the circumstellar environment of evolved stars as seen by vlti / midi
DESCRIPTION
The circumstellar environment of evolved stars as seen by VLTI / MIDI. Keiichi Ohnaka Max-Planck-Institut für Radioastronomie, Infrared Interferometry Group [email protected]. Carbon star, IRC+10216. AGB, AFGL2290. Asymptotic Giant Branch (AGB). Post-AGB Red Rectangle. - PowerPoint PPT PresentationTRANSCRIPT
The circumstellar environment of evolved stars as seen by VLTI / MIDI
Keiichi OhnakaMax-Planck-Institut für Radioastronomie,Infrared Interferometry [email protected]
Asymptotic Giant Branch (AGB) Post-AGB Red Rectangle Teff ~ 3000K
L ~ 104 L
Mass loss ~10-8—10-5 M/yr
Mass loss mechanism
The structure of the outer atmosphere:
Molecule & dust formation
Morphology from AGB to Planetary Nebulae
Carbon star, IRC+10216
AGB, AFGL2290
PN, Cat’s Eye Nebula
Outer atmosphereMolecular layers
Near-infrared
Expanding dust shell
What can interferometry do to study AGB stars?
Mid-infrared
Dust formation
Mira variables: Large variability amplitude
~ 9 mag (in V)
MIDI AMBER
Infrared long-baseline interferometry
B
Spatial resolution = /Bp
N bandBp = 200m 10mas
What’s observed: Visibility, not an image!
MIDI AMBER
Visibility = Amplitude of the (complex) Fourier transform
of the object’s intensity= Fringe contrast
Is it useful?Yes, especially with spectral resolution!
It “contains” information onthe angular size and shape
Point source V = 1 Extended source V < 1Larger size lower V
B p
MIDI: first fringe in December 2002 Open to the community since April 2004
VLTI Interferometric
Laboratory
N band: 8 – 13 m (dust features, molecular bands) Spectral resolution: 30 / 230 Sensitivity @ m: 1 Jy / 10 Jy
UT1 UT3
102m
MIDI observation of the Mira variable RR Sco2003 June, Science Demonstration Time
Unit Telescopes 1 & 3 (8m) Projected baseline = 74—100m Angular resolution @ 10m = ~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
7.5m 13.3m
MIDI observation of RR Sco: spectrally dispersed fringes
Observed N-band visibility of RR Sco
Visibility increases from 8 to 10 m, constant > 10 m UD diameter constant between 8 and 10 m (~18 mas), UD diameter increases > 10 m (~25 mas @ 13 m)
N-band UD diameter (MIDI) twice as large as that in the K band
(VINCI, 3 weeks later) Why?
Observed N-band visibility of RR Sco
Expanding dust shell
Optically thick emission from H2O (pure rotation) + SiO (fundamental) gas
+Dust emission
N band (8—13m)
H2O + SiO gas
K band (2—2.4m)No dust emission
H2O + CO bands Not optically thick Angular size smaller
Modeling H2O + SiO layer
(constant temperature, column densities, radius)
Optically thin dust shell (silicate+corundum) (Inner radius, optical depth)
Basic idea
Ohnaka et al. 2005, A&A, 429, 1067
Angular size larger
H2O+SiO emission
Dust emission (silicate 20% + corundum 80%)
T = 1400 KN(H2O) = 3 x 1021cm-2
N(SiO) = 1 x 1020cm-2
R = 2.3 Rstar
Tin ~ 700 K, Rin = 7--8 Rstar = 0.2 – 0.3 (V band), 0.025 (10m) Comparison with pulsation models
is ongoing
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?
Oxygen-rich dust(silicate) reservoir
Mass loss
AGB, primary star: oxygen-rich, mass loss Circumbinary disk is formed
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
AGB star + main sequence star
Compact silicate disk + extended corundum disk
Silicate
Corundum (Al2O3)
Compact disk
Only present in the outer regionIncrease of the angular size m
Silicate torus (ring) dominantCorundumdominant
Silicate + corundum disk model
15 – 35 Rstar, m) = 1.5 > 35 Rstar,
m) = 0.4
Concluding remarks
First “spectro-interferometric” observation of RR Sco Wavelength dependence of the angular size
Angular size constant between 8 and 10 m, increases longward of 10 m, 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…