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    Radial substructures have now been observed in a wide range of protoplanetary discs (PPDs), from young to old systems; however, their formation is still an area of vigorous debate. Recent magnetohydrodynamic (MHD) simulations have shown that rings and gaps can form naturally in PPDs when non-ideal MHD effects are included. However, these simulations employ ad hoc approximations to the magnitudes of the magnetic diffusivities in order to facilitate ring growth. We replace the parametrization of these terms with a simple chemical network and grain distribution model to calculate the non-ideal effects in a more self-consistent way. We use a range of grain distributions to simulate grain formation for different disc conditions. Including ambipolar diffusion, we find that large grain populations (>1 $\mu$m), and those including a population of very small polyaromatic hydrocarbons (PAHs) facilitate the growth of periodic, stable rings, while intermediate-sized grains suppress ring formation. Including Ohmic diffusion removes the positive influence of PAHs, with only large grain populations still producing periodic ring and gap structures. These results relate closely to the degree of coupling between the magnetic field and the neutral disc material, quantified by the non-dimensional Elsasser number Λ (the ratio of magnetic forces to Coriolis force). For both the ambipolar-only and ambipolar-ohmic cases, if the total Elsasser number is initially of the order of unity along the disc mid-plane, ring and gap structures may develop.

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  2. Abstract

    We use the conjugate angle of radial action (θR), the best representation of the orbital phase, to explore the “midplane,” “north branch,” “south branch,” and “Monoceros area” disk structures that were previously revealed in the LAMOST K giants. The former three substructures, identified by their 3D kinematical distributions, have been shown to be projections of the phase space spiral (resulting from nonequilibrium phase mixing). In this work, we find that all of these substructures associated with the phase spiral show high aggregation in conjugate angle phase space, indicating that the clumping in conjugate angle space is a feature of ongoing, incomplete phase mixing. We do not find theZVZphase spiral located in the “Monoceros area,” but we do find a very highly concentrated substructure in the quadrant of conjugate angle space with the orbital phase from the apocenter to the guiding radius. The existence of the clump in conjugate angle space provides a complementary way to connect the “Monoceros area” with the direct response to a perturbation from a significant gravitationally interactive event. Using test particle simulations, we show that these features are analogous to disturbances caused by the impact of the last passage of the Sagittarius dwarf spheroidal galaxy.

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    Free, publicly-accessible full text available October 1, 2024
  3. Using transdimensional plasmonic materials (TDPM) within the framework of fluctuational electrodynamics, we demonstrate nonlocality in dielectric response alters near-field heat transfer at gap sizes on the order of hundreds of nanometers. Our theoretical study reveals that, opposite to the local model prediction, propagating waves can transport energy through the TDPM. However, energy transport by polaritons at shorter separations is reduced due to the metallic response of TDPM stronger than that predicted by the local model. Our experiments conducted for a configuration with a silica sphere and a doped silicon plate coated with an ultrathin layer of platinum as the TDPM show good agreement with the nonlocal near-field radiation theory. Our experimental work in conjunction with the nonlocal theory has important implications in thermophotovoltaic energy conversion, thermal management applications with metal coatings, and quantum-optical structures. 
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    Free, publicly-accessible full text available August 1, 2024
  4. Calcium ion transporting systems control cytosol Ca2+ levels ([Ca2+]cyt) and generate transient calcium (Ca2+) signatures which are considered to be key to environmental responses. Here, we report an impact of resting [Ca2+]cyt on plants from the functional study of calmodulin regulated Ca2+ pumps or Ca2+-ATPases in Arabidopsis thaliana. The plasma membrane localized pumps ACA8 and ACA10 as well as the vacuole localized pumps ACA4 and ACA11 are found to be critical in maintaining low resting [Ca2+]cyt and be essential for plant survival under chilling and heat-stress conditions. Their loss-of-function mutants aca8 aca10 and aca4 aca11 have autoimmunity at normal temperature, and this deregulated immune activation is enhanced by low temperature leading to chilling lethality. Furthermore, these two mutants have an elevated resting [Ca2+]cyt, and a reduction of external Ca2+ lowers [Ca2+]cyt and represses its autoimmunity and cold susceptibility. The aca8 aca10 and the aca4 aca11 mutants are also susceptible to heat, likely resulting from more closed stomata and higher leaf surface temperature compared to the wild type. These observations support a model in which the regulation of resting [Ca2+]cyt is critical to how plants regulate biotic and abiotic responses. 
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  5. Dynamic allostery emphasizes a role of entropy change manifested as a sole change in protein fluctuations without structural changes. This kind of entropy-driven effect remains largely understudied. The most significant examples involve protein-ligand interactions, leaving protein-protein interactions, which are critical in signaling and other cellular events, largely unexplored. Here we study an example of how protein-protein interaction (binding of Ras to the Ras binding domain [RBD] of the effector protein Raf) affects a subsequent protein association process (Ras dimerization) by quenching Ras internal motions through dynamic allostery. We also investigate the influence of point mutations or ambient temperature, respectively, on the protein dynamics and interaction of two other systems: in adenylate kinase (ADK) and in the EphA2 SAM:Ship2 SAM complex. Based on these examples, we postulate that there are different ways in which dynamic-change-driven protein interactions are manifested and that it is likely a general biological phenomenon. 
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