田中 今日子

<jats:title>Abstract</jats:title> <jats:p>In this study, we demonstrate the dependence of atmospheric dust size on the near-infrared spectra of ten L dwarfs, and constrain the sizes of dust grains in each L dwarf atmosphere. In previous studies, by comparing observed and modeled spectra, it was suggested that the deviations of their spectral shapes from theoretical prediction are general characteristics. Here, we focus on the dust size in brown dwarf atmospheres to understand the observed spectra. We confirm that changing the dust size changes the temperature–pressure structure of the atmosphere, with the shape of the spectrum changing accordingly. At the wavelength at which dust is the main absorber of radiation (the dust-dominated regime), a large dust opacity combined with a medium grain size, e.g., 0.1 <jats:italic>μ</jats:italic>m, results in a low photospheric temperature, and thus a small flux. Conversely, for the wavelength at which gas absorption is dominant (the gas-dominated regime), a large dust opacity modifies the temperature–pressure structure, resulting in a high photospheric temperature, which corresponds to large flux emissions. Taking into account the size effect, we compare the model spectral fluxes in the wavelength range 1–5 <jats:italic>μ</jats:italic>m with the observational ones to constrain the main dust size in the atmosphere of each of the ten L dwarfs observed with <jats:italic>AKARI</jats:italic> and SpeX or CGS4. Ultimately, we reveal that the observed data are reproduced with higher fidelity by models based on a medium dust size of 0.1–3.0 <jats:italic>μ</jats:italic>m for six of these L dwarfs; therefore, we suggest that such atmospheric dust sizes apply to the majority of L dwarfs.</jats:p>

<jats:p>We report water cluster formation in the uniform postnozzle flow of a Laval nozzle at low temperatures of 87.0 and 47.5 K and high supersaturations of lnS ∼ 41 and 104, respectively. Cluster size distributions were measured after soft single-photon ionization at 13.8 eV with mass spectrometry. Critical cluster sizes were determined from cluster size distributions recorded as a function of increasing supersaturation, resulting in critical sizes of 6-15 and 1, respectively. Comparison with previous data for propane and toluene reveals a systematic trend in the nucleation behavior, i.e., a change from a steplike increase to a gradual increase of the maximum cluster size with increasing supersaturation. Experimental nucleation rates of 5 · 1015 cm−3 s−1 and 2 · 1015 cm−3 s−1 for lnS ∼ 41 and 104, respectively, were retrieved from cluster size distributions recorded as a function of nucleation time. These lie 2-3 orders of magnitude below the gas kinetic collision limit assuming unit sticking probability, but they agree very well with a recent prediction by a master equation model based on ab initio transition state theory. The experimental observations are consistent with barrierless growth at 47.5 K, but they hint at a more complex nucleation behavior for the measurement at 87.0 K.</jats:p>

Abstract It is thought that protoplanets formed in protoplanetary disks excite the orbital motion of the surrounding planetesimals, and the bow shocks caused by the highly excited planetesimals heat their icy component evaporating into gas. We have performed model calculations to study the evolution of molecular abundances of the evaporated icy component, which suggests sulfur-bearing molecules can be good tracers of icy planetesimal evaporation. Here we report the result of our ALMA observations of sulfur-bearing molecules towards protoplanetary disks. The lines were undetected but the obtained upper limits of the line fluxes and our model calculations give upper limits of the fractional abundances of x(H₂S) &lt; 10⁻¹¹ and x(SO) &lt; 10⁻¹⁰ in the outer disk. These results are consistent with the molecular abundances in comets in our Solar system.

<p>観測ロケットおよび航空機で得られる微小重力環境を利用し、ガス中蒸発法によりアルミナナノ粒子を作製した。干渉計や赤外分光光度計で、過飽和ガスからアルミナのナノ粒子が生成する過程をその場観察した結果について報告する。</p>