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Accretion Variability as a Guide to Stellar Mass Assembly
No full textVariable accretion in young stellar objects reveals itself photometrically and spectroscopically over a continuum of timescales and amplitudes. Most dramatic are the large outbursts (e.g., FU Ori, V1647 Ori, and EX Lup type events), but more frequent are the less coherent, smaller burst-like variations in accretion rate. Improving our understanding of time-variable accretion directly addresses the fundamental question of how stars gain their masses. We review variability phenomena, as characterized from observations across the wavelength spectrum, and how those observations probe underlying physical conditions. The diversity of observed lightcurves and spectra at optical and infrared wavelengths defies a simple classification of outbursts and bursts into well-defined categories. Mid-infrared and sub-millimeter wavelengths are sensitive to lower-temperature phenomena, and it is currently unclear if observed flux variations probe similar or distinct physics relative to the shorter wavelengths. We highlight unresolved issues and emphasize the value of spectroscopy, multiwavelength studies, and ultimately patience in using variable accretion to understand stellar mass assembly
Dicarbonyl anthracenes and phenanthrenes as singlet fission chromophores
No full textSinglet fission is a highly desired process in photovoltaic devices as it can significantly enhance photoelectric conversion efficiency. Exploitation of this process in photovoltaics is hindered by the lack of appropriate chromophores. We used mixed-reference spin-flip time-dependent density functional theory (TDDFT) to investigate five dicarbonyl anthracenes and phenanthrenes, with the purpose to design singlet fission chromophores. Two molecules were found to be promising candidates. For all the dicarbonyl molecules, the oxygen lone pair orbitals were found to be involved in the excited states that are relevant to singlet fission
Metagenomic, (bio)chemical, and microscopic analyses reveal the potential for the cycling of sulfated EPS in Shark Bay pustular mats
No full textCyanobacteria and extracellular polymeric substances (EPS) in peritidal pustular microbial mats have a two-billion-year-old fossil record. To understand the composition, production, degradation, and potential role of EPS in modern analogous communities, we sampled pustular mats from Shark Bay, Australia and analyzed their EPS matrix. Biochemical and microscopic analyses identified sulfated organic compounds as major components of mat EPS. Sulfur was more abundant in the unmineralized regions with cyanobacteria and less prevalent in areas that contained fewer cyanobacteria and more carbonate precipitates. Sequencing and assembly of the pustular mat sample resulted in 83 high-quality metagenome-assembled genomes (MAGs). Metagenomic analyses confirmed cyanobacteria as the primary sources of these sulfated polysaccharides. Genes encoding for sulfatases, glycosyl hydrolases, and other enzymes with predicted roles in the degradation of sulfated polysaccharides were detected in the MAGs of numerous clades including Bacteroidetes, Chloroflexi, Hydrogenedentes, Myxococcota, Verrucomicrobia, and Planctomycetes. Measurable sulfatase activity in pustular mats and fresh cyanobacterial EPS confirmed the role of sulfatases in the degradation of sulfated EPS. These findings suggest that the synthesis, modification, and degradation of sulfated polysaccharides influence microbial interactions, carbon cycling, and biomineralization processes within peritidal pustular microbial mats
A method to generate initial fault stresses for physics-based ground motion prediction consistent with regional seismicity
No full textNear-field ground motion is the major blind spot of seismic hazard studies, mainly because of the challenges in accounting for source effects. Initial stress heterogeneity is an important component of physics-based approaches to ground motion prediction that represent source effects through dynamic earthquake rupture modeling. We hypothesize that stress heterogeneity on a fault primarily originates from past background seismicity. We develop a new method to generate stochastic stress distributions as a superposition of residual stresses left by previous ruptures that are consistent with regional distributions of earthquake size and hypocentral depth. We validate our method on M_w 7 earthquake models suitable for California, by obtaining a satisfactory agreement with empirical earthquake scaling laws and ground motion prediction equations. To avoid the excessive seismic radiation produced by dynamic models with abrupt arrest at preset rupture borders,
we achieve spontaneous rupture arrest by incorporating a scale-dependent fracture energy adjusted with fracture mechanics theory. Our analyses of rupture and ground motion reveal particular signatures of the initial stress heterogeneity: rupture can locally propagate at supershear speed near the highly-stressed areas; the position of high-stress and low-stress areas due to initial stress heterogeneity determines how the peak ground motion amplitudes and polarization spatially vary along the fault, as low-stress areas slows down the rupture, decrease stress drop, and change the radiation distribution before the rupture arrest. We also find that the medium stratification amplifies the moment rate spectrum at frequencies above 2 Hz, which requires understanding the interaction between site effects and rupture dynamics; therefore, we highlight the need to consider a realistic fault medium on future studies of rupture dynamics. Our approach advances our understanding of the relations between dynamic features of earthquake ruptures and the statistics of regional seismicity, and our capability to model source effects for near-field ground motion prediction studies
Local-Average Free Volume Correlates with Dynamics in Glass Formers
No full textGlass formers exhibit a pronounced slowdown in dynamics, accompanied by progressive heterogeneity as they approach the glass transition. There is intense debate over whether the dramatic slowdown is caused by dynamical heterogeneity and whether the enhanced dynamical heterogeneity originates from structural causes. However, the connection between dynamical heterogeneity and the spatial distribution of the single-particle free volume (a purely static structural quantity) was found to be rather weak, which raises the question of whether dynamic heterogeneity has a purely structural origin. Here, by introducing the concept of local-average free volume, we present numerical evidence that long-time dynamic heterogeneity shows significantly enhanced correlation with the average local free volume over a length scale of a few neighboring shells. Our results resolve the long-standing controversy about whether free volume plays an important role in particle rearrangements associated with the activated hopping relaxation. The concept of “local average” can be applied to other local structural descriptors to better correlate with dynamic heterogeneity in glass-forming liquids
Role of Atomic Structure on Exciton Dynamics and Photoluminescence in NIR Emissive InAs/InP/ZnSe Quantum Dots
No full textThe development of bright, near-infrared-emissive quantum dots (QDs) is a necessary requirement for the realization of important new classes of technology. Specifically, there exist significant needs for brighter, heavy metal-free, near-infrared (NIR) QDs for applications with high radiative efficiency that span diverse applications, including down-conversion emitters for high-performance luminescent solar concentrators. We use a combination of theoretical and experimental approaches to synthesize bright, NIR luminescent InAs/InP/ZnSe QDs and elucidate fundamental material attributes that remain obstacles for development of near-unity NIR QD luminophores. First, using Monte Carlo ray tracing, we identify the atomic and electronic structural attributes of InAs core/shell, NIR emitters, whose luminescence properties can be tailored by synthetic design to match most beneficially those of high-performance, single-band-gap photovoltaic devices based on important semiconductor materials, such Si or GaAs. Second, we synthesize InAs/InP/ZnSe QDs based on the optical attributes found to maximize LSC performance and develop methods to improve the emissive qualities of NIR emitters with large, tunable Stokes ratios, narrow emission linewidths, and high luminescence quantum yields (here reaching 60 ± 2%). Third, we employ atomistic electronic structure calculations to explore charge carrier behavior at the nanoscale affected by interfacial atomic structures and find that significant exciton occupation of the InP shell occurs in most cases despite the InAs/InP type I bulk band alignment. Furthermore, the density of the valence band maximum state extends anisotropically through the (111) crystal planes to the terminal InP surfaces/interfaces, indicating that surface defects, such as unpassivated phosphorus dangling bonds, located on the (111) facets play an outsized role in disrupting the valence band maximum and quenching photoluminescence
Search for a right-handed W boson and a heavy neutrino in proton-proton collisions at √s = 13 TeV
No full textA search is presented for a right-handed W boson (W_R) and a heavy neutrino (N), in a final state consisting of two same-flavor leptons (ee or μμ) and two quarks. The search is performed with the CMS experiment at the CERN LHC using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb⁻¹. The search covers two regions of phase space, one where the decay products of the heavy neutrino are merged into a single large-area jet, and one where the decay products are well separated. The expected signal is characterized by an excess in the invariant mass distribution of the final-state objects. No significant excess over the standard model background expectations is observed. The observations are interpreted as upper limits on the product of W_R production cross sections and branching fractions assuming that couplings are identical to those of the standard model W boson. For N masses m_N equal to half the W_R mass m_WR (m_N = 0.2 TeV), m_WR is excluded at 95% confidence level up to 4.7 (4.8) and 5.0 (5.4) TeV for the electron and muon channels, respectively. This analysis provides the most stringent limits on the W_R mass to date
Spin-Orbit Enhanced Superconductivity in Bernal Bilayer Graphene
No full textIn the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature T꜀ ≈ 30~mK. Here, we show that placing monolayer tungsten diselenide (WSe₂) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in T꜀, and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe₂ as a function of electric field and doping, we establish that superconductivity emerges throughout a region whose normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal a striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe₂ -- suggesting that proximity-induced (Ising) spin-orbit coupling plays a key role in enhancing the pairing. Our results pave the way for engineering robust, highly tunable, and ultra-clean graphene-based superconductors
Amplified J-factors in the Galactic Centre for velocity-dependent dark matter annihilation in FIRE simulations
No full textWe use FIRE-2 zoom cosmological simulations of Milky Way size Galaxy haloes to calculate astrophysical J-factors for dark matter annihilation and indirect detection studies. In addition to velocity-independent (s-wave) annihilation cross-sections 〈σv〉, we also calculate effective J-factors for velocity-dependent models, where the annihilation cross-section is either p-wave (∝ v²/c²) or d-wave (∝ v⁴/c⁴). We use 12 pairs of simulations, each run with dark matter-only (DMO) physics and FIRE-2 physics. We observe FIRE runs produce central dark matter velocity dispersions that are systematically larger than in DMO runs by factors of ∼2.5–4. They also have a larger range of central (∼400 pc) dark matter densities than the DMO runs (ρ_(FIRE)/ρ_(DMO) ≃ 0.5–3) owing to the competing effects of baryonic contraction and feedback. At 3 deg from the Galactic Centre, FIRE J-factors are 3–60 (p-wave) and 10–500 (d-wave) times higher than in the DMO runs. The change in s-wave signal at 3 deg is more modest and can be higher or lower (∼0.3–7), though the shape of the emission profile is flatter (less peaked towards the Galactic Centre) and more circular on the sky in FIRE runs. Our results for s-wave are broadly consistent with the range of assumptions in most indirect detection studies. We observe p-wave J-factors that are significantly enhanced compared to most past estimates. We find that thermal models with p-wave annihilation may be within range of detection in the near future
Connecting galaxy evolution in clusters with their radial profiles and phase space distribution: results from the IllustrisTNG hydrodynamical simulations
No full textWe study the population of galaxies around galaxy clusters in the hydrodynamic simulation suite IllustrisTNG 300-1 to study the signatures of their evolutionary history on observable properties. We measure the radial number density profile, phase space distribution, and splashback radius for galaxies of different masses and colours over the redshift range z = 0−1. The three primary physical effects which shape the galaxy distribution within clusters are the galaxy quenching, angular momentum distribution, and dynamical friction. We find three distinct populations of galaxies by applying a Gaussian mixture model to their distribution in colour and mass. They have distinct evolutionary histories and leave distinct signatures on their distribution around cluster haloes. We find that low-mass red galaxies show the most concentrated distribution in clusters and the largest splashback radius, while high-mass red galaxies show a less concentrated distribution and a smaller splashback radius. Blue galaxies, which mostly quench into the low-mass red population, have the shallowest distribution within the clusters, with those on radial orbits quenched rapidly before reaching pericentre. Comparison with the distribution of galaxies from the Dark Energy Survey survey around Sunyaev–Zeldovich clusters from the Atacama Cosmology Telescope and South Pole Telescope surveys shows evidence for differences in galaxy evolution between simulations and data