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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
Asymptotic Giant Branch
(AGB)
To Planetary Nebulae
AGB
Late evolutionary stage of
low- & intermediate mass
stars (1-8 M8 )
Teff ~ 3000K
L ~ 103 -- 104 L8
3M8
1M8
~2 Rstar = ~600--800 R8(3--4AU)
~200--400 R8(~1AU = Earth’s Orbital Radius)
~0.01--0.1 R8 (~Earth’s radius)
Stellar surface
Hydrogen shell burning
4 H  He
Circumstellar shell
Mass loss, Dust formation
Photosphere
C/O core
Convective mixing
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”)
To interstellar space
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
Mass loss mechanism in AGB stars
Carbon star,
IRC+10216
H
AGB, CIT3
Post-AGB
Red Rectangle
200mas
AGB
K
100mas
J
AGB, AFGL2290
PN, Cat’s Eye Nebula
K
50mas
100mas
Driving mechanism
not well understood
Mass-loss rates
= 10-8—10-5 M8/yr
Dust & Molecule forming
region close to the star
Morphology change from
AGB to planetary nebulae
How and at what stage?
 High Angular Resolution
 IR interferometry
How an IR interferometer works
Spatial resolution
~ l/Bp
N band (8—13 mm)
Bp = 50 m  20 mas
200 m  5 mas
Optical Path
Difference
Ｂp
Ｂ
K band (2 mm)
Bp = 50 m  4 mas
200 m  1 mas
Diffraction Limit (8m)
N band  0.3”
K band  60 mas
Beam combiner
2 Telescopes
 Only visibility
(Amplitude of Fourier
transform of I(x,y)
Delay line to compensate OPD
3 Telescopes
Imaging OK,
but not easy
IR interferometry of Mira stars
Mira variables:
Large variability amplitude
~ 9 mag (in V)
Expanding dust shell
“Warm Molecular layers”,
or “MOLsphere”,
1000—2000K, 2—5 Rstar
Dust formation
Photosphere
Spectro-interferometry
Spatial + Spectral
resolution
Mid-IR (N band)
MIDI
Near-IR (JHK)
AMBER
MIDI observation
Spectrally dispersed fringes extracted from raw data
13.3 mm
8.0 mm
MIDI + VINCI observations of O-rich Mira RR Sco
Dust emission
H2O+SiO emission
Stellar continuum size
(Photospheric size)
MOLsphere
(H2O, SiO, CO)
Photosphere
Dust shell
Multi-epoch MIDI observations of the C-rich Mira star V Oph
O-rich Mira stars
Warm molecular layers
(H2O, SiO)
Spectroscopy + Interferometry
1000--1700K, 2--3 Rstar
Optically thick (tline ~1000)
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
C 2H 2
n4 + n5 band (< 9 mm)
n5 band (> 11 mm)
Dust
Amorphous Carbon
SiC (11.3 mm)
MIDI
Spectro-interferometry
Multi-epoch MIDI observations of the C-rich Mira star V Oph
UT2-UT3
UT2-UT4
UT1-UT4
Same Bp & P.A.
N-band visibilities show
temporal variations
Temporal variation of 8—13mm angular size of V Oph
N-band
Uniform Disk Diameter
Estimated photospheric size
The object appears the smallest at minimum light (when faintest).
N-band angular sizes are remarkably larger than the star itself.
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)
Expanding dust shell
SED + N-band Visibility fitting
Dust shell
Amorphous carbon (featureless)
+ SiC (11.3 mm)
Inner boundary = 2.5 Rstar
 Tdust = 1600K
 Condensation Temperature
Dust shell model compared to MIDI observations
N-band
Uniform Disk Diameter
Estimated photospheric size
N-band spectra
Phase 0.18
Phase 0.49
Phase 0.65
C2H2
n4 + n5 band
SiC
C2H2 n5 band
Interpretation of MIDI data on V Oph (2): C2H2 layers + dust shell
(Ohnaka et al. 2007, A&A, 466, 1099)
(ad hoc) Modeling
Hot and cool C2H2 layers
(constant temperatures, densities)
 Line opacity calculated
analytically
(Band model, Tsuji 1984)
Expanding dust shell
Optically thick emission
from C2H2
n4 + n5 band (< 9 mm)
n5 band (> 11 mm)
C2H2 gas
Optically thin dust shell
(Amorphous carbon + SiC)
 Monte Carlo code
(Ohnaka et al. 2006)
Dust shell
Amorphous carbon (featureless)
+ SiC (11.3 mm)
Inner boundary = 2.5 Rstar
 Tdust = 1600K
 Condensation Temperature
Modeling for 3 epochs
Optically thick emission from C2H2  Angular size larger ( < 9 mm & > 12 mm)
Extended, dense C2H2 layers
in C-rich Mira stars
H2O layers in O-rich Mira stars
Model for post-maximum (phase = 0.18)
Photospheric size
Phase dependence of the C2H2 layers and the dust shell
C2H2 Column Density
C2H2 Radius
Dust Optical Depth
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
Minimum
Shock front
C 2H 2
formation
Dust formation
Diluted
C2H2 layers: less dense,
small
Dust opacity: low
Diluted
Post-Minimum
C 2H 2
formation
C2H2 layers: dense,
extended
Dust opacity: high
New dust formation
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: MIDI+VISIR+AMBER program on 1 Mira
 Non-Mira AGB stars (majority of AGB stars)
Very small variability amplitudes,
but substantial mass loss
Temporal variation of N-band angular size of V Oph
N-band Uniform Disk Diameter
UT2-UT3
UT2-UT4
UT1-UT4
Estimated photospheric size
N-band spectra
Phase 0.18
Phase 0.49
Phase 0.65
C2H2
n4 + n5 band
SiC
C2H2 n5 band
MIDI + VINCI observations of O-rich Mira RR Sco
Dust emission
H2O+SiO emission
Stellar continuum size
 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
Inner radius = 7--8 R*, Tin = 700--800 K,
silicate 20%, corundum 80%