84 research outputs found

### Fragmentation of a dynamically condensing radiative layer

In this paper, the stability of a dynamically condensing radiative gas layer
is investigated by linear analysis. Our own time-dependent, self-similar
solutions describing a dynamical condensing radiative gas layer are used as an
unperturbed state. We consider perturbations that are both perpendicular and
parallel to the direction of condensation. The transverse wave number of the
perturbation is defined by $k$. For $k=0$, it is found that the condensing gas
layer is unstable. However, the growth rate is too low to become nonlinear
during dynamical condensation. For $k\ne0$, in general, perturbation equations
for constant wave number cannot be reduced to an eigenvalue problem due to the
unsteady unperturbed state. Therefore, direct numerical integration of the
perturbation equations is performed. For comparison, an eigenvalue problem
neglecting the time evolution of the unperturbed state is also solved and both
results agree well. The gas layer is unstable for all wave numbers, and the
growth rate depends a little on wave number. The behaviour of the perturbation
is specified by $kL_\mathrm{cool}$ at the centre, where the cooling length,
$L_\mathrm{cool}$, represents the length that a sound wave can travel during
the cooling time. For $kL_\mathrm{cool}\gg1$, the perturbation grows
isobarically.
For $kL_\mathrm{cool}\ll1$, the perturbation grows because each part has a
different collapse time without interaction. Since the growth rate is
sufficiently high, it is not long before the perturbations become nonlinear
during the dynamical condensation. Therefore, according to the linear analysis,
the cooling layer is expected to split into fragments with various scales.Comment: 12 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic

### Self-Sustained Turbulence without Dynamical Forcing: A Two-Dimensional Study of a Bistable Interstellar Medium

In this paper, the nonlinear evolution of a bistable interstellar medium is
investigated using two-dimensional simulations with a realistic cooling rate,
thermal conduction, and physical viscosity. The calculations are performed
using periodic boundary conditions without any external dynamical forcing. As
the initial condition, a spatially uniform unstable gas under thermal
equilibrium is considered. At the initial stage, the unstable gas quickly
segregates into two phases, or cold neutral medium (CNM) and warm neutral
medium (WNM). Then, self-sustained turbulence with velocity dispersion of
$0.1-0.2\;\mathrm{km\;s^{-1}}$ is observed in which the CNM moves around in the
WNM. We find that the interfacial medium (IFM) between the CNM and WNM plays an
important role in sustaining the turbulence. The self-sustaining mechanism can
be divided into two steps. First, thermal conduction drives fast flows
streaming into concave CNM surfaces towards the WNM. The kinetic energy of the
fast flows in the IFM is incorporated into that of the CNM through the phase
transition. Second, turbulence inside the CNM deforms interfaces and forms
other concave CNM surfaces, leading to fast flows in the IFM. This drives the
first step again and a cycle is established by which turbulent motions are
self-sustained.Comment: 14 pages, 15 figures, accepted by The Astrophysical Journa

### Gravitational Fragmentation of Expanding Shells. I. Linear Analysis

We perform a linear perturbation analysis of expanding shells driven by
expansions of HII regions. The ambient gas is assumed to be uniform. As an
unperturbed state, we develop a semi-analytic method for deriving the time
evolution of the density profile across the thickness. It is found that the
time evolution of the density profile can be divided into three evolutionary
phases, deceleration-dominated, intermediate, and self-gravity-dominated
phases. The density peak moves relatively from the shock front to the contact
discontinuity as the shell expands. We perform a linear analysis taking into
account the asymmetric density profile obtained by the semi-analytic method,
and imposing the boundary conditions for the shock front and the contact
discontinuity while the evolutionary effect of the shell is neglected. It is
found that the growth rate is enhanced compared with the previous studies based
on the thin-shell approximation. This is due to the boundary effect of the
contact discontinuity and asymmetric density profile that were not taken into
account in previous works.Comment: 13 pages, 13 figures, to be published in the Astrophysical Journa

### Gravitational Instability of Shocked Interstellar Gas Layers

In this paper we investigate gravitational instability of shocked gas layers
using linear analysis. An unperturbed state is a self-gravitating isothermal
layer which grows with time by the accretion of gas through shock fronts due to
a cloud-cloud collision. Since the unperturbed state is not static, and cannot
be described by a self-similar solution, we numerically solved the perturbation
equations and directly integrated them over time. We took account of the
distribution of physical quantities across the thickness. Linearized
Rankine-Hugoniot relations were imposed at shock fronts as boundary conditions.
The following results are found from our unsteady linear analysis: the
perturbation initially evolves in oscillatory mode, and begins to grow at a
certain epoch. The wavenumber of the fastest growing mode is given by
k=2\sqrt{2\pi G\rho_\mathrm{E} {\cal M\mit}}/c_\mathrm{s}, where
$\rho_\mathrm{E}, c_\mathrm{s}$ and \cal M\mit are the density of parent
clouds, the sound velocity and the Mach number of the collision velocity,
respectively. For this mode, the transition epoch from oscillatory to growing
mode is given by t_g = 1.2/\sqrt{2\pi G\rho_\mathrm{E} {\cal M\mit}}. The
epoch at which the fastest growing mode becomes non-linear is given by
2.4\delta_0^{-0.1}/\sqrt{2\pi G \rho_\mathrm{E}{\cal M\mit}}, where
$\delta_0$ is the initial amplitude of the perturbation of the column density.
As an application of our linear analysis, we investigate criteria for
collision-induced fragmentation. Collision-induced fragmentation will occur
only when parent clouds are cold, or $\alpha_0=5c_\mathrm{s}^2 R/2G M < 1$,
where $R$ and $M$ are the radius and the mass of parent clouds, respectively.Comment: 12 pages, 21 figures, accepted for publication in PAS

### Gravitational Fragmentation of Expanding Shells. I. Linear Analysis

We perform a linear perturbation analysis of expanding shells driven by
expansions of HII regions. The ambient gas is assumed to be uniform. As an
unperturbed state, we develop a semi-analytic method for deriving the time
evolution of the density profile across the thickness. It is found that the
time evolution of the density profile can be divided into three evolutionary
phases, deceleration-dominated, intermediate, and self-gravity-dominated
phases. The density peak moves relatively from the shock front to the contact
discontinuity as the shell expands. We perform a linear analysis taking into
account the asymmetric density profile obtained by the semi-analytic method,
and imposing the boundary conditions for the shock front and the contact
discontinuity while the evolutionary effect of the shell is neglected. It is
found that the growth rate is enhanced compared with the previous studies based
on the thin-shell approximation. This is due to the boundary effect of the
contact discontinuity and asymmetric density profile that were not taken into
account in previous works.Comment: 13 pages, 13 figures, to be published in the Astrophysical Journa

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