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.

The chemical reaction products of molecular hydrogen (H-2) with selenium (Se) are studied by synchrotron x-ray diffraction (XRD) and Raman spectroscopy at high pressures. We find that a common H2Se is synthesized at 0.3 GPa using laser heating. Upon compression at 300 K, a crystal of the theoretically predicted Cccm H3Se has been grown at 4.6 GPa. At room temperature, H3Se shows a reversible phase decomposition after laser irradiation above 8.6 GPa, but remains stable up to 21 GPa. However, at 170 K Cccm H3Se persists up to 39.5 GPa based on XRD measurements, while low-temperature Raman spectra weaken and broaden above 23.1 GPa. At these conditions, the sample is visually nontransparent and shiny suggesting that metallization occurred.

Success in developing remote-sensing methods is largely based on adequate modeling of target-particle shapes. In various terrestrial and cosmic applications, submicrometer- and micrometer-sized dust particles appear to have a highly irregular morphology. Light scattering by such irregularly shaped particles can be computed only with a numerical technique that, in practice, is a time-consuming approach, demanding significant computational resources. In this Letter, we discuss an efficient way to accelerate light-scattering computations through interpolation of the numerical results obtained at different levels of material absorption. We find a nonlinear dependence of reflectance, degree of linear polarization, and linear and circular polarization ratios on the imaginary part of refractive index Im(m). Over the range of Delta Im(m) = 0.05, the dependence can be satisfactorily described with a cubic polynomial function, whose determination requires exact computations at four different values of Im(m). The light-scattering characteristics at other intermediate values of Im(m) can be inferred with great accuracy via interpolation. (C) 2018 Optical Society of America

Hydrogen-rich hydrides attract great attention due to recent theoretical (1) and then experimental discovery of record high-temperature superconductivity in H3S [T-c = 203 K at 155 GPa (2)]. Here we search for stable uranium hydrides at pressures up to 500 GPa using ab initio evolutionary crystal structure prediction. Chemistry of the U-H system turned out to be extremely rich, with 14 new compounds, including hydrogen-rich UH5, UH6, U2H13, UH7, UH8, U2H17, and UH9. Their crystal structures are based on either common face-centered cubic or hexagonal close-packed uranium sublattice and unusual H-8 cubic clusters. Our high-pressure experiments at 1 to 103 GPa confirm the predicted UH7, UH8, and three different phases of UH5, raising confidence about predictions of the other phases. Many of the newly predicted phases are expected to be high-temperature superconductors. The highest-Tc superconductor is UH7, predicted to be thermodynamically stable at pressures above 22 GPa (with T-c = 44 to 54 K), and this phase remains dynamically stable upon decompression to zero pressure (where it has T-c = 57 to 66 K).

The presence of large amounts of dust in the habitable zones of nearby stars is a significant obstacle for future exo-Earth imaging missions. We executed the HOSTS (Hunt for Observable Signatures of Terrestrial Systems) survey to determine the typical amount of such exozodiacal dust around a sample of nearby main sequence stars. The majority of the data have been analyzed and we present here an update of our ongoing work. Nulling interferometry in N band was used to suppress the bright stellar light and to detect faint, extended circumstellar dust emission. We present an overview of the latest results from our ongoing work. We find seven new N band excesses in addition to the high confidence confirmation of three that were previously known. We find the first detections around Sun-like stars and around stars without previously known circumstellar dust. Our overall detection rate is 23%. The inferred occurrence rate is comparable for early type and Sun-like stars, but decreases from 71(-20)(+11) % for stars with previously detected mid- to far-infrared excess to 11(-4)(+9) % for stars without such excess, confirming earlier results at high confidence. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal luminosity function of the dust, we find upper limits on the median dust level around all stars without previously known mid to far infrared excess of 11.5 zodis at 95% confidence level. The corresponding upper limit for Sun-like stars is 16 zodis. An LBTI vetted target list of Sun-like stars for exo-Earth imaging would have a corresponding limit of 7.5 zodis. We provide important new insights into the occurrence rate and typical levels of habitable zone dust around main sequence stars. Exploiting the full range of capabilities of the LBTI provides a critical opportunity for the detailed characterization of a sample of exozodiacal dust disks to understand the origin, distribution, and properties of the dust.