Stellar Evolution as seen through the eyes of VLBI and masers

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Stellar Evolution as seen through the eyes of
VLBI and masers
P.J.Diamond
Jodrell Bank Observatory University of Manchester
Castel San Pietro Terme 19 September 2001
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Stellar Evolution in a flash
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Some definitions

• AGB Asymptotic Giant Branch
• LPV Long Period Variable (P gt 50-100 days)
• Mira variables M-type stars with DV gt 2.5m
  (P100-500d)
• OH/IR star, LPVs with P 500 3000 days. Rarely
  optically detected, bright in the IR thick dust
  shells.
• PNe/PPN Planetary Nebulae, Pre-Planetary Nebulae
• EVN European VLBI Network
• VLBA Very Long Baseline Array
• MERLIN Multi Element Radio Linked Interferometer
  Network

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The Standard Model

• Goldreich Scoville (1976)
• Red giant, spherically symmetric gas outflow. SiO
  exists, mases.
• At r, dust forms, absorb light re-emit as IR,
  momentum coupled to gas, radiation pressure gt
  gas accelerating and heating.
• Cools by H2O formed in lower envelope. Masers
  from H2O.
• UV photons disassociate H2O gt OH, H. OH masers
  form. Gas at terminal velocity.

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Colour-colour diagrams (van der Veen Habing,
AA, 1988, 194, 125
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The Lewis Chronology.
Miras
OH/IR
PPN
PNe
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Interferometry
VLBA provides frequency flexibility and ability
to observe up to 43 GHz. EVN, with big dishes,
provides sensitivity at 18cm and ability to
observe at 6 GHz
MERLIN is instrument of choice for OH and
larger-scale H2O maser studies.
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IRAM image of shell around Carbon star TT
CygOlofsson et al 1999 CO(1-0) _at_ 115GHz
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• Not all circumstellar envelopes are smooth and
  symmetric
• Monnier et al (ApJ, 512, 351, 1999) observed the
  circumstellar dust around the supergiant VY Cma
  using adaptive optics and aperture masking
  techniques

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SiO Masers

• VLBA has enabled routine observations of SiO
  masers
• Early attempts by Moran et al (1979, ApJ, 231,
  L67), Lane (1982, Ph.D), McIntosh (1987, Ph.D)
  demonstrated the difficulty of 7mm VLBI and
  showed that, with the instruments available at
  that time, the SiO emission was clustered in
  regions similar in size to that of the stars.
• Colomer et al (1992, AA, 254, L17) showed that
  modern equipment could detect compact structure
  in SiO masers
• Diamond et al (1994, ApJ, 430, L61) produced the
  first image of SiO masers around the stars TX Cam
  and U Her
• ring-like structures, ordered not random, masers
  are tangentially beamed
• Confirmed by Miyoshi et al (1994, Nature, 371,
  395) and Greenhill et al (1995, ApJ, 449, 365)

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SiO proper motions
Predominant motion is outflow Strong evidence for
shocks dominating the kinematics
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SiO Polarization studies
Tangential vectors confined to narrow inner edge
of ring. Strong evidence of effects of shocks.
Remarkable circular magnetic field structure.
Origin unknown
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H2O

• Proper motions of masers around stars relatively
  undeveloped subject until the advent of the
  VLBA.
• Marvel (Ph.D, 1996) studied the proper motion of
  H2O masers around several stars.
• Demonstrated that masers were expanding as
  expected, some peculiarities showing departures
  from smooth, symmetric outflows

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OH shells thin CSEs Miras
Multiple epoch VLBA observations of the thin
shell source U Her (van Langevelde et al,
2000) Designed to measure parallax and study fine
structure.
?VLBA -17.05?0.85, -9.48?0.73 mas/yr. Parallax
detected ?VLBA 4.2 ? 1.2 mas.

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OH Shells thick CSEs
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OH Shells

• MERLIN observations were the first to show the
  existence of shells of masing gas around AGB
  stars (Booth et al, 1981, Nature, 290, 382)

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OH Polarization
Polzn vectors tangential to circumstellar
envelope. Linear polzn 10-20 Structure
favours a radial field maybe we are viewing a
dipole field end-on. Circular polarization gt
B 1.1 mG
VX Sgr 1612 MHz Szymczak et al, in prep
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OH polarization

• MERLIN observations of NML Cyg (Diamond Etoka,
  in prep)
• In this case
• 1612 MHz field lines are predominantly parallel
  to each other
• 1665 MHz field lines suggest a tangential
  structure

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Getting older

• IRAS 184550448 a dying maser. Lewis et al, 2001

1665 remains largely unaffected. Maybe witnessing
the early stage of expansion of fossil shell
prior to it becoming a planetary nebula. Mass
loss ? 0, UV photodissociating OH.
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OH009.1-0.4 Sevenster Chapman (2001)

• Presence of 1720 MHz OH in CSE
• Suggest is early post-AGB object
• Suggest 1720 MHz collisionally excited in region
  where interaction of remnant AGB wind and hotter,
  fast post-AGB wind cause shocks

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Pre-planetary nebulae

• Zijlstra et al (in press) investigate development
  of bipolar outflows. Observed 10 OH/IR stars with
  irregular OH spectra and unusually large
  expansion velocities.

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Roberts 22 Dyer, Goss Kemball
V_torus 20km/s dV/dx 5 km/s/arcsec
Modelling supports PPN hypothesis
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The Big Picture
Progenitor to Planetary Nebulae? Is axisymmetry
controlled by magnetic field?