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The circumstellar environment of evolved stars as seen by VLTI / MIDI

The circumstellar environment of evolved stars as seen by VLTI / MIDI. Keiichi Ohnaka Max-Planck-Institut für Radioastronomie, Infrared Interferometry Group kohnaka@mpifr-bonn.mpg.de. Carbon star, IRC+10216. AGB, AFGL2290. Asymptotic Giant Branch (AGB). Post-AGB Red Rectangle.

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The circumstellar environment of evolved stars as seen by VLTI / MIDI

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  1. The circumstellar environment of evolved stars as seen by VLTI / MIDI Keiichi Ohnaka Max-Planck-Institut für Radioastronomie, Infrared Interferometry Group kohnaka@mpifr-bonn.mpg.de

  2. Carbon star, IRC+10216 AGB, AFGL2290 Asymptotic Giant Branch (AGB) Post-AGB Red Rectangle Teff ~ 3000K L ~ 104 L8 Mass loss ~10-8—10-5 M8/yr Mass loss mechanism The structure of the outer atmosphere: Molecule & dust formation Morphology from AGB to Planetary Nebulae PN, Cat’s Eye Nebula

  3. AMBER MIDI What can interferometry do to study AGB stars? Mira variables: Large variability amplitude ~ 9 mag (in V) Expanding dust shell Outer atmosphere Molecular layers Dust formation Mid-infrared Near-infrared

  4. Infrared long-baseline interferometry Spatial resolution = l/Bp N band Bp= 200m  10mas What’s observed: Visibility, not an image! Ｂp Visibility = Amplitude of the (complex) Fourier transform of the object’s intensity = Fringe contrast Ｂ MIDI AMBER Point source  V = 1 Extended source  V < 1 Larger size  lower V It “contains” information on the angular size and shape Is it useful? Yes, especially with spectral resolution!

  5. MIDI: first fringe in December 2002 Open to the community since April 2004 N band: 8 – 13 mm (dust features, molecular bands) Spectral resolution: 30 / 230 Sensitivity @ 10mm: 1 Jy / 10 Jy VLTI Interferometric Laboratory

  6. MIDI observation of the Mira variable RR Sco 2003 June, Science Demonstration Time Unit Telescopes 1 & 3 (8m) Projected baseline = 74—100m Angular resolution @ 10mm = ~20mas RR Sco (Phase = 0.6 in 2003 June) P = 284 days, d = 320 pc (Hipparcos) Dust emission not strong  Good for studying the molecular layers UT3 UT1 102m

  7. MIDI observation of RR Sco: spectrally dispersed fringes 7.5 mm 13.3 mm

  8. Observed N-band visibility of RR Sco • Visibility increases from 8 to 10 mm, constant l > 10 mm • UD diameter constant between 8 and 10 mm (~18 mas), UD diameter increases l > 10 mm (~25 mas @ 13 mm) • N-band UD diameter (MIDI) twice as large as that in the K band • (VINCI, 3 weeks later) Why?

  9. Observed N-band visibility of RR Sco

  10. K band (2—2.4mm) No dust emission N band (8—13mm) Basic idea Optically thick emission from H2O (pure rotation) + SiO (fundamental) gas + Dust emission Expanding dust shell H2O + SiO gas  Angular size larger Modeling H2O + SiO layer (constant temperature, column densities, radius) Optically thin dust shell (silicate+corundum) (Inner radius, optical depth) H2O + CO bands Not optically thick  Angular size smaller  Ohnaka et al. 2005, A&A, 429, 1067

  11. T = 1400 K N(H2O) = 3 x 1021cm-2 N(SiO) = 1 x 1020cm-2 R = 2.3 Rstar Dust emission (silicate 20% + corundum 80%) Tin ~ 700 K, Rin = 7--8 Rstar H2O+SiO emission t= 0.2 – 0.3 (V band), 0.025 (10mm) • Comparison with pulsation models is ongoing

  12. MIDI observation of the silicate carbon star Hen 38 Silicate carbon star : carbon-rich photosphere, oxygen-rich circumstellar dust Usually… carbon star, carbon-rich circumstellar dust (amorphous carbon, SiC) M giants (O-rich), oxygen-rich circumstellar dust (silicate, Al2O3: corundum) How can silicate (O-bearing dust) exist around a carbon star?

  13. AGB star + main sequence star AGB, primary star: oxygen-rich, mass loss  Circumbinary disk is formed Oxygen-rich dust (silicate) reservoir Mass loss Primary star becomes a carbon star. Oxygen-rich dust is stored in the disk  Silicate carbon star High-resolution observation in the silicate emission feature is the most direct approach VLTI/MIDI

  14. Compact silicate disk + extended corundum disk Silicate Compact disk Corundum (Al2O3) • Only present in the outer region • Increase of the angular size l > 10 mm

  15. Silicate + corundum disk model Corundum dominant Silicate torus (ring) dominant 15 – 35 Rstar, t(10mm) = 1.5 > 35 Rstar, t(10mm) = 0.4

  16. Concluding remarks First “spectro-interferometric” observation of RR Sco  Wavelength dependence of the angular size Angular size constant between 8 and 10 mm, increases longward of 10 mm, More than twice as large as in the K-band Observed N-band visibilities and spectra can be explained by optically thick emission from H2O + SiO gas & dust emission  Consistent with ISO and previous results Potential to probe the circumstellar environment (molecule and dust) with spatial and spectral information disentangled  Totally new picture of the circumstellar environment Upcoming: more Miras (oxygen-rich, carbon-rich, S-type), Circumstellar dust disks around (post-)AGB stars, Symbiotic stars (Mira + hot companion), etc…

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