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)