Akemi Tamanai

Low-Temperature Opacities

Previous computations of low-temperature Rosseland and Planck mean opacities from Alexander & Ferguson areupdatedandexpanded.Thenewcomputationsincludeamorecompleteequationofstate(EOS)withmoregrain species and updated optical constants. Grains are now explicitly included in thermal equilibrium in the EOS calculation, which allows for a much wider range of grain compositions to be accurately included than was previously the case. The inclusion of high-temperature condensates such as Al2O3 and CaTiO3 significantly affects the total opacityoveranarrowrangeoftemperaturesbeforetheappearanceofthefirstsilicategrains.Thenewopacitytables are tabulated for temperatures ranging from 30,000 to 500 K with gas densities from 10 � 4 to 10 � 19 gc m � 3 .C omparisons with previous Rosseland mean opacity calculations are discussed. At high temperatures, the agreement with OPAL and Opacity Project is quite good. Comparisons at lower temperatures are more divergent as a result of differences in molecular and grain physics included in different calculations. The computation of Planck mean opacities performed with the opacity sampling method is shown to require a very large number of opacity sampling wavelength points; previously published results obtained with fewer wavelength points are shown to be significantly in error. Methods for requesting or obtaining the new tables are provided. Subject heading gs: atomic data — equation of state — methods: numerical — molecular data

THE LIMITING EFFECTS OF DUST IN BROWN DWARF MODEL ATMOSPHERES

We present opacity sampling model atmospheres, synthetic spectra, and colors for brown dwarfs and very low mass stars in the following two limiting cases of dust grain formation: (1) Inefficient gravitational settling (i.e., the dust is distributed according to the chemical equilibrium predictions) and (2) efficient gravitational settling (i.e., the dust forms and depletes refractory elements from the gas, but their opacity does not affect the thermal structure). The models include the formation of over 600 gas-phase species and 1000 liquids and crystals and the opacities of 30 different types of grains including corundum (Al2O3), the magnesium aluminum spinel MgAl2O4, iron, enstatite (MgSiO3), forsterite (Mg2SiO4), amorphous carbon, SiC, and a number of calcium silicates. The models extend from the beginning of the grain formation regime well into the condensation regime of water ice (Teff = 3000-100 K) and encompass the range of log g = 2.5-6.0 at solar metallicity. We find that silicate dust grains can form abundantly in the outer atmospheric layers of red and brown dwarfs with a spectral type later than M8. The greenhouse effects of dust opacities provide a natural explanation for the peculiarly red spectroscopic distribution of the latest M dwarfs and young brown dwarfs. The grainless (cond) models, on the other hand, correspond closely to methane brown dwarfs such as Gliese 229B. We also discover that the λλ5891, 5897 Na I D and λλ7687, 7701 K I resonance doublets play a critical role in T dwarfs, in which their red wings define the pseudocontinuum from the I to the Z bandpass.

Transient dust in warm debris disks - Detection of Fe-rich olivine grains

Context. Debris disks trace remnant reservoirs of leftover planetesimals in planetary systems. In the past years, a handful of “warm” debris disks have been discovered in which emission in excess starts in the mid-infrared. An interesting subset of these warm debris disks shows emission features in mid-infrared spectra, which points towards the presence of μm-sized dust grains, with temperatures above hundreds K. Given the ages of the host stars, the presence of these small grains is puzzling, and raises questions about their origin and survival in time. Aims. This study focuses on determining the mineralogy of the dust around seven debris disks with evidence for warm dust, based on Spitzer/IRS spectroscopic data, to provide new insights into the origin of the dust grains. Methods. We developed and present a new radiative transfer code (Debra) dedicated to spectral energy distribution (SED) modeling of optically thin disks. The Debra code is designed such that it can simultaneously determine dust composition and disk properties. We used this code on the SEDs of seven warm debris disks, in combination with recent laboratory experiments on dust optical properties. Results. We find that most, if not all, debris disks in our sample are experiencing a transient phase, suggesting a production of small dust grains on relatively short timescales. Dust replenishment should be efficient on timescales of months for at least three sources. From a mineralogical point of view, we find that crystalline pyroxene grains (enstatite) have low abundances compared to crystalline olivine grains. The main result of our study is that we find evidence for Fe-rich crystalline olivine grains (Fe/[Mg + Fe] ∼ 0.2) for several debris disks. This finding contrasts with studies of gas-rich protoplanetary disks, where Fe-bearing crystalline grains are usually not observed. Conclusions. These Fe-rich olivine grains, and the overall differences between the mineralogy of dust in Class II disks compared to debris disks suggests that the transient crystalline dust in warm debris disk is of a new generation. We discuss possible crystallization routes to explain our results, and also comment on the mechanisms that may be responsible for the production of small dust grains.

The 10 μm Infrared Band of Silicate Dust: A Laboratory Study Comparing the Aerosol and KBr Pellet Techniques

The profile of the silicate 10 μm IR band contains important information about the evolutional stage of dust in circumstellar environments and the possible ongoing process of planetesimal formation. In order to extract this information, the observed band profiles are compared with calculated or laboratory-measured absorption cross sections of amorphous and crystalline grains with different sizes and compositions. We present in this study the first laboratory measurements of the 10 μm band profiles of nonembedded, i.e., free-flying, particles of amorphous and crystalline Mg2SiO4 (with two different particle shapes), amorphous and crystalline MgSiO3, and crystalline olivine. We compare the spectra with those measured on embedded grains and discuss the potential of the new experimental method for comparison with observed spectra, as well as for future studies of agglomeration and surface manipulation of the grains.

Seed particle formation for silicate dust condensation by SiO nucleation

Context. Dust formation in stellar outflows is initiated by the formation of some seed particles that form the growth centres for macroscopic dust grains. The nature of the seed particles for silicate dust in stellar outflows with an oxygen-rich element mixture is still an open question. Clustering of the abundant SiO molecules has been discussed several times as a possible mechanism and investigated both theoretically and by laboratory experiments. The initial results seemed to indicate, however, that condensation temperatures obtained by model calculations based on this mechanism are significant lower than what is really observed, which renders SiO nucleation unlikely. Aims. This negative result strongly rests on experimental data on the vapour pressure of SiO. The case for SiO nucleation may be not as bad as it previously seemed and needs to be discussed again because new determinations of the vapour pressure of SiO molecules over solid SiO have shown the older data on SiO vapour pressure to be seriously in error. Here we aim to check again the possibility that SiO nucleation triggers the cosmic silicate dust formation in light of improved new data. Methods. First we present results of our measurements of vapour pressure of solid SiO. Second, we use the improved vapour pressure data to recalibrate existing experimental data on SiO nucleation from the literature. Third, we use the recalibrated data on SiO nucleation in a simple model program for dust-driven winds to determine the condensation temperature of silicate in stellar outflows from AGB stars. Results. Our measurements extend the temperature range of measurements for the vapour pressure to lower temperatures and pressures than ever before. This improves the reliability of the required extrapolation from the temperature range where laboratory data can be obtained to the temperature range where circumstellar dust condensation is observed. We determine an analytical fit for the nucleation rate of SiO from recalibrated literature data and show that the onset of nucleation under circumstellar conditions commences at a higher temperature than was previously found. This brings calculated condensation temperatures of silicate dust much closer to the observed condensation temperatures derived from analysis of infrared spectra from dust-enshrouded M stars. Calculated condensation temperatures are still by about 100 K lower than observed ones, but this may be due to the greenhouse effect of silicate dust temperatures, which is not considered in our model calculation. Conclusions. The assumption that the onset of dust formation in late-type stars with oxygen-rich element mixtures is triggered by the cluster formation of SiO is compatible with dust condensation temperatures derived from infrared observations.

Laboratory-based grain-shape models for simulating dust infrared spectra

Context. Analysis of thermal dust emission spectra for dust mineralogy and physical grain properties depends on comparison spectra, which are either laboratory-measured infrared extinction spectra or calculated extinction cross sections based on certain grain models. Often, the agreement between these two kinds of spectra, if available, is not yet satisfactory because of the strong influence of the grain morphology on the spectra. Aims. We investigate the ability of the statistical light-scattering model with a distribution of form factors (DFF) to reproduce measured infrared dust extinction spectra for particles that are small compared to the wavelength, i.e. in the size range of 1 μ ma nd smaller. Methods. We take advantage of new experimental spectra measured for free particles dispersed in air with accompanying information on the grain morphology. For the calculations, we used DFFs that were derived for aggregates of spherical grains, as well as for compact grain shapes corresponding to Gaussian random spheres. In addition we used a fitting algorithm to obtain the best-fit DFFs for the various laboratory samples. In this way we can independently derive information on the shape of the grains from their infrared spectra. Results. With the DFF model, we achieve an adequate fit of the experimental IR spectra. The differences in the IR band profiles between the spectra of particulates with different grain shapes are simply reflected by different DFFs. Irregular particle shapes require a DFF similar to that of a Gaussian Random Sphere with σ = 0.3, whereas roundish grain shapes are best fitted with that of a fractal aggregate of Df = 2.4–1.8. The fitted DFFs generally reproduce the measured spectral shapes quite well. For anisotropic materials, different DFFs are needed for the different crystallographic axes. The implications of this finding are discussed. Conclusions. The use of this model could be a step forward toward more realistic comparison data in infrared spectral analysis of thermal dust emission spectra, provided that these spectra are dominated by emission from submicron grains.

Morphological effects on IR band profiles - Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars

Aims. To trace the source of the unique 13, 19.5, and 28 μm emission features in the spectra of oxygen-rich circumstellar shells around AGB stars, we have compared dust extinction spectra obtained by aerosol measurements. Methods. We have measured the extinction spectra for 19 oxide powder samples of eight different types, such as Ti-compounds (TiO, TiO2 ,T i 2O3 ,T i 3O5 ,A l 2TiO5 ,C aTiO 3), α-, γ-, χ-δ-κ-Al2O3 ,a nd MgAl 2O4 in the infrared region (10–50 μm) paying special attention to the morphological (size, shape, and agglomeration) effects and the differences in crystal structure. Results. Anatase (TiO2) particles with rounded edges are the possible 13, 19.5 and 28 μm band carriers as the main contributor in the spectra of AGB stars, and spherically shaped nano-sized spinel and Al2TiO5 dust grains are possibly associated with the anatase, enhancing the prominence of the 13 μm feature and providing additional features at 28 μm. The extinction data sets obtained by the aerosol and CsI pellet measurements have been made available for public use at http://elbe.astro.uni-jena.de.

Infrared extinction by homogeneous particle aggregates of SiC, FeO and SiO2: Comparison of different theoretical approaches

Abstract Particle shape and aggregation have a strong influence on the spectral profiles of infrared phonon bands of solid dust grains. Calculating these effects is difficult due to the often extreme refractive index values in these bands. In this paper, we use the discrete dipole approximation (DDA) and the T-matrix method to compute the absorption band profiles for simple clusters of touching spherical grains. We invest reasonable amounts of computation time in order to reach high dipole grid resolutions and take high multipolar orders into account, respectively. The infrared phonon bands of three different refractory materials of astrophysical relevance are considered—silicon carbide, wustite and silicon dioxide. We demonstrate that even though these materials display a range of material properties and therefore different strengths of the surface resonances, a complete convergence is obtained with none of the approaches. For the DDA, we find a strong dependence of the calculated band profiles on the exact dipole distribution within the aggregates, especially in the vicinity of the contact points between their spherical constituents. By applying a recently developed method to separate the material optical constants from the geometrical parameters in the DDA approach, we are able to demonstrate that the most critical material properties are those where the real part of the refractive index is much smaller than unity.

Mid-infrared characterization of thiophene-based thin polymer films

Abstract Optical properties of seven regioregular poly(3-alkylthiophene) with different alkyl side chain lengths which are poly(3-butylthiophene-2,5-diyl) (P3BT), poly(3-pentylthiophene-2,5-diyl) (P3PT), poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-heptylthiophene-2,5-diyl) (P3hept), poly(3-octylthiophene-2,5-diyl) (P3OT), poly(3-decylthiophene-2,5-diyl) (P3DT), and poly(3-dodecylthiophene-2,5-diyl) (P3DDT) have been studied in the mid-infrared (IR) spectral region by means of Fourier Transformation Infrared (FTIR) spectroscopy and IR spectroscopic ellipsometry (IRSE). Absorbance spectra obtained in this fingerprint region are potential to characterize the structures formed by organic molecules in thin films due to molecular vibrations in detail. In consequence, the vibrational absorption bands of these seven samples demonstrated that P3PT, P3HT, and P3hept exhibited very similar band profiles, in contrast, the stretching vibration of thiophene rings (≈1465xa0cm −1 : C C) underwent a blue shift in P3BT, P3OT, P3DT and P3DDT. The highest value of the real part ( ɛ 1 ) of the complex dielectric constant was obtained from P3HT on both indium thin oxide (ITO) and silicon (Si) substrates whereas the imaginary part ( e 2 ) was directly affected by increasing in the alkyl side chain lengths in a frequency range around 3000xa0cm −1 . The optical properties of P3PT in the mid-IR region developed an affinity with those of P3HT. Thus, P3PT is particularly a suitable polymer active material candidate for high-performance devices.

The twofold debris disk around HD 113766 A. Warm and cold dust as seen with VLTI/MIDI and Herschel/PACS

Context. Warm debris disks are a sub-sample of the large population of debris disks, and display excess emission in the mid-infrared. Around solar-type stars, very few objects ( 2% of all debris disks) show emission features in mid-IR spectroscopic observations that are attributed to small, warm silicate dust grains. The origin of this warm dust could be explained either by a recent catastrophic collision between several bodies or by transport from an outer belt similar to the Kuiper belt in the solar system. Aims. We present and analyze new far-IR Herschel/PACS photometric observations, supplemented by new and archival ground-based data in the mid-IR (VLTI/MIDI and VLT/VISIR), for one of these rare systems: the 10‐16 Myr old debris disk around HD 113766 A. We improve an existing model to account for these new observations. Methods. We implemented the contribution of an outer planetesimal belt in the Debra code, and successfully used it to model the spectral energy distribution (SED) as well as complementary observations, notably MIDI data. We better constrain the spatial distribution of the dust and its composition. Results. We underline the limitations of SED modeling and the need for spatially resolved observations. We improve existing models and increase our understanding of the disk around HD 113766 A. We find that the system is best described by an inner disk located within the first AU, well constrained by the MIDI data, and an outer disk located between 9‐13 AU. In the inner dust belt, our previous finding of Fe-rich crystalline olivine grains still holds. We do not observe time variability of the emission features over at least an eight-year time span in an environment subjected to strong radiation pressure. Conclusions. The time stability of the emission features indicates that m-sized dust grains are constantly replenished from the same reservoir, with a possible depletion of sub- m-sized grains. We suggest that the emission features may arise from multi-composition aggregates. We discuss possible scenarios concerning the origin of the warm dust observed around HD 113766 A. The compactness of the innermost regions as probed by the MIDI visibilities and the dust composition suggest that we are witnessing the results of (at least) one collision between partially di erentiated bodies, in an environment possibly rendered unstable by terrestrial planetary formation.