Rapid Technique for Liquid Scintillation Counting of Carbon-14-labelled Barium Carbonate

Rapid technique for liquid scintillation counting of carbon-14-labelled barium carbonat

Sub-Nanometer Catalyst Clusters for Propane Dehydrogenation

Propane dehydrogenation (PDH) is used to produce propene, which is the primary building block for many commercial plastics. The catalyst most commonly used for this reaction is platinum. Due to rising demand for propene, an alternative catalyst is being sought due to platinum’s high cost. Alternatives might involve very small platinum particles as well as particles composed of different atoms. For this purpose, we have performed a computational study of the PDH reaction with a 4 atom platinum cluster (Pt4) and several different 4-atom transition metal cluster (TM4) catalysts on a graphene support. We have computed the equilibrium structures of the Pt4 and TM4 clusters on both single-and double-vacancy sites and have calculated the complete PDH reaction pathway for each case. This allowed us to study the effect of the graphene support on catalytic activity. We have also calculated the PDH reaction pathway for larger Ptx clusters, where x = 5-14, in order to study the effect of particle size on catalytic activity. These results help clarify the relationship between the PDH activation energy and the propane binding energy and overall reaction energy and may aid in the design of new potential catalysts for the PDH reaction

Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres

The origin of the near-ultraviolet and optical continuum radiation in flares is critical for understanding particle acceleration and impulsive heating in stellar atmospheres. Radiative-hydrodynamic simulations in 1D have shown that high energy deposition rates from electron beams produce two flaring layers at T~10^4 K that develop in the chromosphere: a cooling condensation (downflowing compression) and heated non-moving (stationary) flare layers just below the condensation. These atmospheres reproduce several observed phenomena in flare spectra, such as the red wing asymmetry of the emission lines in solar flares and a small Balmer jump ratio in M dwarf flares. The high beam flux simulations are computationally expensive in 1D, and the (human) timescales for completing NLTE models with adaptive grids in 3D will likely be unwieldy for a time to come. We have developed a prescription for predicting the approximate evolved states, continuum optical depth, and the emergent continuum flux spectra of radiative-hydrodynamic model flare atmospheres. These approximate prescriptions are based on an important atmospheric parameter: the column mass (m_ref) at which hydrogen becomes nearly completely ionized at the depths that are approximately in steady state with the electron beam heating. Using this new modeling approach, we find that high energy flux density (>F11) electron beams are needed to reproduce the brightest observed continuum intensity in IRIS data of the 2014-Mar-29 X1 solar flare and that variation in m_ref from 0.001 to 0.02 g/cm2 reproduces most of the observed range of the optical continuum flux ratios at the peaks of M dwarf flares.Comment: 29 pages, 9 figures, accepted for publication in the Astrophysical Journa

Temperament in the Classroom

Variance in academic performance that persists when situational variables are held constant suggests that whether students fail or thrive depends not only on circumstance, but also on relatively stable individual differences in how children respond to circumstance. More academically talented children generally outperform their less able peers, but much less is known about how traits unrelated to general intelligence influence academic outcomes. This paper addresses several related questions: What insights can be gleaned from historical interest in the role of temperament in the classroom? What does recent empirical research say about the specific dimensions of temperament most important to successful academic performance? In particular, which aspects of temperament most strongly influence school readiness, academic achievement, and educational attainment? What factors mediate and moderate associations between temperament and academic outcomes? What progress has been made in deliberately cultivating aspects of temperament that matter most to success in school? And, finally, for researchers keenly interested in better understanding how and why temperament influences academic success, in which direction does future progress lie?

A Unified Computational Model for Solar and Stellar Flares

We present a unified computational framework which can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index.Comment: Accepted for publication by the Astrophysical Journa

A White Light Megaflare on the dM4.5e Star YZ CMi

On UT 2009 January 16, we observed a white light megaflare on the dM4.5e star YZ CMi as part of a long-term spectroscopic flare-monitoring campaign to constrain the spectral shape of optical flare continuum emission. Simultaneous U-band photometric and 3350A-9260A spectroscopic observations were obtained during 1.3 hours of the flare decay. The event persisted for more than 7 hours and at flare peak, the U-band flux was almost 6 magnitudes brighter than in the quiescent state. The properties of this flare mark it as one of the most energetic and longest-lasting white light flares ever to be observed on an isolated low-mass star. We present the U-band flare energetics and a flare continuum analysis. For the first time, we show convincingly with spectra that the shape of the blue continuum from 3350A to 4800A can be represented as a sum of two components: a Balmer continuum as predicted by the Allred et al radiative hydrodynamic flare models and a T ~ 10,000K blackbody emission component as suggested by many previous studies of the broadband colors and spectral distributions of flares. The areal coverage of the Balmer continuum and blackbody emission regions vary during the flare decay, with the Balmer continuum emitting region always being significantly (~3-16 times) larger. These data will provide critical constraints for understanding the physics underlying the mysterious blue continuum radiation in stellar flares.Comment: 12 pages, 4 figures, accepted by Astrophysical Journal Letter