Dust formation around M-type stars

IRAS LRS spectra of M Mira variable have shown variation in the appearance of the 9.7 micron silicate feature, which is correlated with the shape of light curve. The LRS spectra of 100 Mira variables have been studied using simple dust shell models containing mixtures of silicate and aluminum oxide dust grains. It has been shown that the aluminum oxide grains account for the observed broad feature around 12 microns and that the variation of the spectra can be interpreted in terms of the variation of the temperature at the inner boundary of silicate dust shell. It has been proposed that silicate mantle growth on aluminum oxide grains is a possible explanation for the results. In this report, the model spectra are calculated taking account of silicate mantle growth, and the physical parameters which may determine the appearance of the 9.7 micron feature in M Mira variables are investigated. In the model calculation it is assumed that aluminum oxide grains are already formed at the bottom of the circumstellar envelope because of their high condensation temperature. The growth of silicate mantle and the motion of gas and grains from r=r(sub 0), where the mantle growth starts, are investigated. Sticking and sputtering processes due to the relative motion of grain to the ambient gas are taken into account. The thermal velocity is assumed to be negligible to the drift velocity. Acceleration by radiation pressure is considered in the gas motion equation. The formal solution is integrated to obtain the emergent spectra. Physical conditions inside r(sub 0) are regarded as boundary conditions. Observed spectra are compared to model spectra to investigate the conditions at the bottom of circumstellar envelope. In modeling the envelope, a parameter C(sub l) is introduced to take account of the density fluctuation of the envelope phenomenologically

Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes

We report on the microscopic surface structure of carbon-fiber-reinforced silicon carbide (C/SiC) composite mirrors that have been improved for the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and residual silicon. Specific microscopic structures are found on the surface of the bare C/SiC mirrors after polishing. These structures are considered to be caused by the different hardness of those materials. The roughness obtained for the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing the roughness to 2 nm rms. The scattering properties of the mirrors were measured at room temperature and also at 95 K. No significant change was found in the scattering properties through cooling, which suggests that the microscopic surface structure is stable with changes in temperature down to cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising candidate for the SPICA telescope.Comment: 25 pages, 6 figure

Impact of the initial disk mass function on the disk fraction

The disk fraction, the percentage of stars with disks in a young cluster, is widely used to investigate the lifetime of the protoplanetary disk, which can impose an important constraint on the planet formation mechanism. The relationship between the decay timescale of the disk fraction and the mass dissipation timescale of an individual disk, however, remains unclear. Here we investigate the effect of the disk mass function (DMF) on the evolution of the disk fraction. We show that the time variation in the disk fraction depends on the spread of the DMF and the detection threshold of the disk. In general, the disk fraction decreases more slowly than the disk mass if a typical initial DMF and a detection threshold are assumed. We find that, if the disk mass decreases exponentially, {the mass dissipation timescale of the disk} can be as short as $1\,{\rm Myr}$ even when the disk fraction decreases with the time constant of ${\sim}2.5\,{\rm Myr}$. The decay timescale of the disk fraction can be an useful parameter to investigate the disk lifetime, but the difference between the mass dissipation of an individual disk and the decrease in the disk fraction should be properly appreciated to estimate the timescale of the disk mass dissipation.Comment: 8 pages, 3 figures, accepted for the publication in PAS

Dust in Hot Plasma of Nearby Dusty Elliptical Galaxies Observed with the Spitzer Space Telescope

We report on mid- and far-IR Spitzer observations of 7 nearby dusty elliptical galaxies by using the Multiband Imaging Photometer (MIPS) and Infrared Spectrograph (IRS). Our sample galaxies are known to contain an excessive amount of interstellar dust against sputtering destruction in hot plasma filling the interstellar space of elliptical galaxies. In order to study the origin and the properties of the excess dust in the hot plasma, we selected galaxies with a wide range of X-ray luminosities but similar optical luminos ities for our Spitzer Guest Observers (GO1) program. The 7 galaxies are detected at the MIPS 24 um, 70 um, and 160 um bands; the far- to mid-IR flux ratios of relatively X-ray-bright elliptical galaxies are lower than those of X-ray-faint galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon (PAH) emission features are detected significantly from 5 of the 7 galaxies; t he emission intensities are weaker as the X-ray luminosity of the galaxy is larger. We have found a correlation between the far- to mid-IR flux ratio and the equivalent width of the PAH emission feature. We have obtained apparent spatial correspondence between mid-IR and X-ray distributions in the outer regions for the three X-ray-brightest galaxies in our sample. Possible interpretations for our observational results are discussed.Comment: 25 pages, 7 figures, accepted for publication in Publications of the Astronomical Society of Japa

Total synthesis of the post-translationally modified polyazole peptide antibiotic goadsporin

The structurally unique polyazole antibiotic goadsporin contains six heteroaromatic oxazole and thiazole rings integrated into a linear array of amino acids that also contains two dehydroalanine residues. An efficient total synthesis of goadsporin is reported in which the key steps are the use of rhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate the four oxazole rings, which demonstrates the power of rhodium carbene chemistry in organic chemical synthesis

High Precision CTE-Measurement of SiC-100 for Cryogenic Space-Telescopes

We present the results of high precision measurements of the thermal expansion of the sintered SiC, SiC-100, intended for use in cryogenic space-telescopes, in which minimization of thermal deformation of the mirror is critical and precise information of the thermal expansion is needed for the telescope design. The temperature range of the measurements extends from room temperature down to $\sim$ 10 K. Three samples, #1, #2, and #3 were manufactured from blocks of SiC produced in different lots. The thermal expansion of the samples was measured with a cryogenic dilatometer, consisting of a laser interferometer, a cryostat, and a mechanical cooler. The typical thermal expansion curve is presented using the 8th order polynomial of the temperature. For the three samples, the coefficients of thermal expansion (CTE), \bar{\alpha}_{#1}, \bar{\alpha}_{#2}, and \bar{\alpha}_{#3} were derived for temperatures between 293 K and 10 K. The average and the dispersion (1 $\sigma$ rms) of these three CTEs are 0.816 and 0.002 ($\times 10^{-6}$/K), respectively. No significant difference was detected in the CTE of the three samples from the different lots. Neither inhomogeneity nor anisotropy of the CTE was observed. Based on the obtained CTE dispersion, we performed an finite-element-method (FEM) analysis of the thermal deformation of a 3.5 m diameter cryogenic mirror made of six SiC-100 segments. It was shown that the present CTE measurement has a sufficient accuracy well enough for the design of the 3.5 m cryogenic infrared telescope mission, the Space Infrared telescope for Cosmology and Astrophysics (SPICA).Comment: in press, PASP. 21 pages, 4 figure