Author
M. Verronen
Bio: M. Verronen is an academic researcher from University of Oulu. The author has contributed to research in topics: Instability. The author has an hindex of 1, co-authored 2 publications receiving 8 citations.
Topics: Instability
Papers
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TL;DR: In this paper, the authors derived a criterion for gravitational instability in a gaseous medium for particulate discs, in which the equilibrium depends on the collisional energy loss, and used it to predict the formation of highly elongated clouds with a relatively long lifetime before decay.
Abstract: Collisionally-induced amplification of density fluctuations can also produce non-axisymmetric local condensations in particulate discs if the optical thickness is between definite values. Gravitational instability occurs above this interval. The theory of both phenomena is derived from collisional equations. The conventional criterion for gravitational instability in a gaseous medium cannot be used for particulate discs, in which the equilibrium depends on the collisional energy loss. These instabilities can produce an unbounded growth in density or a gravitational coagulation of particles, but the typical consequence is the formation of highly elongated clouds which are denser than the background matter and have a relatively long lifetime before decay. The third type of instability, the thermal one, appears at low values of velocity dispersion. It only affects the random motion of particles without producing condensations.
7 citations
TL;DR: In this paper, an improved analytical approximation for the dispersion of impact velocities is constructed, which leads to some modifications in the theory of collisional systems (Hameen-Anttila et al., 1988) which is used as a basis for the calculations.
Abstract: An improved analytical approximation for the dispersion of impact velocities is constructed. This leads to some modifications in the theory of collisional systems (Hameen-Anttila et al., 1988) which is used as a basis for the calculations. A comparison with computer simulations shows that the new formulation improves the accuracy of theory.
1 citations
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TL;DR: In this article, the theory of collisional systems is generalized for an arbitrary geometry and forces acting in the system, mixtures of different particle types, friction, small deviations from the ideal spherical form, axial rotation, finite size of particles and gravitational interactions.
Abstract: The theory of collisional systems is generalized for an arbitrary geometry and forces acting in the system, mixtures of different particle types, friction, small deviations from the ideal spherical form, axial rotation, finite size of particles and gravitational interactions. Terms for the formation of new particles and destruction of old ones are also included, and other unspecified parameters can be introduced. Although some approximations are made to simplify the basic equations and to avoid excessive numerical interactions, a comparison with computer simulations shows a good agreement. The tests were continued up to the optical thicknessτ = 5.
20 citations
TL;DR: The velocity dispersions of the planetesimals that govern the accumulation of the planets can be described by the kinetic formalism of P. Hornung, R. Pellat, and P. Barge as discussed by the authors.
17 citations
TL;DR: In this paper, a quasi-equilibrium solution for the pre-planetary disk is studied in terms of Hameen-Anttila's theory of collisional, self-gravitating systems.
Abstract: Quasi-equilibrium solutions for the pre-planetary disk are studied in terms of Hameen-Anttila's theory (1984) of collisional, self-gravitating systems. The distribution of particle sizes is assumed to follow simple power-law distributions, with a power index in the range of 1.5–5.0. The treatment includes mutual impacts with a velocity dependent coefficient of restitution, as well as gravitational encounters with dynamical friction. The mean gravitational field of the disk is also taken into account. The results indicate that the energy(equi)-partition depends mainly on the index of size distribution, but is also affected by the optical thickness of the system, as well as on the vertical thickness as compared to the particle size. The vertical component of the gravitational field is found to be important, especially when the mass of the system is concentrated on the large particles.
4 citations
TL;DR: In this article, the Navier-Stokes equations for the viscous motion of a mixture of gas and dust in a gravitational field are derived from the statistics of particle orbits and radiative processes in a general form.
Abstract: The equations for the viscous motion of a mixture of gas and dust in a gravitational field are derived from the statistics of particle orbits and radiative processes in a general form which gives the Navier-Stokes equation as a special case. Diffusion, partially elastic collisions and — for larger bodies — the gravitational encounters are included. The results are applied to the evolution of circumstellar discs.
3 citations
TL;DR: In this article, a statistical model for the small-scale turbulence of circumstellar discs was proposed, which yields equations which in stationary systems are mathematically equivalent to those of partially elastic impacts, although the physical content is different.
Abstract: A statistical model for the small-scale turbulence of circumstellar discs yields equations which in stationary systems are mathematically equivalent to those of partially elastic impacts, although the physical content is different. The disc is turbulent but not convective on this scale.
2 citations