1,050 research outputs found
The Effect of the Random Magnetic Field Component on the Parker Instability
The Parker instability is considered to play important roles in the evolution
of the interstellar medium. Most studies on the development of the instability
so far have been based on an initial equilibrium system with a uniform magnetic
field. However, the Galactic magnetic field possesses a random component in
addition to the mean uniform component, with comparable strength of the two
components. Parker and Jokipii have recently suggested that the random
component can suppress the growth of small wavelength perturbations. Here, we
extend their analysis by including gas pressure which was ignored in their
work, and study the stabilizing effect of the random component in the
interstellar gas with finite pressure. Following Parker and Jokipii, the
magnetic field is modeled as a mean azimuthal component, , plus a random
radial component, , where is a random function
of height from the equatorial plane. We show that for the observationally
suggested values of , the tension due to the random
component becomes important, so that the growth of the instability is either
significantly reduced or completely suppressed. When the instability still
works, the radial wavenumber of the most unstable mode is found to be zero.
That is, the instability is reduced to be effectively two-dimensional. We
discuss briefly the implications of our finding.Comment: 10 pages including 2 figures, to appear in The Astrophysical Journal
Letter
Density-PDFs and Lagrangian Statistics of highly compressible Turbulence
We report on probability-density-functions (PDF) of the mass density in
numerical simulations of highly compressible hydrodynamic flows and the
corresponding structure formation of Lagrangian particles advected by the
flows. Numerical simulations were performed with collocation points and
2 million tracer particles integrated over several dynamical times. We propose
a connection between the PDF of the Lagrangian tracer particles and the
predicted log-normal distribution of the density fluctuations in isothermal
systems
A Comparative Study of the Parker Instability under Three Models of the Galactic Gravity
To examine how non-uniform nature of the Galactic gravity might affect length
and time scales of the Parker instability, we took three models of gravity,
uniform, linear and realistic ones. To make comparisons of the three gravity
models on a common basis, we first fixed the ratio of magnetic pressure to gas
pressure at = 0.25, that of cosmic-ray pressure at = 0.4, and
the rms velocity of interstellar clouds at = 6.4 km s, and then
adjusted parameters of the gravity models in such a way that the resulting
density scale heights for the three models may all have the same value of 160
pc. Performing linear stability analyses onto equilibrium states under the
three models with the typical ISM conditions, we calculate the maximum growth
rate and corresponding length scale for each of the gravity models. Under the
uniform gravity the Parker instability has the growth time of 1.2
years and the length scale of 1.6 kpc for symmetric mode. Under the realistic
gravity it grows in 1.8 years for both symmetric and
antisymmetric modes, and develops density condensations at intervals of 400 pc
for the symmetric mode and 200 pc for the antisymmetric one. A simple change of
the gravity model has thus reduced the growth time by almost an order of
magnitude and its length scale by factors of four to eight. These results
suggest that an onset of the Parker instability in the ISM may not necessarily
be confined to the regions of high and .Comment: Accepted for publication in ApJ, using aaspp4.sty, 18 text pages with
9 figure
Mass Transfer, Transiting Stream and Magnetopause in Close-in Exoplanetary Systems with Applications to WASP-12
We study mass transfer by Roche lobe overflow in close-in exoplanetary
systems. The planet's atmospheric gas passes through the inner Lagrangian point
and flows along a narrow stream, accelerating to 100-200\kms velocity before
forming an accretion disk. We show that the cylinder-shaped accretion stream
can have an area (projected in the plane of the sky) comparable to that of the
planet and a significant optical depth to spectral line absorption. Such a
"transiting cylinder" may produce an earlier ingress of the planet transit, as
suggested by recent HST observations of the WASP-12 system. The asymmetric disk
produced by the accretion stream may also lead to time-dependent obscuration of
the star light and apparent earlier ingress. We also consider the interaction
of the stellar wind with the planetary magnetosphere. Since the wind speed is
subsonic/sub-Alfvenic and comparable to the orbital velocity of the planet, the
head of the magnetopause lies eastward relative to the substellar line (the
line joining the planet and the star). The gas around the magnetopause may, if
sufficiently compressed, give rise to asymmetric ingress/egress during the
planet transit, although more works are needed to evaluate this possibility.Comment: 6 pages with 2 figures. Accepted in ApJ. Small changes (add
discussion on asymmetric disks
Parker Instability in a Self-Gravitating Magnetized Gas Disk: I. Linear Stability Analysis
To be a formation mechanism of such large-scale structures as giant molecular
clouds (GMCs) and HI superclouds, the classical Parker instability driven by
external gravity has to overcome three major obstacles: The convective motion
accompanying the instability generates thin sheets than large condensations.
The degree of density enhancement achieved by the instability is too low to
make dense interstellar clouds. The time and the length scales of the
instability are significantly longer and larger than the estimated formation
time and the observed mean separation of the GMCs, respectively. This paper
examines whether a replacement of the driving agent from the external to the
self gravity might remove these obstacles by activating the gravitational
instability in the Galactic ISM disk. The self gravity can suppress the
convective motions, and a cooperative action of the Jeans and the Parker
instabilities can remove all the obstacles confronting the classical version of
the Parker instability. The mass and mean separation of the structures
resulting from the odd-parity undular mode solution are shown to agree better
with the HI superclouds than with the GMCs. We briefly discuss how inclusions
of the external gravity and cosmic rays would modify behaviors of the
odd-parity undular mode solution.Comment: 53 pages, 21 figure
Optical Properties of GaAs Quantum Dots Fabricated by Filling of Self-Assembled Nanoholes
Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines
Testing Molecular-Cloud Fragmentation Theories: Self-Consistent Analysis of OH Zeeman Observations
The ambipolar-diffusion theory of star formation predicts the formation of
fragments in molecular clouds with mass-to-flux ratios greater than that of the
parent-cloud envelope. By contrast, scenarios of turbulence-induced
fragmentation do not yield such a robust prediction. Based on this property,
Crutcher et al. (2009) proposed an observational test that could potentially
discriminate between fragmentation theories. However, the analysis applied to
the data severely restricts the discriminative power of the test: the authors
conclude that they can only constrain what they refer to as the "idealized"
ambipolar-diffusion theory that assumes initially straight-parallel magnetic
field lines in the parent cloud. We present an original, self-consistent
analysis of the same data taking into account the nonuniformity of the magnetic
field in the cloud envelopes, which is suggested by the data themselves, and we
discuss important geometrical effects that must be accounted for in using this
test. We show quantitatively that the quality of current data does not allow
for a strong conclusion about any fragmentation theory. Given the
discriminative potential of the test, we urge for more and better-quality data.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Interactions between downslope flows and a developing cold-air pool
A numerical model has been used to characterize the development of a region of enhanced cooling in an alpine valley with a width of order (Formula presented.) km, under decoupled stable conditions. The region of enhanced cooling develops largely as a region of relatively dry air which partitions the valley atmosphere dynamics into two volumes, with airflow partially trapped within the valley by a developing elevated inversion. Complex interactions between the region of enhanced cooling and the downslope flows are quantified. The cooling within the region of enhanced cooling and the elevated inversion is almost equally partitioned between radiative and dynamic effects. By the end of the simulation, the different valley atmospheric regions approach a state of thermal equilibrium with one another, though this cannot be said of the valley atmosphere and its external environment.Peer reviewe
Three-dimensional simulations of the Parker instability in a uniformly rotating disk
We investigate the effects of rotation on the evolution of the Parker instability by carrying out three-dimensional numerical simulations with an isothermal magnetohydrodynamic code. These simulations extend our previous work on the nonlinear evolution of the Parker instability by J. Kim and coworkers. The initial equilibrium system is composed of exponentially stratified gas and a field (along the azimuthal direction) in a uniform gravity (along the downward vertical direction). The computational box, placed at the solar neighborhood, is set to rotate uniformly around the Galactic center with a constant angular speed. The instability has been initialized by random velocity perturbations. In the linear stage, the evolution is not much different from that without rotation, and the mixed (undular + interchange) mode regulates the system. The interchange mode induces alternating dense and rarefied regions with small radial wavelengths, while the undular mode bends the magnetic field lines in the plane of the azimuthal and vertical directions. In the nonlinear stage, flow motion overall becomes chaotic, as in the case without rotation. However, as the gas in higher positions slides down along field lines forming supersonic flows, the Coriolis force becomes important. As oppositely directed flows fall into valleys along both sides of the magnetic field lines, they experience the Coriolis force toward opposite directions, which twists the magnetic field lines there. Hence, we suggest that the Coriolis force plays a role in randomizing the magnetic field. The three-dimensional density structure formed by the instability is still sheetlike with the short dimension along the radial direction, as in the case without rotation. However, the long dimension is now slightly tilted with respect to the mean field direction. The shape of high-density regions is a bit rounder. The maximum enhancement factor of the vertical column density relative to its initial value is about 1.5, which is smaller than that in the case without rotation. We conclude that uniform rotation does not change our point of view that the Parker instability alone is not a viable mechanism for the formation of giant molecular cloudsopen252
Subsequent chemotherapy reverses acquired tyrosine kinase inhibitor resistance and restores response to tyrosine kinase inhibitor in advanced non-small-cell lung cancer
<p>Abstract</p> <p>Background</p> <p>Patients with advanced or metastatic non-small cell lung cancer (NSCLC) can develop acquired resistance to epidermal growth factor receptor tyrosine kinase inhibitors (TKIs) erlotinib and gefitinib. Here, we report the successful treatment with alternating chemotherapy and TKIs of two cases of advanced NSCLC who developed resistance to TKI.</p> <p>Case presentation</p> <p>Two patients with advanced or metastatic NSCLC were treated with palliative chemotherapy followed by erlotinib/gefitinib. When TKI therapy failed, two cycles of chemotherapy were provided, which were followed by re-challenge with erlotinib or gefitinib.</p> <p>Conclusion</p> <p>NSCLC patients with acquired TKI resistance should be managed aggressively whenever possible. Subsequent chemotherapy and target treatment is one of the reasonable choices for those with an initial dramatic clinical response with erlotinib/gefitinib treatment. Further studies are warranted to substantiate the association of erlotinib /gefitinib treatment with the efficacy of NSCLC patients with acquired TKI failure.</p
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