17,371 research outputs found

    Dynamical cooling of galactic discs by molecular cloud collisions -- Origin of giant clumps in gas-rich galaxy discs

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    Different from Milky-Way-like galaxies, discs of gas-rich galaxies are clumpy. It is believed that the clumps form because of gravitational instability. However, a necessary condition for gravitational instability to develop is that the disc must dissipate its kinetic energy effectively, this energy dissipation (also called cooling) is not well-understood. We propose that collisions (coagulation) between molecular clouds dissipate the kinetic energy of the discs, which leads to a dynamical cooling. The effectiveness of this dynamical cooling is quantified by the dissipation parameter DD, which is the ratio between the free-fall time tff1/Gρdisct_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm disc}} and the cooling time determined by the cloud collision process tcoolt_{\rm cool}. This ratio is related to the ratio between the mean surface density of the disc Σdisc\Sigma_{\rm disc} and the mean surface density of molecular clouds in the disc Σcloud\Sigma_{\rm cloud}. When D<1/3D <1/3 (which roughly corresponds to Σdisc<1/3Σcloud\Sigma_{\rm disc} < 1/3 \Sigma_{\rm cloud}), cloud collision cooling is inefficient, and fragmentation is suppressed. When D>1/3D > 1/3 (which roughly corresponds to Σdisc>1/3Σcloud\Sigma_{\rm disc} > 1/3 \Sigma_{\rm cloud}), cloud-cloud collisions lead to a rapid cooling through which clumps form. On smaller scales, cloud-cloud collisions can drive molecular cloud turbulence. This dynamical cooling process can be taken into account in numerical simulations as a subgrid model to simulate the global evolution of disc galaxies.Comment: MNRAS accepte

    Polyesteramides with mixtures of poly(tetramethylene oxide) and 1,5 pentanediol

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    Segmented polyesteramides have been synthesized from N,N¿-bis(p-carbomethoxybenzoy)butanediamine as crystalline segments and mixtures of poly(tetramethylene oxide) (mol. wt. 1000) and 1,5-pentanediol as soft segments. The polymerization was carried out in the melt at 250°C for 1 h while vacuum was applied. The melting behaviour of the copolymers was studied by differential scanning calorimetry. The mechanical properties were investigated on injection moulded bars, using dynamic mechanical analysis. It was found that the copolymers with more than 50% molar ratio pentanediol showed two glass transition temperatures and two melting temperatures. The glass transition temperatures were not affected by the composition. The melting temperatures increased with pentanediol content. The pentanediol seems in part to be present as adjacent re-entry groups. The under-cooling for these copolymers was very small, which means that these segmented copolymers crystallize very fast

    Insulator-to-metal phase transition in Yb-based Kondo insulators

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    The periodic Anderson lattice model for the crystalline electric field (CEF)split 4f quartet states is used to describe the Yb-based Kondo insulators/semiconductors. In the slave-boson mean-field approximation, we derive the hybridized quasiparticle bands, and find that decreasing the hybridization difference of the two CEF quartets may induce an insulator-to-metal phase transition. The resulting metallic phase has a hole and an electron Fermi pockets. Such a phase transition may be realized experimentally by applying pressure, reducing the difference in hybridization of the two CEF quartets.Comment: 5 pages, 3 figure