84,518 research outputs found

    Characterizing Exoplanet Habitability

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    A habitable exoplanet is a world that can maintain stable liquid water on its surface. Techniques and approaches to characterizing such worlds are essential, as performing a census of Earth-like planets that may or may not have life will inform our understanding of how frequently life originates and is sustained on worlds other than our own. Observational techniques like high contrast imaging and transit spectroscopy can reveal key indicators of habitability for exoplanets. Both polarization measurements and specular reflectance from oceans (also known as "glint") can provide direct evidence for surface liquid water, while constraining surface pressure and temperature (from moderate resolution spectra) can indicate liquid water stability. Observations of variability (that indicates weather) from, as well as mapping of, exoplanets can provide indirect evidence of habitability, and measurements of water vapor or cloud profiles that indicate condensation near a surface could also provide evidence for habitability. Approaches to making the types of measurements that indicate habitability are diverse, and have different considerations for the required wavelength range, spectral resolution, maximum noise levels, stellar host temperature, and observing geometry.Comment: To be published in: Handbook of Exoplanets, 2nd Edition, Hans Deeg and Juan Antonio Belmonte (Eds. in Chief), Springer International Publishing AG, part of Springer Natur

    Constitutive relationships for anisotropic high-temperature alloys

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    A constitutive theory is presented for representing the anisotropic viscoplastic behavior of high temperature alloys that posses directional properties resulting from controlled grain growth or solidification. The theory is an extension of a viscoplastic model that was applied in structural analyses involving isotropic metals. Anisotropy is introduced through the definition of a vector field that identifies a preferential (solidification) direction at each material point. Following the development of a full multiaxial theory, application is made to homogeneously stressed elements in pure shear and to a uniaxially stressed rectangular block in plane stress with the stress direction oriented at an arbitrary angle with the material direction. It is shown that an additional material parameter introduced to characterize the degree of anisotropy can be determined on the basis of simple creep tests

    A continuous damage model based on stepwise-stress creep rupture tests

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    A creep damage accumulation model is presented that makes use of the Kachanov damage rate concept with a provision accounting for damage that results from a variable stress history. This is accomplished through the introduction of an additional term in the Kachanov rate equation that is linear in the stress rate. Specification of the material functions and parameters in the model requires two types of constituting a data base: (1) standard constant-stress creep rupture tests, and (2) a sequence of two-step creep rupture tests

    On iterative solutions for quantum-mechanical bound states

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    Iterative solutions for quantum mechanical bound state

    A continuum deformation theory for metal-matrix composites at high temperature

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    A continuum theory is presented for representing the high temperature, time dependent, hereditary deformation behavior of metallic composites that can be idealized as pseudohomogeneous continua with locally definable directional characteristics. Homogenization of textured materials (molecular, granular, fibrous) and applicability of continuum mechanics in structural applications depends on characteristic body dimensions, the severity of gradients (stress, temperature, etc.) in the structure and the relative size of the internal structure (cell size) of the material. The point of view taken here is that the composite is a material in its own right, with its own properties that can be measured and specified for the composite as a whole
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