2019/08/01

On the roles of molecules
for dust formation
Takaya Nozawa(National Astronomical Observatory of Japan)

Topics of this talk(1) Molecules as precursors of dust grains(2) Molecules as efficient coolants of warm gas

1-1. Molecules in AGB stars
O-rich star (W Hya)
AlO (N=9-8)

C-rich star (IRC+10216)
Si2C

29SiO (v=0, J=8-7)

Takigawa+2016

Cernicharo+2015

1-2. CO and SiO molecules in SN 1987A
CO first overtone: 2.29-2.5 μm
CO fundamental: 4.65-6 μm
SiO fundamental: 7.6-9.5 μm

Mco~4x10-3 Msun

200 day
Liu+1995

415 day

615 day

Wooden+1993

MSiO~7x10-4 Msun

Liu & Dalgarno 1996

1-3. CO and SiO detection in other SNe

Gerardy+2002, SN2000ew at ~100 day

CO (2-1)
SiO (5-4)

Kotak+2009, SN 2004et at ~400 day

Abellan+2017, SN 1987A at ~28 yr

1-4. CO detection in Cassiopeia A SNR
Herschel PACS

AKARI IRC spectra

CO (4.5 μm)
dust (21 μm)

Rho+2012
Cas A at ~330 yr
Wallstrom+2013

1-5. Observations of molecules in AGB/SNe(1) O-rich (C/O < 1) star : CO, SiO, AlO, MgO, …
C-rich (C/O > 1) star : CO, CS, HCN, SI2C, SiC2, …
(2) N-MIR emissions by CO and SiO ro-vibration
100-600 days in several SNe(3) FIR-submm emission by CO and SiO rotation
28 yr in SN 1987A(4) CO in Cas A (330 yr) ➜ survive? or re-formation?

〇 CO : inhibiting dust formation〇 SiO, AlO, MgO, Si2C, SiC2:
precursor molecules for dust formation

2-1. Theory of nucleation and grain growth
1. Nucleation

2. Grain growth(formation of stable clusters)・ Steady-state nucleation rate・ Growth rate of dust grains
number of stable clusters
formed per volume per sec

2-2. Base of nucleation theory〇 Nucleation : the first-order phase transition(condensation from gas-phase to solid-phase)- described by the change of Gibbs free energy- Need to specify the chemical reaction
ex. silicate : Mg(g) + SiO(g) + 2O(g) ==> MgSiO3(s)gas-phaseatoms

molecules(small clusters)

stableclusters

dustgrains

2-3. Non-steady-state (kinetic) nucleation
c1c2
J2c3
J3

αn-1c1cn-1
Jnβncn〇 Master equationssticking coefficient, ηs = 1

energy barrier : ~ 10cn

2-3-1. Concept of non-steady-state nucleation〇 steady-state nucleation rate: Js
gas atomsnc (~10)bulk dust〇 non-steady-state nucleation rate: J＊
gas atomsclusters

bulk dustnc (= 10-103)

2-4. Reaction coefficients of carbon clusters
Arrhenius form

Cherchneff & Dwek (2010)

2-5. Drawbacks of chemical nucleation〇 Unknown reaction coefficients- no data for gas species and clusters of interest➜ most of reaction coefficients assumed- experimental data mainly <300 K➜ Extrapolation to high-T may
involve large uncertainties〇 Incomplete chemical network- need to treat the destruction processes- if important (bottleneck) reactions are missed,
the results may change dramatically2-6. Hybrid nucleation model〇 Hybrid nucleation model- formation of diatomic molecules➜ simple chemistry- nucleation theory➜ kinetics and thermodynamics

Nozawa et al. in prep.

SN-origin presolar SiC
Grains (Nittler 2003)

2-7. Insufficient data on reaction coefficients
Reaction coefficients〇 Wanted reaction data
- C-bering grains➜ C3, C4, … Si2C, SiC2
- silicate/oxide grains➜ MgO, SiO2, AlO, …

Andreazza+2009- Fe-bearing grains➜ Fe2, FeO, FeS, …

〇 Quantum chemistry calculation

Babb+2019, ApJ, 876, 38

3-1. Theory of nucleation and grain growth
1. Nucleation

2. Grain growth(formation of stable clusters)

・ steady-state nucleation rate

・ Growth rate of dust grains
number of stable clusters
formed per volume per sec

3-2. Scaling relation of average grain radius
Nozawa & Kozasa (2013)

CMgSiO3〇 Λon = τsat/τcoll ∝ τcool/τgrow➜ the ratio of supersaturation timescale to gas collisiontimescale at the onset time (ton) of dust formation- τsat ∝ τcool : timescale of nucleation- τcoll ∝ τgrow : timescale of grain growth3-3. Sizes of newly formed grains(1) larger τcool/τgrow
grain growth proceeds much
more rapidly than nucleation➜ larger grains

https://ja.wikipedia.org/wiki/雪だるま(2) smaller τcool/τgrow (> 10)τgrow is moderately shorter
than timescale of nucleation➜ smaller grains

http://asahi-ginza.jugem.jp/?eid=395(3) τcool/τgrow < 1➜ no dust formation
https://ja.wikipedia.org/wiki/雪3-4. Temperature evolution of gas〇 Supernovae〇 Stellar winds (AGB stars)〇 Others

3-5. Cooling rates around 1000 K
Cooling rate by emission lines??
Cooling function indiffuse medium

Gas cooling by COvibration would begreat in dense gas

Cooling function in dense medium

4. Conclusions(1) Molecules as precursors of dust grains- important for nucleation process of dust grains- hybrid nucleation model➜ following only formation of diatomic molecules- wanted data :reaction rate coefficients of AlO, MgO, Fe2, FeO, …
(2) Molecules as efficient coolants of warm gas- accelerate cooling, affecting size of newly formed dust
- wanted data :Einstein coefficients of CO/SiO ro-vibration transition