Physical Conditions of Supernova Ejecta as Viewed from
the Measured Sizes of Presolar Al2O3 Grains
Takaya Nozawa1, Shigeru Wakita1, Yasuhiro Hasegawa2, Takashi Kozasa3

takaya.nozawa@nao.ac.jp

1National Astronomical Observatory of Japan (NAOJ), 2California Institute of Technology (CIT), 3Hokkaido University○ Abstract A few particles of presolar Al2O3 grains with sizes above 0.5 μm are believed to have been produced in the ejecta of corecollapse supernovae (SNe). In order to clarify the formation condition of such large Al2O3 grains, we investigate the condensation of Al2O3
grains for wide ranges of the gas density and cooling rate. We first show that the average radius and condensation efficiency of newly formed
Al2O3 grains are successfully described by a non-dimensional quantity Λon, defined as the ratio of the timescale on which the supersaturation
ratio increases to the collision timescale of reactant gas species at dust formation. Then we find that the formation of submicron-sized Al2O3
grains requires at least 10 times higher gas densities than those predicted by one-dimensional (1-D) SN models. This indicates that presolar
Al2O3 grains identified as having their origin in SNe might be formed in dense gas clumps, allowing us to propose that the measured sizes ofpresolar grains can be a powerful tool for constraining the physical conditions in which they formed. We also briefly discuss the survival of
newly formed Al2O3 grains against subsequent destruction by the reverse shock at the phase of the SN remnant (SNR).

○ Presolar Al2O3 grains
・ About 10% of presolar Al2O3 grains are considered to have originated fromcore-collapse SNe (so-called Group 3/4 grains, Nittler et al. 1997, 2008).
・ Some of SN-origin presolar Al2O3 grains have radii larger than 0.25 µm.
(see Fig. 1, Choi et al. 1998; Nittler et al. 1998)○ Theoretical predictions about sizes of Al 2O3 grains (see Fig. 2)
・ The calculated radii of newly formed Al2O3 grains are smaller than 0.01 µm.
➜ Such small grains may be completely destroyed by the reverse shocks
before injection into interstellar medium (ISM) (Nozawa et al. 2007).

Fig. 1 – SEM images of (presolar) Al2O3
grains from samples of Choi et al. (1998).
S-C122, a grain in (B), is inferred as a SN
source from its anomalous O isotopic
composition and has a size about 1 µm.

Fig. 2 – Size distributions of dust grains formed in
the ejecta of a core-collapse SN, obtained from dust
formation calculations (Nozawa et al. 2003). Notethat the radii of Al2O3 grains (red) are < 0.01 µm.

○ Calculations of dust formation

Fig. 3 – Dependence of the final
average radii aave,∞ and condensation
efficiencies fcon,∞ of newly formed
Al2O3 grains on Λon. The results forfour different γ (γ = 1.1, 1.3, 1.5, and1.7) are shown in each color but they
are plotted as almost the same curve.
The hatched regions depict theexpected ranges of Λon for theformation of Al2O3 grains in the Alrich region, referring to 1D models ofa Type II-P SN (cyan; Kozasa et al.
2009) and a Type IIb SN (Nozawa etal. 2010). The solid vertical line
denotes the minimum value of Λon(=3x104) necessary for explaining themeasured sizes (radius of …
0.25 μm) ofpresolar Al2O3 grains.

・ Formula of non-steady-state dust formation (Nozawa & Kozasa 2013)
・ Evolutions of number density c(t) and temperature T(t) of the gas(c0 and γ : parameters)○ Results of dust formation calculations (Fig. 3)
・ Average radius aave,∞ and condensation efficiency fcon,∞ of newly
formed Al2O3 grains are scaled by a quantity Λon ≡ τsat / τcoll;τsat : timescale on which the supersaturation ratio increases
τcoll : collision timescale of reactant gas species, Λon ∝ (τcoll)-1 ∝ c(t)・ Λon should be higher than 3x10 4 in order to produce Al2O3 grains
with radii larger than 0.25 μm (diameters of > …
0.5 µm).
➜ Formation of submicron-sized Al2O3 grains requires 10 times
higher gas density than those inferred by 1-D SN models.
➜ SN-origin presolar Al2O3 might be formed in dense clumps.

Fig. 4 – Time evolutions of the positions(upper) and the ratios of radii to the initial
ones (lower) of Al2O3 grains in an SNR. The
results are plotted for the initial grain radii ofaini = 0.01, 0.02, 0.05, 0.1, and 0.25 μm. In
the upper panel, the thick solid curves depict
the trajectories of the forward shock (FS) andreverse shock (RS). The shaded region
between the FS and RS is the shock-heated
hot gas, in which dust grains are destroyed bysputtering. Small grains of aini < 0.02 µm arecompletely destroyed in the shocked gas.
Grains with 0.02 µm < aini < 0.1 µm areeroded in the hot gas and are finally destroyed
as soon as they encounter the cool dense shellcreated behind the forward shock after 5x104
years. On the other hand, Al2O3 grains withaini > 0.1 µm can be ejected from SNe without
reducing their sizes significantly○ Can newly formed Al2O3 grains survive in SNRs? (Fig. 4)
・ Calculations of dust destruction by reverse shocks (Nozawa et al. 2007)
・ Once submicron-sized Al2O3 grains are produced in dense clumps,
they are likely to survive the destruction in the shock-heated gas.
➜ The formation of large Al2O3 grains is indispensable in order that
Al2O3 grains endure shock destruction and are injected into ISM.

○ Conclusion
・ The average radius and condensation efficiency of newly formed Al2O3 grains are uniquelydescribed by the non-dimensional quantity Λon ≡ τsat / τcoll.
・ Presolar Al2O3 grains with radii above 0.25 µm can be formed only in the gas with morethan 10 times higher density than those estimated by 1-D SN models.
➜ indicating the presence of dense clumps in the inhomogeneous SN ejecta
・ The measured sizes of presolar grains are powerful probes for constraining the physicalconditions and structure in which they formed.

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