The near-circumstellar environment of TX Cam

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_at_nrao.edu
• Jodrell Bank Observatory
• Jodrell Bank, Univ. Manchester, UK
• pdiamond_at_jb.man.ac.uk, yiannis_at_jb.man.ac.uk

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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)
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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

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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)
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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

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6 frames
23 Jun 19977
23 Nov 1997
22 May 1998
28 Oct 1998
7
28/10/98
22/5/98
(Gonidakis et al. 2003)
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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 gt mean inner-shell radius

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Mean-shell kinematics

• For M1-1.2 M? and D0.39 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

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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)
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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)

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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 (v1,J1-0) for B10-100 G
• Both Zeeman, and non-Zeeman inferred B-field
  magnitudes (with significant milliGauss/Gauss
  differences).
• Standard model Zeeman interpretation
• B orientation depends on (lt55 deg , gt55
  deg ?)

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Global polarization morphology
05 Dec 1994

• Significant linear polarization higher circular
  polarization at VLBI resolution (up to 30-40 for
  isolated features median 3-5)
• Ordered global polarization morphology gt
  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
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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 gt 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 gt
  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.

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Tangential vectors generally confined to narrow
inner edge of ring.
Remarkable circular magnetic field structure.
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E-vector reversals at inner-shell boundary
(Soker Clayton 1999)
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28/10/98
22/5/98
(Gonidakis et al. 2003)
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Summary

• First direct measurement of NCSE kinematics in an
  LALPV star
• Ballistic deceleration and saw-tooth radial
  velocity profile confirmed gt supporting evidence
  for pulsation shock model of LPV dynamics
• LPV kinematics set by interaction of pulsation
  and gas infall time-scales gt 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 10s
  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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23/6/97
23/11/97
28/10/98
22/5/98
(Gonidakis et al. 2003)