Gravitational Settling of ^{22}Ne in Liquid White Dwarf Interiors--Cooling and Seismological Effects

We assess the impact of the trace element ^{22}Ne on the cooling and seismology of a liquid C/O white dwarf (WD). Due to this elements' neutron excess, it sinks towards the interior as the liquid WD cools. The subsequent gravitational energy released slows the cooling of the WD by 0.25--1.6 Gyrs by the time it has completely crystallized, depending on the WD mass and the adopted sedimentation rate. The effects will make massive WDs or those in metal rich clusters (such as NGC 6791) appear younger than their true age. Our diffusion calculations show that the ^{22}Ne mass fraction in the crystallized core actually increases outwards. The stability of this configuration has not yet been determined. In the liquid state, the settled ^{22}Ne enhances the internal buoyancy of the interior and changes the periods of the high radial order g-modes by approximately 1%. Though a small adjustment, this level of change far exceeds the accuracy of the period measurements. A full assessment and comparison of mode frequencies for specific WDs should help constrain the still uncertain ^{22}Ne diffusion coefficient for the liquid interior.Comment: 26 pages (11 text pages with 15 figures); to appear in The Astrophysical Journa

Efficient low-power terahertz generation via on-chip triply-resonant nonlinear frequency mixing

Achieving efficient terahertz (THz) generation using compact turn-key sources operating at room temperature and modest power levels represents one of the critical challeges that must be overcome to realize truly practical applications based on THz. Up to now, the most efficient approaches to THz generation at room temperature -- relying mainly on optical rectification schemes -- require intricate phase-matching set-ups and powerful lasers. Here we show how the unique light-confining properties of triply-resonant photonic resonators can be tailored to enable dramatic enhancements of the conversion efficiency of THz generation via nonlinear frequency down-conversion processes. We predict that this approach can be used to reduce up to three orders of magnitude the pump powers required to reach quantum-limited conversion efficiency of THz generation in nonlinear optical material systems. Furthermore, we propose a realistic design readily accesible experimentally, both for fabrication and demonstration of optimal THz conversion efficiency at sub-W power levels

The Borexino Thermal Monitoring & Management System and simulations of the fluid-dynamics of the Borexino detector under asymmetrical, changing boundary conditions

A comprehensive monitoring system for the thermal environment inside the Borexino neutrino detector was developed and installed in order to reduce uncertainties in determining temperatures throughout the detector. A complementary thermal management system limits undesirable thermal couplings between the environment and Borexino's active sections. This strategy is bringing improved radioactive background conditions to the region of interest for the physics signal thanks to reduced fluid mixing induced in the liquid scintillator. Although fluid-dynamical equilibrium has not yet been fully reached, and thermal fine-tuning is possible, the system has proven extremely effective at stabilizing the detector's thermal conditions while offering precise insights into its mechanisms of internal thermal transport. Furthermore, a Computational Fluid-Dynamics analysis has been performed, based on the empirical measurements provided by the thermal monitoring system, and providing information into present and future thermal trends. A two-dimensional modeling approach was implemented in order to achieve a proper understanding of the thermal and fluid-dynamics in Borexino. It was optimized for different regions and periods of interest, focusing on the most critical effects that were identified as influencing background concentrations. Literature experimental case studies were reproduced to benchmark the method and settings, and a Borexino-specific benchmark was implemented in order to validate the modeling approach for thermal transport. Finally, fully-convective models were applied to understand general and specific fluid motions impacting the detector's Active Volume.Comment: arXiv admin note: substantial text overlap with arXiv:1705.09078, arXiv:1705.0965

The Detonation Mechanism of the Pulsationally-Assisted Gravitationally-Confined Detonation Model of Type Ia Supernovae

We describe the detonation mechanism comprising the "Pulsationally Assisted" Gravitationally Confined Detonation (GCD) model of Type Ia supernovae SNe Ia. This model is analogous to the previous GCD model reported in Jordan et al.(2008); however, the chosen initial conditions produce a substantively different detonation mechanism, resulting from a larger energy release during the deflagration phase. The resulting final kinetic energy and nickel-56 yields conform better to observational values than is the case for the "classical" GCD models. In the present class of models, the ignition of a deflagration phase leads to a rising, burning plume of ash. The ash breaks out of the surface of the white dwarf, flows laterally around the star, and converges on the collision region at the antipodal point from where it broke out. The amount of energy released during the deflagration phase is enough to cause the star to rapidly expand, so that when the ash reaches the antipodal point, the surface density is too low to initiate a detonation. Instead, as the ash flows into the collision region (while mixing with surface fuel), the star reaches its maximally expanded state and then contracts. The stellar contraction acts to increase the density of the star, including the density in the collision region. This both raises the temperature and density of the fuel-ash mixture in the collision region and ultimately leads to thermodynamic conditions that are necessary for the Zel'dovich gradient mechanism to produce a detonation. We demonstrate feasibility of this scenario with three 3-dimensional (3D), full star simulations of this model using the FLASH code. We characterized the simulations by the energy released during the deflagration phase, which ranged from 38% to 78% of the white dwarf's binding energy. We show that the necessary conditions for detonation are achieved in all three of the models.Comment: 22 pages, 8 figures; Ap

Corrigendum to “Phylogeography of the Variable Antshrike (Thamnophilus caerulescens), a South American passerine distributed along multiple environmental gradients” [Mol. Phylogenet. Evol. 148 (2020) 106810](S1055790320300828)(10.1016/j.ympev.2020.106810)

© 2020 Elsevier Inc. The authors regret: (a) The information of Table 2 has incorrect values in column 6. The following table has the correct values. (b) Also, there is a minor typo in the section ‘Data accessibility’, in the word: ‘alignmentsare’. The correct words are: ‘alignments are’. The authors would like to apologize for any inconvenience caused

Phylogeography of the Variable Antshrike (Thamnophilus caerulescens), a South American passerine distributed along multiple environmental gradients

© 2020 Elsevier Inc. The Neotropics show a wealth of distributional patterns shared by many co-distributed species. A distinctive pattern is the so-called “circum-Amazonian distribution,” which is observed in species that do not occur in Amazonia but rather along a belt of forested habitats spanning south and east of Amazonia, the Andean foothills, and often into the Venezuelan Coastal Range and the Tepuis. Although this pattern is widespread across animals and plants, its underlying biogeographic mechanisms remain poorly understood. The Variable Antshrike (Thamnophilus caerulescens) is a sexually dimorphic suboscine passerine that exhibits extreme plumage variation and occurs along the southern portion of the circum-Amazonian belt. We describe broad-scale phylogeographic patterns of T. caerulescens and assess its demographic history using DNA sequences from the mitochondrion and ultraconserved elements (UCEs). We identified three genomic clusters: a) northern Atlantic Forest; b) southeastern Cerrado and central-southern Atlantic Forest, and c) Chaco and Andes. Our results were consistent with Pleistocene divergence followed by gene flow, mainly between the latter two clusters. There were no genetic signatures of rapid population expansions or bottlenecks. The population from the northern Atlantic Forest was the most genetically divergent group within the species. The demographic history of T. caerulescens was probably affected by series of humid and dry periods throughout the Quaternary that generated subtle population expansions and contractions allowing the intermittent connection of habitats along the circum-Amazonian belt. Recognizing the dynamic history of climate-mediated forest expansions, contractions, and connections during the South American Pleistocene is central toward a mechanistic understanding of circum-Amazonian distributions

Fast Rossi-alpha Measurements of Plutonium using Organic Scintillators

In this work, Rossi-alpha measurements were simultaneously performed with a $^3$He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt$\%$ $^{239}$Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate $k_\text{eff}$ = 0.624 and $\alpha$ = 52.3 $\pm$ 2.5 ns for the measured assembly. The organic scintillator system estimated $\alpha$ = 47.4 $\pm$ 2.0 ns, having a 9.37$\%$ error (though the 1.09 standard deviation confidence intervals overlapped). The $^3$He system estimated $\alpha$ = 37 $\mu$s. The known slowing down time of the $^3$He system is 35-40 $\mu$s, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 $\mu$s.Comment: PHYSOR 2020: Transition to a Scalable Nuclear Future Cambridge, United Kingdom, March 29th-April 2nd, 202

[Comparison of socio-demographic characteristics of patients with PTSD in the four countries of former Yugoslavia following the war].

OBJECTIVE: To determine the socio-demographic characteristics of survivors who presented for treatment in therapeutic centers in the four Balkan countries involved in the study by summarizing the results of STOP study ("Treatment Seeking and Treatment Outcomes in People Suffering from PTSD Following the War and Migration in the Balkans"). STOP study was performed in the following countries: Bosnia and Herzegovina (specialized center Sarajevo), Croatia (centers in Zagreb and Rijeka), Serbia and Montenegro (center in Belgrade), Germany (center in Dresden) and United Kingdom (center in London). METHODS: The authors applied a socio-demographic questionnaire in patients with PTSD who presented for treatment in the centers. RESULTS: The authors present the differences among the socio-demographic characteristics of survivors who presented for treatment in therapeutic centers in the four Balkan countries involved in the study according to: gender, status (refugee, war veterans, refugee and war veteran, civilian), ethnic background, employment status, marital status and household monthly net income. CONCLUSIONS: Comparative statistical analysis of data obtained through interviews in four countries shows significant differences across the centers concerning socio-economic characteristics of the patients

Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

On 2017 August 17, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively-powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the Universe.Comment: Accepted to Scienc

Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger

On 2017 August 17, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from $11,000^{+3400}_{-900}$ K to $9300^{+300}_{-300}$ K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.Comment: 33 pages, 5 figures, 2 tables, Accepted to Scienc