The Umov effect manifests itself as an inverse correlation between the light-scattering maximum of positive polarization P-max and the geometric albedo A of the target. In logarithmic scales, Pmax is linearly dependent on A. This effect has been long known in the optics of particulate surfaces and, recently, it was extended for the case of single-scattering dust particles whose size is comparable to the wavelength of the incident light. In this work, we investigate the effect of irregular shape on the Umov effect in single-scattering particles. Using the discrete dipole approximation (DDA), we model light scattering by two different types of irregularly shaped particles. Despite significant differences in their morphology, both types of particles reveal remarkably similar diagrams of log (P-max) versus log (A). Moreover, in a power-law size distribution r(-n) with n = 2.5-3.0, the Umov diagrams in both types of particles nearly coincide. This suggests little dependence on the shape of target particles in the retrieval of their reflectance using the Umov effect. (C) 2017 Optical Society of America

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (similar to 11 W/m/K) is almost two times smaller than that of bridgmanite (similar to 2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth. (C) 2017 Elsevier B.V. All rights reserved.

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable Xe(Fe; Fe/Ni)(3) and XeNi3 compounds at thermodynamic conditions representative of Earth's core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

We have obtained new images of the protoplanetary disk orbiting TW Hya in visible, total intensity light with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), using the newly commissioned BAR5 occulter. These HST/STIS observations achieved an inner working angle of similar to 0.'' 2, or 11.7 au, probing the system at angular radii coincident with recent images of the disk obtained by ALMA and in polarized intensity near-infrared light. By comparing our new STIS images to those taken with STIS in 2000 and with NICMOS in 1998, 2004, and 2005, we demonstrate that TW Hya's azimuthal surface brightness asymmetry moves coherently in position angle. Between 50 au and 141 au we measure a constant angular velocity in the azimuthal brightness asymmetry of 22 degrees. 7 yr(-1) in a counterclockwise direction, equivalent to a period of 15.9. yr assuming circular motion. Both the (short) inferred period and lack of radial dependence of the moving shadow pattern are inconsistent with Keplerian rotation at these disk radii. We hypothesize that the asymmetry arises from the fact that the disk interior to 1 au is inclined and precessing owing to a planetary companion, thus partially shadowing the outer disk. Further monitoring of this and other shadows on protoplanetary disks potentially opens a new avenue for indirectly observing the sites of planet formation.

Raman spectroscopy in diamond anvil cells has been employed to study phase boundaries and transformation kinetics of H2O ice at high pressures up to 16 GPa and temperatures down to 15 K. Ice I formed at nearly isobaric cooling of liquid water transforms on compression to high-density amorphous (HDA) ice at 1.1-3 GPa at 15-100 K and then crystallizes in ice VII with the frozen-in disorder (ice VII') which remains stable up to 14.1 GPa at 80 K and 15.9 GPa at 100 K. Unexpectedly, on decompression of ice VII', it transforms to ice VIII in its domain of metastability, and then it relaxes into low-density amorphous (LDA) ice on a subsequent pressure release and warming up. On compression of ice I at 150-170 K, ice IX is crystallized and no HDA ice is found; further compression of ice IX results in the sequential phase transitions to stable ices VI and VIII. Cooling ice I to 210 K at 0.3 GPa transforms it to a stable ice II. Our extensive investigations provide previously missing information on the phase diagram of water, especially on the kinetic paths that result in formation of phases which otherwise are not accessible; these results are keys for understanding the phase relations including the formation of metastable phases. Our observations inform on the ice modifications that can occur naturally in planetary environments and are not accessible for direct observations. Published by AIP Publishing.

The electrical conductivity and Raman spectroscopy measurements have been performed on MoS2 at high pressures up to 80 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a mixed phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a different Raman phonon mode appears in the mixed state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge-density wave state, the presence of which is indicative for an emergence of superconductivity at higher pressures.

We present coronagraphic long slit spectra of AU Mic's debris disk taken with the STIS instrument aboard the Hubble Space Telescope. Our spectra are the first spatially-resolved, scattered light spectra of the system's disk, which we detect at projected distances between approximately 10 and 45 au. Our spectra cover a wavelength range between 5200 and 10200 angstrom. We find that the color of AU Mic's debris disk is bluest at small (12-17 au) projected separations. These results both confirm and quantify the findings qualitatively noted by Krist et al. and are different than IR observations that suggested a uniform blue or gray color as a function of projected separation in this region of the disk. Unlike previous literature, which reported that the color of AU Mic's disk became increasingly more blue as a function of projected separation beyond similar to 30 au, we find the disk's optical color between 35 and 45 au to be uniformly blue on the southeast side of the disk and decreasingly blue on the northwest side. We note that this apparent change in disk color at larger projected separations coincides with several fast, outward moving "features" that are passing through this region of the southeast side of the disk. We speculate that these phenomenon might be related and that the fast moving features could be changing the localized distribution of sub-micron-sized grains as they pass by, thereby reducing the blue color of the disk in the process. We encourage follow-up optical spectroscopic observations of AU Mic to both confirm this result and search for further modifications of the disk color caused by additional fast moving features propagating through the disk.

The Comment of Pace et al. [Phys. Rev. B 98, 106101 (2018)] claims that structural analysis and nomenclature of Zhang et al. [Zhang, Xu, Wang, Jiang, Gorelli, Greenberg, Prakapenka, and Goncharov, Phys. Rev. B 97, 064107 (2018)] are incorrect, that this compound is not metallic at high pressures and 200 K, and that the compound instead decomposes. In this Reply we argue that there are no experimental data that can discriminate between theoretically predicted Cccm H3Se and I4/mcm (H2Se)(2)H-2 advocated by Pace et al. The difference in nomenclature is due to different naming conventions. We find the name "H3Se" more convenient to apply in the limit of high pressure. We also substantiate the initial claims of the stability up to 40 GPa at 170 K of the H3Se compound after synthesis at 4.6 GPa and argue that the pressure induced metallization above 23 GPa is a plausible explanation of the reported visual observations and Raman spectroscopy results.

Dense fluid metallic hydrogen occupies the interiors of Jupiter, Saturn, and many extrasolar planets, where pressures reach millions of atmospheres. Planetary structure models must describe accurately the transition from the outer molecular envelopes to the interior metallic regions. We report optical measurements of dynamically compressed fluid deuterium to 600 gigapascals (GPa) that reveal an increasing refractive index, the onset of absorption of visible light near 150 GPa, and a transition to metal-like reflectivity (exceeding 30%) near 200 GPa, all at temperatures below 2000 kelvin. Our measurements and analysis address existing discrepancies between static and dynamic experiments for the insulator-metal transition in dense fluid hydrogen isotopes. They also provide new benchmarks for the theoretical calculations used to construct planetary models.

We present new analyses of ALMA 12 m and Atacama Compact Array (ACA) observations at 233 GHz (1.3 mm) of the Proxima Centauri system with sensitivities of 9.5 and 47 mu Jy beam(-1), respectively, taken from 2017 January 21 through April 25. These analyses reveal that the star underwent a significant flaring event during one of the ACA observations on 2017 March 24. The complete event lasted for approximately 1. minute and reached a peak flux density of 100 +/- 4 mJy, nearly a factor of 1000 times brighter than the star's quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency F-nu proportional to nu(alpha) with alpha = -1.77 +/- 0.45, and a lower limit on the fractional linear polarization of vertical bar Q/I vertical bar = 0.19 +/- 0.02. Because the ACA observations do not show any quiescent excess emission, we conclude that there is no need to invoke the presence of a dust belt at 1-4 au. We also posit that the slight excess flux density of 101 +/- 9 mu Jy observed in the 12 m observations, compared to the photospheric flux density of 74 +/- 4 mu Jy extrapolated from infrared wavelengths, may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby well-studied M dwarf flare star. If this is true, then the need for warm dust at similar to 0.4 au is also removed.