Transcript The near-circumstellar environment of TX Cam Athol Kemball (NRAO) Yiannis Gonidakis (JBO)

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
[email protected]
Jodrell Bank Observatory
Jodrell Bank, Univ. Manchester, UK
[email protected], [email protected]
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 nearcircumstellar 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 largeamplitude, long-period
variables (LALPV)
• Stellar pulsation drives shocks
into the NCSE
Variation of  Ceti continuum photosphere
with stellar phase at 11 m by ISI (Weiner,
Hale & Townes 2003)
• Shock emerges at premaximum and propagates
outwards; gas subsequently
decelerates and falls back
towards star (double-lined, Sshaped 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
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 ~
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 lightcurve plot versus
epochs
23 Jun 19977
23 Nov 1997
6 frames
22 May 1998
28 Oct 1998
22/5/98
28/10/98
(Gonidakis et al. 2003)
Mean-shell kinematics
• Choose to characterize the
gross shell kinematics by the
evolution of the mean innershell radius with pulsation
phase
• Inner shell does not take an
analytic mathematical form;
irregular at almost all epochs
• Use robust estimator: fit innershell 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:
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
g  1.86  0.26  107 km s 2
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)
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.
  1, J  1  0 (43.122027 GHz )
  2, J  1  0 (42.820539 GHz )
  1, J  2  1 (86.243350 GHz ) ...
3
• Non-paramagnetic molecule, simple rotor:  B  10  N
• Magnetic transitions overlap in frequency, as defined by the
splitting ratio: r   Z
Z
 D
 Z  Zeeman splitting
 D  Doppler linewidth
• Zeeman splitting (v=1,J=1-0) for B=10-100 G: rZ ~ 0.005  0.05
• Both Zeeman, and non-Zeeman inferred B-field magnitudes
(with significant milliGauss/Gauss differences).
• Standard model Zeeman interpretation:
• B orientation depends on

B  mc
(<55 deg ||, >55 deg )
05 Dec 1994
Global polarization morphology
• Significant linear
polarization; higher circular
polarization at VLBI
resolution (up to 30-40% for
isolated features; median 35%)
24 May 1997
• 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
23 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)
22/5/98
28/10/98
(Gonidakis et al. 2003)
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
23/6/97
22/5/98
23/11/97
28/10/98
(Gonidakis et al. 2003)