We present Hubble Space Telescope Space Telescope Imaging Spectrograph far-UV spectra of the edge-on disk around 49 Ceti, one of the very few debris disks showing submillimeter CO emission. Many atomic absorption lines are present in the spectra, most of which arise from circumstellar gas lying along the line-of-sight to the central star. We determined the line-of-sight C I column density, estimated the total carbon column density, and set limits on the OI column density. Surprisingly, no line-of-sight CO absorption was seen. We discuss possible explanations for this non-detection, and present preliminary estimates of the carbon abundances in the line-of-sight gas. The C/Fe ratio is much greater than the solar value, suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of beta Pictoris.

The high-pressure behavior of manganese-rich carbonate, rhodochrosite, has been characterized up to 62GPa by synchrotron-based midinfrared spectroscopy and X-ray diffraction. Modifications in both the infrared spectra and the X-ray diffraction patterns were observed above similar to 35GPa, indicating the presence of a high-pressure phase transition at these pressures. We found that rhodochrosite adopts a structure close to CaCO3-VI with a triclinic unit cell (a=2.87 angstrom, b=4.83 angstrom, c=5.49 angstrom, =99.86 degrees, =94.95 degrees, and =90.95 degrees at 62GPa). Using first-principles calculations based on density functional theory, we confirmed these observations and assigned modes in the new infrared signature of the high-pressure phase. These results suggest that high-pressure metastable phase of calcite may play an important role in carbon storage and transport in the deep Earth.

We report on the first nulling interferometric observations with the Large Binocular Telescope Interferometer (LBTI), resolving the N' band (9.81-12.41 mu m) emission around the nearby main-sequence star eta Crv (F2V, 1-2 Gyr). The measured source null depth amounts to 4.40% +/- 0.35% over a field-of-view of 140 mas in radius (similar to 2.6AU for the distance of eta Crv) and shows no significant variation over 35 degrees of sky rotation. This relatively low null is unexpected given the total disk to star flux ratio measured by the Spitzer Infrared Spectrograph (IRS; similar to 23% across the N' band), suggesting that a significant fraction of the dust lies within the central nulled response of the LBTI (79 mas or 1.4AU). Modeling of the warm disk shows that it cannot resemble a scaled version of the solar zodiacal cloud unless it is almost perpendicular to the outer disk imaged by Herschel. It is more likely that the inner and outer disks are coplanar and the warm dust is located at a distance of 0.5-1.0AU, significantly closer than previously predicted by models of the IRS spectrum (similar to 3AU). The predicted disk sizes can be reconciled if the warm disk is not centrosymmetric, or if the dust particles are dominated by very small grains. Both possibilities hint that a recent collision has produced much of the dust. Finally, we discuss the implications for the presence of dust for the distance where the insolation is the same as Earth's (2.3AU).

The phase diagram of H2O is extremely complex; in particular, it is believed that a second critical point exists deep below the supercooled water (SCW) region where two liquids of different densities coexist. The problem, however, is that SCW freezes at temperatures just above this hypothesized liquid liquid critical point (LLCP), so direct experimental verification of its existence has yet to be realized. Here, we report two anomalies in the complex dielectric constant during warming in the form of a peak anomaly near T-p = 203 K and a sharp minimum near T-m = 210 K from ice samples prepared from SCW under hydrostatic pressures of up to 760 MPa. The same features were observed about 4 K higher in heavy ice. T-p is believed to be associated with the nucleation process of metastable cubic ice Ic, and T-m, is the transitioning of ice Ic to either ice Ih or II depending on the pressure. Given that T-p and T-m, are nearly isothermal, present up to at least 620 MPa, and end as a critical point near 33-50 MPa, it is deduced that two types of SCWs with different density concentrations exist, which affects the surface energy of ice Ic nuclei in the "no man's land" region of the phase diagram. Our results are consistent with the LLCP theory and suggest that a metastable critical point exists in the region of 33-50 MPa and T-c >= 210 K.

We present resolved images of the HR 4796A debris disk using the Magellan adaptive optics system paired with Clio-2 and VisAO. We detect the disk at 0.77 mu m, 0.91 mu m, 0.99 mu m, 2.15 mu m, 3.1 mu m, 3.3 mu m, and 3.8 mu m. We find that the deprojected center of the ring is offset from the star by 4.76 +/- 1.6 AU and that the deprojected eccentricity is 0.06 +/- 0.02, in general agreement with previous studies. We find that the average width of the ring is 14(-2)(+3)% (11.1(-1.6)(+2.4) AU), also comparable to previous measurements. Combining our new scattered light data with archival Hubble Space Telescope images at similar to 0.5-2 mu m, along with previously unpublished Spitzer/MIPS thermal emission data and all other literature thermal data, we set out to constrain the chemical composition of the dust grains. After testing 19 individual root compositions and more than 8400 unique mixtures of these compositions, we find that good fits to the scattered light alone and thermal emission alone are discrepant, suggesting that caution should be exercised if fitting to only one or the other. When we fit to both data sets simultaneously, we find that silicates and organics are generally the most favored, while large abundances of water ice are usually not favored. These results suggest the HR 4796A dust grains are similar to interstellar dust and solar system comets, though improved modeling is necessary to place better constraints on the exact chemical composition of the dust.

Rocky planets are thought to comprise compounds of Mg and O as these are among the most abundant elements, but knowledge of their stable phases may be incomplete. MgO is known to be remarkably stable to very high pressure and chemically inert under reduced condition of the Earth's lower mantle. However, in exoplanets oxygen may be a more abundant constituent. Here, using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we show that MgO and oxygen react at pressures above 96 GPa and T = 2150 K with the formation of I4/mcm MgO2. Raman spectroscopy detects the presence of a peroxide ion (O-2(2-)) in the synthesized material as well as in the recovered specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO. Our finding suggests that MgO2 may be present together or instead of MgO in rocky mantles and rocky planetary cores under highly oxidized conditions.

Habitable zone dust levels are a key unknown that must be understood to ensure the success of future space missions to image Earth analogs around nearby stars. Current detection limits are several orders of magnitude above the level of the solar system's zodiacal cloud, so characterization of the brightness distribution of exo-zodi down to much fainter levels is needed. To this end, the Large Binocular Telescope Interferometer (LBTI) will detect thermal emission from habitable zone exo-zodi a few times brighter than solar system levels. Here we present a modeling framework for interpreting LBTI observations, which yields dust levels from detections and upper limits that are then converted into predictions and upper limits for the scattered light surface brightness. We apply this model to the HOSTS survey sample of nearby stars; assuming a null depth uncertainty of 10(-4) the LBTI will be sensitive to dust a few times above the solar system level around Sun-like stars, and to even lower dust levels for more massive stars.

Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) - the major mineral of Earth's lower mantle have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m*K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg(0.94)Pe(0.06)SiO(3) bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth's lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure temperature conditions. (C) 2015 Elsevier B.V. All rights reserved.

The Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS) on the Large Binocular Telescope Interferometer will survey nearby stars for faint emission arising from similar to 300K dust (exozodiacal dust), and aims to determine the exozodiacal dust luminosity function. HOSTS results will enable planning for future space telescopes aimed at direct spectroscopy of habitable zone terrestrial planets, as well as greater understanding of the evolution of exozodiacal disks and planetary systems. We lay out here the considerations that lead to the final HOSTS target list. Our target selection strategy maximizes the ability of the survey to constrain the exozodi luminosity function by selecting a combination of stars selected for suitability as targets of future missions and as sensitive exozodi probes. With a survey of approximately 50 stars, we show that HOSTS can enable an understanding of the statistical distribution of warm dust around various types of stars and is robust to the effects of varying levels of survey sensitivity induced by weather conditions.

We present multi-epoch non-redundant masking observations of the T Cha transition disk, taken at the Very Large Telescope and Magellan in the II, Ks, and L' bands. T Cha is one of a small number of transition disks that host companion candidates discovered by high-resolution imaging techniques, with a putative companion at a position angle of 78 degrees, separation of 62 mas, and contrast of Delta L' = 5.1 mag. We find comparable binary parameters in our re-reduction of the initial detection images, and similar parameters in the 2011 L', 2013 NaCo L', and 2013 NaCo Ks data sets. We find a close-in companion signal in the 2012 NaCo L' data set that cannot be explained by orbital motion, and a non-detection in the 2013 MagAO/Clio2 L' data. However, Monte Carlo simulations show that the best fits to the 2012 NaCo and 2013 MagAO/Clio2 followup data may be consistent with noise. There is also a significant probability of false non-detections in both of these data sets. We discuss physical scenarios that could cause the best fits, and argue that previous companion and scattering explanations are inconsistent with the results of the much larger data set presented here.