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 Jodrell Bank Observatory Jodrell Bank, Univ. Manchester, UK

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)

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

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)

VLBA monitoring of the SiO masers towards TX Cam TX Cam is an isolated Mira variable: mass ~ M O ; mass loss rate ~10 -6 M O /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 frames 23 Jun Nov Oct May 1998

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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

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

Global component proper motions (Bessell, Scholz & Wood 1996) (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

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)

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= 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  )

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 05 Dec May Jan 1999

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.

IAU206 Tangential vectors generally confined to narrow inner edge of ring. Remarkable circular magnetic field structure.

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

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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

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