Non-linear Thin Shell Instability Research Articles

Nonlinear thin shell instabilities in molecular clouds

Observations of molecular clouds point to the existence of supersonic, turbulent flows. Therefore, any theory which attempts to describe molecular cloud evolution and star formation must include a consideration of the dynamics of colliding flows. Previous studies have considered the collision of supersonic streams or clouds. The resultant instabilities provide a mechanism which may give rise to observable cloud morphologies and enhance the star formation rate. One such instability is the nonlinear thin shell instability (NTSI) of a shock-bounded slab. This process is driven by ram pressure and efficient cooling. In this study, I use numerical simulations to examine the head-on collision of supersonic gas streams in a cold, molecular gas. A dense slab forms in the collision midplane and is prone to a number of instabilities, including the NTSI. The thermodynamic processes involved are found to have a controlling influence upon the instability and fragmentation of the slab. Although some minimal amount of cooling is needed to drive the instability, too rapid a cooling rate gives rise to smaller wavelength instabilities which wipe out the NTSI. The growth rate of the NTSI in a gas undergoing molecular cooling corresponds to a timescale of order 10 12 s, in general agreement with the theoretical value for an isothermal gas. The NTSI may provide a viable mechanism for the instigation of rapid star formation.

Instability of isothermal stellar wind bowshocks

We present hydrodynamical simulations illustrating the instability of stellar wind bowshocks in the limit of an isothermal equation of state. In this limit, the bowshock is characterized by a thin dense shell bounded on both sides by shocks. In a time-averaged sense the shape of this bowshock shell roughly matches the steady state solution of Wilkin (1996)[ApJ, 459, L31], although the apex of the bowshock can deviate in or out by a factor of two or more. The shape of the bowshock is distorted by large amplitude kinks with a characteristic wavelength of order the standoff distance from the star. The instability is driven by a strong shear flow within the shock-bounded shell, suggesting an origin related to the nonlinear thin-shell instability. This instability occurs when both the forward bowshock and the reverse wind shock are effectively isothermal and the star is moving through the interstellar medium with a Mach number greater than a few. This work therefore suggests that ragged, clumpy bowshocks should be expected to surround stars with a slow, dense wind (which leads to rapid cooling behind the reverse wind shock), whose velocity with respect to the surrounding interstellar medium is of order 60kms−1 (leading both to rapid cooling behind the forward bowshock and sufficiently high Mach numbers to drive the instability).

The Morphology of Planetary Nebulae: Simulations with Time‐evolving Winds

We have carried out simulations of planetary nebulae (PNs) within the general framework of the interacting stellar winds models, wherein the fast wind from a central star sweeps up the slow wind emitted in a previous epoch. Our two-dimensional simulations take into account the evolution of the velocity and mass-loss rate of the fast wind. This leads to considerably more structure on smaller scales than was seen in those nebulae where the fast wind velocity is held constant. The nebula evolves through an initial momentum-conserving phase before entering the more commonly encountered energy-conserving stage. Both stages are prone to instabilities, which may be the precursor of knots, filaments, and other microstructures that are now commonly seen in Hubble Space Telescope images of PNs. In particular, we note the occurrence of the nonlinear thin shell instability in the early stages, and the formation of Rayleigh-Taylor filaments in the energy-conserving stage. The growth of small-scale structure in the momentum-conserving stage is sensitive to the ratio of the initial wind momenta; the lifetime of the nebula in this stage depends also on the evolution of the wind properties. The overall size scale of the system is determined mainly by the evolution of the fast wind properties. If the evolution is not taken into account when computing kinematic ages, then the ages may be underestimated.

Evolution of cold shock-bounded slabs

The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray