expanded knowledge of physical properties in the masing region. • inference of the B-field magnitude, orientation, spatial distribution, energy density and dynamical influence. • Tag or identify individual maser components in kinematic studies, such as proper motion. • Verify and/or expand basic maser polarization theory

  4. Atmosphere dynamics of late-type, evolved stars • Central stars are large-amplitude, long-period variables (LALPV) • Stellar pulsation drives shocks into the NCSE • Shock emerges at pre-maximum and propagates outwards; gas subsequently decelerates and falls back towards star (double-lined, S-shaped velocity profile) • Material levitated above hydrostatic stellar atmosphere by outward shock propagation • Subsequent radiation pressure on dust couples to the gas and accelerates it outwards Variation of  Ceti continuum photosphere with stellar phase at 11 m by ISI (Weiner, Hale & Townes 2003) Spectroscopic velocity signature of 1.6 m CO  = 3 absorption (Hinkle, Hall & Ridgway 1982 ff)

  5. VLBA monitoring of the SiO masers towards TX Cam • TX Cam is an isolated Mira variable: mass ~ 1-1.5 MO; mass loss rate ~10-6MO/yr; distance 390 pc; period 557 days (80 weeks) • Imaged at 2 to 4 week intervals (~85 epochs obtained) • AAVSO visual light-curve plot versus epochs

  6. 23 Jun 19977 23 Nov 1997 6 frames 22 May 1998 28 Oct 1998

  7. 22/5/98 28/10/98 (Gonidakis et al. 2003)

  8. Mean-shell kinematics • Choose to characterize the gross shell kinematics by the evolution of the mean inner-shell radius with pulsation phase • Inner shell does not take an analytic mathematical form; irregular at almost all epochs • Use robust estimator: fit inner-shell radius as peak in radial intensity gradient for range of position angles => mean inner-shell radius

  9. Mean-shell kinematics • For M~1-1.2 M and D=0.39 kpc; at mean radius of SiO measured here, expect gravitational acceleration: gSiO= -1.73 ± 0.16 x 10-7 km s-2 • Confirmed ballistic deceleration during phases 0.7 to 1.5 • New inner shell appears at phase ~1.5-1.6

  10. Global component proper motions (Humphreys et al. 2002) • Outer components falling back from earlier pulsation cycles • Confirms expected saw-tooth radial velocity profile • Significant local departures from globally ordered flow (Bessell, Scholz & Wood 1996)

  11. Individual component proper motions (N,S,E,W) • Velocities exceed upper limits from expected shock damping in radio photosphere, as deduced from upper limits on continuum stellar variability (~5 km s-2) (Reid and Menten 1997)

  12. SiO maser polarization • Maser action in several vibrationally excited rotational transitions, e.g. • Non-paramagnetic molecule, simple rotor: • Magnetic transitions overlap in frequency, as defined by the splitting ratio: • Zeeman splitting (v=1,J=1-0) for B=10-100 G: • Both Zeeman, and non-Zeeman inferred B-field magnitudes (with significant milliGauss/Gauss differences). • Standard model Zeeman interpretation: • B orientation depends on (<55 deg ||, >55 deg )

  13. 05 Dec 1994 Global polarization morphology • Significant linear polarization; higher circular polarization at VLBI resolution (up to 30-40% for isolated features; median 3-5%) • Ordered global polarization morphology => electric vector generally tangential to the inner maser ring • Significant local anisotropy, particularly in the outer shell with 90° changes in E-vector orientation common 24 May 1997 23 Jan 1999

  14. Global polarization morphology • Possible origins for tangential alignment: • Radiation from central star defines radial quantization axis; combined with assumption of radiative pumping for SiO region => preferential polarization axis tangential to sphere • Global ordered longitudinal B-field within a permitted range of polar axis orientations • Local shock compression at inner shell radius => enhanced tangential B-field and characteristic associated radial B-field signature • Global B-field magnitude in AGB stars remains controversial: models with both global or local dynamical influence proposed.

  15. Tangential vectors generally confined to narrow inner edge of ring. Remarkable circular magnetic field structure.

  16. E-vector reversals at inner-shell boundary (Soker & Clayton 1999)

  17. 22/5/98 28/10/98 (Gonidakis et al. 2003)

  18. Summary • First direct measurement of NCSE kinematics in an LALPV star: • Ballistic deceleration and saw-tooth radial velocity profile confirmed => supporting evidence for pulsation shock model of LPV dynamics • LPV kinematics set by interaction of pulsation and gas infall time-scales => significant inter-cycle variability expected • Representative proper motions of ~5-10 km s-2; exceeds limits from radio continuum stellar variability • Ordered B-field morphology; generally tangential to inner shell with E-vector position angle reversals at shell boundary • Observations favor shock compression of B-field, enhancing tangential component and producing a radial signature • Post-shock B-field magnitudes may be several 10’s G; orders of magnitude greater than the thermal energy density • Global B-field magnitudes in AGB stars still unclear • Spherical symmetry is unsustainable in models of LPV atmospheres; strong asymmetry already evident at tip of AGB before onset of post-AGB and PPN evolution • C

  19. 23/6/97 23/11/97 22/5/98 28/10/98 (Gonidakis et al. 2003)

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