The exact nature of the luminous fast blue optical transient AT 2018cow is still debated. In this first of a two-paper series, we present a detailed analysis of three Hubble Space Telescope (HST) observations of AT 2018cow covering & SIM;50-60 days post-discovery in combination with other observations throughout the first two months and derive significantly improved constraints of the late thermal properties. By modeling the spectral energy distributions (SEDs), we confirm that the UV-optical emission over 50-60 days was still a smooth blackbody (i.e., optically thick) with a high temperature (T BB & SIM; 15,000 K) and small radius (R BB & LSIM; 1000 R & ODOT;). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of L BB & PROP; t -2.40 but faded much faster at t -3.06 after day 13. Reexamining possible late-time power sources, we disfavor significant contributions from radioactive decay based on the required 56Ni mass and lack of UV line blanketing in the HST SEDs. We argue that the commonly proposed interaction with circumstellar material may face significant challenges in explaining the late thermal properties, particularly the effects of the optical depth. Alternatively, we find that continuous outflow/wind driven by a central engine can still reasonably explain the combination of a receding photosphere, optically thick and rapidly fading emission, and intermediate-width lines. However, the rapid fading may have further implications on the power output and structure of the system. Our findings may support the hypothesis that AT 2018cow and other "Cow-like transients" are powered mainly by accretion onto a central engine.

In this second of a two-paper series, we present a detailed analysis of three Hubble Space Telescope observations taken & SIM;2-4 yr post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at & SIM;2-3 yr post-discovery revealed an exceptionally blue (L & nu; & PROP; & nu; 1.99) underlying source with relatively stable optical brightness, fading in the near-UV was observed at year 4, indicating flattening in the spectrum (to L & nu; & PROP; & nu; 1.64). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation, based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with known radio-producing circumstellar material (CSM) as well as magnetar spin down with B & SIM; 1015 G as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., a previously ejected hydrogen envelope) or a magnetar with B & LSIM; 1014 G. Finally, we find that a highly inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source.

The isotopic analysis of moderately volatile elements such as K have gained significant interest in recent years as they possess the potential to help us better understand solar system formation. Even so, the precise K isotopic composition of CI chondrites, the most chemically primitive chondrite, has remained elusive. As the K elemental composition of CI chondrites matches well with the solar photosphere, it is possible that their K isotopic composition represents the solar system initial value. Here, we investigate the CI chondrite K isotopic composition in order to determine the precise CI chondrite, and thus possibly solar system initial, & delta;41K value. In addition, we investigate the K isotope compositions of several other chondrite groups, evaluate all available chondrite K isotope data together, and use these data along with data from a range of other isotope systems to assess if nucleosynthetic variations, volatility related processes, or parent body processes can best explain the range of isotope variations. The & delta;41K composition of all nine CI chondrite pieces analyzed in this study show limited variation, ranging from -0.29%o to -0.17%o. When combined with the previous CI analysis, an overall mean CI & delta;41K value of -0.21 & PLUSMN; 0.05%o (2SE) is obtained. This K isotope composition is distinct from the Bulk Silicate Earth value of -0.43 & PLUSMN; 0.17%o (2SD), heavier than almost all other chondrite groups, and may represent the solar system initial K isotope composition. When comparing all chondrites broadly, ordinary chondrites show the lightest mean K isotope composition of -0.76 & PLUSMN; 0.06%o (H = -0.71 & PLUSMN; 0.12%o, L = -0.77 & PLUSMN; 0.04%o, LL = -0.81 & PLUSMN; 0.12%o), enstatite chondrites the middle composition of -0.39 & PLUSMN; 0.11%o (EH = -0.34 & PLUSMN; 0.05%o, EL = -0.45 & PLUSMN; 0.20%o), and carbonaceous chondrites the heaviest composition of -0.31 & PLUSMN; 0.08%o. For the carbonaceous chondrite groups CK (-0.42 & PLUSMN; 0.11%o), CR (-0.46 & PLUSMN; 0.05%o), and CV (-0.38 & PLUSMN; 0.07%o) chondrites show lighter & delta;41K compositions compared to CO (-0.20 & PLUSMN; 0.10%o), CM (-0.23 & PLUSMN; 0.11%o), and CI (-0.21 & PLUSMN; 0.05%o) chondrites. When these K isotope group averages are compared against the averages for other mass-dependent moderately volatile element isotope systems (& delta;87Rb, & delta;66Zn, & delta;71Ga, & delta;128Te) and mass-independent isotope systems (& epsilon;54Cr, & epsilon;64Ni, & epsilon;50Ti, & UDelta;17O, & epsilon;40K, and & epsilon;66Zn,), a range of correlations are observed. Across all chondrite groups & delta;41K shows correlations with & delta;87Rb, & delta;66Zn, and & delta;71Ga, and correlations with & epsilon;54Cr, & epsilon;64Ni, & epsilon;50Ti, & epsilon;40K, and & epsilon;66Zn. When comparing the CCs only, correlations are observed between & delta;41K and all four of the other moderately volatile elements assessed, while the mass-independent isotope systems show no strong correlations. Regarding the K isotope variations, these observations, along with other textural and chemical data, can be best explained by inherited isotopic variations form different precursor reservoirs (the cause of which is difficult to conclusively determine, though most likely related to the NC-CC dichotomy), and volatility related fractionation processes for the carbonaceous chondrite groups (most likely due to component mixing).

Coral reefs are highly diverse ecosystems of immense ecological, economic, and aesthetic importance built on the calcium-carbonate based skeletons of stony corals. The formation of these skeletons is threatened by increasing ocean temperatures and acidification, and a deeper understanding of the molecular mechanisms involved may assist efforts to mitigate the effects of such anthropogenic stressors. In this study, we focused on the role of the predicted bicarbonate transporter SLC4 gamma, which was suggested in previous studies to be a product of gene duplication and to have a role in coral-skeleton formation. Our comparative-genomics study using 30 coral species and 15 outgroups indicates that SLC4 gamma is present throughout the stony corals, but not in their non-skeleton-forming relatives, and apparently arose by gene duplication at the onset of stony coral evolution. Our expression studies show that SLC4 gamma, but not the closely related and apparently ancestral SLC4 gamma, is highly upregulated during coral development coincident with the onset of skeleton deposition. Moreover, we show that juvenile coral polyps carrying CRISPR/Cas9-induced mutations in SLC4 gamma are defective in skeleton formation, with the severity of the defect in individual animals correlated with their frequencies of SLC4 gamma mutations. Taken together, the results suggest that the evolution of the stony corals involved the neofunctionalization of the newly arisen SLC4 gamma for a unique role in the provision of concentrated bicarbonate for calcium-carbonate deposition. The results also demonstrate the feasibility of reverse-genetic studies of ecologically important traits in adult corals.

In order to characterize rhenium transport via infiltration of fluids in the Earth's interior, the solubility and solution mechanisms of ReO2 in aqueous fluids were determined to 900 & DEG;C and about 1710 MPa by using an externally-heated hydrothermal diamond anvil cell. In order to shed light on how Re solubility and solution mechanisms in aqueous fluids can be affected by interaction of Re with other solutes, compositions ranged from the comparatively simple ReO2-H2O system to compositionally more complex Na2O-ReO2-SiO2-H2O fluids. Fluids in the ReO2-SiO2-H2O, SiO2-H2O, Na2O-SiO2-H2O, and Na2O-ReO2-H2O systems also were examined. The presence of Na2O enhances the ReO2 solubility so that in Na2O-ReO2-H2O fluids, for example, Re solubility is increased by a factor of 10-15 compared with the Re solubility in Na2O-free ReO2-H2O fluids. The SiO2 component in ReO2-SiO2-H2O causes reduction of ReO2 solubility compared with ReO2-H2O fluids. The ReO2 solubility in the Na-bearing Na2O-ReO2-SiO2-H2O fluids is greater than that in fluids in both the ReO2-H2O and ReO2-SiO2-H2O systems. Rhenium is dissolved in aqueous fluid as ReO4-complexes with Re in fourfold coordination with oxygen. Some, or all, of the oxygen in these complexes is replaced by OH-groups depending on whether Na2O also is present. It is proposed that during dehydration of hydrated subduction zone mineral assemblages in the upper mantle, the alkali/alkaline earth ratio of the source of the released aqueous fluid affects the extent to which Re (and other HFSE) can be transported into an overlying peridotite mantle wedge. The infiltration of such fluids will, in turn, affect the Re content (and Re/Os ratio) of magma formed by partial melting of this peridotite wedge.