Dust Envelopes around Oxygen-rich AGB stars Kyung-Won Suh Dept. of Astronomy & Space Science Chungbuk National University, Korea Introduction Radiative transfer models - Models and comparison with the observations ★ Pulsation Phase-Dependent Dust Shell Models for O-rich AGB stars 1) Low mass-loss rate O-rich AGB (LMOA) stars 2) High mass-loss rate O-rich AGB (HMOA) stars ★ Axi-symmetric dust envelope Models for O-rich AGB stars (tentative results) Summary

AGB stars  Large amplitude, long period pulsation.  Strong stellar winds with high mass-loss rates. - Pulsation driven shock & Radiation pressure on dust grains  Thick dust envelopes – Main sources of dust grains in galaxies

Radiative Transfer Models for AGB stars  Stellar Parameters The flux from the central star ( core: degenerate C,N,O shell: H, He burning ) Usually, blackbody model is enough for AGB stars T*, R*, (L*)  Dust Envelope Parameters Opacity, Envelope structure (Shape, Location, Optical depth,,,) For Spherically symmetric dust shell models: Dust opacity functions (silicate, uniform 0.1  m spheres), The optical depth,  (r) : dust density distribution, R c : dust shell inner radius, and the outer radius (10000 Rc)  T c : Inner shell dust temperature (may not be same as the dust formation temperature) For Axi-symmetric dust envelope models: + Shape parameters (degree of flattening, viewing angle, etc.)

Optical properties of dust grains – amorphous and crystalline silicates

AGB stars on IR 2-color diagrams

Low mass-loss rate O-rich AGB stars (LMOA stars) No Evident Crystalline Silicate features Mass-loss Rate: 1 × ∼ 1 × M  /yr

High mass-loss rate O-rich AGB stars (HMOA stars) Prominent Crystalline Silicate features at 33.3, 40.6, 43.3  m Mass-loss Rate: 1 × ∼ 1 × M  /yr

Mass-loss rates of O-rich AGB stars Mass-loss Rate: 5.0 × ∼ 1.0 × M  /yr

Models for AGB stars (LMOA stars)

Models for AGB stars (LMOA stars) small grains (0.01  m) vs. large grains (0.1  m )

Dust Shell Models for AGB stars 1.Continuous model: Dust shell is continuous (  (r)  r -2 ) from the dust shell inner radius (R c ) up to R c. 2. Superwind model: There is a density-enhanced region for the overall continuous dust shell.

Pulsating AGB stars (LMOA stars) Continuous vs. Superwind models

Pulsating AGB stars (LMOA stars) Mass-loss Rate: 7.6 × ∼ 1.1 × M  /yr

Pulsating AGB stars (LMOA stars) Mass-loss Rate: 8.6 × ∼ 1.6 × M  /yr

Pulsating AGB stars (LMOA stars) Continuous vs. Superwind models

Pulsating AGB stars (LMOA stars) The superwind model results compared with continuous dust shell models. With T c =1000 K, the 10 times density-enhanced region from 5 to 205 R c produces a similar SED to the ISO spectra of the both stars. Namely, the superwind dust shell with the density enhancement mimics a continuous shell with a lower inner shell dust temperature. If the superwind model is right, the dust formation temperature can be as high as 1000 K for LMOA stars.

AGB stars (HMOA stars) – Pulsations

Dust shell models for HMOA stars

Pulsating AGB stars (HMOA stars) Crystalline Silicate features at 33.3, 40.6, 43.3  m Mass-loss Rate: 4.1 × ∼ 4.3 × M  /yr

P ulsating AGB stars (HMOA stars) Crystalline Silicate features at 33.3, 40.6, 43.3  m Mass-loss Rate: 6.1 × ∼ 6.8 × M  /yr

Dust shell models for Pulsating HMOA stars (from Suh 2004) Dust Model One Dust grains form at 1000 K and instantaneously evaporate at T > 1000 K. Dust Model Two Dust grains form only at 1000 K and there is no dust evaporation at any phase (because the dust evaporation requires much higher temperature than 1000 K). Dust Model Three Dust formation temperature is higher than 1000 K at higher luminosity (or mass-loss rate). The dust formation process does not cease at any phase and there is no dust evaporation.

Dust shell models for Pulsating HMOA stars Dust Model OneDust Model TwoDust Model Three

Dust shell models for Pulsating HMOA stars  The 3 different dust models produce similar fitting with the observations. Dust Model OneDust Model Two

Dust formation and Crystallization in Pulsating AGB stars Crystallization Dust grains may spend enough time for annealing after their formation in AGB stars.  A HMOA star with a higher inner shell dust temperature provides better conditions for crystallization.  Only HMOA stars show crystalline features.

The IR two-color diagram

Axi-symmetric dust envelope models for AGB stars – The Model SEDs (using 2-Dust with multi dust components; Suh 2005 in preparation)

Axi-symmetric dust envelope Model for HMOA stars - The model SEDs

Axi-symmetric dust envelope Model for HMOA stars - Model Images   (equ)=30=10*   (pol) Edge-on views of an axi-symmetric dust envelope model (disk-like shape) at different wavelengths 0.8  m 10  m 25  m 60  m

AGB stars and Planetary Nebulae

Summary O-rich AGB stars at their last stage of stellar evolution lose their mass to ISM by large amplitude pulsation and dust formation in outer envelopes. We find that the dust shell structures (e.g., the inner shell dust temperatures) change as well as the central stars depending on the phase of pulsation. 1. LMOA stars with thin dust shells : T c ~ 400 K (min) – 700 K (max) ; R c ~ R * (too large?)  Superwind models: T c ~ 1000 K ; R c ~ 10 R * 2. HMOA stars with thick dust shells : T c ~ 1000 K (min) – 1300 K (max) ; R c ~ R * (depending on the dust model) 3. We expect that the dust annealing process (T > 1000K) driven by pulsation could be a mechanism for crystallizing the dust grains in inner regions of the dust shells around HMOA stars with thick dust shells. 4. New axi-symmetric dust envelope models are necessary. New IR observations (Spitzer, SOFIA, Astro-F) will provide better data for understanding the dust grains and the envelope structure.