New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-

Extension of a de novo TIM barrel with a rationally designed secondary structure element

The ability to construct novel enzymes is a major aim in de novo protein design. A popular enzyme fold for design attempts is the TIM barrel. This fold is a common topology for enzymes and can harbor many diverse reactions. The recent de novo design of a four-fold symmetric TIM barrel provides a well understood minimal scaffold for potential enzyme designs. Here we explore opportunities to extend and diversify this scaffold by adding a short de novo helix on top of the barrel. Due to the size of the protein, we developed a design pipeline based on computational ab initio folding that solves a less complex sub-problem focused around the helix and its vicinity and adapt it to the entire protein. We provide biochemical characterization and a high-resolution X-ray structure for one variant and compare it to our design model. The successful extension of this robust TIM-barrel scaffold opens opportunities to diversify it towards more pocket like arrangements and as such can be considered a building block for future design of binding or catalytic sites

Mineralogical characterisation of naturally floatable gangue in Merensky Reef ore flotation.

During the recovery of the valuable platinum group minerals and base metal sulphides in the Merensky Reef ore by flotation, a certain amount of naturally floatable gangue (NFG) is recovered, which dilutes the concentrate. Although talc is the only silicate mineral well known for it naturally floatable properties, the proportion of NFG recovered in batch flotation tests of the Merensky ore, however, cannot be solely accounted for by the mass of pure talc present in the feed ore. Using methodology developed at the University of Cape Town to decouple the NFG from entrained gangue recovered in flotation, a series of concentrate samples have been analysed in this study using quantitative mineralogical analysis methods. Results show the major diluents of concentrate grade are orthopyroxene, talc, clinopyroxene and plagioclase in decreasing order of abundance. Orthopyroxene is however, by far the greatest diluent of concentrate grade even though the particles recovered are classified as liberated or high grade middlings. On closer examination, these orthopyroxene particles show a preferential surface association to talc and vice versa which is consistent with the low temperature alteration of anhydrous orthopyroxene to a hydrous phase, namely talc. These composite orthopyroxene particles with partial talc rims are therefore thought to be one of the main contributors to NFG in the Merensky Reef ore

Electrochemical interactions of some platinum group minerals with flotation reagents

The recovery of base metal sulfides has been emphasized in the study of the flotation of platinum group minerals (PGMs). However, in some ore bodies such as the Platreef of the Bushveld Complex, sulfide minerals do not make up the majority of the more significant PGM mineralization. It is therefore important to develop a fundamental understanding of how non-sulfide and sulfide PGMs respond respectively to the current reagents being used in the flotation of these minerals. The interactions between platinum and palladium sulfides and tellurides with the thiol collectors, ethyl xanthate and diethyl dithiophosphate, have been studied using the electrochemical techniques of mineral rest potential and voltammetry measurements. The differences in the interaction between the two types of minerals with these reagents were of key interest. Rest potential measurements indicated stronger electrochemical interactions of the minerals with ethyl xanthate than diethyl dithiophosphate. These measurements also indicated that there are different forms of oxidized collector present on the mineral surfaces after interaction with ethyl xanthates. Voltammetry measurements support these observations and suggest differences of adsorption on Pt and Pd sulfides than on Pt and Pd tellurides. Voltammetry also showed that there was minimal electrochemical interaction between the Pt and Pd minerals and diethyl dithiophosphate

An investigation into electrochemical interactions between platinum group minerals and xanthate: Voltammetric study

In the flotation of platinum group minerals (PGMs) containing significant amounts of tellurides and arsenides it is generally assumed that these minerals will respond in the same way as sulphides to collectors such as xanthates which are typically used in sulphide flotation. This paper presents the results of a fundamental study which has been conducted to provide a greater insight into the comparative interactions between synthetic moncheite (PtTe2) and cooperite (PtS) with sodium ethyl xanthate (SEX). Cyclic voltammetry has been used to investigate these interactions in the absence and presence of sodium ethyl xanthate (SEX) in aerated and de-aerated solutions. Reduction potentials of the SEX/dixanthogen couple have been measured and compared to published data. Reduction potentials of possible reactions of the minerals have been predicted from thermodynamic calculations and used to attempt to interpret the potentials measured in the cyclic voltammetry investigations. Results have shown differences in the oxidation behaviour of the minerals as well as in the SEX interaction with the minerals. The effect of pH was investigated in the case of the Pt minerals and fractional surface coverages were calculated with a view to ultimately correlate these results with flotation behaviour. Proposals are made with respect to the various reactions occurring under the conditions studied

An investigation into the electrochemical interactions between platinum group minerals and sodium ethyl xanthate and sodium diethyl dithiophosphate collectors: Mixed potential study

In the flotation of platinum group minerals (PGMs) occurring in the Platreef ore body, it is often assumed that the interactions between thiol collectors and tellurides of Pt and Pd are similar to those occurring with the equivalent sulphides. In order to gain a greater insight into this assumption a rest(mixed) potential study was conducted to investigate the interactions occurring between two thiol collectors, viz. sodium ethyl xanthate (SEX) and sodium diethyl dithiophosphate (SEDTP), and selected PGMs, viz. moncheite (PtTe2), merenskyite (PdTe2), cooperite (PtS) and vysotskite (PdS) as well as pure platinum and palladium. The results have shown that all these minerals reacted with the collectors in question but that the rest potential of the Pt minerals are more anodic than those of the Pd minerals. Moreover when SEX was used, dixanthogen is more likely to form on PtS than PtTe2 but the reverse is the case for PdTe2 and PdS. In the case of SEDTP it was found that the dithiolate (SEDTP2) did not form on any of the mineral surfaces. The results were compared to those already reported in a previous investigation of the same systems using cyclic voltammetry. The information obtained in these two investigations complement each other and may provide a more fundamental understanding of the role of these collectors in recovering the respective minerals

In vitro biodegradation testing of Mg-alloy EZK400 and manufacturing of implant prototypes using PM (powder metallurgy) methods

The study is focussing towards Metal Injection Moulding (MIM) of Mg-alloys for biomedical implant applications. Especially the influence of the sintering processing necessary for the consolidation of the finished part is in focus of this study. In doing so, the chosen high strength EZK400 Mg-alloy powder material was sintered using different sintering support bottom plate materials to evaluate the possibility of iron impurity pick up during sintering. It can be shown that iron pick up took place from the steel bottom plate into the specimen. Despite the fact that a separating boron nitrite (BN) barrier layer was used and the Mg-Fe phase diagram is not predicting any significant solubility to each other. As a result of this study a new bottom plate material not harming the sintering and the biodegradation performance of the as sintered material, namely a carbon plate material, was found. Keywords: Magnesium, Mg alloy, Sintering, MIM, Powder metallurgy, Biodegradable implant, EZK40