58,185 research outputs found
Producing the Deuteron in Stars: Anthropic Limits on Fundamental Constants
Stellar nucleosynthesis proceeds via the deuteron (D), but only a small
change in the fundamental constants of nature is required to unbind it. Here,
we investigate the effect of altering the binding energy of the deuteron on
proton burning in stars. We find that the most definitive boundary in parameter
space that divides probably life-permitting universes from probably
life-prohibiting ones is between a bound and unbound deuteron. Due to neutrino
losses, a ball of gas will undergo rapid cooling or stabilization by electron
degeneracy pressure before it can form a stable, nuclear reaction-sustaining
star. We also consider a less-bound deuteron, which changes the energetics of
the and reactions. The transition to endothermic and
reactions, and the resulting beta-decay instability of the deuteron, do not
seem to present catastrophic problems for life.Comment: 19 pages, 5 figures. Accepted to JCAP. Revised to match the published
version; corrected to better take into account free neutron
Was SN1997ff at z~1.7 magnified by gravitational lensing?
The quest for the cosmological parameters has come to fruition with the
identification of a number of supernovae at a redshift of . Analyses of
the brightness of these standard candles reveal that the Universe is dominated
by a large cosmological constant. The recent identification of the
SN1997ff in the northern Hubble Deep Field has provided further evidence for
this cosmology. Here we examine the case for gravitational lensing of SN1997ff
due to the presence of galaxies lying along our line of sight. We find that,
while the alignment of SN1997ff with foreground masses was not favorable for it
to be multiply imaged and strongly magnified, two galaxies did lie close enough
to result in significant magnification: for the case where these
elliptical galaxies have velocity dispersion . Given the small
difference between supernova brightnesses in different cosmologies, detailed
modeling of the gravitational lensing properties of the intervening matter is
therefore required before the true cosmological significance of SN1997ff can be
deduced.Comment: 3 pages, 2 figures. Accepted for publication in MNRAS. Missing
reference adde
A Fully Unsupervised Texture Segmentation Algorithm
This paper presents a fully unsupervised texture segmentation algorithm by using a modified discrete wavelet frames decomposition and a mean shift algorithm. By fully unsupervised, we mean the algorithm does not require any knowledge of the type of texture present nor the number of textures in the image to be segmented. The basic idea of the proposed method is to use the modified discrete wavelet frames to extract useful information from the image. Then, starting from the lowest level, the mean shift algorithm is used together with the fuzzy c-means clustering to divide the data into an appropriate number of clusters. The data clustering process is then refined at every level by taking into account the data at that particular level. The final crispy segmentation is obtained at the root level. This approach is applied to segment a variety of composite texture images into homogeneous texture areas and very good segmentation results are reported
Investigating dark matter substructure with pulsar timing: I. Constraints on ultracompact minihalos
Small-scale dark matter structure within the Milky Way is expected to affect
pulsar timing. The change in gravitational potential induced by a dark matter
halo passing near the line of sight to a pulsar would produce a varying delay
in the light travel time of photons from the pulsar. Individual transits
produce an effect that would either be too rare or too weak to be detected in
30-year pulsar observations. However, a population of dark matter subhalos
would be expected to produce a detectable effect on the measured properties of
pulsars if the subhalos constitute a significant fraction of the total halo
mass. The effect is to increase the dispersion of measured period derivatives
across the pulsar population. By statistical analysis of the ATNF pulsar
catalogue, we place an upper limit on this dispersion of . We use this to place strong upper limits on the number density of
ultracompact minihalos within the Milky Way. These limits are completely
independent of the particle nature of dark matter.Comment: 9 pages, 5 figues, includes erratum published in MNRA
Microlensing in phase space II: Correlations analysis
Applications of the phase space approach to the calculation of the
microlensing autocorrelation function are presented. The continuous propagation
equation for a random star field with a Gaussian velocity distribution is
solved in the leading non-trivial approximation using the perturbation
technique. It is shown that microlensing modulations can be important in the
interpretation of optical and shorter-wavelength light curves of pulsars, power
spectra of active galactic nuclei and coherence estimates for quasi-periodic
oscillations of dwarf novae and low-mass X-ray binaries. Extra scatter in the
brightness of type Ia supernovae due to gravitational microlensing is shown to
be of order up to 0.2 stellar magnitudes depending on the extent of the light
curves.Comment: Accepted for publication in MNRAS. 17 pages, 8 figures. The first
part of this little series is available at
http://www.arxiv.org/abs/astro-ph/0604302 . Replaced to add a link to the
first par
Chromatographic separation and identification of some peptides in partial hydroylsates of gelatin
Recently we have been engaged in a study of the chemical structure of collagen and gelatin with the object of determining the sequence of the amino acid residues in the polypeptide chains of these proteins. In the course of this study we have made considerable progress in the chromatographic analysis of complex mixtures of peptides and we have isolated and identified several simple peptides which occur in partial hydrolysates of gelatin. The initial separation of the mixture into zones of one or more peptides has been made on a column of ion exchange resin; further separation of the peptides in each zone has been achieved by chromatographing in the form of dinitrophenyl (DNP) peptides on columns of silicic acid-Celite. It is to be hoped that the particular combination of chromatographic methods which has been successfully used in the present study will be helpful in the resolution of the complex mixtures which result from the partial hydrolysis of other proteins
A mechanical fastening technique development for application in space Final report
Mechanical fastening technique development for aerospace applicatio
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