Excitation Of Spiral Waves Research Articles

Spiral waves in driven strongly coupled Yukawa systems.

Spiral wave formations are ubiquitous in nature. In the present paper, the excitation of spiral waves in the context of driven two-dimensional dusty plasma (Yukawa system) has been demonstrated at particle level using molecular-dynamics simulations. The interaction amidst dust particles is modeled by the Yukawa potential to take account of the shielding of dust charges by the lighter electron and ion species. The spatiotemporal evolution of these spiral waves has been characterized as a function of the frequency and amplitude of the driving force and dust neutral collisions. The effect of strong coupling has been studied, which shows that the excited spiral wave structures get clearer as the medium gets more strongly coupled. The radial propagation speed of the spiral wave is observed to remain unaltered with the coupling parameter. However, it is found to depend on the screening parameter of the dust medium and decreases when it is increased. In the crystalline phase (with screening parameter κ>0.58), the spiral wavefronts are shown to be hexagonal in shape. This shows that the radial propagation speed depends on the interparticle spacing.

Non-linear dynamics of the corotation torque

The excitation of spiral waves by an external perturbation in a disc deposits angular momentum in the vicinity of the corotation resonance (the radius where the speed of a rotating pattern matches the local rotation rate). We use matched asymptotic expansions to derive a reduced model that captures non-linear dynamics of the resulting torque and fluid motions. The model is similar to that derived for forced Rossby wave critical layers in geophysical fluid dynamics. Using the model we explore the saturation of the corotation torque, which occurs when the background potential (specific) vorticity is redistributed by the disturbance. We also consider the effects of dissipation. If there is a radial transport of potential vorticity, the corotation torque does not saturate. The main application is to the creation, growth and migration of protoplanets within discs like the primordial solar nebula. The disturbance also nucleates vortices in the vicinity of corotation, which may spark further epochs of planet formation.

Star complexes and evolutionary processes in spiral galaxies

The concept of star complexes as fundamental building blocks of the structure of spiral galaxies is reviewed. Basic observational data on these huge aggregates of stars and gas, which are the largest cells of star formation in galaxies are presented. A scenario for formation and evolution of disk galaxies is considered. A particular emphasis is placed upon the role of dark matter and star-gas complexes in galactic evolution. A simple time-scale argument indicates that the central part of the gaseous protogalactic cloud should undergo rapid collapse well before the main bulk of the cloud collapses, the galactic bulge and halo being the result. The galactic disk is formed from the rest of the gas which is the outer part of the cloud having larger momentum. Fragmentation in the rotationally supported disk starts a few billion years after formation of the bulge-halo system. At that time its density and temperature achieve the sufficient magnitude, characteristic Jeans mass being 10 6 – 10 7 M⊙. This is just the mass of the observed gas superclouds which are considered to be progenitors of star complexes. They constitute ensemble of relatively small number of members (a few hundreds in a galaxy similar to the Milky Way) where fluctuations of their space density are large enough to produce rapid arising of collective processes leading to formation of a mini-bar in the central region of the galaxy. Also excitation of spiral waves in the ensemble of supercloud-star complexes in the disk takes place. The star forming disks of galaxies are marginally stable, and because of this the intermittent two-armed spiral structure can arise in them. It is essential for these considerations that gas-star complexes have masses an order of magnitude higher than giant molecular clouds usually referred to in similar studies. The rotating central bar selects and amplifies the wave mode corresponding to the two-armed grand design spiral pattern. In floculent spiral galaxies concentration of mass to the center is lower, they have no bars, and their fragmented arms may be star-gas complexes sheared by differential rotation. The irregular galaxies are too small and they have a rigid body rotation to have a spiral structure. Thus an explanation for the absence of a spiral structure in dwarf disk galaxies can be given, as well as for obligatory presence of a bar, a mini-bar or at least weak deviations from axial symmetry in the central parts of grand design spiral galaxies. A process of interaction of massive gaseous clouds with the spiral shock fronts is also described using numerical hydrodynamic models; some physical characteristics of star complexes are analysed on this basis.