The effects of preaggregation silicate crystallinity and postaggregation silicate crystallinity o... more The effects of preaggregation silicate crystallinity and postaggregation silicate crystallinity on the 11.3μm structure in comet dust emission have been calculated. Of the order of 10 - 20% crystallinity provides the best agreement to date with observation but the fits are not yet fully satisfactory.
Proceedings of the International Astronomical Union, 2008
We investigate the evolution of dust formed in Population III supernovae (SNe) by considering its... more We investigate the evolution of dust formed in Population III supernovae (SNe) by considering its transport and processing by sputtering within the SN remnants (SNRs). We find that the fate of dust grains within SNRs heavily depends on their initial radii aini. For Type II SNRs expanding into the ambient medium with density of nH,0 = 1 cm−3, grains of aini < 0.05 μm are detained in the shocked hot gas and are completely destroyed, while grains of aini > 0.2 μm are injected into the surrounding medium without being significantly destroyed. Grains with aini = 0.05–0.2 μm are finally trapped in the dense shell behind the forward shock. We show that the grains piled up in the dense shell enrich the gas up to 10−6–10−4 Z, high enough to form low-mass stars with 0.1–1 M. In addition, [Fe/H] in the dense shell ranges from −6 to −4.5, which is in good agreement with the ultra-metal-poor stars with [Fe/H] < −4. We suggest that newly formed dust in a Population III SN can have great ...
We present recent advances in theoretical studies of the formation and evolution of dust in primo... more We present recent advances in theoretical studies of the formation and evolution of dust in primordial supernovae (SNe) that are considered to be the main sources of dust in the early universe. Being combined with the results of calculations of dust formation in the ejecta of Population III SNe, the investigations of the evolution of newly formed dust within supernova remnants (SNRs) show that smaller grains are predominantly destroyed by sputtering in the shocked gas, while larger grains are injected into the ambient medium. The mass of dust grains surviving the destruction in SNRs reaches up to 0.1--15 $M_\odot$, which is high enough to account for the content of dust observed for the host galaxies of quasars at $z > 5$. In addition, the transport of dust formed in the ejecta causes the formation of low-mass stars in the dense shell of primordial SNRs and affects the elemental composition of those stars. We also show that the flat extinction curve is expected in the high-redshi...
The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has b... more The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13µm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13µm feature when the mass loss rate is less than 2 × 10-5M·/yr.
ABSTRACT We construct a semi-analytical model which includes a novel implementation of dust size ... more ABSTRACT We construct a semi-analytical model which includes a novel implementation of dust size evolution and the resulting H2 formation on dust grains in the early universe. We investigate the cosmic star formation rate and the reionization history, taking into acount the initial mass function transition from Pop III to Pop II stars due to dust cooling. We derive the star formation efficiencies for various host dark matter halo masses and discuss effects of the value of critical dust-to-gas mass ratio, Dcr, on the reionization history. We conclude that the reionization completes before z = 6 in the case of the fiducail value cr = 7.4×10-8.
— We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in th... more — We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon‐rich asymptotic giant branch (AGB) stars employing a newly‐derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core‐graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core‐graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 &lt; n̈0.1 (M5/ζ)−1/2L41/4 &lt; 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s−1, M5 the mass loss rate in 10−5 M⊙ year−1, L4 the stellar luminosity in 104 L⊙, and M/ζ is the effective mass loss rate taking account of non‐spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M/ζ = 10−5‐10−3 M⊙ year−1 correspond to 0.01 &lt; Pc &lt; 0.9 dyn cm−2, implying that the observed TiC core‐graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1 &lt; ∍ ≲ 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon‐rich AGB stars. Brief discussion is given on other types of graphite spherules.
The effects of preaggregation silicate crystallinity and postaggregation silicate crystallinity o... more The effects of preaggregation silicate crystallinity and postaggregation silicate crystallinity on the 11.3μm structure in comet dust emission have been calculated. Of the order of 10 - 20% crystallinity provides the best agreement to date with observation but the fits are not yet fully satisfactory.
Proceedings of the International Astronomical Union, 2008
We investigate the evolution of dust formed in Population III supernovae (SNe) by considering its... more We investigate the evolution of dust formed in Population III supernovae (SNe) by considering its transport and processing by sputtering within the SN remnants (SNRs). We find that the fate of dust grains within SNRs heavily depends on their initial radii aini. For Type II SNRs expanding into the ambient medium with density of nH,0 = 1 cm−3, grains of aini < 0.05 μm are detained in the shocked hot gas and are completely destroyed, while grains of aini > 0.2 μm are injected into the surrounding medium without being significantly destroyed. Grains with aini = 0.05–0.2 μm are finally trapped in the dense shell behind the forward shock. We show that the grains piled up in the dense shell enrich the gas up to 10−6–10−4 Z, high enough to form low-mass stars with 0.1–1 M. In addition, [Fe/H] in the dense shell ranges from −6 to −4.5, which is in good agreement with the ultra-metal-poor stars with [Fe/H] < −4. We suggest that newly formed dust in a Population III SN can have great ...
We present recent advances in theoretical studies of the formation and evolution of dust in primo... more We present recent advances in theoretical studies of the formation and evolution of dust in primordial supernovae (SNe) that are considered to be the main sources of dust in the early universe. Being combined with the results of calculations of dust formation in the ejecta of Population III SNe, the investigations of the evolution of newly formed dust within supernova remnants (SNRs) show that smaller grains are predominantly destroyed by sputtering in the shocked gas, while larger grains are injected into the ambient medium. The mass of dust grains surviving the destruction in SNRs reaches up to 0.1--15 $M_\odot$, which is high enough to account for the content of dust observed for the host galaxies of quasars at $z > 5$. In addition, the transport of dust formed in the ejecta causes the formation of low-mass stars in the dense shell of primordial SNRs and affects the elemental composition of those stars. We also show that the flat extinction curve is expected in the high-redshi...
The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has b... more The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13µm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13µm feature when the mass loss rate is less than 2 × 10-5M·/yr.
ABSTRACT We construct a semi-analytical model which includes a novel implementation of dust size ... more ABSTRACT We construct a semi-analytical model which includes a novel implementation of dust size evolution and the resulting H2 formation on dust grains in the early universe. We investigate the cosmic star formation rate and the reionization history, taking into acount the initial mass function transition from Pop III to Pop II stars due to dust cooling. We derive the star formation efficiencies for various host dark matter halo masses and discuss effects of the value of critical dust-to-gas mass ratio, Dcr, on the reionization history. We conclude that the reionization completes before z = 6 in the case of the fiducail value cr = 7.4×10-8.
— We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in th... more — We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon‐rich asymptotic giant branch (AGB) stars employing a newly‐derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core‐graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core‐graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 &lt; n̈0.1 (M5/ζ)−1/2L41/4 &lt; 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s−1, M5 the mass loss rate in 10−5 M⊙ year−1, L4 the stellar luminosity in 104 L⊙, and M/ζ is the effective mass loss rate taking account of non‐spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M/ζ = 10−5‐10−3 M⊙ year−1 correspond to 0.01 &lt; Pc &lt; 0.9 dyn cm−2, implying that the observed TiC core‐graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1 &lt; ∍ ≲ 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon‐rich AGB stars. Brief discussion is given on other types of graphite spherules.
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Papers by takashi kozasa