2017/12/19

超新星起源プレソーラー
SiC粒子の形成
野沢 貴也（Takaya Nozawa）
国立天文台 理論研究部(National Astronomical Observatory of Japan)
Collaborators:
Shigeru Wakita (NAOJ), Yasuhiro Hasegawa (JPL),
Sachiko Amari, Takashi Yoshida (U. of Tokyo),
Kyoko, K. Tanaka, Yuki Kimura (Hokkaido Univ.),
Hidekazu Tanaka (Tohoku 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/28Si rich, with excesses in 44Ca (44Ti), 49Ti (49V)➜ originated from core-collapse supernovae

Nittler 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

Umeda & Nomoto 2002

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

300 K

Temperature; T104 K

3-1. Formation of SiC molecules
Element# abundance

He0.3

C0.3

O0.001

Ne0.001

Mg0.002

Si0.2

S

0.19

Ar0.006

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?

Yes, the formation of SiC grains is possible !

3-4. Formation process of SiC grains〇 Formation of SiC grains is possible !!
‐ Condensation temperature is higher for higher gas density‐ Dust growth proceeds on a short timescale through rapid
accretion of molecules‐ Coagulation makes the average radius by about a factor of 2

3-5. Size distribution of SiC grains formed〇 Timescale of collision－ Timescale of gas accretion onto 0.001um-sized grains－ Timescale of coagulation between 0.001um-sized grains－ Timescale of gas expansion

3-6. Size distribution of SiC grains formed‐ Radius of newly formed grains is larger for higher gas density‐ In the density range considered in this study, grain radius is notlarge enough to reproduce ones observed in presolar SiC grains

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 (but with small radii of < 0.1 µm…)## calculations for more dense cases needed to## achieve the formation of SiC grains as large as## presolar Type-X SiC grains (a > 0.1 µm)