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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

AN INITIAL MASS FUNCTION FOR INDIVIDUAL STARS IN GALACTIC DISKS. I. CONSTRAINING THE SHAPE OF THE INITIAL MASS FUNCTION

We derive a semi-empirical galactic initial mass function (IMF) from observational constraints. We assume that the IMF, {psi}(m), is a smooth function of the stellar mass m. The mass dependence of the proposed IMF is determined by five parameters: the low-mass slope {gamma}, the high-mass slope -{Gamma} (taken to be -1.35), the characteristic mass m{sub ch} ({approx} the peak mass of the IMF), and the lower and upper limits on the mass, m{sub l} and m{sub u} (taken to be 0.004 and 120 M{sub sun}, respectively): {psi}(m)dln m {proportional_to} m{sup -}{Gamma}{l_brace}1 - exp [- (m/m{sub ch}){sup {gamma}}+{Gamma}]{r_brace}dln m. The values of {gamma} and m{sub ch} are derived from two integral constraints: (1) the ratio of the number density of stars in the range m = 0.1-0.6 M{sub sun} to that in the range m = 0.6-0.8 M{sub sun} as inferred from the mass distribution of field stars in the local neighborhood and (2) the ratio of the number of stars in the range m = 0.08-1 M{sub sun} to the number of brown dwarfs in the range m = 0.03-0.08 M{sub sun} in young clusters. The IMF satisfying the above constraints is characterized by the parameters {gamma} = 0.51 and m{sub ch} = 0.35 M{sub sun} (which corresponds to a peak mass of 0.27 M{sub sun} ). This IMF agrees quite well with the Chabrier IMF for the entire mass range over which we have compared with data, but predicts significantly more stars with masses <0.03 M{sub sun}; we also compare with other IMFs in current use and give a number of important parameters implied by the IMFs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SIMILARITY BETWEEN THE C{sup 18}O (J = 1-0) CORE MASS FUNCTION AND THE INITIAL MASS FUNCTION (IMF) IN THE S140 REGION

We present the results of C{sup 18}O(J = 1-0) mapping observations of a 20' x 18' area in the Lynds 1204 molecular cloud associated with the Sharpless 2-140 (S140) H II region. The C{sup 18}O cube ({alpha}-{delta}-v{sub LSR}) data show that there are three clumps of sizes {approx}1 pc in the region. Two of these have peculiar redshifted velocity components at their edges, which can be interpreted as the results of the interaction between the cloud and the Cepheus Bubble. From the C{sup 18}O cube data, clumpfind identified 123 C{sup 18}O cores, which have mean radius, velocity width in FWHM, and LTE mass of 0.36 {+-} 0.07 pc, 0.37 {+-} 0.09 km s{sup -1}, and 41 {+-} 29 M{sub sun}, respectively. Considering the uncertainty in the C{sup 18}O abundance, all the cores in S140 are most likely to be gravitationally bound. We derived a C{sup 18}O core mass function (CMF), which shows a power-law-like behavior above a turnover at 30 M{sub sun}. The best-fit power-law index of -2.1 {+-} 0.2 is quite consistent with that of the initial mass function (IMF) and the C{sup 18}O CMF in the OMC-1 region as found by Ikeda and Kitamura. Kramer et al. estimated the power-law index of -1.65 in S140 from the C{sup 18}O(J = 2-1) data, which is inconsistent with this study. The C{sup 18}O(J = 2-1) data are spatially limited to the central part of the cloud and are likely to be biased toward high-mass cores, leading to the flatter CMF. Consequently, this study and our previous study strongly support that the power-law form of the IMF has already been determined at the density of {approx}< 10{sup 3}-10{sup 4} cm{sup -3}, traced by the C{sup 18}O(J = 1-0) line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE FERMI LARGE AREA TELESCOPE ON ORBIT: EVENT CLASSIFICATION, INSTRUMENT RESPONSE FUNCTIONS, AND CALIBRATION

The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy {gamma}-ray telescope, covering the energy range from 20 MeV to more than 300 GeV. During the first years of the mission, the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the instrument response functions (IRFs), the description of the instrument performance provided for data analysis. In this paper, we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A marked correlation function for constraining modified gravity models

Future large scale structure surveys will provide increasingly tight constraints on our cosmological model. These surveys will report results on the distance scale and growth rate of perturbations through measurements of Baryon Acoustic Oscillations and Redshift-Space Distortions. It is interesting to ask: what further analyses should become routine, so as to test as-yet-unknown models of cosmic acceleration? Models which aim to explain the accelerated expansion rate of the Universe by modifications to General Relativity often invoke screening mechanisms which can imprint a non-standard density dependence on their predictions. This suggests density-dependent clustering as a `generic' constraint. This paper argues that a density-marked correlation function provides a density-dependent statistic which is easy to compute and report and requires minimal additional infrastructure beyond what is routinely available to such survey analyses. We give one realization of this idea and study it using low order perturbation theory. We encourage groups developing modified gravity theories to see whether such statistics provide discriminatory power for their models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Modifying the variational principle in the action-integral-functional derivation of time-dependent density-functional theory

According to a recent paper by G. Vignale [Phys. Rev. A 77, 062511 (2008)], the problems arising in the original derivation of time-dependent density-functional theory based on the Runge-Gross (RG) action-integral functional (AIF) are because of an incorrect variational principle (VP). This argument and the proposed modification of the VP are critically analyzed. The more fundamental problem, though, is the indefiniteness of the RG AIF. In contrast to a widely held belief the indefiniteness is not eliminated in the variational procedure, which unwittingly is corroborated by Vignale's initial point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Density and pair-density scaling for deriving the Euler equation in density-functional and pair-density-functional theory

A link between density and pair density functional theories is presented. Density and pair density scaling are used to derive the Euler equation in both theories. Density scaling provides a constructive way of obtaining approximations for the Pauli potential. The Pauli potential (energy) of the density functional theory is expressed as the difference of the scaled and original exchange-correlation potentials (energies).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Generalization of internal density-functional theory and Kohn-Sham scheme to multicomponent self-bound systems, and link with traditional density-functional theory

We generalize the recently developed ''internal'' density-functional theory (DFT) and Kohn-Sham scheme to multicomponent systems. We obtain a general formalism, applicable for the description of multicomponent self-bound systems (such as molecular systems where the nuclei are treated explicitly, atomic nuclei and mixtures of {sup 3}He and {sup 4}He droplets), where the fundamental translational symmetry has been treated correctly. The main difference with traditional DFT is the explicit inclusion of center-of-mass correlations in the functional. A large part of the paper is dedicated to the application to molecular systems, which permits us to clarify the approximations that underly traditional DFT.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Antisymmetric wave functions for mixed fermion states and energy convexity

We show how ensembles or mixed states can be described in terms of pure states that for Fermions lead to wave functions that are antisymmetric with respect to interchange of particle coordinates (and spin). The pure states are constructed in an augmented Hilbert space spanned by products of ensemble states projected onto mutually nonoverlapping coordinates that prevents the appearance of interference terms under antisymmetrization. As a demonstration of this new formalism, and under the assumptions of a positive interparticle interaction and a corresponding energy that is extensive in the number of particle pairs (pair extensive), we prove the convexity relation, E v [ N - 1 ] + E v [ N + 1 ] ≤ 2 E v [ N ] , where E v [ N ] denotes the total ground-state energy of N Fermions (electrons) under an external potential v ( r ) .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Bypassing the Energy Functional in Density Functional Theory: Direct Calculation of Electronic Energies from Conditional Probability Densities

Density functional calculations can fail for want of an accurate exchange-correlation approximation. The energy can instead be extracted from a sequence of density functional calculations of conditional probabilities (CP DFT). Simple CP approximations yield usefully accurate results for two-electron ions, the hydrogen dimer, and the uniform gas at all temperatures. CP DFT has no self-interaction error for one electron, and correctly dissociates H2, both major challenges. For warm dense matter, classical CP DFT calculations can overcome the convergence problems of Kohn-Sham DFT.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Symmetry-mode analysis for local structure investigations using pair distribution function data

Symmetry-adapted distortion modes provide a natural way of describing distorted structures derived from higher-symmetry parent phases. Structural refinements using symmetry-mode amplitudes as fit variables have been used for at least ten years in Rietveld refinements of the average crystal structure from diffraction data; more recently, this approach has also been used for investigations of the local structure using real-space pair distribution function (PDF) data. Here, the value of performing symmetry-mode fits to PDF data is further demonstrated through the successful application of this method to two topical materials: TiSe2, where a subtle but long-range structural distortion driven by the formation of a charge-density wave is detected, and MnTe, where a large but highly localized structural distortion is characterized in terms of symmetry-lowering displacements of the Te atoms. Here, the analysis is performed using fully open-source code within the DiffPy framework via two packages developed for this work: isopydistort, which provides a scriptable interface to the ISODISTORT web application for group theoretical calculations, and isopytools, which converts the ISODISTORT output into a DiffPy-compatible format for subsequent fitting and analysis. These developments expand the potential impact of symmetry-adapted PDF analysis by enabling high-throughput analysis and removing the need for any commercial software.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Inverter Reliability Estimation for Advanced Inverter Functionality

In the near future, grid operators are expected to regularly use advanced distributed energy resource (DER) functions, defined in IEEE 1547-2018, to perform a range of grid-support operations. Many of these functions adjust the active and reactive power of the device through commanded or autonomous modes, which will produce new stresses on the grid-interfacing power electronics components, such as DC/AC inverters. In previous work, multiple DER devices were instrumented to evaluate additional component stress under multiple reactive power setpoints. We utilize quasi-static time-series simulations to determine voltage-reactive power mode (volt-var) mission profile of inverters in an active power system. Mission profiles and loss estimates are then combined to estimate the reduction of the useful life of inverters from different reactive power profiles. It was found that the average lifetime reduction was approximately 0.15% for an inverter between standard unity power factor operation and the IEEE 1547 default volt-var curve based on thermal damage due to switching in the power transistors. For an inverter with an expected 20-year lifetime, the 1547 volt-var curve would reduce the expected life of the device by 12 days. This framework for determining an inverter's useful life from experimental and modeling data can be applied to any failure mechanism and advanced inverter operation.

component degradation↗
DOE OSTI2017

Systems analysis in Cellvibrio japonicus resolves predicted redundancy of β-glucosidases and determines essential physiological functions: Functional analysis of C. japonicus β-glucosidases

Degradation of polysaccharides forms an essential arc in the carbon cycle, provides a percentage of our daily caloric intake, and is a major driver in the renewable chemical industry. Microorganisms proficient at degrading insoluble polysaccharides possess large numbers of carbohydrate active enzymes, many of which have been categorized as functionally redundant. Here we present data that suggests that carbohydrate active enzymes that have overlapping enzymatic activities can have unique, non-overlapping biological functions in the cell. Our comprehensive study to understand cellodextrin utilization in the soil saprophyte Cellvibrio japonicus found that only one of four predicted β-glucosidases is required in a physiological context. Gene deletion analysis indicated that only the cel3B gene product is essential for efficient cellodextrin utilization in C. japonicus and is constitutively expressed at high levels. Interestingly, expression of individual β-glucosidases in Escherichia coli K-12 enabled this non-cellulolytic bacterium to be fully capable of using cellobiose as a sole carbon source. Furthermore, enzyme kinetic studies indicated that the Cel3A enzyme is significantly more active than the Cel3B enzyme on the oligosaccharides but not disaccharides. Finally, our approach for parsing related carbohydrate active enzymes to determine actual physiological roles in the cell can be applied to other polysaccharide-degradation systems.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Block Krylov Subspace Methods for Functions of Matrices II: Modified Block FOM

We analyze an expansion of the generalized block Krylov subspace framework of [Electron. Trans. Numer. Anal., 47 (2017), pp. 100--126]. This expansion allows the use of low-rank modifications of the matrix projected onto the block Krylov subspace and contains, as special cases, the block GMRES method and the new block Radau--Arnoldi method. Within this general setting, we present results that extend the interpolation property from the nonblock case to a matrix polynomial interpolation property for the block case, and we relate the eigenvalues of the projected matrix to the latent roots of these matrix polynomials. Some error bounds for these modified block FOM methods for solving linear systems are presented. We then show how cospatial residuals can be preserved in the case of families of shifted linear block systems. This result is used to derive computationally practical restarted algorithms for block Krylov approximations that compute the action of a matrix function on a set of several vectors simultaneously. Finally, we prove some error bounds and present numerical results showing that two modifications of FOM, the block harmonic and the block Radau--Arnoldi methods for matrix functions, can significantly improve the convergence behavior.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

Sensitivity of Surface Temperature to Oceanic Forcing via q -Flux Green’s Function Experiments. Part I: Linear Response Function

This paper explores the use of the linear response function (LRF) to relate the mean sea surface temperature (SST) response to prescribed ocean heat convergence ( q flux) forcings. Two methods for constructing the LRF based on the fluctuation–dissipation theorem (FDT) and Green’s function (GRF) are examined. A 900-yr preindustrial simulation by the Community Earth System Model coupled with a slab ocean model (CESM–SOM) is used to estimate the LRF using FDT. For GRF, 106 pairs of CESM–SOM simulations with warm and cold q-flux patches are performed. FDT is found to have some skill in estimating the SST response to a q-flux forcing when the local SST response is strong, but it fails in inverse estimation of the q-flux forcing for a given SST pattern. In contrast, GRF is shown to be reasonably accurate in estimating both SST response and q-flux forcing. Possible degradation in FDT may be attributed to insufficient data sampling, significant departure of the SST distribution from Gaussianity, and the nonnormality of the constructed operator. The GRF-based LRF is then used to (i) generate a global surface temperature sensitivity map that shows the q-flux forcing in higher latitudes to be 3–4 times more effective than low latitudes in producing global surface warming, and (ii) identify the most excitable SST mode (neutral vector) that shows marked resemblance to the interdecadal Pacific oscillation (IPO). The latter discovery suggests that the IPO-like fluctuation exists in the absence of the coupling to the ocean dynamics. Coupling to the ocean dynamics in CESM, on the other hand, only enhances the spectral power of the IPO at interannual time scales.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Functionalized Porous Aromatic Frameworks as High‐Performance Adsorbents for the Rapid Removal of Boric Acid from Water

Abstract This study demonstrates that functionalized, highly porous polymers are promising for the adsorptive capture of boric acid, a neutral contaminant that is difficult to remove from seawater using conventional reverse osmosis membranes. Appending N ‐methyl‐d‐glucamine (NMDG) to the pore walls of high‐surface‐area porous aromatic frameworks (PAFs) yields the adsorbents PAF‐1‐NMDG and P2‐NMDG in a simple two‐step synthesis. The boron‐selective PAFs demonstrate adsorption capacities that are up to 70% higher than those of a commercial boron‐selective resin, Amberlite IRA743, and markedly faster adsorption rates, owing to their higher NMDG loadings and greater porosities relative to the resin. Remarkably, PAF‐1‐NMDG is able to reduce the boron concentration in synthetic seawater from 2.91 to <0.5 ppm in less than 3 min at an adsorbent loading of only 0.3 mg mL −1 . The boron adsorption rate constants of both frameworks, determined via a pseudo‐second‐order rate model, represent the highest values reported in the literature—in most cases orders of magnitude higher than those of other boron‐selective adsorbents. The frameworks can also be readily regenerated via mild acid/base treatment and maintain constant boron adsorption capacities for at least 10 regeneration cycles. These results highlight the numerous advantages of PAFs over traditional porous polymers in water treatment applications.

Kamcev, Jovan↗
DOE OSTI2016

Water‐Based Assembly of Polymer–Metal Organic Framework (MOF) Functional Coatings

Metal organic frameworks (MOFs) have gained attention for their porosity, size selectivity, and structural diversity. There is a need for MOF‐based coatings, particularly in applications such as separations, electronics, and energy; yet forming thin, functional, conformal coatings is prohibitive because MOFs exist as a powder. Layer‐by‐layer assembly, a versatile thin film coating approach, offers a unique solution to this problem, but this approach requires MOFs that are water‐dispersible and bear a surface charge. Here, these issues are addressed by examining water‐based dispersions of MIL‐101(Cr) that facilitate the formation of robust polymer–MOF hybrid coatings. Specifically, the substrate to be coated is alternately exposed to an aqueous solution of poly(styrene sulfonate) and nano MIL‐101(Cr) dispersion, yielding linear film growth and coatings with a MOF content as high as 77 wt%. This approach is surface‐agnostic, in which the coating is successfully applied to silicon, glass, flexible plastic, and even cotton fabric, conformally coating individual fibers. By contrast, prior attempts at forming MOF‐coatings are severely limited to a handful of surfaces, require harsh chemical treatment, and are not conformal. The approach presented here unambiguously confirms that MOFs can be conformally coated onto complex and unusual surfaces, opening the door for a wide variety of applications.

De, Souvik↗