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

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 $D$, which is the ratio between the free-fall time $t_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm disc}}$ and the cooling time determined by the cloud collision process $t_{\rm cool}$. This ratio is related to the ratio between the mean surface density of the disc $\Sigma_{\rm disc}$ and the mean surface density of molecular clouds in the disc $\Sigma_{\rm cloud}$. When $D <1/3$ (which roughly corresponds to $\Sigma_{\rm disc} < 1/3 \Sigma_{\rm cloud}$), cloud collision cooling is inefficient, and fragmentation is suppressed. When $D > 1/3$ (which roughly corresponds to $\Sigma_{\rm disc} > 1/3 \Sigma_{\rm cloud}$), 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

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

Energy Loss in Nuclear Drell-Yan Process

By means of the nuclear parton distributions which can be used to provide a good explanation for the EMC effect in the whole x range, we investigate the energy loss effect in nuclear Drell-Yan process. When the cross section of lepton pair production is considered varying with the center-of-mass energy of the nucleon-nucleon collision, we find that the nuclear Drell-Yan(DY) ratio is suppressed due to the energy loss, which balances the overestimate of the DY ratio only in consideration of the effect of nuclear parton distributions.Comment: 10 pages, LaTeX, 1 ps figures, To appear in Eur. Phys. J.