Low Temperature Crystallization of Dust

Tetsuo Yamamoto, Kyoko K. Tanaka(ILTS, Hokkaido U.)

Hiroshi Kimura (CPS, Kobe U.)

(Tanaka+2010, ApJ, 717, 586)

Staub in Planetensystemen/惑星系の「うちゅうじん」Japanese-German Workshop

September 27-October 01, 2010

2010年10月1日金曜日

Crystalline silicate in various kinds of objects

• Observed in– evolved stars

• C-rich giant stars, post AGB stars

– Herbig Ae/Be stars– T Tauri stars– young MS stars– Comets– ZL dust– IDPs– other galaxies (ULIRG)

• Not observed in – ISM & molecular clouds– Protostars

(Molster et al. 1999)

2010年10月1日金曜日

Crystallization: Transition from amorphous to crystalline state

• L : latent heat of crystallization•must overcome energy barrier Ec for crystallization

internal energy : E(crystal) < E(amorphous)

2010年10月1日金曜日

Energy source to overcome Ec

But other energy sources:

Annealing induced by fluctuation of thermal energy (Hallenbeck et al. 1998, Fabian et al. 2000, Kamitsuji et al. 2005,

and many others)

• irradiation of electrons or high energy particles (Carrez et al. 2002, Y. Kimura et al. 2008)

• heat of chemical reactions

(Yamamoto & Chigai 2005, Yamamoto+2010)

2010年10月1日金曜日

Basic idea of crystallization by heat of reactions

0) Suppose a silicate grain coated with a mantle including chemically reactive molecules (radicals).

1) Moderate heating induces chemical reactions in the mantle2) heat of reactions leads to crystallization

(Yamamoto & Chigai 2005, Yamamoto+2010)2010年10月1日金曜日

Series of experiments by Kaito et al.(Kamitsuji et al. 2005; Kaito et al. 2007a,b)

Exp.1

Exp.2

Exp.3

amorphous silicate

amorphous carbon

amorphous carbon

**

**

*

**

*

CH4amorphous silicate

graphite

forsterite

graphite

amorphous silicate

amorphous silicate

forsterite

forsterite

T~1000K

T~870K

T~300K

~100nm

~10nm

heating in vacuum

exposedto the air

heating in vacuum

2010年10月1日金曜日

graphitization of C

HREM image of the sample and its magnification of the interface region

crystallization of Mg silicate

Kaito et al. 2007b

2010年10月1日金曜日

Picture of crystallization in Exp. 3 by Kaito et al. (2007b)

1. Release of heat of chemical reactions C(oxidation of CH4 ) in C mantle

2. Induce graphitization in the C mantle

3. Latent heat deposited by graphitization leads further temperature elevation

4. Induce crystallization of silicate from the interface of C mantle and silicate core

5. Cessation of crystallization due to cooling

**

** *

*アモルファsil

C

CH

2010年10月1日金曜日

Modelling of Exp. 3

Time variation of temperature of the particle

Rate of generation of heat of reactions in mantle

Crystal growth

ahsil.

organics

Ttrigger =Ed

ln !0"= 200 K

!Ed

104 K

"for " = 103 yr

4

3#$cp

dT

dt=

4

3#[(a + h)2 ! a3]% ! !air ! !rad + Hc + Hsil

%̇ = !dnrad

dtEr

dnrad

dt= !!re

!Ear/kTnrad

dasil

dt= a0!e!Esil/kT (1 ! e!qsil(Tm!T )/kT )

J =4#!

3a30

#&'

#kTexp

$!

E

kT!

4&3'3T

27q2k(Tm ! T )2

%

da

dt= a0! exp

!!

E

kT

" $1 ! exp

!!

q(T ! Tm)

kT 2

"%

Vcr =

& t

0dt" (V0 ! Vcr)J

4

3#a3(t, t")

1

Ttrigger =Ed

ln !0"= 200 K

!Ed

104 K

"for " = 103 yr

4

3#$cp

dT

dt=

4

3#[(a + h)2 ! a3]% ! !air ! !rad + Hc + Hsil

%̇ = !dnrad

dtEr

dnrad

dt= !!re

!Ear/kTnrad

dasil

dt= a0!e!Esil/kT (1 ! e!qsil(Tm!T )/kT )

J =4#!

3a30

#&'

#kTexp

$!

E

kT!

4&3'3T

27q2k(Tm ! T )2

%

da

dt= a0! exp

!!

E

kT

" $1 ! exp

!!

q(T ! Tm)

kT 2

"%

Vcr =

& t

0dt" (V0 ! Vcr)J

4

3#a3(t, t")

1

Ttrigger =Ed

ln !0"= 200 K

!Ed

104 K

"for " = 103 yr

4

3#$cp

dT

dt=

4

3#[(a + h)2 ! a3]% ! !air ! !rad + Hc + Hsil

%̇ = !dnrad

dtEr

dnrad

dt= !!re

!Ear/kTnrad

dasil

dt= a0!e!Esil/kT (1 ! e!qsil(Tm!T )/kT )

J =4#!

3a30

#&'

#kTexp

$!

E

kT!

4&3'3T

27q2k(Tm ! T )2

%

da

dt= a0! exp

!!

E

kT

" $1 ! exp

!!

q(T ! Tm)

kT 2

"%

Vcr =

& t

0dt" (V0 ! Vcr)J

4

3#a3(t, t")

1

4

3!a3"c

dT

dt=

4

3![(a + h)3 ! a3]#̇ ! !air ! !rad + Hsi + Hc

1

2010年10月1日金曜日

Key physical quantities

• Stored nergy densiy : Q = nrad(0) Er

• Time scale of the reactions : τ

• Ambient gas pressure

2010年10月1日金曜日

Feature of crystallization

T/K

2000

0

cooling by heatcooling by heatconductionconductioninto ambient airinto ambient air

heat of reactionheat of reaction

latent heat oflatent heat ofcrystallizationcrystallization

10

1

C crystallization

silicate crystallization

thic

knes

s/nm

10-8 10-5 time/s

ｎＡ０Er＝102７ Kcm-3

　　　 ＝５ -8sｎＡ０Er＝102７ Kcm-3

　　　 ＝５ -8s

ｎＡ０Er＝102７ Kcm-3

　　　 ＝５ -8s

( nrad (0) Er =1027 K cm-3 ; tr = 10-8 s, gas density = 10-3 g cm-3 = 1 atm )

2010年10月1日金曜日

( nrad (0) Er = 0.9 ×1027 K cm-3 ; tr = 10-8 s, gas density = 10-3 g cm-3 = 1 atm )

T/K

2000

0

10

1

thickness/nm

10-8 10-5 time/s

C crystallization

Feature of crystallizationFeature of crystallizationFeature of crystallization

Feature of crystallization (2)small deposition of heat of reaction

2010年10月1日金曜日

0

1000

2000

3000

4000

10 -8 10 -6 10 -410 -8

10 -7

10 -6

10 -5

10 -8 10 -6 10 -4 10 -8 10 -6 10 -4t [s]

(a) Q=1.1*1027 Kcm-3

crystal silicate

graphite

t [s]

crystal silicate

graphite

t [s]

graphite

(b) Q=1.0*10 27 Kcm -3 (c) Q=0.9*1027 Kcm -3

Exp.3 Exp.3

Exp.3

T[K]

thickness[cm]

(P = 1 atm)2010年10月1日金曜日

0

1000

2000

3000

4000

10 -8 10 -6 10 -410 -8

10 -7

10 -6

10 -5

10 -8 10 -6 10 -4 10 -8 10 -6 10 -4t [s]

(a) τ =5*10-9 s

crystal silicate

graphite

t [s]

crystal silicate

graphite

t [s]

Exp.3 Exp.3

Exp.3

(b) τ =5*10-8 s (c) τ =2*10-7 s

graphite

T[K]

thickness[cm]

(P = 1 atm)2010年10月1日金曜日

10 -10 10 -5

no crystallization

partial crystal

crystallization

air density [gcm-3]

Q[1027 Kcm-3]

10 -10 10 -5

10-5

1

air density [g cm-3]

no crystallization

partial crystal

crystallization

τ[s]

Crystallization conditions

Radical concentration >1 - 10 % leads to substantial crystallization

Low gas density, large energy deposition, fast reactionseasy crystallization

2010年10月1日金曜日

Crystallization degree in steady accretion disks(Q = nrad(0) Er = 1027 K cm-3 )

Cry

stal

lizat

ion

degr

ee

1

0.5

1 r / AU

Dr =10000K3000K

1000K

0.1

partial crystallization

complete crystallization l

0

10

no crystallization

Crystalline region is much extended than by annealing. 2010年10月1日金曜日

• Crystallization triggered by chemical reactions may explain ubiquity of crystalline silicate in various objects

‣ Similar phenomenon

- Wigner energy release known in nuclear reactor engineering

- Sudden release of energy stored in graphite moderator irradiated by neutrons upon moderate heating (Spontaneous energy release)

Concluding remarks (1/3)

2010年10月1日金曜日

• The present mechanism does not depend on the details of the chemistry but depends only on the amount of reactive molecules times heat of reactions, Q = nrad(0) Er, and reaction timescale τ.

Concluding remarks (2/3)

2010年10月1日金曜日

• Similarity of ice compositions in molecular clouds and comets:

- Present mechanism can explain the coexistence of crystalline silicate and ice of IS composition in comets without mixing.

• Search for crystalline silicate at low temperature environments is encouraged by future high resolution observations of disks.

Concluding remarks (3/3)

2010年10月1日金曜日