CLOUDY modeling of weird far-IR emission in the central zone of the Helix Nebula [abstract]

Abstract only availableStars like the sun will evolve into objects that are comprised of a very hot white dwarf central star surrounded by a cloud of gas and dust, known as planetary nebulae. In PNe, UV radiation from the star creates an ionized region bounded by neutral gas and molecules. Hence, they are expected to have an onion-layer structure of concentric shells in which the level of ionization decreases with distance from the central star. The central zone of the Helix Nebula appears as a hole in the classic optical image, yet has been found to produce both He II and [OIV] emission. Strong emission has been observed at 60-100 microns, but is not detected in the mid-IR, or at 160 or 180 microns. The emission in this region is suspected to be due to cool dust grains. We present the results of modeling using the photo-ionization code, CLOUDY, to match this observed emission. Our best fit models fill the central zone with 1 micron sized grains, with a gas-to-dust ratio of 500. These grains are composed of both graphite and astronomical silicate, with little sensitivity to specific grain composition. Additional models were run to limit the distribution of grain sizes. Grains between 0.5-2 microns, with a power law distribution of a^(-3.5) fit within error, as did Gaussian distributions centered at 1 micron, with FWHM up to 0.6 microns, indicating that the dust grains must fall within a fairly narrow range of sizes. As previously predicted, the resulting grain temperatures in the central zone were around 30K. The effects of radiation pressure and Poynting-Robertson drag on dust grains surrounding the central star were also calculated; however, these calculations cannot explain the predicted size distribution. Thus, we also discuss the nature and origin of the dust grains in this region.MU Undergraduate Research Scholars Progra

NanoRocks: Design and Performance of an Experiment Studying Planet Formation on the International Space Station

In an effort to better understand the early stages of planet formation, we have developed a 1.5U payload that flew on the International Space Station (ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016. This payload, named NanoRocks, ran a particle collision experiment under long-term microgravity conditions. The objectives of the experiment were (a) to observe collisions between mm-sized particles at relative velocities of $<$1~cm/s, and (b) to study the formation and disruption of particle clusters for different particle types and collision velocities. Four types of particles were used: mm-sized acrylic, glass, and copper beads, and 0.75 mm-sized JSC-1 lunar regolith simulant grains. The particles were placed in sample cells carved out of an aluminum tray. This tray was attached to one side of the payload casing with three springs. Every 60~s, the tray was agitated and the resulting collisions between the particles in the sample cells were recorded by the experiment camera. During the 18 months the payload stayed on ISS, we obtained 158 videos, thus recording a great number of collisions. The average particle velocities in the sample cells after each shaking event were around 1 cm/s. After shaking stopped, the inter-particle collisions damped the particle kinetic energy in less than 20~s, reducing the average particle velocity to below 1 mm/s, and eventually slowing them to below our detection threshold. As the particle velocity decreased, we observed the transition from bouncing to sticking collisions. We recorded the formation of particle clusters at the end of each experiment run. This paper describes the design and performance of the NanoRocks ISS payload.Comment: 8 pages, 8 figure

Regolith behavior under asteroid-level gravity conditions: low-velocity impact experiments

The dusty regolith covering the surfaces of asteroids and planetary satellites differs in size, shape, and composition from terrestrial soil particles and is subject to very different environmental conditions. Experimental studies of the response of planetary regolith in the relevant environmental conditions are thus necessary to facilitate future Solar System exploration activities. We combined the results and provided new data analysis elements for a series of impact experiments into simulated planetary regolith in low-gravity conditions using two experimental setups: the Physics of Regolith Impacts in Microgravity Experiment (PRIME) and the COLLisions Into Dust Experiment (COLLIDE). Results of these experimental campaigns found that there is a significant change in the regolith behavior with the gravity environment. In a 10-2g environment (Lunar g levels), only embedding of the impactor was observed and ejecta production was produced for most impacts at > 20 cm/s. Once at microgravity levels (<10-4g), the lowest impact energies also produced impactor rebound. In these microgravity conditions, ejecta started to be produced for impacts at > 10 cm/s. The measured ejecta speeds were lower than the ones measured at reduced-gravity levels, but the ejected masses were higher. The mean ejecta velocity shows a power-law dependence on the impact energy with an index of ~0.7. When projectile rebound occurred, we observed that its coefficients of restitution on the bed of regolith simulant decrease by a factor of 10 with increasing impact speeds from ~5 cm/s up to 100 cm/s. We could also observe an increased cohesion between the JSC-1 grains compared to the quartz sand targets

Experimental investigations of the lunar photoelectron environment and related dust dynamics

Airless bodies in space are exposed to a variety of charging environments in which a balance of currents due to plasma processes determines the surface charge. In the inner solar system, photoelectron emission is the dominant charging process on sunlit surfaces due to the intense solar UV radiation. This results in a positive surface potential with a photoelectron sheath above the surface. Conversely, the unlit side of the body will charge negatively due the collection of the fast-moving solar wind electrons. The interaction of charged dust grains with these positively and negatively charged surfaces, and with the photoelectron and plasma sheaths, may explain the occurrence of dust lofting, levitation and transport above the lunar surface and on other airless bodies. This dust has been recognized as a potentially great hazard to future exploration of dusty planetary surfaces, due to its abrasive and adhesive nature. An initial investigation explores the mechanisms that control adhesion of dust grains to insulating and conducting surfaces. Unfortunately, there is little known about the mechanisms of adhesion on widely varying surface types, but van der Waals and electrostatic forces are the dominant forces that are taken into consideration in this study, which measures the adhesive forces between ≤ 25 μm JSC-1 lunar simulant grains and various surfaces vacuum using a centrifugal force detachment method. UV irradiation effects on surface adhesion were also examined. In order to better understand the plasma processes at work on sunlit surfaces, we have performed laboratory experiments to study the physics of photoelectron sheaths above both conducting and insulating surfaces in vacuum. The first set of experiments determines the characteristics of photoelectron sheaths generated over a conducting Zr surface that is large in comparison to the Debye length of the sheath. These characteristics are derived from cylindrical Langmuir probe measurements, and are compared with the results from a 1D PIC-code simulation to gain a greater understanding of the sheath physics. To study the photoelectron sheath above an insulating material, a portion of this conducting surface is covered with insulating material. CeO2 is used both in powdered and solid disk form, and un-sieved JSC-1 is used to represent planetary surfaces. Electron densities and temperatures of the photoelectron plasma are measured with a single-sided planar Langmuir probe. The measurements taken above the CeO2 are compared with those taken above the Zr to observe the differences in photoemission, and to determine how the insulating surface modifies the structure of the photoelectron sheath. The densities above the surfaces are only found to have a modest dependence on the surrounding surface bias, and the plasma potentials measured above the insulating surfaces are significantly different than those above the Zr, due to the fact that the insulating materials float to an equilibrium potential independent of the surrounding surface bias. These measurements indicate that plasma probes above a planetary body can accurately determine potentials and densities above the surfaces, valuable information for understanding the charging environment of spacecraft and other objects

Cross-Coupling Biarylation of Nitroaryl Chlorides Through High Speed Ball Milling

Solvent-free reaction using a high-speed ball milling technique has been applied to the classical Ullmann coupling reaction. Cross-coupling biarylation of several nitroaryl chlorides was achieved in good yields when performed in custom-made copper vials through continuous shaking without additional copper or solvent. Cross-coupling products were obtained almost pure and NMR-ready. These reactions were cleaner than solution phase coupling which require longer reaction time in high boiling solvents, and added catalysts as well as lengthy extraction and purification steps. Gram quantities of cross biaryl compounds have been synthesized with larger copper vials, a proof that this method can be used to reduce industrial waste and for sustainability

Pathogenic Mechanism of the FIG4 Mutation Responsible for Charcot-Marie-Tooth Disease CMT4J

CMT4J is a severe form of Charcot-Marie-Tooth neuropathy caused by mutation of the phosphoinositide phosphatase FIG4/SAC3. Affected individuals are compound heterozygotes carrying the missense allele FIG4-I41T in combination with a null allele. Analysis using the yeast two-hybrid system demonstrated that the I41T mutation impairs interaction of FIG4 with the scaffold protein VAC14. The critical role of this interaction was confirmed by the demonstration of loss of FIG4 protein in VAC14 null mice. We developed a mouse model of CMT4J by expressing a Fig4-I41T cDNA transgene on the Fig4 null background. Expression of the mutant transcript at a level 5× higher than endogenous Fig4 completely rescued lethality, whereas 2× expression gave only partial rescue, providing a model of the human disease. The level of FIG4-I41T protein in transgenic tissues is only 2% of that predicted by the transcript level, as a consequence of the protein instability caused by impaired interaction of the mutant protein with VAC14. Analysis of patient fibroblasts demonstrated a comparably low level of mutant I41T protein. The abundance of FIG4-I41T protein in cultured cells is increased by treatment with the proteasome inhibitor MG-132. The data demonstrate that FIG4-I41T is a hypomorphic allele encoding a protein that is unstable in vivo. Expression of FIG4-I41T protein at 10% of normal level is sufficient for long-term survival, suggesting that patients with CMT4J could be treated by increased production or stabilization of the mutant protein. The transgenic model will be useful for testing in vivo interventions to increase the abundance of the mutant protein

Experimental investigations of the lunar photoelectron environment and related dust dynamics

Airless bodies in space are exposed to a variety of charging environments in which a balance of currents due to plasma processes determines the surface charge. In the inner solar system, photoelectron emission is the dominant charging process on sunlit surfaces due to the intense solar UV radiation. This results in a positive surface potential with a photoelectron sheath above the surface. Conversely, the unlit side of the body will charge negatively due the collection of the fast-moving solar wind electrons. The interaction of charged dust grains with these positively and negatively charged surfaces, and with the photoelectron and plasma sheaths, may explain the occurrence of dust lofting, levitation and transport above the lunar surface and on other airless bodies. This dust has been recognized as a potentially great hazard to future exploration of dusty planetary surfaces, due to its abrasive and adhesive nature. An initial investigation explores the mechanisms that control adhesion of dust grains to insulating and conducting surfaces. Unfortunately, there is little known about the mechanisms of adhesion on widely varying surface types, but van der Waals and electrostatic forces are the dominant forces that are taken into consideration in this study, which measures the adhesive forces between ≤ 25 μm JSC-1 lunar simulant grains and various surfaces vacuum using a centrifugal force detachment method. UV irradiation effects on surface adhesion were also examined. In order to better understand the plasma processes at work on sunlit surfaces, we have performed laboratory experiments to study the physics of photoelectron sheaths above both conducting and insulating surfaces in vacuum. The first set of experiments determines the characteristics of photoelectron sheaths generated over a conducting Zr surface that is large in comparison to the Debye length of the sheath. These characteristics are derived from cylindrical Langmuir probe measurements, and are compared with the results from a 1D PIC-code simulation to gain a greater understanding of the sheath physics. To study the photoelectron sheath above an insulating material, a portion of this conducting surface is covered with insulating material. CeO 2 is used both in powdered and solid disk form, and un-sieved JSC-1 is used to represent planetary surfaces. Electron densities and temperatures of the photoelectron plasma are measured with a single-sided planar Langmuir probe. The measurements taken above the CeO2 are compared with those taken above the Zr to observe the differences in photoemission, and to determine how the insulating surface modifies the structure of the photoelectron sheath. The densities above the surfaces are only found to have a modest dependence on the surrounding surface bias, and the plasma potentials measured above the insulating surfaces are significantly different than those above the Zr, due to the fact that the insulating materials float to an equilibrium potential independent of the surrounding surface bias. These measurements indicate that plasma probes above a planetary body can accurately determine potentials and densities above the surfaces, valuable information for understanding the charging environment of spacecraft and other objects