20 research outputs found
Evaporation and condensation of spherical interstellar clouds. Self-consistent models with saturated heat conduction and cooling
Shortened version: The fate of IS clouds embedded in a hot tenuous medium
depends on whether the clouds suffer from evaporation or whether material
condensates onto them. Analytical solutions for the rate of evaporative mass
loss from an isolated spherical cloud embedded in a hot tenuous gas are deduced
by Cowie & McKee (1977). In order to test the validity of the analytical
results for more realistic IS conditions the full hydrodynamical equations must
be treated. Therefore, 2D numerical simulations of the evolution of IS clouds
%are performed with different internal density structures and surrounded by a
hot plasma reservoir. Self-gravity, interstellar heating and cooling effects
and heat conduction by electrons are added. Classical thermal conductivity of a
fully ionized hydrogen plasma and saturated heat flux are considered. Using
pure hydrodynamics and classical heat flux we can reproduce the analytical
results. Heat flux saturation reduces the evaporation rate by one order of
magnitude below the analytical value. The evolution changes totally for more
realistic conditions when interstellar heating and cooling effects stabilize
the self-gravity. Evaporation then turns into condensation, because the
additional energy by heat conduction can be transported away from the interface
and radiated off efficiently from the cloud's inner parts. I.e. that the
saturated heat flux consideration is inevitable for IS clouds embedded in hot
tenuous gas. Various consequences are discussed in the paper.Comment: 16 pages, 24 figures, accepted in Astronomy and Astrophysic
The evolution of interstellar clouds in a streaming hot plasma including heat conduction
To examine the evolution of giant molecular clouds in the stream of a hot
plasma we performed two-dimensional hydrodynamical simulations that take full
account of self-gravity, heating and cooling effects and heat conduction by
electrons. We use the thermal conductivity of a fully ionized hydrogen plasma
proposed by Spitzer and a saturated heat flux according to Cowie & McKee in
regions where the mean free path of the electrons is large compared to the
temperature scaleheight. Significant structural and evolutionary differences
occur between simulations with and without heat conduction. Dense clouds in
pure dynamical models experience dynamical destruction by Kelvin-Helmholtz (KH)
instability. In static models heat conduction leads to evaporation of such
clouds. Heat conduction acting on clouds in a gas stream smooths out steep
temperature and density gradients at the edge of the cloud because the
conduction timescale is shorter than the cooling timescale. This diminishes the
velocity gradient between the streaming plasma and the cloud, so that the
timescale for the onset of KH instabilities increases, and the surface of the
cloud becomes less susceptible to KH instabilities. The stabilisation effect of
heat conduction against KH instability is more pronounced for smaller and less
massive clouds. As in the static case more realistic cloud conditions allow
heat conduction to transfer hot material onto the cloud's surface and to mix
the accreted gas deeper into the cloud.Comment: 19 pages, 12 figures, accepted in Astronomy and Astrophysic