219 research outputs found
Small Structures via Thermal Instability of Partially Ionized Plasma. I. Condensation Mode
(Shortened) Thermal instability of partially ionized plasma is investigated
by linear perturbation analysis. According to the previous studies under the
one fluid approach, the thermal instability is suppressed due to the magnetic
pressure. However, the previous studies did not precisely consider the effect
of the ion-neutral friction, since they did not treat the flow as two fluid
which is composed of ions and neutrals. Then, we revisit the effect of the
ion-neutral friction of the two fluid to the growth of the thermal instability.
According to our study, (1) The instability which is characterized by the mean
molecular weight of neutrals is suppressed via the ion-neutral friction only
when the magnetic field and the friction are sufficiently strong. The
suppression owing to the friction occurs even along the field line. If the
magnetic field and the friction are not so strong, the instability is not
stabilized. (2) The effect of the friction and the magnetic field is mainly
reduction of the growth rate of the thermal instability of weakly ionized
plasma. (3) The effect of friction does not affect the critical wavelength
lambdaF for the thermal instability. This yields that lambdaF of the weakly
ionized plasma is not enlarged even when the magnetic field exists. We insist
that the thermal instability of the weakly ionized plasma in the magnetic field
can grow up even at the small length scale where the instability under the
assumption of the one fluid plasma can not grow owing to the stabilization by
the magnetic field. (4) The wavelength of the maximum growth rate of the
instability shifts shortward according to the decrement of the growth rate,
because the friction is effective at rather larger scale. Therefore, smaller
structures are expected to appear than those without the ion-neutral friction.Comment: To appear in Ap
Spectra from Forming Region of the First Galaxies : The Effect of Aspherical Deceleration
Ly line emission from the Loeb-Rybicki (LR) halo, which is the
expanding HI IGM (intergalactic medium) around the first star clusters and the
ionized interstellar medium, is investigated by solving a radiative transfer
problem. While the initial scattering optical depth is for the
Ly photons, most of the Ly photons can escape when the
cumulative frequency-shift due to the expansion of the HI IGM becomes
significantly large. The current paper improves upon previous treatments of the
scattering processes and the opacity for the Ly transfer. Confirming
the previous results of the LR halo, we investigate the effect of the
aspherical expansion of the IGM. The asphericity is hypothesized to follow the
initial stage of the gravitational deceleration to form the large scale
filamentary structure of the Universe. According to our results, the effect of
the asphericity lets the peak wavelength of the line profile shift to longer
wavelengths and the FWHM of the profile become wider than those of the
spherically expanding model. To detect these features is meaningful if we are
interested in the initial evolution of the large scale structure, since they
reflect the dynamical properties of the IGM at that time. Furthermore, given
the recent discovery of the high redshift cosmological reionization, we briefly
comment on the effects of the redshift and the cosmological parameters on the
line profile.Comment: 18 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Is Thermal Instability Significant in Turbulent Galactic Gas?
We investigate numerically the role of thermal instability (TI) as a
generator of density structures in the interstellar medium (ISM), both by
itself and in the context of a globally turbulent medium. Simulations of the
instability alone show that the condenstion process which forms a dense phase
(``clouds'') is highly dynamical, and that the boundaries of the clouds are
accretion shocks, rather than static density discontinuities. The density
histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak
at low densities plus a slope change at high densities. Final static situations
may be established, but the equilibrium is very fragile: small density
fluctuations in the warm phase require large variations in the density of the
cold phase, probably inducing shocks into the clouds. This result suggests that
such configurations are highly unlikely. Simulations including turbulent
forcing show that large- scale forcing is incapable of erasing the signature of
the TI in the density PDFs, but small-scale, stellar-like forcing causes
erasure of the signature of the instability. However, these simulations do not
reach stationary regimes, TI driving an ever-increasing star formation rate.
Simulations including magnetic fields, self-gravity and the Coriolis force show
no significant difference between the PDFs of stable and unstable cases, and
reach stationary regimes, suggesting that the combination of the stellar
forcing and the extra effective pressure provided by the magnetic field and the
Coriolis force overwhelm TI as a density-structure generator in the ISM. We
emphasize that a multi-modal temperature PDF is not necessarily an indication
of a multi-phase medium, which must contain clearly distinct thermal
equilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap
The Minimum Stellar Mass in Early Galaxies
The conditions for the fragmentation of the baryonic component during merging
of dark matter halos in the early Universe are studied. We assume that the
baryonic component undergoes a shock compression. The characteristic masses of
protostellar molecular clouds and the minimum masses of protostars formed in
these clouds decrease with increasing halo mass. This may indicate that the
initial stellar mass function in more massive galaxies was shifted towards
lower masses during the initial stages of their formation. This would result in
an increase of the number of stars per unit halo mass, i.e., the efficiency of
star formation.Comment: 18 pages, 7 figure
Effect of Dust Extinction on Estimating Star Formation Rate of Galaxies: Lyman Continuum Extinction
We re-examine the effect of Lyman continuum ( \AA)
extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it
affects the estimation of the global star formation rate (SFR) of galaxies. To
clarify the first issue, we establish two independent methods for estimating a
parameter of LCE (), which is defined as the fraction of Lyman continuum
photons contributing to hydrogen ionization in an H {\sc ii} region. One of
those methods determines from the set of Lyman continuum flux, electron
density and metallicity. In the framework of this method, as the metallicity
and/or the Lyman photon flux increase, is found to decrease. The other
method determines from the ratio of infrared flux to Lyman continuum flux.
Importantly, we show that f \la 0.5 via both methods in many H {\sc ii}
regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}
regions absorbs significant amount of Lyman continuum photons directly. To
examine the second issue, we approximate to a function of only the
dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the
Galactic H {\sc ii} regions. We find that a characteristic , which is
defined as averaged over a galaxy-wide scale, is 0.3 for the nearby spiral
galaxies. This relatively small indicates that a typical increment
factor due to LCE for estimating the global SFR () is large () for the nearby spiral galaxies. Therefore, we conclude that the effect of
LCE is not negligible relative to other uncertainties of estimating the SFR of
galaxies.Comment: 18 papges, 11 figures, accepted by Ap
Phase transitions in biological membranes
Native membranes of biological cells display melting transitions of their
lipids at a temperature of 10-20 degrees below body temperature. Such
transitions can be observed in various bacterial cells, in nerves, in cancer
cells, but also in lung surfactant. It seems as if the presence of transitions
slightly below physiological temperature is a generic property of most cells.
They are important because they influence many physical properties of the
membranes. At the transition temperature, membranes display a larger
permeability that is accompanied by ion-channel-like phenomena even in the
complete absence of proteins. Membranes are softer, which implies that
phenomena such as endocytosis and exocytosis are facilitated. Mechanical signal
propagation phenomena related to nerve pulses are strongly enhanced. The
position of transitions can be affected by changes in temperature, pressure, pH
and salt concentration or by the presence of anesthetics. Thus, even at
physiological temperature, these transitions are of relevance. There position
and thereby the physical properties of the membrane can be controlled by
changes in the intensive thermodynamic variables. Here, we review some of the
experimental findings and the thermodynamics that describes the control of the
membrane function.Comment: 23 pages, 15 figure
Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment
This paper presents a possible way to understand dissolution of OB
associations (or groups). Assuming rapid escape of parental cloud gas from
associations, we show that the shadow of the formation environment for
associations can be partially imprinted on the velocity dispersion at their
dissolution. This conclusion is not surprising as long as associations are
formed in a multiphase interstellar medium, because the external pressure
should suppress expansion caused by the internal motion of the parental clouds.
Our model predicts a few km s as the internal velocity dispersion.
Observationally, the internal velocity dispersion is km s which
is smaller than our prediction. This suggests that the dissipation of internal
energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte
The Hall effect in star formation
Magnetic fields play an important role in star formation by regulating the
removal of angular momentum from collapsing molecular cloud cores. Hall
diffusion is known to be important to the magnetic field behaviour at many of
the intermediate densities and field strengths encountered during the
gravitational collapse of molecular cloud cores into protostars, and yet its
role in the star formation process is not well-studied. We present a
semianalytic self-similar model of the collapse of rotating isothermal
molecular cloud cores with both Hall and ambipolar diffusion, and similarity
solutions that demonstrate the profound influence of the Hall effect on the
dynamics of collapse.
The solutions show that the size and sign of the Hall parameter can change
the size of the protostellar disc by up to an order of magnitude and the
protostellar accretion rate by fifty per cent when the ratio of the Hall to
ambipolar diffusivities is varied between -0.5 <= eta_H / eta_A <= 0.2. These
changes depend upon the orientation of the magnetic field with respect to the
axis of rotation and create a preferred handedness to the solutions that could
be observed in protostellar cores using next-generation instruments such as
ALMA.
Hall diffusion also determines the strength and position of the shocks that
bound the pseudo and rotationally-supported discs, and can introduce subshocks
that further slow accretion onto the protostar. In cores that are not initially
rotating Hall diffusion can even induce rotation, which could give rise to disc
formation and resolve the magnetic braking catastrophe. The Hall effect clearly
influences the dynamics of gravitational collapse and its role in controlling
the magnetic braking and radial diffusion of the field merits further
exploration in numerical simulations of star formation.Comment: 22 pages, 10 figures, accepted by MNRA
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