Post on 14-Jan-2016
description
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
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 ~ 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 frames
23 Jun 19977 23 Nov 1997
28 Oct 199822 May 1998
28/10/9822/5/98
(Gonidakis et al. 2003)
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:
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
271026.086.1 skmg
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=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 )
linewidthDoppler
splittingZeeman
r
D
Z
D
ZZ
...)243350.86(12,1
)820539.42(01,2
)122027.43(01,1
GHzJ
GHzJ
GHzJ
05.0005.0~ Zr
NB 310
cmB
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 1994
24 May 1997
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 magneticfield structure.
E-vector reversals at inner-shell boundary
(Soker & Clayton 1999)
28/10/9822/5/98
(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
• C
23/6/97 23/11/97
28/10/9822/5/98
(Gonidakis et al. 2003)