10,835 research outputs found
X-ray Development of the Classical Nova V2672 Ophiuchi with Suzaku
We report the Suzaku detection of a rapid flare-like X-ray flux amplification
early in the development of the classical nova V2672 Ophiuchi. Two
target-of-opportunity ~25 ks X-ray observations were made 12 and 22 days after
the outburst. The flux amplification was found in the latter half of day 12.
Time-sliced spectra are characterized by a growing supersoft excess with
edge-like structures and a relatively stable optically-thin thermal component
with Ka emission lines from highly ionized Si. The observed spectral evolution
is consistent with a model that has a time development of circumstellar
absorption, for which we obtain the decline rate of ~10-40 % in a time scale of
0.2 d on day 12. Such a rapid drop of absorption and short-term flux
variability on day 12 suggest inhomogeneous ejecta with dense blobs/holes in
the line of sight. Then on day 22 the fluxes of both supersoft and thin-thermal
plasma components become significantly fainter. Based on the serendipitous
results we discuss the nature of this source in the context of both short- and
long-term X-ray behavior.Comment: To appear in PASJ; 9 pages, 5 figures, 2 table
Host-Parasite Co-evolution and Optimal Mutation Rates for Semi-conservative Quasispecies
In this paper, we extend a model of host-parasite co-evolution to incorporate
the semi-conservative nature of DNA replication for both the host and the
parasite. We find that the optimal mutation rate for the semi-conservative and
conservative hosts converge for realistic genome lengths, thus maintaining the
admirable agreement between theory and experiment found previously for the
conservative model and justifying the conservative approximation in some cases.
We demonstrate that, while the optimal mutation rate for a conservative and
semi-conservative parasite interacting with a given immune system is similar to
that of a conservative parasite, the properties away from this optimum differ
significantly. We suspect that this difference, coupled with the requirement
that a parasite optimize survival in a range of viable hosts, may help explain
why semi-conservative viruses are known to have significantly lower mutation
rates than their conservative counterparts
On the 3-D structure and dissipation of reconnection-driven flow-bursts
The structure of magnetic reconnection-driven outflows and their dissipation
are explored with large-scale, 3-D particle-in-cell (PIC) simulations. Outflow
jets resulting from 3-D reconnection with a finite length x-line form fronts as
they propagate into the downstream medium. A large pressure increase ahead of
this ``reconnection jet front'' (RJF), due to reflected and transmitted ions,
slows the front so that its velocity is well below the velocity of the ambient
ions in the core of the jet. As a result, the RJF slows and diverts the
high-speed flow into the direction perpendicular to the reconnection plane. The
consequence is that the RJF acts as a thermalization site for the ion bulk flow
and contributes significantly to the dissipation of magnetic energy during
reconnection even though the outflow jet is subsonic. This behavior has no
counterpart in 2-D reconnection. A simple analytic model predicts the front
velocity and the fraction of the ion bulk flow energy that is dissipated
Four-way regulation of mosquito yolk protein precursor genes by juvenile hormone-, ecdysone-, nutrient-, and insulin-like peptide signaling pathways.
Anautogenous mosquito females require a meal of vertebrate blood in order to initiate the production of yolk protein precursors by the fat body. Yolk protein precursor gene expression is tightly repressed in a state-of-arrest before blood meal-related signals activate it and expression levels rise rapidly. The best understood example of yolk protein precursor gene regulation is the vitellogenin-A gene (vg) of the yellow fever mosquito Aedes aegypti. Vg-A is regulated by (1) juvenile hormone signaling, (2) the ecdysone-signaling cascade, (3) the nutrient sensitive target-of-rapamycin signaling pathway, and (4) the insulin-like peptide (ILP) signaling pathway. A plethora of new studies have refined our understanding of the regulation of yolk protein precursor genes since the last review on this topic in 2005 (Attardo et al., 2005). This review summarizes the role of these four signaling pathways in the regulation of vg-A and focuses upon new findings regarding the interplay between them on an organismal level
Two-scale structure of the electron dissipation region during collisionless magnetic reconnection
Particle in cell (PIC) simulations of collisionless magnetic reconnection are
presented that demonstrate that the electron dissipation region develops a
distinct two-scale structure along the outflow direction. The length of the
electron current layer is found to decrease with decreasing electron mass,
approaching the ion inertial length for a proton-electron plasma. A surprise,
however, is that the electrons form a high-velocity outflow jet that remains
decoupled from the magnetic field and extends large distances downstream from
the x-line. The rate of reconnection remains fast in very large systems,
independent of boundary conditions and the mass of electrons.Comment: Submitted to Physical Review Letters, 4 pages, 4 figure
Simulation of fluid flows during growth of organic crystals in microgravity
Several counter diffusion type crystal growth experiments were conducted in space. Improvements in crystal size and quality are attributed to reduced natural convection in the microgravity environment. One series of experiments called DMOS (Diffusive Mixing of Organic Solutions) was designed and conducted by researchers at the 3M Corporation and flown by NASA on the space shuttle. Since only limited information about the mixing process is available from the space experiments, a series of ground based experiments was conducted to further investigate the fluid dynamics within the DMOS crystal growth cell. Solutions with density differences in the range of 10 to the -7 to 10 to the -4 power g/cc were used to simulate microgravity conditions. The small density differences were obtained by mixing D2O and H2O. Methylene blue dye was used to enhance flow visualization. The extent of mixing was measured photometrically using the 662 nm absorbance peak of the dye. Results indicate that extensive mixing by natural convection can occur even under microgravity conditions. This is qualitatively consistent with results of a simple scaling analysis. Quantitave results are in close agreement with ongoing computational modeling analysis
Is the magnetic field in the heliosheath laminar or a turbulent bath of bubbles?
All the current global models of the heliosphere are based on the assumption
that the magnetic field in the heliosheath, in the region close to the
heliopause is laminar. We argue that in that region the heliospheric magnetic
field is not laminar but instead consists of magnetic bubbles. Recently, we
proposed that the annihilation of the "sectored" magnetic field within the
heliosheath as it is compressed on its approach to the heliopause produces the
anomalous cosmic rays and also energetic electrons. As a product of the
annihilation of the sectored magnetic field, densely-packed magnetic
islands/bubbles are produced. These magnetic islands/bubbles will be convected
with the ambient flows as the sector region is carried to higher latitudes
filling the heliosheath. We further argue that the magnetic islands/bubbles
will develop upstream within the heliosheath. As a result, the magnetic field
in the heliosheath sector region will be disordered well upstream of the
heliopause. We present a 3D MHD simulation with very high numerical resolution
that captures the north-south boundaries of the sector region. We show that due
to the high pressure of the interstellar magnetic field a north-south asymmetry
develops such that the disordered sectored region fills a large portion of the
northern part of the heliosphere with a smaller extension in the southern
hemisphere. We suggest that this scenario is supported by the following changes
that occur around 2008 and from 2009.16 onward: a) the sudden decrease in the
intensity of low energy electrons detected by Voyager 2; b) a sharp reduction
in the intensity of fluctuations of the radial flow; and c) the dramatic
differences in intensity trends between GCRs at V1 and 2. We argue that these
observations are a consequence of V2 leaving the sector region of disordered
field during these periods and crossing into a region of unipolar laminar
field.Comment: 36 pages, 15 figures, submitted to Ap
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