68 research outputs found
Constitutional Law - Fifth Amendment Takings Clause - Monetary Compensation for Temporary Regulatory Taking Is Required for the Period During Which the Regulation Is Effective - First English Evangelical Lutheran Church of Glendale v. County of Los Angeles, 482 U.S. 304 (1987).
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
Coronal winds powered by radiative driving
A two-component phenomenological model developed originally for zeta Puppis
is revised in order to model the outflows of late-type O dwarfs that exhibit
the weak-wind phenomenon. With the theory's standard parameters for a generic
weak-wind star, the ambient gas is heated to coronal temperatures ~ 3 x 10^{6}K
at radii > 1.4 R, with cool radiativly-driven gas being then confined to dense
clumps with filling factor ~ 0.02. Radiative driving ceases at radius ~ 2.1R
when the clumps are finally destroyed by heat conduction from the coronal gas.
Thereafter, the outflow is a pure coronal wind, which cools and decelerates
reaching infinity with terminal velocity ~ 980$ km/ s.Comment: 10 pages, 4 figure
Stationary models for the extra-planar gas in disc galaxies
The kinematics of the extra-planar neutral and ionised gas in disc galaxies
shows a systematic decline of the rotational velocity with height from the
plane (vertical gradient). This feature is not expected for a barotropic gas,
whilst it is well reproduced by baroclinic fluid homogeneous models. The
problem with the latter is that they require gas temperatures (above K)
much higher than the temperatures of the cold and warm components of the
extra-planar gas layer. In this paper, we attempt to overcome this problem by
describing the extra-planar gas as a system of gas clouds obeying the Jeans
equations. In particular, we consider models having the observed extra-planar
gas distribution and gravitational potential of the disc galaxy NGC 891: for
each model we construct pseudo-data cubes and we compare them with the HI data
cube of NGC 891. In all cases the rotational velocity gradients are in
qualitative agreement with the observations, but the synthetic and the observed
data cubes of NGC 891 show systematic differences that cannot be accommodated
by any of the explored models. We conclude that the extra-planar gas in disc
galaxies cannot be satisfactorily described by a stationary Jeans-like system
of gas clouds.Comment: 14 pages, 7 figures, accepted for pubblication in MNRA
Two-component model of the interaction of an interstellar cloud with surrounding hot plasma
We present a two-component gasdynamic model of an interstellar cloud embedded
in a hot plasma. It is assumed that the cloud consists of atomic hydrogen gas,
interstellar plasma is quasineutral. Hydrogen atoms and plasma protons interact
through a charge exchange process. Magnetic felds and radiative processes are
ignored in the model. The influence of heat conduction within plasma on the
interaction between a cloud and plasma is studied. We consider the extreme case
and assume that hot plasma electrons instantly heat the plasma in the
interaction region and that plasma flow can be described as isothermal. Using
the two-component model of the interaction of cold neutral cloud and hot
plasma, we estimate the lifetime of interstellar clouds. We focus on the clouds
typical for the cluster of local interstellar clouds embedded in the hot Local
Bubble and give an estimate of the lifetime of the Local interstellar cloud
where the Sun currently travels. The charge transfer between highly charged
plasma ions and neutral atoms generates X-ray emission. We assume typical
abundance of heavy ions for the Local Bubble plasma and estimate the X-ray
emissivity due to charge exchange from the interface between cold neutral cloud
and hot plasma. Our results show that charge exchange X-ray emission from the
neutral-plasma interfaces can be a non-negligible fraction of the observed
X-ray emission.Comment: 9 pages, 7 figure
The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies
(Abridged)
We study the effects of interstellar clouds on the dynamical and chemical
evolution of gas-rich dwarf galaxies. In particular, we focus on two model
galaxies similar to IZw18 and NGC1569 in comparison to models in which a smooth
initial distribution of gas is assumed. We use a 2-D hydrodynamical code
coupled with a series of routines able to trace the chemical products of SNeII,
SNeIa and intermediate-mass stars. Clouds are simulated by adding overdense
regions in the computational grid, whose locations are chosen randomly and
whose density profiles match observed ones. The clouds are inherently
dynamically coupled to the diffuse gas, and they experience heat conduction
from a hot surrounding gas. Due to dynamical processes and thermal evaporation,
the clouds survive only a few tens of Myr. Due to the additional cooling agent,
the internal energy of cloudy models is typically reduced by 20 - 40% compared
with models of diffuse gas alone. The clouds delay the development of
large-scale outflows by mass loading, therefore helping to retain a larger
amount of gas inside the galaxy. However, their bullet effect can pierce the
expanding supershell and create holes through which the superbubble can vent
freshly produced metals. Moreover, assuming a pristine chemical composition for
the clouds, their interaction with the superbubble dilutes the gas, reducing
the metallicity. The resulting final metallicity is therefore generally lower
(by ~ 0.2 - 0.4 dex) than the one attained by diffuse models.Comment: 13 pages, 11 figures, A&A accepte
Do High-Velocity Clouds trace the Dark Matter subhalo population?
Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos
form the building blocks which merge hierarchically to more massive galaxies.
Since intergalactic gas is accreted by massive galaxies, observable e.g. as
high- velocity clouds (HVCs) around the Milky Way, with extremely low
metallicities, these can be suggested to represent the baryonic content of
primordial Dark Matter (DM) subhalos. Another possibility of their origin is
that they stem from disrupted satellite galaxies, but in this case, these gas
clouds move unaccompanied by a bound DM structure. Since HVCs are observed with
long gas tails and with irregular substructures, numerical models are performed
aiming at exploring their structure and compare them with observations. If HVCs
are engulfed by DM subhalos, their gas must leave the DM gravitational
potential and reflect this in their dynamics. On the other hand, the evolution
and survival of pure gas models must be tested to distinguish between
DM-dominated and DM-free clouds and to allow conclusions on their origin. The
models demonstrate that purely baryonic HVCs with low masses are disrupted by
ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive
ones survive, losing their initially spherical shape and develop significant
substructures including cometary elongations in the column density distribution
("head-tail structure"). On the contrary, HVCs with DM subhalos survive with
more than 90% of their gas mass still bound and spherically shaped, approaching
the Galactic disk like bullets. In addition, we find that velocity gradients
along the cometary head-tail structures does not necessarily offer a
possibility to distinguish between DM-dominated and purely gaseous HVCs.
Comparison of models with observations let us conclude that HVCs are not
embedded in a DM substructure and do not trace the cosmological subhalo
population.Comment: Accepted for publication in A&
The role of the Rayleigh-Taylor instability in ram pressure stripped disk galaxies
Ram pressure stripping, i.e. the removal of a galaxy's gas disk due to its
motion through the intracluster medium of a galaxy cluster, appears to be a
common phenomenon. Not every galaxy, however, is completely stripped of its gas
disk. If the ram pressure is insufficiently strong, only the outer parts of the
gas disk are removed, and the inner gas disk is retained by the galaxy. One
example of such a case is the Virgo spiral NGC 4402. Observations of NGC 4402
(Crowl et al. 2005) reveal structures at the leading edge of the gas disk,
which resemble the characteristic finger-like structures produced by the
Rayleigh-Taylor (RT) instability. We argue, however, that the RT instability is
unlikely to be responsible for these structures. We demonstrate that the
conditions under which a galaxy's disk gas experiences ram pressure stripping
are identical to those that lead to RT instability. If the galaxy's gravity
prevents ram pressure stripping of the inner disk, it also prevents the RT
instability. In contrast, the stripped gas could still be subject to RT
instability, and we discuss consequences for the stripped gas.Comment: accepted by A&
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
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