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We consider the penetration of spherical dense cloud into a planar layer of gas that is in gravitational equilibrium in a constant field of gravity with acceleration W. We take into account the compressibility of the medium in contrast to the classical problem of a drop falling on the surface of an incompressible fluid. Our goal is to study the formation of cumulative jets in relation to the conditions on the borders of H ii regions. We determine those features of motion that would distinguish jet streams from inhomogeneities of a different origin. Our simulation has shown that cumulative jets may arise in the presence of an acceleration layer W. The gas in jet moves in the direction opposite to the initial velocity Vc. At the same time, there are both a cumulative jet and tails, teardrop-shaped condensations, caused, respectively, by gas inflow in the wake behind the cloud and the classical Rayleigh–Taylor instability. We assume the model, according to which the H ii RCW 82 region formed in an initially homogeneous cloud. In framework of this model, we estimated the characteristic time of a cumulative jet formation ts ≈ 0.15 Myr. This is less than the present age of the H ii RCW 82 region, which is about 0.4 Myr. From the obtained estimates, it follows that the conditions at the periphery of the H ii RCW 82 region are favourable for the manifestation of cumulative effects on scales of ∼1.4 × 1017 cm, which are significantly smaller than the layer thickness.

The propagation of plane, cylindrical, and spherical waves in a thermally unstable gas–dust medium has been simulated numerically. As applied to the photodissociation regions near O and B stars, we take into account the interaction of ultraviolet radiation with dust grains and large polycyclic aromatic hydrocarbon molecules as well as the gas cooling through the excitation of СII ions and OI atoms and the deexcitation of rotational levels of CO molecules. The instability regions have been determined. The perturbation growth times corresponding to them are ~103−105 yr. We show that wave breaking occurs irrespective of the geometry of motion, while a perturbation in the form of a single pulse gives rise to a sequence of shock waves. The post-shock gas velocity is approximately 0.1−0.5 of the sound velocity, so that the autowaves can contribute noticeably to the observed velocity dispersion of the gas near the boundaries of HII regions. Two-dimensional simulations suggest that the presence of multiple shocks in a thermally unstable medium can accelerate significantly the destruction of preexisting isolated condensations.

The evolution of perturbations in a two-layer spherical shell and a plane layer with a two-step density distribution has been simulated numerically. The clumps formed by instability are shown to have qualitatively different structures, depending on the ratio of the densities in the inner and outer layers of the shell. Inhomogeneities bordered by a dense gas are formed in shells with an outwardly decreasing density. If, however, a denser gas is in the outer layer, then cores surrounded by a more rarefied material appear. These results are used to analyze the expansion of the HII region RCW 82. Since the inhomogeneities observed in the 13CO emission in the outer parts of this region have sharply delineated boundaries, our calculations argue for the model of the expansion of a shell with an outwardly decreasing density. The interaction of an accelerating shell with clumps in front of it has also been investigated. The deformations of a clump during its penetration into the shell and the formation of a groove in the shell gradually fillingwith a cold gas have been revealed. Thereafter, the shell material collapses to form a cumulative jet. As applied to the HII region RCW 82, we conclude that the existence of jets is possible if there are inhomogeneities with a scale of ∼1018 cm in the interstellar medium. The lack of data on such jet flows at the boundary of this region is an additional argument for the model where the inhomogeneous structure of the boundaries is attributable to the development of Rayleigh-Taylor instability.

The growth of two-dimensional axisymmetric perturbations in the motion of a neutral shell formed in the interstellar medium when an ionization-shock front exits at the surface of a cloud is simulated numerically. The perturbations are assumed to emerge when the shock ahead of the ionization front reaches the cloud boundary. For long-wavelength perturbations, the accumulation of mass has been found to take place in radially oriented condensations in the shape of a rod pointed toward the star and widened at the opposite end as a result of instability. The shell fragmentation is accompanied by supersonic spouting of a hot plasma into a low-density medium. Flow nonstationarity is shown to affect significantly the gas density and velocity distributions both inside and in the immediate vicinity of the condensation. As one recedes from the ionization front, the density of charged particles changes only slightly, which is inconsistent with the power law of density decrease with increasing distance from the condensation center commonly used in interpreting observations.

The process of formation and acceleration of a neutral gas envelope during the propagation of an ionization-shock front in a spherical gas cloud is numerically modeled. The velocity and acceleration of the envelope as well as its mass variation with time are determined. The results obtained are compared with some approximate formulas known in the literature. The growth of small velocity perturbations in the dense gas and the effects of mass and momentum accumulation in “finger-like” condensations are studied in the framework of a two-dimensional model.

We numerically model the formation and acceleration of a neutral gas shell as an ionization-shock front propagates in a spherical cloud by taking into account the photoionization and radiative heating of the gas, the spectral radiative transfer. We suggest and implement an approximation of the cooling function that allows calculations to be performed in a wide range of gas ionization fractions and temperatures. The total mass, average velocity, and thickness of the shell have been determined. The results are compared with approximate formulas known in the literature. Based on the parameters of the shell found, we estimate its acceleration, characteristic scales, and the growth times of unstable perturbations. We analyze the influence of the cloud particle density, cloud radius, stellar temperature, and radiation spectrum on the integrated characteristics of the neutral gas in the layer between the ionization and shock fronts. The distribution of matter in the shell and its thickness are shown to differ significantly from those used in approximate models.

The two-dimensional nonstationary motion of a cold dense shell accelerated under the action of rarefied hot gas pressure is modeled. The influence of radiative processes on manifestations of cumulative effects predicted by the inertia model is analyzed. It is assumed that the major medium component is hydrogen with small admixtures of oxygen, nitrogen, carbon, silicon, and iron. Radiative energy loss caused by photorecombinations and excitation followed by impurity ion metastable level deexcitation is taken into account. An approximation to the cooling function is suggested and implemented. This approximation can be used to perform calculations over wide ionization degree and gas temperature ranges. The formation of a shell during gas expansion ionized and heated by a source of ultraviolet radiation is studied. The characteristic time of shell appearance and its gas dynamic parameters are determined. The distribution of plasma temperature is shown to be nonmonotonic and have a maximum close to the ionization front. An increase in small perturbations of the velocity of a shell is shown to cause the formation of radial fibers and the concentration of gas mass and momentum in them. The structure of condensates formed is, however, much more complex than that predicted by the model of a thin layer of incompressible matter. In particular, condensation includes “fingerlike” thickening and a more extended region with a lower density. It also follows from the calculations that radiative cooling contributes to shell expansion in the radial direction but does not change the integral characteristics of condensations substantially.

This paper aims to resolve the problem of formation of young objects observed in the RCW 82 H {\small II} region. In the framework of a classical trigger model the estimated time of fragmentation is larger than the estimated age of the H {\small II} region. Thus the young objects could not have formed during the dynamical evolution of the H {\small II} region. We propose a new model that helps resolve this problem. This model suggests that the H {\small II} region RCW 82 is embedded in a cloud of limited size that is denser than the surrounding interstellar medium. According to this model, when the ionization--shock front leaves the cloud it causes the formation of an accelerating dense gas shell. In the accelerated shell, the effects of the Rayleigh--Taylor (R-T) instability dominate and the characteristic time of the growth of perturbations with the observed magnitude of about 3 pc is 0.14 Myr, which is less than the estimated age of the H {\small II} region. The total time $t_{\sum}$, which is the sum of the expansion time of the H {\small II} region to the edge of the cloud, the time of the R-T instability growth, and the free fall time, is estimated as $0.44, Comment: 14 pages, 3 figures