Space Density Research Articles

Predicting the Yield of Small Transiting Exoplanets around Mid-M and Ultracool Dwarfs in the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey

We simulate the yield of small (0.5–4.0 R ⊕) transiting exoplanets around single mid-M and ultracool dwarfs (UCDs) in the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey. We consider multiple approaches for simulating M3–T9 sources within the survey fields, including scaling local space densities and using Galactic stellar population synthesis models. These approaches independently predict ∼100,000 single mid-M dwarfs and UCDs brighter than a Roman F146 magnitude of 21 that are within the survey fields. Assuming planet occurrence statistics previously measured for early-to-mid-M dwarfs, we predict that the survey will discover small transiting planets around these sources, each to a significance of 7.1σ or greater. Significant departures from this prediction would test whether the occurrence rates of small planets increase or decrease around mid-M dwarfs and UCDs compared to early-M dwarfs. We predict the detection of habitable, terrestrial planets (R p < 1.23 R ⊕) in the survey. However, atmospheric characterization of these planets will be challenging with current or near-future space telescope facilities due to the faintness of the host stars. Nevertheless, accurate statistics for the occurrence of small planets around mid-M dwarfs and UCDs will enable direct tests of predictions from planet formation theories and will determine our understanding of planet demographics around the objects at the bottom of the main sequence. This understanding is critical given the prevalence of such objects in our galaxy, whose planets may therefore comprise the bulk of the galactic census of exoplanets.

Prospects of detecting soft X-ray emission from typical WHIM filaments around massive clusters and the coma cluster soft excess

ABSTRACT While hot ICM in galaxy clusters makes these objects powerful X-ray sources, the cluster’s outskirts and overdense gaseous filaments might give rise to much fainter sub-keV emission. Cosmological simulations show a prominent ‘focusing’ effect of rich clusters on the space density of the warm-hot intergalactic medium (WHIM) filaments up to a distance of $\sim 10\, {\rm Mpc}$ (∼ turnaround radius, rta) and beyond. Here, we use Magneticum simulations to characterize their properties in terms of integrated emission measure for a given temperature and overdensity cut and the level of contamination by the more dense gas. We suggest that the annuli $(\sim 0.5-1)\times \, r_{ta}$ around massive clusters might be the most promising sites for the search of the gas with overdensity ≲ 50. We model spectral signatures of the WHIM in the X-ray band and identify two distinct regimes for the gas at temperatures below and above $\sim 10^6\, {\rm K}$. Using this model, we estimate the sensitivity of X-ray telescopes to the WHIM spectral signatures. We found that the WHIM structures are within reach of future high spectral resolution missions, provided that the low-density gas is not extremely metal-poor. We then consider the Coma cluster observed by SRG/eROSITA during the CalPV phase as an example of a nearby massive object. We found that beyond the central r ∼ 40 arcmin ($\sim 1100\, {\rm kpc}$) circle, where calibration uncertainties preclude clean separation of the extremely bright cluster emission from a possible softer component, the conservative upper limits are about an order of magnitude larger than the levels expected from simulations.

On the Energy‐Dependent Deep (L &lt; 3.5) Penetration of Radiation Belt Electrons

AbstractDeep penetration of outer radiation belt electrons to low L (&lt;3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons.

Overcoming the Ambient Manufacturability-Performance Bottleneck in Perovskite Nanocrystal Emitters for Efficient Light-Emitting Diodes.

Colloidal perovskite nanocrystals (NCs) have risen rapidly in luminescence efficiency and color purity. However, their high performance requires careful and complex pre-treatment of precursors and precise regulation of the reaction atmosphere; otherwise, their emission will be weak and broad. To overcome these limitations, we develop a facile ligand exchange method using a new type of bidentate ligand, which is obtained by reacting cheap sulfur with tributylphosphine (S-TBP). During ligand exchange, the double bond between P and S atoms breaks and a single bond is formed between them, after which S-TBP switches into a bidentate ligand and binds to a perovskite NC at two points. With short-chain S-TBP ligands that have high spatial position resistance, both NC spacing and surface ligand density can be reduced, thereby improving carrier injection and transport. On the NC surface after ligand exchange, halogen vacancies were substantially filled, leading to a PbSP (Pb, S, and P elements) component-dominated shell that greatly decreases trap density and enhances material stability. The resulting perovskite NCs are stable and bright with a photoluminescence quantum yield of ~96%, and an external quantumefficiency of 22%. Note that our ligand-exchange strategy remains effective even when scaling up, which should accelerate commercialization.

Overcoming the Ambient Manufacturability‐Performance Bottleneck in Perovskite Nanocrystal Emitters for Efficient Light‐Emitting Diodes

AbstractColloidal perovskite nanocrystals (NCs) have risen rapidly in luminescence efficiency and color purity. However, their high performance requires careful and complex pre‐treatment of precursors and precise regulation of the reaction atmosphere; otherwise, their emission will be weak and broad. To overcome these limitations, we develop a facile ligand exchange method using a new type of bidentate ligand, which is obtained by reacting cheap sulfur with tributylphosphine (S‐TBP). During ligand exchange, the double bond between P and S atoms breaks and a single bond is formed between them, after which S‐TBP switches into a bidentate ligand and binds to a perovskite NC at two points. With short‐chain S‐TBP ligands that have high spatial position resistance, both NC spacing and surface ligand density can be reduced, thereby improving carrier injection and transport. On the NC surface after ligand exchange, halogen vacancies were substantially filled, leading to a PbSP (Pb, S, and P elements) component‐dominated shell that greatly decreases trap density and enhances material stability. The resulting perovskite NCs are stable and bright with a photoluminescence quantum yield of ≈96 %, and an external quantum efficiency of 22 %. Note that our ligand‐exchange strategy remains effective even when scaling up, which should accelerate commercialization.

Trastornos respiratorios en la especie porcina resultantes de la producción en sistema intensivo: Revisión

The expansion of pig production is due to the intensification of the animal’s breeding process. Outdoor farming systems have been replaced by intensive systems with limited space and high animal density. Due to confinement, a greater number of animals in a smaller space, in addition to greater animal turnover, there was also an increase in the frequency of certain diseases, including respiratory diseases, due to their form of transmission, pathogenesis and epidemiology.

Comparing transferability in neural network approaches and linear models for machine-learning interaction potentials

Atomic simulations using machine learning interatomic potential (MLIP) have gained a lot of popularity owing to their accuracy in comparison to conventional empirical potentials. However, the transferability of MLIP to systems outside the training set poses a significant challenge. Here, we compare the transferability of three MLIP approaches: (i) neural network potentials (NNP), (ii) physical LassoLars interactions potential (PLIP) and (iii) linear potentials with Belher-Parrinello descriptors, trained over a small but diverse configuration of zinc oxide polymorphs. We compared the obtained models with density functional theory reference results for physical properties including bulk lattice parameters, surface energies, and vibrational density of states and showed the superiority of both NNP and PLIP models. However, the NNP model performed poorly when compared to the other two linear models for the structural optimization of nanoparticles and molecular dynamics simulation of liquid phases, which are systems outside the training set. While providing less accurate prediction for solid Zinc Oxides phases, both linear models appear more transferable than NNP when testing for nanoscale systems and liquid phases. Our results are finally rationalized by a combination of different statistical analysis including spread in force evaluation, information imbalance, convex hull calculation, and density in descriptor space.

Itinerant ferromagnetism entrenched by the anisotropy of spin–orbit coupling in a dipolar Fermi gas

We investigate the itinerant ferromagnetism in a dipolar Fermi atomic system with the anisotropic spin–orbit coupling (SOC), which is traditionally explored with isotropic contact interaction. We first study the ferromagnetism transition boundaries and the properties of the ground states through the density and spin-flip distribution in momentum space, and we find that both the anisotropy and the magnitude of the SOC play an important role in this process. We propose a helpful scheme and a quantum control method which can be applied to conquering the difficulties of previous experimental observation of itinerant ferromagnetism. Our further study reveals that exotic Fermi surfaces and an abnormal phase region can exist in this system by controlling the anisotropy of SOC, which can provide constructive suggestions for the research and the application of a dipolar Fermi gas. Furthermore, we also calculate the ferromagnetism transition temperature and novel distributions in momentum space at finite temperature beyond the ground states from the perspective of experiment.

HylleraasMD: A Domain Decomposition-Based Hybrid Particle-Field Software for Multiscale Simulations of Soft Matter

We present HylleraasMD (HyMD), a comprehensive implementation of the recently proposed Hamiltonian formulation of hybrid particle-field molecular dynamics. The methodology is based on a tunable, grid-independent length-scale of coarse graining, obtained by filtering particle densities in reciprocal space. This enables systematic convergence of energies and forces by grid refinement, also eliminating nonphysical force aliasing. Separating the time integration of fast modes associated with internal molecular motion from slow modes associated with their density fields, we enable the first time-reversible, energy-conserving hybrid particle-field simulations. HyMD comprises the optional use of explicit electrostatics, which, in this formalism, corresponds to the long-range potential in particle-mesh Ewald. We demonstrate the ability of HyMD to perform simulations in the microcanonical and canonical ensembles with a series of test cases, comprising lipid bilayers and vesicles, surfactant micelles, and polypeptide chains, comparing our results to established literature. An on-the-fly increase of the characteristic coarse-grain length significantly speeds up dynamics, accelerating self-diffusion and leading to expedited aggregation. Exploiting this acceleration, we find that the time scales involved in the self-assembly of polymeric structures can lie in the tens to hundreds of picoseconds instead of the multimicrosecond regime observed with comparable coarse-grained models.

M.2 NVMe SSD의 신뢰성 향상을 위한 히트싱크 열 설계

M.2 NVMe SSD (Non-Volatile Memory express Solid-State Drive), which have higher computational speed and reliability than conventional devices, have come to be widely used. Recent studies have reported that M.2 NVMe SSD are beginning to have thermal issues due to the increasing heat generation occurring with the high chip density and high-performance operation in a limited space. Thermal issues in the controller and memory units of M.2 NVMe SSD lead to increased failure rates and decreased data retention times. In this study, we propose a compact and optimized thermal solution for commercial M.2 NVMe SSD installed between the mainboard and GPU (Graphic Processing Unit). A thermal and fluid dynamics simulation of an M.2 NVMe SSD, including the heatsink, was performed, and the Genetic Algorithm method was used to optimize the heatsink size.

Effect of excess lithium on the structure and microwave dielectric properties of LiMg0.5Ti0.5O2 ceramics

AbstractHerein, Li1+xMg0.5Ti0.5O2 (0.000 ≤ x ≤ 0.075) ceramics with excess lithium were prepared by solid‐state reaction method to compensate for lithium volatilization, thus optimizing dielectric properties. The second phase Mg2TiO4 decreases with excess lithium content and eventually disappears at x = 0.05. Of these, Li1.0375Mg0.5Ti0.5O2 illustrates the best dielectric properties with εr = 16.58, Q × f = 120712 GHz (@9.18 GHz), and τf = −16.31 ppm/°C. From the point of view of non‐intrinsic factors, excess lithium at optimum ratios slightly facilitates the sintering of the ceramics, improves densification, and increases the grain size, resulting in improved dielectric properties. For intrinsic factors, a slight increase in bond ionicity results in a slight improvement in εr. First‐principles calculations demonstrate that suitable excess lithium increases electron cloud density in the internal space of Li/Mg/TiO6 octahedron and increases band gap, thus optimizing the dielectric properties. The same results were obtained from the perspective of lattice vibrations. The modified Li1.0375Mg0.5Ti0.5O2 ceramics offer great potential for future microwave communication technology.

Characterization methods for natural fractures distribution in shale and tight reservoirs

Aperture distribution and spatial arrangement are fundamental characteristics of natural fractures. Since it is impossible to directly measure the aperture of all existing fractures, a power law (fractal model) has been commonly used to describe fracture apertures, which has also been extended to estimate fracture spacing and macrofractures density. Although a power law (fractal model) has commonly been used to describe fracture apertures, data points in small and large fracture size ranges tend to deviate from this distribution. In this study, a Variable Shape Distribution (VSD) method was derived from a fractal theory to model fracture size distribution. The VSD method was compared with seven other commonly used models. Eleven fracture datasets collected from five countries on four continents were regressed using these methods. The scale-free methods of the VSD and power law models showed more advantages than the scale-dependent methods. The VSD method was found to best fit all variable aperture size distribution patterns, with an average coefficient of determination of 0.99. The average fractal dimension for all samples calculated by the VSD method (1.36) was slightly higher than that of the power law (1.06). This study is unique in showing that the VSD method can solve a boundary characterization problem that has been widely neglected in previous studies. With the assumption of an even fracture distribution, the VSD method was further applied to estimate average fracture spacing, predict a threshold fracture length that controls the linkage of fractures, and predict fractures outside of the measurement range. This study not only proposes an advanced fractal method to model aperture size distributions in shale and tight rocks but also helps to better understand the importance of fractal dimensions and boundary characterizations.

지역활성화를 위한 준공업지역의 상업 공간 분포 특성 비교 연구 - 성수동2가와 문래동을 중심으로 -

Mullae-dong and Seongsu-dong (2ga) are the same semi-industrial and urban regeneration areas that are turning into commercial spaces. But Seongsu-dong is higher in the total of de facto population and is full of energetic young people in their 20s to 40s compared to Mullae-dong. This study aims to suggest the new direction of regeneration for the Mullae-dong by comparing the differences in the pattern of spatial distribution that can increase the activity and mobility of population based on the diversity of space types and the density of commercial spaces. This study compared the diversity, density and clustering of commercial spaces in two areas using Q-GIS. For analyzing, the de facto population vector data and commercial space point data were overlapped, and then the spatial data was re-extracted. The result shows that in spite of the high population density of young people, Mullae-dong is less energetic place than Seongsu-dong due to the lack of diversity and the low density of commercial spaces. It means that Mullae-dong is needed to revitalize commercial districts for the regional regeneration.

Wave and Particle Analysis of Z‐Mode and O‐Mode Emission in the Jovian Inner Magnetosphere

AbstractWe report some of the most intense Z‐mode and O‐mode observations obtained by the Juno spacecraft while in orbit about Jupiter in a low to mid‐latitude region near the inner edge of the Io torus. We have been able to estimate the density of the plasma in this region based on the lower frequency cutoff of the observed Z‐mode emission. The results are compatible with the electron density measurements of the Jovian Auroral Distributions Experiment (JADE), on board the Juno spacecraft, if we account for unmeasured cold plasma. Direction‐finding measurements indicate that the Z‐ and O‐mode emission have distinct source regions. We have also used the measured phase space density of the JADE and the Jupiter energetic particle detector instruments to calculate estimated local growth rates of the observed O‐mode and Z‐mode emission assuming a loss cone instability and quasilinear analysis. The results suggest the emissions were observed near, but not within, a source region, and the free energy source is consistent with a loss cone. We have thus carried out the quasilinear wave analysis of the assumed remote Z‐ and O‐mode wave growths. It is shown that the remotely generated waves, propagated through an inhomogeneous medium to the satellite location, may account for the observed wave characteristics. The importance of Z‐mode in accelerating electrons in the inner Jovian magnetosphere makes these new wave mode confirmations at Jupiter of particular interest.

Electric-Field Control of Spin Polarization above Room Temperature in Single-Layer A-Type Antiferromagnetic Semiconductor

Two-dimensional (2D) antiferromagnets have drawn great interest for absence of stray fields in antiferromagnetic (AFM) spintronics. However, it remains challenging to manipulate their spin polarization above room temperature for practical applications. Herein, a general strategy is reported to realize the control of spin polarization above room temperature in 2D A-type AFM semiconductors by external electric field based on first-principles calculations, exemplified by transition metal monohalide MnCl and carbide MXenes Cr2CX2 (X = F, Cl, OH). It shows that 100% spin polarization can be induced around Fermi level with spin splitting gap related to the spatial distribution of spin density in real space. Meanwhile, the Neél temperature of 2D MnCl and Cr2CF2 remains above room temperature under external electric field up to 0.6 V/Å. This study exhibits the potential for application of 2D AFM semiconductors in electric-field-controlled spintronics.

Nature of Interlayer Bonds in Two-Dimensional Materials

The role of interlayer bonds in the two-dimensional (2D) materials "beyond graphene" and so-called van der Waals heterostructures is vital, and understanding the nature of these bonds in terms of strength and type is essential due to a wide range of their prospective technological applications. However, this issue has not yet been properly addressed in the previous investigations devoted to 2D materials. In our work, by using first-principles calculations we perform a systematic study of the interlayer bonds and charge redistribution of several representative 2D materials that are traditionally referred as van der Waals systems. Our results demonstrate that one can distinguish three main types of inter-layer couplings in the considered 2D structures: one atom thick membranes bonded by London dispersion forces (graphene, hBN), systems with leading electrostatic interaction between layers (diselenides, InSe and bilayer silica) and materials with so-called dative or coordination chemical bonds between layers (ditelurides). We also propose a protocol for recognising the leading type of interlayer bonds in a system that includes comparison of interlayer distances, binding energies and redistribution of the charge densities in interlayer space. Such an approach is computationally cheap and can be used to further prediction of chemical and physical properties such as charge density waves (CDW), work function and chemical stability at ambient conditions.

Influence of the Heterogeneity of Spatial Distribution of Peripheral Radio Equipment of 4G/5G Networks on the Level of Electromagnetic Background Created in Buildings

Known results of studies of the radio frequency electromagnetic background generated in buildings by electromagnetic radiation from the peripheral equipment of 4G/5G mobile communications , including equipment for massive machine-type communication /Internet of things ( mMTC / IoT ) and user’s devices for wireless telephony and data transmission , are based on the use of a model of uniform random distribution of this equipment over the building internal space . The adequacy of this model for the analysis of a number of 4G/5G scenarios is questioned due to the fact that , excluding user’s devices , the main part of the mMTC / IoT peripheral radio equipment located in buildings is distributed along the walls of the premises with a tendency to concentrate them near the intersections of flat surfaces of walls , floors and ceilings . Paper presents the results of comparative estimates of the average intensity of electromagnetic background inside building for different types of distribution of the set of point sources of RF electromagnetic fields over the building interior space : for their uniform distribution over internal space and for two types of heterogeneity of this distribution : for uniform distribution of sources over building internal surfaces and with their uniform distribution along the lines of intersection of these surfaces . Results of this analysis indicate a relatively small effect of the considered types of heterogeneity in the spatial distribution of radiation sources on the estimates of the electromagnetic background average intensity created by these sources inside the building , when using a technique based on the representation of the internal space of a building as a set of solid angles , which internal space separate sections are characterized by the constancy of the sources average space density and by the average conditions for radio waves propagation in the direction of the observation point .

Where are the people? Counting people in millions of street-level images to explore associations between people’s urban density and urban characteristics

A thorough understanding of how urban space characteristics, such as urban equipment or network topology, affect people’s density in urban spaces is essential to well-informed urban policy making. Hitherto, studies have primarily examined how the characteristics of the urban space impacts the number of people visiting different parts of the urban area (e.g., the city center). However, these studies almost without exception have used relatively small data sets, targeting specific neighborhoods or places. As a result, their findings are confined to specific areas and it is unclear to what extent their findings generalize to other urban areas. This study addresses this gap. We propose a new computer vision-based method to study how the urban space is associated with people’s urban density in outdoor urban spaces. Specifically, our method uses a pre-trained object detection model to identify and count people as well as urban-related objects, such as presence of cars, and benches in millions street-level images collected throughout the Netherlands. Importantly, each street-level image is geo-located. Therefore, for each detected person and object its location is known. In turn, we regress urban space characteristics and urban-related objects on the number of people identified as a proxy for density in urban spaces. Our results show that higher numbers of people tend to be observed in places with smaller blocks, suggesting that compact urban development may be an effective way to increase people’s density. Moreover, we find that the presence of food places and bicycles is associated with more people, indicating that urban planners could study the location of these amenities to attract more visitors to urban spaces and exploring the causality effects in this relationship. Our methodology offers a complementary way to monitor how the urban space is used over the time and to assess the effectiveness of urban interventions and policies.

Effects of polymethacrylimide foam reinforced aluminum honeycomb sandwich under quasi‐static compression

AbstractIn this study, aluminum honeycomb cells were filled with polymethacrylimide (PMI) foam of high strength and stiffness, and carbon fiber reinforced composite (CFRC) skins were added to create a composite sandwich structure. Nine types of specimens with varying cell node spacing and foam density were constructed, and stiffness equations were derived. To compare the performances of aluminum honeycomb and PMI foam sandwich structures, their compressive responses were examined using edgewise and flatwise compression tests. Moreover, the failure was analyzed using finite element methods (FEM). Analytical and numerical models were used to investigate the size effects of rib width, rib angle, and node spacing.

Parametric analysis of pitch angle scattering and losses of relativistic electrons by oblique EMIC waves

This study analyzes the effects of electromagnetic ion cyclotron (EMIC) waves on relativistic electron scattering and losses in the Earth’s outer radiation belt. EMIC emissions are commonly observed in the inner magnetosphere and are known to reach high amplitudes, causing significant pitch angle changes in primarily &gt;1 MeV electrons via cyclotron resonance interactions. We run test-particle simulations of electrons streaming through helium band waves with different amplitudes and wave normal angles and assess the sensitivity of advective and diffusive scattering behaviors to these two parameters, including the possibility of very oblique propagation. The numerical analysis confirms the importance of harmonic resonances for oblique waves, and the very oblique waves are observed to efficiently scatter both co-streaming and counter-streaming electrons. However, strong finite Larmor radius effects limit the scattering efficiency at high pitch angles. Recently discussed force-bunching effects and associated strong positive advection at low pitch angles are, surprisingly, shown to cause no decrease in the phase space density of precipitating electrons, and it is demonstrated that the transport of electrons into the loss cone balances out the scattering out of the loss cone. In the case of high-amplitude obliquely propagating waves, weak but non-negligible losses are detected well below the minimum resonance energy, and we identify them as the result of non-linear fractional resonances. Simulations and theoretical analysis suggest that these resonances might contribute to subrelativistic electron precipitation but are likely to be overshadowed by non-resonant effects.