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) T eff ~ 3000K L ~ 10 3 -- 10 4 L  Late evolutionary stage of low- & intermediate mass stars (1-8 M  ) AGB To Planetary Nebulae Main sequence 1M1M 3M3M

3. C/O core He H Helium shell burning (3 4 He  12 C) Thermally unstable, run-away reaction “Thermal pulse” or “Helium shell flash” Hydrogen shell burning 4 H  He Photosphere Circumstellar shell Mass loss, Dust formation Convective mixing ~0.01--0.1 R  (~Earth’s radius) ~200--400 R  (~1AU = Earth’s Orbital Radius) ~2 R star = ~600--800 R  (3--4AU) Stellar surface To interstellar space 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 3. Enrichment of ISM via mass loss Major “Dust Factory”, together with supernovae O-rich photosphere  C-rich photosphere  “Carbon stars” (C 2, CN, HCN, C 2 H 2 features in optical/IR spectra) s-process elements (Ba, La, Eu, Tc, etc)

5. Driving mechanism not well understood Mass-loss rates = 10 -8 —10 -5 M  /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 AGB, CIT3 J 100mas AGB Post-AGB Red Rectangle 200mas PN, Cat’s Eye Nebula AGB, AFGL2290 50mas K H Carbon star, IRC+10216 100mas K Mass loss mechanism in AGB stars

6. 2 Telescopes  Only visibility (Amplitude of Fourier transform of I(x,y) How an IR interferometer works Ｂ Spatial resolution  /B p N band (8—13  m) B p = 50 m  20 mas 200 m  5 mas K band (2  m) B p = 50 m  4 mas 200 m  1 mas Diffraction Limit (8m) N band  0.3” K band  60 mas ＢpＢp Beam combiner Optical Path Difference Delay line to compensate OPD 3 Telescopes  Imaging OK, but not easy

7. Expanding dust shell IR interferometry of Mira stars Mid-IR (N band) Dust formation Mira variables: Large variability amplitude ~ 9 mag (in V) Photosphere Spectro-interferometry Spatial + Spectral resolution Near-IR (JHK) “Warm Molecular layers”, or “MOLsphere”, 1000—2000K, 2—5 Rstar MIDI AMBER

8. 8.0  m 13.3  m MIDI observation Spectrally dispersed fringes extracted from raw data

9. MIDI + VINCI observations of O-rich Mira RR Sco H 2 O+SiO emission Dust emission Stellar continuum size (Photospheric size) Photosphere MOLsphere (H 2 O, SiO, CO) Dust shell

10. Multi-epoch MIDI observations of the C-rich Mira star V Oph O-rich Mira stars Warm molecular layers (H 2 O, SiO) 1000--1700K, 2--3 Rstar Optically thick (  line  ~1000) Spectroscopy + Interferometry 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 2 H 2     band (< 9  m)  band (> 11  m) Dust Amorphous Carbon SiC (11.3  m) MIDI Spectro-interferometry

11. Multi-epoch MIDI observations of the C-rich Mira star V Oph UT1-UT4 UT2-UT4 UT2-UT3 Same Bp & P.A. N-band visibilities show temporal variations

12. Estimated photospheric size Temporal variation of 8—13  m angular size of V Oph N-band Uniform Disk Diameter N-band angular sizes are remarkably larger than the star itself. The object appears the smallest at minimum light (when faintest).

13. Dust Shell Modeling Optically thin dust shell (Amorphous carbon + SiC)  Monte Carlo code (Ohnaka et al. 2006) SED + N-band Visibility fitting Interpretation of MIDI data on V Oph (1) Dust shell model Dust shell Amorphous carbon (featureless) + SiC (11.3  m) Inner boundary = 2.5 Rstar  Tdust = 1600K  Condensation Temperature Expanding dust shell

14. Estimated photospheric size Dust shell model compared to MIDI observations N-band Uniform Disk Diameter N-band spectra Phase 0.18 Phase 0.49 Phase 0.65 C 2 H 2    band C 2 H 2  band SiC Estimated photospheric size

15. Optically thick emission from C 2 H 2     band (< 9  m)  band (> 11  m) (ad hoc) Modeling Hot and cool C 2 H 2 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) Interpretation of MIDI data on V Oph (2): C 2 H 2 layers + dust shell (Ohnaka et al. 2007, A&A, 466, 1099) C 2 H 2 gas Dust shell Amorphous carbon (featureless) + SiC (11.3  m) Inner boundary = 2.5 Rstar  Tdust = 1600K  Condensation Temperature Expanding dust shell

16. Modeling for 3 epochs Optically thick emission from C 2 H 2  Angular size larger ( 12  m) Extended, dense C 2 H 2 layers in C-rich Mira stars H 2 O layers in O-rich Mira stars

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

18. Phase dependence of the C 2 H 2 layers and the dust shell C 2 H 2 Radius C 2 H 2 Column Density Dust Optical Depth

19. How to explain the phase dependence Shock front C 2 H 2 formation Series of “snapshots” of a dynamical atmosphere (shock wave passage), Nowotny et al. (2005) Dust formation Diluted New dust formation C 2 H 2 formation C 2 H 2 layers: dense, extended Dust opacity: high C 2 H 2 layers: less dense, small Dust opacity: low C 2 H 2 layers: dense, extended Dust opacity: high dM/dt = 10 -6 M  /yr dM/dt = 10 -8 M  /yr (V Oph) Post-Maximum Minimum Post-Minimum

20. Conclusion & Outlook C-rich version of the warm molecular layers (C 2 H 2 ) 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 C 2 H 2 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. Estimated photospheric size Temporal variation of N-band angular size of V Oph N-band Uniform Disk Diameter N-band spectra Phase 0.18 Phase 0.49 Phase 0.65 C 2 H 2    band C 2 H 2  band SiC UT1-UT4 UT2-UT4 UT2-UT3 Estimated photospheric size

22. MIDI + VINCI observations of O-rich Mira RR Sco Warm molecular layer makes the star appear larger in MIR than in NIR Dust shell emission is responsible for the size increase beyond 10  m H 2 O+SiO emission Dust emission Stellar continuum size silicate 20%, corundum 80% ~1400K, 2.3 R *, column densities = 10 20 --10 21 cm -2 Inner radius = 7--8 R *, Tin = 700--800 K, (Large-amplitude pulsation may explain the formation of warm H 2 O layers)