Dust Envelopes around Oxygen-rich AGB stars

Kyung-Won Suh

Dept. of Astronomy & Space Science

Chungbuk National University, Korea

E-mail: kwsuh@chungbuk.ac.kr

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

Low mass-loss rate O-rich

AGB (LMOA) stars

Carbon stars High mass-loss rate O-rich

AGB (HMOA) stars

Optical depth (10 m) 0.001 - 3 0.1 - 1 3- 40

mB 0.3 - 1 1 1-3

Pulsation Period (days) 100 - 500 300 - 700 500 - 2000

Mass Loss Rate (Mo/yr) 10-8 - 10-6 10-7- 10-5 10-6 - 10-4

Dust grains (Main) Silicates

(emission features)

Amorphous

Carbon

Silicates

(absorption features)

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 starsT*, 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,

Rc : dust shell inner radius, and the outer radius (10000 Rc)

Tc : 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

Wavelength (m)10 20 30 40 50 60 70 80

Qab

s

10-3

10-2

10-1

100

Suh 1999 (cool dust) Suh 1999 (warm dust)

Crystal Enstatite (Jager et al. 1998)

Mixed (Crystal 10%)

Crystal Forsterite (Jager et al. 1998)

Mixed (Crystal 10%)

AGB stars on IR 2-color diagrams

[12 - 25]0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

AMCSilicate

[25

- 60

]

Oxygen-rich starsCarbon-rich starsSilicate carbon stars

K - 120 2 4 6 8 10 12 14 16 18 20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

[12-

25]

Silicate (1000 K)

AMC (1000 K)

Oxygen-rich starsCarbon starsSilicate carbon stars

OS

IRCS

VCS

Low mass-loss rate O-rich AGB stars (LMOA stars)

Wavelength (m)10 100

F

(W m

-2)

10-11

10-10

10-9

10-8

ISOIRAS LRSIRAS PSC

Mira (o Ceti)

Envelope ( 10 = 0.1, = 0.1)

Envelope ( 10 = 0.1, = 0)

No Evident Crystalline Silicate features

Mass-loss Rate: 1×10-8∼1×10-6 M/yr

High mass-loss rate O-rich AGB stars (HMOA stars)

Wavelength (m)10 10010-13

10-12

10-11

ISO SWSIRAS LRSIRAS PSC

OH21.5+0.5

Envelope ( = 30, = 0.2)

F

(W

m-2

)

Envelope and disk ( = 30, = 0.2, = 0.1)

Prominent Crystalline Silicate features at 33.3, 40.6, 43.3 m

Mass-loss Rate: 1×10-6∼1×10-4 M/yr

Mass-loss rates of O-rich AGB starsMass-loss Rate: 5.0×10-8∼1.0×10-4 M/yr

Models for AGB stars (LMOA stars)

Wavelength (m)1 10 100

Models for typical LMOA stars

L*=1.0E4 Lo, T*=2900 K, Tc=553 K, 10=0.04

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04

F

L*=4.5E3 Lo, T*=2500 K, Tc=420 K, 10=0.027

L*=4.5E3 Lo, T*=2900 K, Tc=420 K, 10=0.027

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

Wavelength (m)1 10 100

Models for typical LMOA stars

L*=1.0E4 Lo, T*=2900 K, Tc=553 K, 10=0.04

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04

F

L*=4.5E3 Lo, T*=2500 K, Tc=420 K, 10=0.027

L*=4.5E3 Lo, T*=2500 K, Tc=1000 K, 10=0.027 (small)

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04 (small)

L*=4.5E3 Lo, T*=2500 K, Tc=1000 K, 10=0.027

Dust Shell Models for AGB stars 1.Continuous model: Dust shell is continuous ((r) r-2) from the dust sh

ell inner radius (Rc) up to 10000 Rc.

2. Superwind model: There is a density-enhanced region for the overall

continuous dust shell.

R ( RC )1 10 100 1000 10000

Dus

t Num

ber

Den

sity

R1

R2

Pulsating AGB stars (LMOA stars)

Wavelength (m)1 10 100

Superwind models for typical LMOA stars

L*=1.0E4 Lo, T*=2900 K, Tc=553 K, 10=0.04, Rc=36.8 R*

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04, Rc=10.9 R*

F

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04 (SW 10-110)

Continuous dust shell models for typical LMOA stars

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.04 (SW 30-130)

Rc=10.8 R* ; density is 10 times enhanced at 10 - 110 RC

Rc=10.8 R* ; density is 10 times enhanced at 30 - 130 RC

Continuous vs. Superwind models

Pulsating AGB stars (LMOA stars)

Wavelength (m)10 10010-14

10-13

10-12

10-11

10-10

Z Cyg (IRAS 20000+4954)

L*=4.5E3 Lo, T*=2500 K, Tc=420 K, 10=0.027

L*=1.0E4 Lo, T*=2900 K, Tc=553 K, 10=0.04

L*=6.0E3 Lo, T*=2650 K, Tc=463 K, 10=0.03

ISO SWS01_03 (1996-08-05T18:18:37)= 0.55

ISO SWS01_04 (1996-10-08T04:30:57)= 0.79

ISO SWS01_02 (1996-11-24T11:29:59)= 0.97

ISO SWS01_01 (1997-01-24T00:26:47)= 1.20

ISO SWS01_06 (1997-03-21T05:11:56)= 1.42

ISO SWS01_05 (1997-05-15T02:16:42)= 1.63

IRAS PSC

IRAS LRS

F

(W m

-2)

Mass-loss Rate: 7.6×10-8∼1.1×10-7 M/yr

Pulsating AGB stars (LMOA stars)

Wavelength (m)10 100

F

(W m

-2)

10-11

10-10

10-9

10-8

o Ceti (IRAS 02168-0312)

L*=1.0E4 Lo, T*=2900 K, Tc=654 K, 10=0.020

L*=7.0E3 Lo, T*=2670 K, Tc=578 K, 10=0.015

L*=4.0E3 Lo, T*=2500 K, Tc=484 K, 10=0.010

ISO (SWS: 1997-02-09, LWS: 1997-07-04)IRAS LRSIRAS PSCEpchtein et al. 1980 (1979. 07. 13.)

Mass-loss Rate: 8.6×10-8∼1.6×10-7 M/yr

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

Wavelength (m)10 100

F

(W m

-2)

10-11

10-10

10-9

10-8

o Ceti (IRAS 02168-0312)

L*=1.0E4 Lo, T*=2900 K, Tc=654 K, 10=0.020

ISO (SWS: 1997-02-09, LWS: 1997-07-04)IRAS LRSIRAS PSCEpchtein et al. 1980 (1979. 07. 13.)

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.020

L*=1.0E4 Lo, T*=2900 K, Tc=1000 K, 10=0.020 (SW 5-205)

Pulsating AGB stars (LMOA stars)

The superwind model results compared with continuous dust shell models. With Tc=1000 K, the 10 times density-enhanced region from 5 to 205 Rc 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.

JD 2440000+2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

0.5

1.0

1.5

Jones et al. 1990 ISO SWS01ISO PHT40

F( 5

10-1

0 W

m-2)

IRAS

OH127.8+0.0 (M Band)Period : 1541 ± 16.51 (day)Amplitude : 0.61 ± 0.07 (5 10-10 Wm-2)

Gehrz et al. 1985Ney & Merrill 1980Grasdalen et al. 1983 Persi et al. 1990

JD 2440000+2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

F (

51

0-10 W

m-2)

0.0

0.5

1.0

1.5

Lepine et al. 1995 Olivier et al. 2001 ISO SWS01

IRAS

OH26.5+0.6 (L Band)Period : 1559 ± 7.20 (day)Amplitude : 0.55± 0.03 (5 10-10 Wm-2)

Ney & Merrill 1980Evans II & Beckwith 1977Lebofsky et al. 1978 Engels 1982

Forrest et al. 1978 Werner et al. 1980 Grasdalen et al. 1983Jones et al. 1990

AGB stars (HMOA stars) – Pulsations

Dust shell models for HMOA stars

Wavelength (m)10 100

Models for typical HMOA stars

L*=3.6E4 Lo, T*=2000 K, Tc=1379 K, 10=19

F

L*=3.6E4 Lo, T*=2000 K, Tc=1000 K, 10=10

L*=3.6E4 Lo, T*=2000 K, Tc=1000 K, 10=19

L*=3.6E4 Lo, T*=2300 K, Tc=1000 K, 10=19

Pulsating AGB stars (HMOA stars)

Wavelength (m)10 100

F

(

W m

-2 )

10-12

10-11

10-10

10-9

OH127.8+0.0 (IRAS 01304+6211)

L*=1.0x104 Lo , Tc=1000 K, 10 = 17, =0

L*=2.7x104 Lo , Tc=1182 K, 10 = 15

L*=3.6x104 Lo , Tc=1200 K, 10 = 13.6

L*=1.0x104 Lo , Tc=1000 K, 10 = 17, =0.2

Dust Model Two (=0)

Minimum phase for all dust models

L*=2.7x104 Lo , Tc=1270 K, 10 = 17.2

L*=3.6x104 Lo , Tc=1365 K, 10 = 17.3

Dust Model Three (=0)

L*=2.7x104 Lo , Tc=1000 K, 10 = 11

L*=3.6x104 Lo , Tc=1000 K, 10 = 10

Dust Model One (=0)

ISO (SWS: 1998-01-11, LWS: 1997-07-21)IRAS LRSIRAS PSCJones et al. 1990 (1987-09-13 ~ 1988-12-07)Jones et al. 1990 (1988-5-22)Ney & Merrill 1980 (1976-09-08)

Crystalline Silicate features at 33.3, 40.6, 43.3 m

Mass-loss Rate: 4.1×10-5∼4.3×10-5 M/yr

Pulsating AGB stars (HMOA stars)

Wavelength (m)10 100

F

(

W m

-2 )

10-13

10-12

10-11

10-10

10-9

OH26.5+0.6 (IRAS 18348-0526) L*=1.0x104 Lo , Tc= 1000 K, 10 = 22, = 0

L*=1.0x104 Lo , Tc= 1000 K, 10 = 22, = 0.2

Forrest et al. 1978 (1976-05 ~ 1976-06)Forrest et al. 1978 (1975-04 ~ 1975-05)Engels 1982 (1976-02-28 ~ 1980-10-20)Engels 1982 (1977-08-23)IRAS PSC

ISO (SWS, LWS: 1996-10-11)

Minimum phase for all dust models

L*=2.7x104 Lo , Tc=1000 K, 10 = 17

L*=3.6x104 Lo , Tc=1000 K, 10 = 15

Dust Model One (=0)

L*=2.7x104 Lo , Tc=1121 K, 10 = 20

L*=3.6x104 Lo , Tc=1139 K, 10 = 17.3

Dust Model Two (=0)

L*=2.7x104 Lo , Tc=1191 K, 10 = 23

L*=3.6x104 Lo , Tc=1328 K, 10 = 24

Dust Model Three (=0)

Crystalline Silicate features at 33.3, 40.6, 43.3 m

Mass-loss Rate: 6.1×10-5∼6.8×10-5 M/yr

Dust shell models for Pulsating HMOA stars (from Suh 2004)

Dust Model OneDust grains form at 1000 K and instantaneously evaporate at T > 1000 K.

Dust Model TwoDust 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 ThreeDust 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 One Dust Model Two Dust Model Three

Dust shell models for Pulsating HMOA stars

The 3 different dust models produce similar fitting with the observations.

Dust Model One

Pulsation Phase0.00 0.25 0.50 0.75 1.00 1.25 1.50

r (x

10

-4 p

c)

1.0

1.2

1.4

1.6

1.8

2.0

Max

Dust Model Two for OH26.5+0.6 (a HMOA star)

Dust formation radius (1000 K)

Rc formed at minimum phase

Dust shell inner radius (Rc)

1139 K

Min

Dust 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

[12 - 25]0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

[25

- 60

]

Z Cyg OH127.8+0.0

o Ceti OH26.5+0.6

Min

Max

Min

Max

Min

Max

Min

Max

LMOA stars HMOA stars (using Dust Model Three)

The IR two-color diagram

K - L0 1 2 3 4 5 6 7 8 9

0.5

1.0

1.5

2.0

2.5

3.0

3.5

[12-

25]

HMOA stars (using Dust Model Three)

Z Cyg

o Ceti OH26.5+0.6

OH127.8+0.0

LMOA stars

Min

Max

Min

Max

Min

Max

Min

Max

Axi-symmetric dust envelope models for AGB stars – The Model SEDs

(using 2-Dust with multi dust components; Suh 2005 in preparation)

Wavelength (m)1 10 100

10-13

10-12

10-11

10-10

10-9

2-dust models (A=9, F=1)

F

(W m

-2)

L*=1.0E4 Lo, T*=2000 K, Tc=1131 K, 10=30 (90d)

L*=1.0E4 Lo, T*=2000 K, Tc=1131 K, 10=30 (45d)

L*=1.0E4 Lo, T*=2000 K, Tc=1131 K, 10=30 (0d) L*=1.0E4 Lo, T*=2000 K, Tc=1131 K, 10=30 (Shell)

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 m10 m

25 m

60 m

AGB stars and Planetary Nebulae

SummaryO-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 : Tc ~ 400 K (min) – 700 K (max) ; Rc ~ 30 - 40 R* (too large?)

Superwind models: Tc ~ 1000 K ; Rc ~ 10 R*

2. HMOA stars with thick dust shells : Tc~ 1000 K (min) – 1300 K (max) ; Rc ~ 4 - 6 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.