3,573 research outputs found
Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques
DNA supercoiling plays an important role in a variety of cellular processes.
The torsional stress related with supercoiling may be also involved in gene
regulation through the local structure and dynamics of the double helix. To
check this possibility steady torsional stress was applied to DNA in the course
of all-atom molecular dynamics simulations. It is found that small static
untwisting significantly reduces the torsional persistence length () of
GC-alternating DNA. For the AT-alternating sequence a smaller effect of the
opposite sign is observed. As a result, the measured values are similar
under zero stress, but diverge with untwisting. The effect is traced to
sequence-specific asymmetry of local torsional fluctuations, and it should be
small in long random DNA due to compensation. In contrast, the stiffness of
special short sequences can vary significantly, which gives a simple
possibility of gene regulation via probabilities of strong fluctuations. These
results have important implications for the role of local DNA twisting in
complexes with transcription factors.Comment: 8 pages, 5 figures, to appear in Phys. Rev. Let
Free Space Optical Polarization De-multiplexing and Multiplexing by means of Conical Refraction
Polarization de-multiplexing and multiplexing by means of conical refraction
is proposed to increase the channel capacity for free space optical
communication applications. The proposed technique is based on the
forward-backward optical transform occurring when a light beam propagates
consecutively along the optic axes of two identical biaxial crystals with
opposite orientations of their conical refraction characteristic vectors. We
present experimental proof of usefulness of the conical refraction
de-multiplexing and multiplexing technique by increasing in one order of
magnitude the channel capacity at optical frequencies in a propagation distance
of 4m
The Effects of Variations in Nuclear Interactions on Nucleosynthesis in Thermonuclear Supernovae
The impact of nuclear physics uncertainties on nucleosynthesis in
thermonuclear supernovae has not been fully explored using comprehensive and
systematic studies with multiple models. To better constrain predictions of
yields from these phenomena, we have performed a sensitivity study by
post-processing thermodynamic histories from two different hydrodynamic,
Chandrasekhar-mass explosion models. We have individually varied all input
reaction and, for the first time, weak interaction rates by a factor of ten and
compared the yields in each case to yields using standard rates. Of the 2305
nuclear reactions in our network, we find that the rates of only 53 reactions
affect the yield of any species with an abundance of at least 10^-8 M_sun by at
least a factor of two, in either model. The rates of the 12C(a,g), 12C+12C,
20Ne(a,p), 20Ne(a,g) and 30Si(p,g) reactions are among those that modify the
most yields when varied by a factor of ten. From the individual variation of
658 weak interaction rates in our network by a factor of ten, only the stellar
28Si(b+)28Al, 32S(b+)32P and 36Ar(b+)36Cl rates significantly affect the yields
of species in a model. Additional tests reveal that reaction rate changes over
temperatures T > 1.5 GK have the greatest impact, and that ratios of
radionuclides that may be used as explosion diagnostics change by a factor of
less than two from the variation of individual rates by a factor of 10.
Nucleosynthesis in the two adopted models is relatively robust to variations in
individual nuclear reaction and weak interaction rates. Laboratory measurements
of a limited number of reactions would help to further constrain predictions.
As well, we confirm the need for a consistent treatment for relevant stellar
weak interaction rates since simultaneous variation of these rates (as opposed
to individual variation) has a significant effect on yields in our models.Comment: accepted by A&A, 14 pages, 5 figures, 2 table
White Dwarfs constrain Dark Forces
The white dwarf luminosity function, which provides information about their
cooling, has been measured with high precision in the past few years.
Simulations that include well known Standard Model physics give a good fit to
the data. This leaves little room for new physics and makes these astrophysical
objects a good laboratory for testing models beyond the Standard Model. It has
already been suggested that white dwarfs might provide some evidence for the
existence of axions. In this work we study the constraints that the white dwarf
luminosity function puts on physics beyond the Standard Model involving new
light particles (fermions or bosons) that can be pair-produced in a white dwarf
and then escape to contribute to its cooling. We show, in particular, that we
can severely constrain the parameter space of models with dark forces and light
hidden sectors (lighter than a few tens of keV). The bounds we find are often
more competitive than those from current lab searches and those expected from
most future searches.Comment: 25 pages, 8 figures, equivalent to published versio
Reactions of ethanol over CeO2 and Ru/CeO2 catalysts
The reaction of ethanol has been investigated on Ru/CeO2 in steady state conditions as well as with temperature programmed desorption (TPD). High resolution transmission electron microscopy (HRTEM) images indicated that the used catalyst contained Ru particles with a mean size of ca. 1.5 nm well dispersed on CeO2 (of about 12–15 nm in size). Surface uptake of ethanol was measured by changing exposure to ethanol followed by TPD. Saturation coverage is found to be between 0.25 and 0.33 of a monolayer for CeO2 that has been prior heated with O2 at 773 K. The main reactions of ethanol on CeO2 during TPD are: re-combinative desorption of ethanol; dehydrogenation to acetaldehyde; and dehydration to ethylene. The dehydration to ethylene occurs mainly in a small temperature window at about 700 K and it is attributed to ethoxides adsorbed on surface-oxygen defects. The presence of Ru considerably modified the reaction of ceria towards ethanol. It has switched the desorption products to CO, CO2, CH4 and H2. These latter products are typical reforming products. Ethanol steam reforming (ESR) conducted on Ru/CeO2 indicated that optimal reaction activity is at about 673 K above which CO2 production declines (together with that of H2) due to reverse water gas shift. This trend was well captured during ethanol TPD where CO2 desorbed about 50 K below than CO on both oxidized and reduced Ru/CeO2 catalysts.Peer ReviewedPostprint (author's final draft
A Wide-Field View of Leo II -- A Structural Analysis Using the SDSS
Using SDSS I data, we have analysed the stellar distribution of the Leo II
dwarf spheroidal galaxy (distance of 233 kpc) to search for evidence of tidal
deformation. The existing SDSS photometric catalogue contains gaps in regions
of high stellar crowding, hence we filled the area at the centre of Leo II
using the DAOPHOT algorithm applied to the SDSS images. The combined
DAOPHOT-SDSS dataset contains three-filter photometry over a 4x4 square degree
region centred on Leo II. By defining a mask in three-filter colour-magnitude
space, we removed the majority of foreground field stars. We have measured the
following Leo II structural parameters: a core radius of r_c = 2.64 +/- 0.19
arcmin (178 +/- 13 pc), a tidal radius of r_t = 9.33 +/- 0.47 arcmin (632 +/-
32 pc) and a total V-band luminosity of L_V = (7.4 +/- 2.0) times 10^5 L_sun
(M_V = -9.9 +/- 0.3). Our comprehensive analysis of the Leo II structure did
not reveal any significant signs of tidal distortion. The internal structure of
this object contains only mild isophotal twisting. A small overdensity was
discovered appoximately 4.5 tidal radii from the Leo II centre, however we
conclude it is unlikely to be material tidally stripped from Leo II based on
its stellar population, and is most likely a foreground overdensity of stars.
Our results indicate that the influence of the Galactic graviational field on
the structure of Leo II has been relatively mild. We rederived the
mass-to-light ratio of this system using existing kinematic data combined with
our improved structural measurements, and favour the scenario in which Leo II
is strongly dominated by dark matter with (M/L)_V ~ 100 in solar units.Comment: 41 pages, 15 figures, accepted for publication in the Astronomical
Journa
Bistable phase control via rocking in a nonlinear electronic oscillator
We experimentally demonstrate the effective rocking of a nonlinear electronic
circuit operating in a periodic regime. Namely, we show that driving a Chua
circuit with a periodic signal, whose phase alternates (also periodically) in
time, we lock the oscillation frequency of the circuit to that of the driving
signal, and its phase to one of two possible values shifted by pi, and lying
between the alternating phases of the input signal. In this way, we show that a
rocked nonlinear oscillator displays phase bistability. We interpret the
experimental results via a theoretical analysis of rocking on a simple
oscillator model, based on a normal form description (complex Landau equation)
of the rocked Hopf bifurcationComment: 7 pages, 10 figure
Coordinated analysis of two graphite grains from the CO3.0 LAP 031117 meteorite: First identification of a CO Nova graphite and a presolar iron sulfide subgrain
Presolar grains constitute remnants of stars that existed before the formation of the solar system.
In addition to providing direct information on the materials from which the solar system formed, these grains provide ground-truth information for models of stellar evolution and nucleosynthesis.
Here we report the in-situ identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 5-3+7 ppm in this meteorite. One of the grains (LAP-141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP-149) is extremely 13C-rich and 15N-poor, with one of the lowest 12C/13C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar
nucleosynthesis and dust condensation models indicates an origin in the ejecta of a low-mass CO nova. Grain LAP-149 is the first putative nova grain that quantitatively best matches nova model
predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.Peer ReviewedPostprint (author's final draft
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