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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sources NASA NTRS DOE OSTI

DOE OSTI2023

2D k -th nearest neighbour statistics: a highly informative probe of galaxy clustering

ABSTRACT Beyond standard summary statistics are necessary to summarize the rich information on non-linear scales in the era of precision galaxy clustering measurements. For the first time, we introduce the 2D k-th nearest neighbour (kNN) statistics as a summary statistic for discrete galaxy fields. This is a direct generalization of the standard 1D kNN by disentangling the projected galaxy distribution from the redshift-space distortion signature along the line-of-sight. We further introduce two different flavours of 2D kNNs that trace different aspects of the galaxy field: the standard flavour which tabulates the distances between galaxies and random query points, and a ‘DD’ flavour that tabulates the distances between galaxies and galaxies. We showcase the 2D kNNs’ strong constraining power both through theoretical arguments and by testing on realistic galaxy mocks. Theoretically, we show that 2D kNNs are computationally efficient and directly generate other statistics such as the popular two-point correlation function (2PCF), voids probability function, and counts-in-cell statistics. In a more practical test, we apply the 2D kNN statistics to simulated galaxy mocks that fold in a large range of observational realism and recover parameters of the underlying extended halo occupation distribution (HOD) model that includes velocity bias and galaxy assembly bias. We find unbiased and significantly tighter constraints on all aspects of the HOD model with the 2D kNNs, both compared to the standard 1D kNN, and the classical redshift-space 2PCF.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Robust cosmological inference from non-linear scales with k -th nearest neighbour statistics

ABSTRACT We present the methodology for deriving accurate and reliable cosmological constraints from non-linear scales ($\lt 50\, h^{-1}$ Mpc) with k-th nearest neighbour (kNN) statistics. We detail our methods for choosing robust minimum scale cuts and validating galaxy–halo connection models. Using cross-validation, we identify the galaxy–halo model that ensures both good fits and unbiased predictions across diverse summary statistics. We demonstrate that we can model kNNs effectively down to transverse scales of $r_{\rm p}\sim 3\, h^{-1}$ Mpc and achieve precise and unbiased constraints on the matter density and clustering amplitude, leading to a 2 per cent constraint on σ8. Our simulation-based model pipeline is resilient to varied model systematics, spanning simulation codes, halo finding, and cosmology priors. We demonstrate the effectiveness of this approach through an application to the Beyond-2p mock challenge. We propose further explorations to test more complex galaxy–halo connection models and tackle potential observational systematics.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Full forward model of galaxy clustering statistics with AbacusSummit light cones

ABSTRACT Novel summary statistics beyond the standard 2-point correlation function (2PCF) are necessary to capture the full astrophysical and cosmological information from the small-scale (r < 30h−1Mpc) galaxy clustering. However, the analysis of beyond-2PCF statistics on small scales is challenging because we lack the appropriate treatment of observational systematics for arbitrary summary statistics of the galaxy field. In this paper, we develop a full forward modelling pipeline for a wide range of summary statistics using the large high-fidelity AbacusSummit light cones that account for many systematic effects as well as remain flexible and computationally efficient to enable posterior sampling. We apply our forward model approach to a fully realistic mock galaxy catalog and demonstrate that we can recover unbiased constraints on the underlying galaxy–halo connection model using two separate summary statistics: the standard 2PCF and the novel k-th nearest neighbour (kNN) statistics, which are sensitive to correlation functions of all orders. We will demonstrate its strong constraining power on extended galaxy–halo connection models and cosmology in follow up papers. We expect this to become a powerful approach when applying to upcoming surveys such as DESI where we can leverage a multitude of summary statistics across a wide redshift range to maximally extract information from the non-linear scales.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1982

The structure of the antenna pattern of an adaptive array

The antenna pattern of a receiving adaptive array is investigated for the case of its operation in an environment of one desirable narrow-band signal and (K-1) narrow-band jammers. Results show that the adapted (voltage) antenna pattern of the array is a linear combination of k (or less) basis-patterns, each of which is a function of one source only. It is determined that these basis-patterns have a simple physical meaning in that the k-th basis-pattern is the pattern formed by the array when the transmission of source k is considered a desired signal and all other sources are turned off.

Zohar, S.↗

NASA NTRS1983

Numerical computations on one-dimensional inverse scattering problems

An approximate method to determine the index of refraction of a dielectric obstacle is presented. For simplicity one dimensional models of electromagnetic scattering are treated. The governing equations yield a second order boundary value problem, in which the index of refraction appears as a functional parameter. The availability of reflection coefficients yield two additional boundary conditions. The index of refraction by a k-th order spline which can be written as a linear combination of B-splines is approximated. For N distinct reflection coefficients, the resulting N boundary value problems yield a system of N nonlinear equations in N unknowns which are the coefficients of the B-splines.

Dunn, M. H.↗

NASA NTRS1984

Numerical computations on one-dimensional inverse scattering problems

Dunn, M. H.↗

NASA NTRS1997

The Local Discontinuous Galerkin Method for Time-Dependent Convection-Diffusion Systems

In this paper, we study the Local Discontinuous Galerkin methods for nonlinear, time-dependent convection-diffusion systems. These methods are an extension of the Runge-Kutta Discontinuous Galerkin methods for purely hyperbolic systems to convection-diffusion systems and share with those methods their high parallelizability, their high-order formal accuracy, and their easy handling of complicated geometries, for convection dominated problems. It is proven that for scalar equations, the Local Discontinuous Galerkin methods are L(sup 2)-stable in the nonlinear case. Moreover, in the linear case, it is shown that if polynomials of degree k are used, the methods are k-th order accurate for general triangulations; although this order of convergence is suboptimal, it is sharp for the LDG methods. Preliminary numerical examples displaying the performance of the method are shown.

Cockburn, Bernardo↗

NASA NTRS2008

K/TH in Achondrites and Interpretation of Grand Data for the Dawn Mission

The Dawn mission will explore 4 Vesta [1], a highly differentiated asteroid believed to be the parent body of the howardite, eucrite and diogenite (HED) meteorite suite [e.g. 2]. The Dawn spacecraft is equipped with a gamma-ray and neutron detector (GRaND), which will enable measurement and mapping of elemental abundances on Vesta s surface [3]. Drawing on HED geochemistry, Usui and McSween [4] proposed a linear mixing model for interpretation of GRaND data. However, the HED suite is not the only achondrite suite representing asteroidal basaltic crusts; others include the mesosiderites, angrites, NWA 011, and possibly Ibitira, each of which is thought to have a distinct parental asteroid [5]. Here we critically examine the variability of GRaND-analyzed elements, K and Th, in HED meteorites, and propose a method based on the K-Th systematics to distinguish between HED and the other differentiated achondrites. Maps of these elements might also recognize incompatible element enriched areas such as mapped locally on the Moon (KREEP) [6], and variations in K/Th ratios might indicate impact volatilization of K. We also propose a new mixing model using elements that will be most reliably measured by GRaND, including K.

Usui, T.↗

DOE OSTI2020

Materials Data on K3Th by Materials Project

K3Th is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K is bonded to eight equivalent K and four equivalent Th atoms to form distorted KK8Th4 cuboctahedra that share corners with twelve equivalent KK8Th4 cuboctahedra, edges with eight equivalent ThK12 cuboctahedra, edges with sixteen equivalent KK8Th4 cuboctahedra, faces with four equivalent ThK12 cuboctahedra, and faces with fourteen equivalent KK8Th4 cuboctahedra. All K–K bond lengths are 4.26 Å. All K–Th bond lengths are 4.26 Å. Th is bonded to twelve equivalent K atoms to form ThK12 cuboctahedra that share corners with twelve equivalent ThK12 cuboctahedra, edges with twenty-four equivalent KK8Th4 cuboctahedra, faces with six equivalent ThK12 cuboctahedra, and faces with twelve equivalent KK8Th4 cuboctahedra.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on KTh3 by Materials Project

KTh3 is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. K is bonded to twelve equivalent Th atoms to form KTh12 cuboctahedra that share corners with six equivalent KTh12 cuboctahedra, corners with twelve equivalent ThK4Th8 cuboctahedra, edges with eighteen equivalent ThK4Th8 cuboctahedra, faces with eight equivalent KTh12 cuboctahedra, and faces with twelve equivalent ThK4Th8 cuboctahedra. There are six shorter (3.71 Å) and six longer (3.77 Å) K–Th bond lengths. Th is bonded to four equivalent K and eight equivalent Th atoms to form distorted ThK4Th8 cuboctahedra that share corners with four equivalent KTh12 cuboctahedra, corners with fourteen equivalent ThK4Th8 cuboctahedra, edges with six equivalent KTh12 cuboctahedra, edges with twelve equivalent ThK4Th8 cuboctahedra, faces with four equivalent KTh12 cuboctahedra, and faces with sixteen equivalent ThK4Th8 cuboctahedra. There are a spread of Th–Th bond distances ranging from 3.47–3.95 Å.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on K3Th by Materials Project

K3Th is Uranium Silicide-like structured and crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. there are two inequivalent K sites. In the first K site, K is bonded to eight K and four equivalent Th atoms to form distorted KK8Th4 cuboctahedra that share corners with twelve equivalent KK8Th4 cuboctahedra, edges with eight equivalent KK8Th4 cuboctahedra, edges with eight equivalent ThK12 cuboctahedra, faces with four equivalent ThK12 cuboctahedra, and faces with ten equivalent KK8Th4 cuboctahedra. There are four shorter (3.98 Å) and four longer (4.36 Å) K–K bond lengths. All K–Th bond lengths are 4.36 Å. In the second K site, K is bonded in a square co-planar geometry to eight equivalent K and four equivalent Th atoms. All K–Th bond lengths are 3.98 Å. Th is bonded to twelve K atoms to form ThK12 cuboctahedra that share corners with four equivalent ThK12 cuboctahedra, edges with eight equivalent ThK12 cuboctahedra, edges with sixteen equivalent KK8Th4 cuboctahedra, faces with four equivalent ThK12 cuboctahedra, and faces with eight equivalent KK8Th4 cuboctahedra.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on K3Th by Materials Project

K3Th is alpha bismuth trifluoride structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. there are two inequivalent K sites. In the first K site, K is bonded in a distorted body-centered cubic geometry to four equivalent K and four equivalent Th atoms. All K–K bond lengths are 4.15 Å. All K–Th bond lengths are 4.15 Å. In the second K site, K is bonded in a distorted body-centered cubic geometry to eight equivalent K and six equivalent Th atoms. All K–Th bond lengths are 4.79 Å. Th is bonded in a body-centered cubic geometry to fourteen K atoms.

36 MATERIALS SCIENCE↗