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Register a new userDistributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations
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We present 0.8 to 2.4 micron spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the first time the presence of CO2 ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption bands near 2 microns. Several additional, weaker CO2 ice absorptions have also been detected. No CO2 absorption is seen in Oberon spectra, and the strengths of the CO2 ice bands decline with planetocentric distance from Ariel through Titania. We use the CO2 absorptions to map the longitudinal distribution of CO2 ice on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres. We also examine H2O ice absorptions in the spectra, finding deeper H2O bands on the leading hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon. Potential mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices are considered.
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We have utilized the NASA IRTF 3m SpeX instruments high resolution spectral mode (Rayner et al. 2003) to observe and characterize the near-infrared flux emanating from the unusual Kepler lightcurve system KIC8462852. By comparing the resulting 0.8 to 4.2 um spectrum to a mesh of model photospheric spectra, the 6 emission line analysis of the Rayner et al. 2009 catalogue, and the 25 system collection of debris disks we have observed to date using SpeX under the Near InfraRed Debris disk Survey (NIRDS; Lisse et al. 2016), we have been able to additionally characterize the system. Within the errors of our measurements, this star looks like a normal solar abundance main sequence F1V to F3V dwarf star without any obvious traces of significant circumstellar dust or gas. Using Connelley & Greenes (2014) emission measures, we also see no evidence of significant ongoing accretion onto the star nor any stellar outflow away from it. Our results are inconsistent with large amounts of static close-in obscuring material or the unusual behavior of a YSO system, but are consistent with the favored episodic models of a Gyr old stellar system favored by Boyajian et al. (2015). We speculate that KIC8462852, like the approximately 1.4 Gyr old F2V system {eta} Corvi (Wyatt et al. 2005, Chen et al. 2006, Lisse et al. 2012), is undergoing a Late Heavy Bombardment, but is only in its very early stages.
It has been implicitly assumed that ices on grains in molecular clouds and proto planetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition, but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a proto-planetary disk is shown. These results have important implications for the chemical evolution of molecules, non-thermal desorption, collision of icy grains, and sintering.
HST/STIS observations of Uranus in 2015 show that the depletion of upper tropospheric methane has been relatively stable and that the polar region has been brightening over time as a result of increased aerosol scattering. This interpretation is confirmed by near-IR imaging from HST and from the Keck telescope using NIRC2 adaptive optics imaging. Our analysis of the 2015 spectra, as well as prior spectra from 2012, shows that there is a factor of three decrease in the effective upper tropospheric methane mixing ratio between 30deg N and 70deg N. The absolute value of the deep methane mixing ratio, which probably does not vary with latitude, is lower than our previous estimate, and depends significantly on the style of aerosol model that we assume, ranging from a high of 3.5$pm$0.5% for conservative non-spherical particles with a simple Henyey-Greenstein phase function to a low of 2.7%$pm$0.3% for conservative spherical particles. Our previous higher estimate of 4$pm$0.5% was a result of a forced consistency with occultation results of Lindal et al. (1987, JGR 92, 14987-15001). That requirement was abandoned in our new analysis because new work by Orton et al. (2014, Icarus 243, 494-513) and by Lellouch et al. (2015, Astron. & AstroPhys. 579, A121) called into question the occultation results. For the main cloud layer in our models we found that both large and small particle solutions are possible for spherical particle models. At low latitudes the small-particle solution has a mean particle radius near 0.3 $mu$m, a real refractive index near 1.65, and a total column mass of 0.03 mg/cm$^2$, while the large-particle solution has a particle radius near 1.5 $mu$m, a real index near 1.24, and a total column mass 30 times larger. The pressure boundaries of the main cloud layer are between about 1.1 and 3 bars, within which H$_2$S is the most plausible condensable.
We present the first results of AKARI Infrared Camera near-infrared spec- troscopic survey of the Large Magellanic Cloud (LMC). We detected absorption features of the H2O ice 3.05 um and the CO2 ice 4.27 um stretching mode toward seven massive young stellar objects (YSOs). These samples are for the first time spectroscopically confirmed to be YSOs. We used a curve-of-growth method to evaluate the column densities of the ices and derived the CO2/H2O ratio to be 0.45 pm 0.17. This is clearly higher than that seen in Galactic massive YSOs (0.17 pm 0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO2 ice abundance.
The formation of solid calcium carbonate (CaCO3) from aqueous solutions or slurries containing calcium and carbon dioxide (CO2) is a complex process of considerable importance in the ecological, geochemical and biological areas. Moreover, the demand for powdered CaCO3 has increased considerably recently in various fields of industry. The aim of this study was therefore to synthesize fine particles of calcite with controlled morphology by hydrothermal carbonation of calcium hydroxide at high CO2 pressure (initial PCO2=55 bar) and at moderate and high temperature (30 and 90 degrees C). The morphology of precipitated particles was identified by transmission electron microscopy (TEM/EDS) and scanning electron microscopy (SEM/EDS). In addition, an X-ray diffraction analysis was performed to investigate the carbonation efficiency and purity of the solid product. Carbonation of dispersed calcium hydroxide in the presence of supercritical (PT=90 bar, T=90 degrees C) or gaseous (PT=55 bar, T=30 degrees C) CO2 led to the precipitation of sub-micrometric isolated particles (<1$mu$m) and micrometric agglomerates (<5$mu$m) of calcite. For this study, the carbonation efficiency (Ca(OH)2-CaCO3 conversion) was not significantly affected by PT conditions after 24 h of reaction. In contrast, the initial rate of calcium carbonate precipitation increased from 4.3 mol/h in the 90bar-90 degrees C system to 15.9 mol/h in the 55bar-30 degrees C system. The use of high CO2 pressure may therefore be desirable for increasing the production rate of CaCO3, carbonation efficiency and purity, to approximately 48 kg/m3h, 95% and 96.3%, respectively in this study. The dissipated heat for this exothermic reaction was estimated by calorimetry to be -32 kJ/mol in the 90bar-90 degrees C system and -42 kJ/mol in the 55bar-30 degrees C system.
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