2018/08/13

Formation of SiC grainsin the ejecta of core-collapse
supernovae
Takaya Nozawa(National Astronomical Observatory of Japan)

Collaborators:
Sachiko Amari, Takashi Yoshida (U. of Tokyo),
Shigeru Wakita (ELSI), Yasuhiro Hasegawa (JPL),
Kyoko, K. Tanaka, Hidekazu Tanaka (Tohoku Univ.),
Yuki Kimura (Hokkaido Univ.)

1-1. Presolar grains○ Presolar grains‐ discovered in primitive meteorites‐ showing peculiar isotopic compositions(different from the solar system’s materials)‐ thought to have formed in stars before
the Sun was formed➜ offering key information on
http://presolargrains.net/
・ nuclear processes in the parent stars
・ physical conditions in which they formed‐ abundance (volume fraction): ~100-500 ppm (~0.1-0.5 %)‐ mineral composition
graphite, SiC, TiC, Si3N4, Al2O3, MgAl2O4, Mg2SiO4, MgSiO3 …

1-2. Presolar Type X SiC grains○ Presolar SiC grains (~10 ppm)

Nittler & Hoppe 2005

〇 Type X SiC grains (~0.1 ppm, size: 0.1-20 µm)‐ 12C and 28Si rich, with excesses in 44Ca (44Ti), 49Ti (49V)➜ originated from core-collapse supernovaeNittler 2003

1-3. Long-lasting questions〇 FactA fraction of presolar SiC grains is highly likely
SN-origin〇 Question
How do these SiC grains form in the ejecta ofcore-collapse supernovae?
## Any theoretical studies have not yet realized the## formation of SiC grains in supernovae

2-1. Elemental abundances in the SN ejecta
44Ti

28Si rich

12C rich

Si isotopes

C (N) isotopes

Yoshida 2007; 4 Msun He-core model (Mstar ~ 18 Msun)

O-rich (O/C >> 1)
29, 30Si rich

29, 30Si solar
14N rich

Needed unrealistic mixing of the ejecta

2-2. Possible formation site of SiC grains in SNe
16O

4He

○ Moderately energetic SNe

12C

Ekin = (3-5)x1051 erg(normal SNe : Ekin ~ 1x1051 erg)

28Si

explosive burning happens at theboundary between O-rich regionand He-rich region➜ producing 12C-rich, 28Si-rich,(44Ti-rich) region## 3(4He) ➔ 12C## 12C + n(4He) ➜ α-elements

Pignatari+2013

2-3. No formation of SiC in the calculations〇 Condensation temperature‐C grains : Tcon = 1700-2200 K‐SiC grains: Tcon = 1300-1800 K➜ In C/Si-rich gas, C grains first
condense to use up the gasphase C atoms〇 Formation path of grains

cool downMore efficient destruction ofC grains (clusters)?
UV radiation, high-energy e,
chemical reaction, …

➜ this may not do a good job

2-4. Formation of SiC grains via molecules〇 Previous works

rate coefficients of radiative
association of molecules ki,j

➜ accretion of Si/C atoms〇 This work
‐accretion of SiC molecules‐coagulation
Andreazza+2009

3-1. Formation of SiC molecules
Element# abundance

He0.3C0.3

O0.001

Ne0.001

Mg

0.002

Si0.2

S

0.19

Ar

0.006

Pignatari+2013

3-2. Equation of dust formation〇 Master equations of dust growth

taking account of nucleation and grain growth through gas
accretion and coagulation between grains simultaneously!

3-3. Can SiC grains form in the SN ejecta?
behavior of dust formation
size distribution of dust

Yes, the formation of SiC grains is possible !

3-4. Size distribution of SiC grains formed‐ Radius of newly formed grains is larger for higher gas density‐ For the gas densities considered in this study, grain radiicannot reach ones (> 0.1 µm) observed in presolar SiC grains

3-5. Dependence of grain size on density
Average radius of dust grains

Mass fraction of dust grains‐ Radius of newly formed grains is larger for shorter τ_cool‐ Radius and mass of SiC grains are lower than those of C grains
by factors of ~5 and ~10, respectively.

4. Summary of this talkWe investigate the formation of SiC grains in theejecta of supernovae, self-consistently treating‐formation of SiC molecules‐growth of SiC grains via accretion of molecules‐growth of SiC grains via grain-grain coagulation

We have realized, for the first time, the formationof SiC grains in the ejecta of supernovae➜ Formation of large presolar Type-X SiC grains
above 0.1 µm is still difficult

Any idea, please !!