Aki Takigawa

Injection of Short-Lived Radionuclides into the Early Solar System from a Faint Supernova with Mixing Fallback

Several short-lived radionuclides (SLRs), some of which should have formed just prior to or soon after the solar system formation, were present in the early solar system. Stellar nucleosynthesis has been proposed as the mechanism for the production of SLRs in the solar system, but no appropriate stellar source has been found that explains the abundances of all solar system SLRs. In this study, we propose a faint supernova with mixing and fallback as a stellar source of SLRs with mean lives of <5 Myr (26Al,41Ca,53Mn, and 60Fe) in the solar system. In such a supernova, the inner region of the exploding star experiences mixing, a small fraction of mixed materials is ejected, and the rest undergoes fallback onto the core. The modeled SLR abundances agree well with their solar system abundances if mixing fallback occurs within the C/O-burning layer. In some cases, the initial solar system abundances of the SLRs can be reproduced within a factor of 2. The dilution factor of supernova ejecta to the solar system materials is ~10−4, and the time interval between the supernova explosion and the formation of the oldest solid materials in the solar system is ~1 Myr. If the dilution occurred due to spherically symmetric expansion, a faint supernova should have occurred near the solar system-forming region in a star cluster.

ANISOTROPIC EVAPORATION OF FORSTERITE AND ITS IMPLICATION FOR DUST FORMATION CONDITIONS IN CIRCUMSTELLAR ENVIRONMENTS

Crystalline silicates are observed in many protoplanetary disks and some dust shells around evolved stars. The peak positions of infrared (IR) spectra of forsterite, which is the most abundant circumstellar silicate, vary with dust temperature, composition, size, and crystallinity. However, there is another important factor that affects IR spectra, which is the shape with a specific crystallographic orientation called the crystallographically anisotropic shape. We focused on anisotropic evaporation of crystalline forsterite as one of the possible processes that change the crystallographically anisotropic shape of forsterite grains, and carried out evaporation experiments of single crystals of forsterite in hydrogen gas (0.01-10 Pa) and at temperatures of 1150-1660°C. Forsterite evaporated anisotropically in all experimental conditions, and the anisotropy depended on temperature and hydrogen gas pressure. The results enabled us to calculate crystallographically anisotropic shapes of heated forsterite as a function of temperature and hydrogen pressure, and their corresponding IR spectra. Distinctly, different sets of peak positions were seen in IR spectra of grains with different combination of shapes and their orientation reflecting the heating conditions. The results were applied to the IR spectrum of a protoplanetary disk, HD100546, which suggests that forsterite dust particles that experienced evaporation exist with dominant secondarily fragmented forsterite formed by small-body collisions. We propose that detailed IR spectroscopy of forsterite, and probably other anisotropic crystals, is a new tool to estimate temperature and pressure conditions of circumstellar environments where dust formed.

Crystallographically Anisotropic Shape of Forsterite: New Probe for Evaluating Dust Formation History from Infrared Spectroscopy

Crystalline dust has been observed by infrared spectroscopy around dust-enshrouded asymptotic giant branch stars, in protoplanetary disks, and from some comets. Crystalline materials often have a specific shape related to a specific crystallographic orientation (crystallographically anisotropic shape), which reflects the anisotropic nature of crystals, and their infrared spectral features depend on crystallographically anisotropic shapes. The crystallographically anisotropic shape is thus a potentially powerful probe to evaluate circumstellar dust-forming conditions quantitatively. In order to assess the possibility to determine the crystallographically anisotropic shape from infrared spectra, we calculated mass absorption coefficients for ellipsoidal forsterite particles, the most abundant circumstellar crystalline silicate, elongated and flattened along the crystallographic a-, b-, and c-axes with various aspect ratios in the wavelength range of 9-70 ?m. It was found that differences in infrared features caused by various crystallographicaly anisotropic shapes are distinguishable from each other irrespective of the effects of temperature, size, chemical composition, and grain edges of forsterite in the range of 9-12 ?m and 15-20 ?m. We thus concluded that the crystallographically anisotropic shape of forsterite can be deduced from peak features in infrared spectra. We also showed that the crystallographically anisotropic shapes formed by evaporation and condensation of forsterite can be distinguished from each other and the temperature condition for evaporation can be evaluated from the peak features. We applied the present results to the infrared spectrum of a protoplanetary disk HD100546 and found that a certain fraction (~25%) of forsterite dust may have experienced high-temperature evaporation (>1600?K).

Dust formation and wind acceleration around the aluminum oxide–rich AGB star W Hydrae

AlO and 29SiO distributions around W Hya showed that AlO efficiently forms dust and contributes to the wind acceleration. Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but ~40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide–rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae.

The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA

Our current understanding of the chemistry and mass-loss processes in Sun-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere1. Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions2, but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimetre continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hydrae. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time longer than commonly assumed. A shocked hot layer will be an important ingredient in current models of stellar convection, pulsation and chemistry at the late stages of stellar evolution.The atmosphere of evolved star W Hya has been resolved with ALMA and shown to be shock heated. These observations provide important empirical constraints for the understanding of circumstellar structure, convection, chemistry and pulsation.

High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum

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Morphologies, Isotopes, Crystal Structures, and Microstructures of Presolar Al 2 O 3 Grains: a NanoSIMS, EBSD, EDS, CL, and FIB-TEM study

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Contribution of a

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Morphology and crystal structures of solar and presolar Al2O3 in unequilibrated ordinary chondrites

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EVAPORATION AND CONDENSATION KINETICS OF CORUNDUM: THE ORIGIN OF THE 13 μ m FEATURE OF OXYGEN-RICH AGB STARS

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