Temporal variations of the circumstellar environment of the Mira star V Oph

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Temporal variations of the circumstellar
environment of the Mira star V Oph

• Keiichi Ohnaka
• Max-Planck-Institut für Radioastronomie
• ESO Santiago Seminar
• 10 January 2008

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Asymptotic Giant Branch (AGB)
Late evolutionary stage of low- intermediate
mass stars (1-8 M8 )
To Planetary Nebulae
AGB
Teff 3000K L 103 -- 104 L8
3M8
Main sequence
1M8
3
2 Rstar 600--800 R8(3--4AU)
200--400 R8(1AU Earths Orbital Radius)
0.01--0.1 R8 (Earths radius)
Circumstellar shell Mass loss, Dust
formation
Stellar surface
Hydrogen shell burning 4 H ? He
Photosphere
C/O core
Convective mixing
To interstellar space
He
H
Helium shell burning (3 4He ? 12C) Thermally
unstable, run-away reaction Thermal pulse or
Helium shell flash
Carbon mixed up to surface by convection
O-rich photosphere ? C-rich (Carbon Star)
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Why AGB stars are important?
1. Majority of the stellar population
2. Nucleosynthesized material mixed to the
stellar surface

• O-rich photosphere ? C-rich photosphere
  
• Carbon stars
  
• (C2, CN, HCN, C2H2 features in optical/IR spectra)

s-process elements (Ba, La, Eu, Tc, etc)
3. Enrichment of ISM via mass loss
Major Dust Factory, together with
supernovae
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Mass loss mechanism in AGB stars
H
Driving mechanism not well understood
Mass-loss rates 10-810-5 M8/yr
Dust Molecule forming region close to
the star
PN, Cats Eye Nebula
Morphology change from AGB to planetary
nebulae How and at what stage?
? High Angular Resolution ? IR interferometry

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How an IR interferometer works
Spatial resolution l/Bp N band (813 mm) Bp
50 m ? 20 mas 200 m ? 5 mas K band (2
mm) Bp 50 m ? 4 mas 200 m ? 1
mas Diffraction Limit (8m) N band ? 0.3 K
band ? 60 mas
Optical Path Difference
Bp
B
Beam combiner

• 2 Telescopes
• Only visibility
• (Amplitude of Fourier
• transform of I(x,y)

• 3 Telescopes
• Imaging OK,
• but not easy

Delay line to compensate OPD
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IR interferometry of Mira stars
Mira variables Large
variability amplitude 9 mag (in V)
Expanding dust shell
Warm Molecular layers, or MOLsphere,
10002000K, 25 Rstar
Dust formation
Photosphere
Spectro-interferometry
Spatial Spectral resolution
Near-IR (JHK)
Mid-IR (N band)
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MIDI observation Spectrally dispersed fringes
extracted from raw data
8.0 mm
13.3 mm
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MIDI VINCI observations of O-rich Mira RR Sco
Dust shell
MOLsphere (H2O, SiO, CO)
Photosphere
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Multi-epoch MIDI observations of the C-rich Mira
star V Oph
C-rich Mira stars
Circumstellar material close to the star ? Dust
or gas ? (or both?)
Little mid-IR interferometry on optically bright
(not so dusty) C-rich Miras ? V Oph


C2H2 n4
n5 band (lt 9 mm) n5 band (gt 11 mm)
MIDI Spectro-interferometry
Dust Amorphous Carbon SiC
(11.3 mm)
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Multi-epoch MIDI observations of the C-rich Mira
star V Oph
UT2-UT3
UT2-UT4
UT1-UT4
N-band visibilities show temporal variations

Same Bp P.A.
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Temporal variation of 813mm angular size of V Oph
N-band Uniform Disk
Diameter
The object appears the smallest at minimum light
(when faintest).
N-band angular sizes are remarkably larger than
the star itself.
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Interpretation of MIDI data on V Oph (1) Dust
shell model

• Dust Shell Modeling
• Optically thin dust shell
• (Amorphous carbon SiC)
• ? Monte Carlo code
• (Ohnaka et al. 2006)
• SED N-band Visibility fitting

Expanding dust shell
Dust shell Amorphous carbon (featureless) SiC
(11.3 mm)
Inner boundary 2.5 Rstar ? Tdust 1600K
? Condensation Temperature
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Dust shell model compared to MIDI observations
N-band Uniform Disk
Diameter
N-band spectra
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Interpretation of MIDI data on V Oph (2) C2H2
layers dust shell (Ohnaka et al. 2007, AA,
466, 1099)
Optically thick emission from C2H2
n4 n5 band (lt 9 mm) n5
band (gt 11 mm)

• (ad hoc) Modeling
• Hot and cool C2H2 layers
• (constant temperatures, densities)
• ? Line opacity calculated
• analytically
• (Band model, Tsuji 1984)
• Optically thin dust shell
• (Amorphous carbon SiC)
• ? Monte Carlo code
• (Ohnaka et al. 2006)

Expanding dust shell
C2H2 gas
Dust shell Amorphous carbon (featureless) SiC
(11.3 mm)
Inner boundary 2.5 Rstar ? Tdust 1600K
? Condensation Temperature
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Modeling for 3 epochs
Optically thick emission from C2H2 ? Angular
size larger ( lt 9 mm gt 12 mm)
Extended, dense C2H2 layers in C-rich Mira stars

H2O layers in O-rich Mira stars
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Model for post-maximum (phase 0.18)
Photospheric size
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Phase dependence of the C2H2 layers and the dust
shell
C2H2 Column Density
Dust Optical Depth
C2H2 Radius
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How to explain the phase dependence
Series of snapshots of a dynamical
atmosphere (shock wave passage), Nowotny et al.
(2005)
dM/dt 10-6 M8/yr
Post-Maximum
dM/dt 10-8 M8/yr (V Oph)
C2H2 layers dense, extended Dust opacity
high
Shock front
C2H2 formation
Dust formation
C2H2 layers less dense, small
Dust opacity low
Minimum
Diluted
Diluted
Post-Minimum
C2H2 layers dense,
extended Dust opacity high
C2H2 formation
New dust formation
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Conclusion Outlook

• C-rich version of the warm molecular layers
  (C2H2)

• Phase dependence of the mid-IR angular size
  
• The object appears the smallest at minimum
  light.

• Observed N-band visibilities and spectra can be
  explained by
• the C2H2 layers dust shell model.
  

• Dust formation zone not well constrained
  (baselines were too long).
• Better (u,v) coverage with ATs.
  
• O-rich Miras MIDI/AT program on 3 Miras
  
• C-rich Miras MIDIVISIRAMBER program on 1 Mira
  

• Non-Mira AGB stars (majority of AGB stars)
• Very small variability amplitudes,
• but substantial mass loss

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Temporal variation of N-band angular size of V Oph
N-band Uniform Disk Diameter
N-band spectra
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MIDI VINCI observations of O-rich Mira RR Sco

• Warm molecular layer makes the star appear
  larger in MIR than in NIR

1400K, 2.3 R, column densities 1020--1021
cm-2
(Large-amplitude pulsation may explain the
formation of warm H2O layers)

• Dust shell emission is responsible for the size
  increase beyond 10 mm

silicate 20, corundum 80
Inner radius 7--8 R, Tin 700--800 K,