19,202 research outputs found
Phase-field approach to heterogeneous nucleation
We consider the problem of heterogeneous nucleation and growth. The system is
described by a phase field model in which the temperature is included through
thermal noise. We show that this phase field approach is suitable to describe
homogeneous as well as heterogeneous nucleation starting from several general
hypotheses. Thus we can investigate the influence of grain boundaries,
localized impurities, or any general kind of imperfections in a systematic way.
We also put forward the applicability of our model to study other physical
situations such as island formation, amorphous crystallization, or
recrystallization.Comment: 8 pages including 7 figures. Accepted for publication in Physical
Review
Males Feeding Females During Incubation. I. Required by Microclimate or Constrained by Nest Predation?
Nest attentiveness (percentage of time spent on the nest) during incubation represents a parent-offspring conflict; incubating birds must balance a trade-off between caring for embryos by staying on the nest versus caring for themselves by getting off the nest to forage. For species in which females are the sole incubator, males can potentially affect this trade-off and increase nest attentiveness by feeding incubating females on the nest (incubation feeding). Increased nest attentiveness may be required when local microclimate conditions are harsh and thereby require greater incubation feeding (microclimate hypothesis). Alternatively, incubation feeding may be constrained by risk of attracting nest predators (nest predation hypothesis), which in turn may constrain female nest attentiveness because of energy limitation. We show that incubation feeding rates are much greater among cavity-nesting than among coexisting open-nesting birds. Under the microclimate hypothesis, the greater incubation feeding rates of cavity-nesting birds generate the prediction that microclimate should be harsher than for open-nesting birds. Our results reject this hypothesis because we found the opposite pattern; cavity-nesting birds experienced more moderate (less variable) microclimates that were less often below temperatures (i.e., 16 degrees C) that can negatively impact eggs compared with open-nesting species. In contrast, incubation feeding rates were highly negatively correlated with nest predation both within and between the two nest types, supporting the nest predation hypothesis. Incubation feeding in turn was positively correlated with nest attentiveness. Thus, nest predation may indirectly affect female incubation behavior by directly affecting incubation feeding by the male
Estimating the masses of extra-solar planets
All extra-solar planet masses that have been derived spectroscopically are
lower limits since the inclination of the orbit to our line-of-sight is unknown
except for transiting systems. It is, however, possible to determine the
inclination angle, i, between the rotation axis of a star and an observer's
line-of-sight from measurements of the projected equatorial velocity (v sin i),
the stellar rotation period (P_rot) and the stellar radius (R_star). This
allows the removal of the sin i dependency of spectroscopically derived
extra-solar planet masses under the assumption that the planetary orbits lie
perpendicular to the stellar rotation axis. We have carried out an extensive
literature search and present a catalogue of v sin i, P_rot, and R_star
estimates for exoplanet host stars. In addition, we have used Hipparcos
parallaxes and the Barnes-Evans relationship to further supplement the R_star
estimates obtained from the literature. Using this catalogue, we have obtained
sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper
1-sigma two-tailed confidence limits to be placed on the derived sin i's along
with the transit probability for each planet to be determined. While a small
proportion of systems yield sin i's significantly greater than 1, most likely
due to poor P_rot estimations, the large majority are acceptable. We are
further encouraged by the cases where we have data on transiting systems, as
the technique indicates inclinations of ~90 degrees and high transit
probabilities. In total, we estimate the true masses of 133 extra-solar
planets. Of these, only 6 have revised masses that place them above the 13
Jupiter mass deuterium burning limit. Our work reveals a population of
high-mass planets with low eccentricities and we speculate that these may
represent the signature of different planetary formation mechanisms at work.Comment: 40 pages, 6 tables, 2 figures. Accepted for publication in the
Monthly Notices of the Royal Astronomical Society after editing of Tables 1 &
6 for electronic publication. Html abstract shortened for astro-ph submissio
Application of a new screening model to thermonuclear reactions of the rp process
A new screening model for astrophysical thermonuclear reactions was derived
recently which improved Salpeter's weak-screening one. In the present work we
prove that the new model can also give very reliable screening enhancement
factors (SEFs) when applied to the rp process. According to the results of the
new model, which agree well with Mitler's SEFs, the screened rp reaction rates
can be, at most, twice as fast as the unscreened ones.Comment: 8 RevTex pages + 7 ps figures. (Revised version). Accepted for
publication in Journal of Physics
Treating and Preventing Influenza in Aged Care Facilities: A Cluster Randomised Controlled Trial
PMCID: PMC3474842This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
On the Backbending Mechanism of Cr
The mechanism of backbending in Cr is investigated in terms of the
Projected Shell Model and the Generator Coordinate Method. It is shown that
both methods are reasonable shell model truncation schemes. These two quite
different quantum mechanical approaches lead to a similar conclusion that the
backbending is due to a band crossing involving an excited band which is built
on simultaneously broken neutron and proton pairs in the ``intruder'' subshell
. It is pointed out that this type of band crossing is usually known
to cause the second backbending in rare-earth nuclei.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities
A prominence eruption on 7 June 2011 produced spectacular curtains of plasma
falling through the lower corona. At the solar surface they created an
incredible display of extreme ultraviolet brightenings. The aim is to identify
and analyze some of the local instabilities which produce structure in the
falling plasma. The structures were investigated using SDO/AIA 171A and 193A
images in which the falling plasma appeared dark against the bright coronal
emission. Several instances of the Rayleigh-Taylor instability were
investigated. In two cases the Alfven velocity associated with the dense plasma
could be estimated from the separation of the Rayleigh-Taylor fingers. A second
type of feature, which has the appearance of self-similar branching horns, is
also discussed.Comment: 6 pages, 7 figures, submitted A&A. Movies are at
http://www.mps.mpg.de/data/outgoing/innes/arcs_movie.avi and
http://www.mps.mpg.de/data/outgoing/innes/horns_movie.av
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