21,396 research outputs found
The Pseudo-continuum Bound-free Opacity of Hydrogen and its Importance in Cool White Dwarf Atmospheres
We investigate the importance of the pseudo-continuum bound-free opacity from
hydrogen atoms in the atmospheres of cool white dwarfs. This source of
absorption, when calculated by the occupation probability formalism applied in
the modeling of white dwarf atmospheres with ,
dominates all other sources of opacity at optical wavelengths. This is
unrealistic and not observed. On the other hand, a significant flux suppression
in the blue part of the spectra of cool white dwarfs has been reported, and
mainly interpreted as a result of the pseudo-continuum absorption from atomic
hydrogen. We investigate this problem by proposing a new, more realistic
approach to calculating this source of opacity. We show that this absorption is
orders of magnitude smaller than that predicted by current methods. Therefore,
we rule out the pseudo-continuum opacity as a source of the flux deficiency
observed in the spectra of cool white dwarfs.Comment: 11 pages, 5 gigures, accepted for publication in the Astrophysical
Journa
Helix untwisting and bubble formation in circular DNA
The base pair fluctuations and helix untwisting are examined for a circular
molecule. A realistic mesoscopic model including twisting degrees of freedom
and bending of the molecular axis is proposed. The computational method, based
on path integral techniques, simulates a distribution of topoisomers with
various twist numbers and finds the energetically most favorable molecular
conformation as a function of temperature. The method can predict helical
repeat, openings loci and bubble sizes for specific sequences in a broad
temperature range. Some results are presented for a short DNA circle recently
identified in mammalian cells.Comment: The Journal of Chemical Physics, vol. 138 (2013), in pres
The pulsar spectral index distribution
The flux density spectra of radio pulsars are known to be steep and, to first
order, described by a power-law relationship of the form S_{\nu} \propto
\nu^{\alpha}, where S_{\nu} is the flux density at some frequency \nu and
\alpha is the spectral index. Although measurements of \alpha have been made
over the years for several hundred pulsars, a study of the intrinsic
distribution of pulsar spectra has not been carried out. From the result of
pulsar surveys carried out at three different radio frequencies, we use
population synthesis techniques and a likelihood analysis to deduce what
underlying spectral index distribution is required to replicate the results of
these surveys. We find that in general the results of the surveys can be
modelled by a Gaussian distribution of spectral indices with a mean of -1.4 and
unit standard deviation. We also consider the impact of the so-called
"Gigahertz-peaked spectrum" pulsars. The fraction of peaked spectrum sources in
the population with significant turn-over at low frequencies appears to be at
most 10%. We demonstrate that high-frequency (>2 GHz) surveys preferentially
select flatter-spectrum pulsars and the converse is true for lower-frequency
(<1 GHz) surveys. This implies that any correlations between \alpha and other
pulsar parameters (for example age or magnetic field) need to carefully account
for selection biases in pulsar surveys. We also expect that many known pulsars
which have been detected at high frequencies will have shallow, or positive,
spectral indices. The majority of pulsars do not have recorded flux density
measurements over a wide frequency range, making it impossible to constrain
their spectral shapes. We also suggest that such measurements would allow an
improved description of any populations of pulsars with 'non-standard' spectra.Comment: 8 pages, 5 figures. Accepted by MNRA
Optimal conversion of Bose condensed atoms into molecules via a Feshbach resonance
In many experiments involving conversion of quantum degenerate atomic gases
into molecular dimers via a Feshbach resonance, an external magnetic field is
linearly swept from above the resonance to below resonance. In the adiabatic
limit, the fraction of atoms converted into molecules is independent of the
functional form of the sweep and is predicted to be 100%. However, for
non-adiabatic sweeps through resonance, Landau-Zener theory predicts that a
linear sweep will result in a negligible production of molecules. Here we
employ a genetic algorithm to determine the functional time dependence of the
magnetic field that produces the maximum number of molecules for sweep times
that are comparable to the period of resonant atom-molecule oscillations,
. The optimal sweep through resonance indicates that
more than 95% of the atoms can be converted into molecules for sweep times as
short as while the linear sweep results in a
conversion of only a few percent. We also find that the qualitative form of the
optimal sweep is independent of the strength of the two-body interactions
between atoms and molecules and the width of the resonance
Design study of general aviation collision avoidance system
The selection and design of a time/frequency collision avoidance system for use in general aviation aircraft is discussed. The modifications to airline transport collision avoidance equipment which were made to produce the simpler general aviation system are described. The threat determination capabilities and operating principles of the general aviation system are illustrated
J-factors of short DNA molecules
The propensity of short DNA sequences to convert to the circular form is
studied by a mesoscopic Hamiltonian method which incorporates both the bending
of the molecule axis and the intrinsic twist of the DNA strands. The base pair
fluctuations with respect to the helix diameter are treated as path
trajectories in the imaginary time path integral formalism. The partition
function for the sub-ensemble of closed molecules is computed by imposing chain
ends boundary conditions both on the radial fluctuations and on the angular
degrees of freedom. The cyclization probability, the J-factor, proves to be
highly sensitive to the stacking potential, mostly to its nonlinear parameters.
We find that the J-factor generally decreases by reducing the sequence length (
N ) and, more significantly, below N = 100 base pairs. However, even for very
small molecules, the J-factors remain sizeable in line with recent experimental
indications. Large bending angles between adjacent base pairs and anharmonic
stacking appear as the causes of the helix flexibility at short length scales.Comment: The Journal of Chemical Physics - May 2016 ; 9 page
A search for rotating radio transients and fast radio bursts in the Parkes high-latitude pulsar survey
Discoveries of rotating radio transients and fast radio bursts (FRBs) in
pulsar surveys suggest that more of such transient sources await discovery in
archival data sets. Here we report on a single-pulse search for dispersed radio
bursts over a wide range of Galactic latitudes (|b| < ) in data
previously searched for periodic sources by Burgay et al. We re-detected 20 of
the 42 pulsars reported by Burgay et al. and one rotating radio transient
reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into
account this result, and other recent surveys at Parkes, we corrected for
detection sensitivities based on the search software used in the analyses and
the different backends used in these surveys and find that the all-sky FRB
event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be FRBs day sky, where the
uncertainties represent a confidence interval. While this rate is lower
than inferred from previous studies, as we demonstrate, this combined event
rate is consistent with the results of all systematic FRB searches at Parkes to
date and does not require the need to postulate a dearth of FRBs at
intermediate latitudes.Comment: Accepted, 10 pages, 6 figure
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