VLBI observations of two 43GHz SiO masers in R Cas

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VLBI observations of two 43-GHz SiO masers in R
Cas

• Jiyune Yi
• Korea VLBI Network ( KVN ) group
• Korea Astronomy and Space Science Institute
• In collaboration with
• R. Booth 1,2 and J. Conway 1
• 1.Onsala Space Observatory, Sweden
• 2. Hartebeesthoek Radio Astronomy Observatory

8th EVN Symposium 2006
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Asymptotic Giant Branch
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Stellar masers Evolved stars

• SiO, H2O and OH masers form in the extended
  stellar atmosphere
• circumstellar envelope of evolved star (AGB
  stars)

?High resolution studies of SiO masers ? unique
tool to study extended stellar
atmosphere of AGB stars
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SiO maser in AGB star
adopted by J. Hron, original idea by T. Le Bertre
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Scientific goalsVLBA observations of SiO masers

• To find significant constraints on SiO maser
  modellings ? evidence of stellar phase dependence
• To provide highly plausible inputs for new models
  
• To extend our understanding on the physical and
  dynamical properties of CSEs ? positions of
  individual maser clumps measured down to
  sub-milliarcsecond accuracy
• To put confidence in non-standard VLBI
  techniques, (both observations and calibrations)

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Technical challenge

• To track the delay across the 301 MHz frequency
  gap between the v1 and v2 transitions
• ?Simultaneous observations of the two maser
  transitions required
• To determine the relative position of the masers
• in the two transitions
• ?Imaging the two maser maps relative to each
  other using cross-phase referencing

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4 epochs of VLBA observations R Cas
R Cas Light curve (courtesy,AAVSO)
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Epoch I (F 0.25)
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Epoch II (F 0.68)
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Epoch III (F 0.95)
10 mas 1.07 AU
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Epoch IV (F 0.23)
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R Cas image at 671 nm (Weigelt et al. 1996)
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R Cas photospheric size measured by optical/IR

• Weigelt et al. 1996
• 36 mas(700 nm), 49 mas(714)
• Hofmann et al. 2000
• 44 mas(671), 37 mas(700)
• 49 mas(714), 30 mas(1045)
• Mennesson et al. 2002
• 24.78 mas (F 0.09) at 2.16 mm
• 31.09 mas (F 0.17) at 3.79 mm
• ?

• 700 nm
• R Cas

• 714 nm
• R Cas

Weigelt et al. 1996
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Stellar photospheric size versus SiO maser shell
size of R Cas

• Angular diameter lt 30 mas, at near IR continuum
• Angular diameter gt 30 mas, at visible
• ?
• Comparison with 3.8 2.2 mm radii, R(3.8)
  R(2.2)
• at F0.17 0.09, respectively
• (obs. by Mennesson et al. 2002)

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Summary R Cas

• Instead of ring disruption at near maser minimum
  (Epoch II) both masers formed circular rings.
• Both maser rings expanded and contracted
  depending on the stellar phase. At maser maximum,
  both masers showed many coincident masers.
• Outward-extending flare-like structure of
  emission survived over 2 epochs ( 0.3 stellar
  phase) .
• SiO maser shell diameters estimated around 1 2
  stellar diameter.
• Asymmetry found at Epoch I , asymmetric ejection
  of material directed away from us ?
• Models which are predominantly collisionally
  pumped are in good agreement with our results.
• Missing flux (typically more than 50 ) density
  found,
• estimated a lower limit of the structure,
  34 mas

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TX Cam maps at 4 epochs (Jiyune Yi et al. 2005)
10 mas 3.8 AU
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Analysis of MASER ring radius
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Comparison with models
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Comparison with models

• Ring shape ?
• Disruption of the ring structure at maser
  minimum
• development of the ring afterwards
• Ring radius ?
• Expansion and contraction along the stellar
  cycle
• Ratio of the ring radius, v2/v1 ?
• 96 (III) vs 94 (M) 91 (IV) vs 92
  (M)
• relatively smaller R at IV ? v2, contracting
  while
• v1, constant
• Ring thickness (2575 percentiles) , v1 vs v2
  ?
• 15.6 vs 14.9 (III) 19.5 vs 18.6
  (IV) of the ring radius ?? twice thicker in the
  v1 ring (M)

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Constraint on SiO maser models

• To excite the lowest vibrational state ,v1
• Required temperature gt1800 K
• Unable to have spatial coincidence of masers in
  various v-states by
• radiative pumping

3600 K
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Velocity field of the masers
Epoch III
V1
V2
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Spoke-like features
Epoch IV
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Radial Spokes
Rectangles ? spokes of gas flowing outward at
different angle Thick rect. ? the brightest
spokes which we observe
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Models of SiO masers in M-Miras (Humphreys et
al. 2002 Gray Humphreys 2000)
86 GHz v1,J1-0 v2,J1-0
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Comparison with other observations
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43.1-GHz SiO maser NIR observations in S
Orionis ?SiO maser at 2 photospheric radii
(Boboltz Wittkowski 2005)