Hiroichi Hasegawa

Formation of dust grains in the ejecta of SN 1987A

Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolometric light curve around its maximum and the early emergence of X-rays and γ-rays, results in the sequential formation of Al 2 O 3 , MgSiO 3 and Fe 3 O 4 grains respectively in the ejecta at 1.0 M ⊙ ≤M r ≤4.4 M ⊙ as the gas cools down

Grain Formation through Nucleation Process in Astrophysical Environment

General picture of grain formation is preoented based on the nucleation theory. Grain formation process is described by a growth equation of grain radius and an equation of monomer consumption due to the growth of grains. These equations are characterized by two parameters. One depends on the physical conditions of the system and the other reflects the nature of grain materials. An overall feature of the grain formation process is illustrated by the use of an analytic expression of the solutions. After the vapor cools down to the saturated state, a waiting time is necessary until the grain formation begins effectively. Size distribution is relatively sharp in general. The representative size is closely related to the parameter which depends on the physical conditions. Growth by coalescence is not effective until the monomer sticking process is almost completed. The results are applied to the condensation in the primordial solar nebula. It is shown how the picture based on the chemical equilibrium calculations should be modified.

Extinction coefficients of amorphous carbon grains from 2100 Å to 340 μm

Extinction measurements were made for three kinds of amorphous carbon grains in the range 2100 Å-340 μm. Absolute values of extinction coefficients from different sources vary by almost constant factors. Wavelength dependences of extinction curves show a λ−1 or less steep fall off, against a λ−2 or steeper fall off of graphite grains. Small humps are found around 6.3, 8, 13.3 and 90 μm, although their origins are not yet clear. The infrared properties of amorphous carbon grains suggest that they could supply a significant amount of far-infrared emission noted in recent astronomical observations.

Grain formation in the expanding gas flow around cool luminous stars

We studied grain formation process and flow structure around cool luminous mass-loss stars. The nucleation and growth theory of Yamamoto and Hasegawa was extended to the case of expanding gas flow.The envelope was assumed to be steady, spherically symmetric, in thermal and radiative equilibrium, optically thin, and driven by radiation pressure on grains. For oxygen rich stars, Mg-silicate was found to be the first condensate which can drive the gas effectively. The following stellar parameters were chosen; stellar massM*=1M⊙, effective temperatureT*=3000K, stellar luminosityL* from 7.5×103 to 2.0×104L⊙, and mass-loss rate |M| from 1.0×10−6 to 1.0×10−4M⊙yr−1.Main results of calculations are as follows; (1) grain condensation temperatureTc≊980∼1080 K; (2) total gas pressure at the condensation pointPt≊6×10−11∼6×10−9 atm; (3) scale parameterAc≊103∼6×104; and (4) final grain sizerf=400Å∼1μm. For the smaller |M| or the largerL*, these values are the smaller. We recognized two types of flow solutions (1) Dust driven flow for large |M|, which reaches the sonic point near the condensation point; and (2) Modified Parker flow for small |M| for which grain sizerf is almost independent of |M|.A comparison with observational results ofL* and gas terminal velocityV∞ suggests that Mg-silicate grains are of submicron size, which are effective for interstellar extinction in visible and infrared. Fe-grains condense in the rarefied outflow with a size probably smaller than 100Å, which may contribute for interstellar ultraviolet extinction. The envelope has three-layer structure inner dense region with small outflow velocity, grain formation layer and outer rarefied region with fast outflow velocity.No flow solutions exist forM* greater than a critical stellar massM*cr for a given stellar luminosityL* and mass-loss rate |M|.For example, critical stellar massM*cr=1.8M⊙ forL*=104L⊙,T*=3000 K, and |M|=10-5M⊙yr-1.

The extinction coefficients in mid- and farinfrared of silicate and iron-oxide minerals of interest for astronomical observations

Extinction measurements were made for some silicate and iron-oxide mineral grains in mid- and far-infrared region. For far-infrared region, high temperature magnesium silicates such as olivine and pyroxenes show the absorption spectra of steep dependence as λ−3 (λ being the wavelength) with some peak structure, but the spectrum of magnetite shows λ−1 dependence.

The heterogeneous condensation of interstellar ice grains

Processes of heterogeneous condensation are investigated. We consider the situation that vapor and solid grains of which material is other than that of the vapor coexist and the saturation of the vapor increases with time. If the contact angle of the condensate to the solid is small (≳90°), the condensate will be formed as mantles coating the solid grains but only a fraction of the solid grains will be coated by mantles. The ratio of core-mantle grains to bare solid grains in number will be smaller as the density of the vapor and the time scale of saturation increase are large. Actual calculations are made by adopting H2O ice as the mantle material and magnesium silicate as the core material.

Mid- and far-infrared extinction coefficients of hydrous silicate minerals

Extinction coefficients were measured for three kind of hydrous silicate minerals, montmorillonite, chlorite and serpentine, from 7 to 140 μm. The infrared extinction coefficients of these minerals show (1) a few broad bands in the mid-infrared region and (2) a less steep wavelength-dependence in the far-infrared region, in contrast to those of high-temperature magnesium silicates. In the far-infrared region, montmorillonite shows a λ−0.8±0.1 dependence (λ, the wavelength) without any band structure, chlorite has a double maxima structure around 80 μm, and serpentine shows a rather steep dependence with a small peak at 77 μm.The changes of mid-infrared spectra by heating were measured. Change in chlorite spectrum is the most significant. Many fine features appear by heating and then they disappear. Above 900°C one broad feature remains around 10 μm. Fine features of the montmorillonite spectrum disappear by heating. For serpentine, many new peaks appear and the spectrum resembles the spectrum of olivin. In near-infrared a band around 2.72 μm disappears by heating.Extinction coefficients at very low temperatures were measured in the far-infrared region. For montmorillonite and serpentine, the spectrum is the same as that at room temperature. The double peaks of chlorite around 80 μm become higher.

Mid-infrared extinction coefficients of amorphous silicates

AbstractMid-infrared extinction coefficients of five natural amorphous silicates and seven synthetic glasses were measured. Three bands at about 10, 12, and 20 μm were seen for all the measured samples. The quantities of these bands are found to have good correlations with the SiO2 content of the samples. The correlations are the most remarkable for the 10 μm band. As the SiO2 content decreases, the peak wavelengthλm shifts to longer side, the peak heightKm decreases and the full width of half maximumW increases. A quantityλmKmW is constant within 15%. Empirical formula

Thermal radiation from dust grains formed in the ejecta of SN 1987A

\lambda_m (\mu m) = {11.10-2.30 x 10^-2} {[SiO_2 wt.\%]} \pm 0.15