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  1. Forest response to rising CO 2 drives zonally asymmetric rainfall change over tropical land

    Understanding how anthropogenic CO 2 emissions will influence future precipitation is critical for sustainably managing ecosystems, particularly for drought-sensitive tropical forests. Although tropical precipitation change remains uncertain, nearly all models from the Coupled Model Intercomparison Project Phase 5 predict a strengthening zonal precipitation asymmetry by 2100, with relative increases over Asian and African tropical forests and decreases over South American forests. Here we show that the plant physiological response to increasing CO 2 is a primary mechanism responsible for this pattern. Applying a simulation design in the Community Earth System Model in which CO 2 increases are isolated over individualmore » continents, we demonstrate that different circulation, moisture and stability changes arise over each continent due to declines in stomatal conductance and transpiration. The sum of local atmospheric responses over individual continents explains the pan-tropical precipitation asymmetry. Our analysis suggests that South American forests may be more vulnerable to rising CO 2 than Asian or African forests.« less
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  2. Is HESS J1912+101 Associated with an Old Supernova Remnant?

    HESS J1912+101 is a shell-like TeV source that has no clear counterpart in multiwavelength. Using CO and H i data, we reveal that V {sub LSR} ∼ +60 km s{sup −1} molecular clouds (MCs), together with shocked molecular gas and high-velocity neutral atomic shells, are concentrated toward HESS J1912+101. The prominent wing profiles up to V {sub LSR} ∼ +80 km s{sup −1} seen in {sup 12}CO ( J = 1–0 and J = 3–2) data, as well as the high-velocity expanding H i shells up to V {sub LSR} ∼ +100 km s{sup −1}, exhibit striking redshifted-broadening relative tomore » the quiescent gas. These features provide compelling evidences for large-scale perturbation in the region. We argue that the shocked MCs and the high-velocity H i shells may originate from an old supernova remnant (SNR). The distance to the SNR is estimated to be ∼4.1 kpc based on the H i self-absorption method, which leads to a physical radius of 29.0 pc for the ∼(0.7–2.0) × 10{sup 5} years old remnant with an expansion velocity of ≳40 km s{sup −1}. The +60 km s{sup −1} MCs and the disturbed gas are indeed found to coincide with the bright TeV emission, supporting the physical association between them. Naturally, the shell-like TeV emission comes from the decay of neutral pions produced by interactions between the accelerated hadrons from the SNR and the surrounding high-density molecular gas.« less
  3. THE ROLE OF THE DIFFUSIVE PROTONS IN THE GAMMA-RAY EMISSION OF SUPERNOVA REMNANT RX J1713.7–3946—A TWO-ZONE MODEL

    RX J1713.7−3946 is a prototype in the γ-ray-bright supernova remnants (SNRs) and is in continuing debates on its hadronic versus leptonic origin of the γ-ray emission. We explore the role played by the diffusive relativistic protons that escape from the SNR shock wave in the γ-ray emission, apart from the high-energy particles’ emission from the inside of the SNR. In the scenario that the SNR shock propagates in a clumpy molecular cavity, we consider that the γ-ray emission from the inside of the SNR may arise either from the inverse Compton scattering or from the interaction between the trapped energetic protons and themore » shocked clumps. The dominant origin between them depends on the electron-to-proton number ratio. The diffusive protons that escaped from the shock wave during the expansion history can provide an outer hadronic γ-ray component by bombarding the surrounding dense matter. The broadband spectrum can be well explained by this two-zone model, in which the γ-ray emission from the inside governs the TeV band, while the outer emission component substantially contributes to the GeV γ-rays. The two-zone model can also explain the TeV γ-ray radial brightness profile that significantly stretches beyond the nonthermal X-ray-emitting region. In the calculation, we present a simplified algorithm for Li and Chen's “accumulative diffusion” model for escaping protons and apply the Markov Chain Monte Carlo method to constrain the physical parameters.« less
  4. Exploring the structural changes on excitation of a luminescent organic bromine-substituted complex by in-house time-resolved pump-probe diffraction

    The structural changes accompanying the excitation of the luminescent dibromobenzene derivative, 1,4-dibromo-2,5-bis(octyloxy)benzene, have been measured by in-house monochromatic time-resolved (TR) diffraction at 90 K. Results show an increment of the very short intermolecular Br•••Br contact distance from 3.290 Å to 3.380 Å. These calculations show the Br…Br interaction to be strongly repulsive in both the Ground and Excited states but significantly relaxed by the lengthening of the contact distance on excitation. The stability of the crystals is attributed to the many weak C-H···Br and C-H···π intermolecular interactions. Our study described is the first practical application of In-House Time-Resolved diffraction, mademore » possible by the continuing increase in the brightness of X-ray sources and the sensitivity of our detectors.« less
  5. INTERACTION BETWEEN THE SUPERNOVA REMNANT HB 3 AND THE NEARBY STAR-FORMING REGION W3

    We performed millimeter observations of CO lines toward the supernova remnant (SNR) HB 3. Substantial molecular gas around −45 km s{sup −1} is detected in the conjunction region between the SNR HB 3 and the nearby W3 complex. This molecular gas is distributed along the radio continuum shell of the remnant. Furthermore, the shocked molecular gas indicated by line wing broadening features is also distributed along the radio shell and inside it. By both morphological correspondence and dynamical evidence, we confirm that the SNR HB 3 interacts with the −45 km s{sup −1} molecular cloud (MC), in essence, with the nearby H ii region/MC complexmore » W3. The redshifted line wing broadening features indicate that the remnant is located at the nearside of the MC. With this association, we could place the remnant at the same distance as the W3/W4 complex, which is 1.95 ± 0.04 kpc. The spatial distribution of aggregated young stellar object candidates shows a correlation with the shocked molecular strip associated with the remnant. We also find a binary clump of CO at ( l = 132.°94, b = 1.°12) around −51.5 km s{sup −1} inside the projected extent of the remnant, and it is associated with significant mid-infrared emission. The binary system also has a tail structure resembling the tidal tails of interacting galaxies. According to the analysis of CO emission lines, the larger clump in this binary system is about stable, and the smaller clump is significantly disturbed.« less
  6. Revealing pMDI Spray Initial Conditions: Flashing, Atomisation and the Effect of Ethanol

    Sprays from pressurised metered-dose inhalers are produced by a transient discharge of a multiphase mixture. Small length and short time scales have made the investigation of the governing processes difficult. Consequently, a deep understanding of the physical processes that govern atomisation and drug particle formation has been elusive. X-ray phase contrast imaging and quantitative radiography were used to reveal the internal flow structure and measure the time-variant nozzle exit mass density of 50 µL metered sprays of HFA134a, with and without ethanol cosolvent. Internal flow patterns were imaged at a magnification of 194 pixels/mm and 7759 frames per second withmore » 150 ps temporal resolution. Spray projected mass was measured with temporal resolution of 1 ms and spatial resolution 6 µm × 5 µm. The flow upstream of the nozzle comprised large volumes of vapour at all times throughout the injection. The inclusion of ethanol prevented bubble coalescence, altering the internal flow structure and discharge. Radiography measurements confirmed that the nozzle exit area is dominantly occupied by vapour, with a peak liquid volume fraction of 13%. Vapour generation in pMDIs occurs upstream of the sump, and the dominant volume component in the nozzle exit orifice is vapour at all times in the injection. Furthermore, the flow in ethanol-containing pMDIs has a bubbly structure resulting in a comparatively stable discharge, whereas the binary structure of propellant-only flows results in unsteady discharge and the production of unrespirable liquid masses.« less
  7. Center for Gyrokinetic/MHD Hybrid Simulation of Energetic Particle Physics in Toroidal Plasmas (CSEPP). Final report

    At Colorado University-Boulder the primary task is to extend our gyrokinetic Particle-in-Cell simulation of tokamak micro-turbulence and transport to the area of energetic particle physics. We have implemented a gyrokinetic ion/massless fluid electron hybrid model in the global {delta} f-PIC code GEM, and benchmarked the code with analytic results on the thermal ion radiative damping rate of Toroidal Alfven Eigenmodes (TAE) and with mode frequency and spatial structure from eigenmode analysis. We also performed nonlinear simulations of both a single-n mode (n is the toroidal mode number) and multiple-n modes, and in the case of single-n, benchmarked the code onmore » the saturation amplitude vs. particle collision rate with analytical theory. Most simulations use the f method for both ions species, but we have explored the full-f method for energetic particles in cases where the burst amplitude of the excited instabilities is large as to cause significant re-distribution or loss of the energetic particles. We used the hybrid model to study the stability of high-n TAEs in ITER. Our simulations show that the most unstable modes in ITER lie in the rage of 10 < n < 20. Thermal ion pressure effect and alpha particles non-perturbative effect are important in determining the mode radial location and stability threshold. The thermal ion Landau damping rate and radiative damping rate from the simulations are compared with analytical estimates. The thermal ion Landau damping is the dominant damping mechanism. Plasma elongation has a strong stabilizing effect on the alpha driven TAEs. The central alpha particle pressure threshold for the most unstable n=15 mode is about {beta}{sub {alpha}}(0) = 0.7% for the fully shaped ITER equilibrium. We also carried nonlinear simulations of the most unstable n = 15 mode and found that the saturation amplitude for the nominal ITER discharge is too low to cause large redistribution or loss of alpha particles. To include kinetic electron effects in the hybrid model we have studied a kinetic electron closure scheme for the fluid electron model. The most important element of the closure scheme is a complete Ohm’s law for the parallel electric field E{sub ||}, derived by combining the quasi-neutrality condition, the Ampere’s equation and the v{sub ||} moment of the gyrokinetic equations. A discretization method for the closure scheme is studied in detail for a three-dimensional shear-less slab plasma. It is found that for long-wavelength shear Alfven waves the kinetic closure scheme is both more accurate and robust than the previous GEM algorithm using the split-scheme, whereas for the ion-gradient-driven instability the previous algorithm is more efficient. This kinetic electron closure scheme will be implemented in GEM in the future. We have studied the beam driven Reverse Shear Alfven Eigenmodes (RSAE) observed in DIII-D discharge 142111. For this purpose a new scheme for obtaining the electric potential is implemented, i.e., by solving the gyrokinetic moment (GKM) equation, which is essentially the equation for {partial_derivative}{phi}}/{partial_derivative}t used in GEM’s split-weight scheme, and then integrating in time. Due to charge-neutrality the ExB motions of the equilibrium densities of all species cancel each other and do not cause charge separation if there is no finite Larmor radius effect. The advantage of solving the GKM equation is that this lowest-order cancellation can be made explicit. The GKM approach is found to be more accurate and robust. GEM simulations have reproduced many features of RSAE seen in the experiment, such as frequency chirping and the chirping range. It has been reported by other simulation codes that the shearing direction of the mode structure in the poloidal plane disagrees with observation. We found that the mode structure, including the shearing in the poloidal plane, is in general sensitive to the beam distribution. Using the same beam density profile as in other codes but with a slowing-down distribution in velocity, GEM simulations reproduce the mode shearing direction seen in the experiment. We have carried out extensive nonlinear simulations of RSAE.« less
  8. Saturation of Alfvén modes in tokamaks

    Here, the growth of Alfvén modes driven unstable by a distribution of high energy particles up to saturation is investigated with a guiding center code, using numerical eigenfunctions produced by linear theory and a numerical high energy particle distribution, in order to make detailed comparison with experiment and with models for saturation amplitudes and the modification of beam profiles. Two innovations are introduced. First, a very noise free means of obtaining the mode-particle energy and momentum transfer is introduced, and secondly, a spline representation of the actual beam particle distribution is used.
  9. A NEW GENERATION OF PARSEC-COLIBRI STELLAR ISOCHRONES INCLUDING THE TP-AGB PHASE

    We introduce a new generation of PARSEC–COLIBRI stellar isochrones that includes a detailed treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase, covering a wide range of initial metallicities (0.0001 < Z {sub i} < 0.06). Compared to previous releases, the main novelties and improvements are use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in themore » stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first-overtone modes; and new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the “C-star island” that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6-Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.« less
  10. Self-powered and broadband photodetectors based on graphene/ZnO/silicon triple junctions

    A self-powered photodetector with ultrahigh sensitivity, fast photoresponse, and wide spectral detectivity covering from 1000 nm to 400 nm based on graphene/ZnO/Si triple junctions has been designed, fabricated, and demonstrated. In this device, graphene serves as a transparent electrode as well as an efficient collection layer for photogenerated carriers due to its excellent tunability of Fermi energy. The ZnO layer acts as an antireflection layer to trap the incident light and enhance the light absorption. Furthermore, the insertion of the ZnO layer in between graphene and Si layers can create build-in electric field at both graphene/ZnO and ZnO/Si interfaces, which can greatlymore » enhance the charge separation of photogenerated electron and hole pairs. As a result, the sensitivity and response time can be significantly improved. It is believed that our methodology for achieving a high-performance self-powered photodetector based on an appropriate design of band alignment and optical parameters can be implemented to many other material systems, which can be used to generate unique optoelectronic devices for practical applications.« less
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