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

Temporal variations of the circumstellar environment of the Mira star V Oph. Keiichi Ohnaka Max-Planck-Institut f ü r Radioastronomie ESO Santiago Seminar 10 January 2008. Asymptotic Giant Branch (AGB). To Planetary Nebulae. Late evolutionary stage of low- & intermediate mass

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

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  1. 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

  2. Asymptotic Giant Branch (AGB) To Planetary Nebulae Late evolutionary stage of low- & intermediate mass stars (1-8M8) AGB Teff ~ 3000K L ~ 103 -- 104L8 3M8 Main sequence 1M8

  3. ~2 Rstar = ~600--800 R8(3--4AU) ~200--400 R8(~1AU = Earth’s Orbital Radius) ~0.01--0.1 R8 (~Earth’s 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”)

  4. 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

  5. Post-AGB Red Rectangle AGB, CIT3 Carbon star, IRC+10216 200mas AGB K 100mas 100mas J AGB, AFGL2290 K 50mas Mass loss mechanism in AGB stars H Driving mechanism not well understood Mass-loss rates = 10-8—10-5 M8/yr Dust & Molecule forming region close to the star PN, Cat’s Eye Nebula Morphology change from AGB to planetary nebulae How and at what stage?  High Angular Resolution  IR interferometry

  6. How an IR interferometer works Spatial resolution ~ l/Bp N band (8—13 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

  7. AMBER MIDI 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 Near-IR (JHK) Mid-IR (N band)

  8. MIDI observation Spectrally dispersed fringes extracted from raw data 8.0 mm 13.3 mm

  9. Dust emission H2O+SiO emission Stellar continuum size (Photospheric size) MIDI + VINCI observations of O-rich Mira RR Sco Dust shell MOLsphere (H2O, SiO, CO) Photosphere

  10. O-rich Mira stars Spectroscopy + Interferometry Warm molecular layers (H2O, SiO) 1000--1700K, 2--3 Rstar Optically thick (tline~1000) 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 + n5band (< 9 mm) n5 band (> 11 mm) MIDI Spectro-interferometry Dust Amorphous Carbon SiC (11.3 mm)

  11. 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.

  12. Estimated photospheric size Temporal variation of 8—13mm 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.

  13. 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

  14. Estimated photospheric size Estimated photospheric size Phase 0.18 Phase 0.49 SiC Phase 0.65 C2H2 n4 + n5 band C2H2 n5 band Dust shell model compared to MIDI observations N-band Uniform Disk Diameter N-band spectra

  15. Interpretation of MIDI data on V Oph (2): C2H2 layers + dust shell (Ohnaka et al. 2007, A&A, 466, 1099) Optically thick emission from C2H2 n4 + n5 band (< 9 mm) n5 band (> 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

  16. 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

  17. Model for post-maximum (phase = 0.18) Photospheric size

  18. Phase dependence of the C2H2 layers and the dust shell C2H2 Column Density Dust Optical Depth C2H2 Radius

  19. 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

  20. 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

  21. UT2-UT3 UT2-UT4 UT1-UT4 Estimated photospheric size Estimated photospheric size Phase 0.18 Phase 0.49 SiC Phase 0.65 C2H2 n4 + n5 band C2H2 n5 band Temporal variation of N-band angular size of V Oph N-band Uniform Disk Diameter N-band spectra

  22. Dust emission H2O+SiO emission Stellar continuum size 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,

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