21,942 research outputs found
Environmental conditions during early life accelerate the rate of senescence in a short-lived passerine bird
Environmental conditions experienced in early life may shape subsequent phenotypic traits including life history. We investigated how predation risk caused by domestic cats (Felis silvestris catus) and local breeding density affected patterns of reproductive and survival senescence in Barn Swallows (Hirundo rustica) breeding semicolonially in Denmark. We recorded the abundance of cats and the number of breeding pairs at 39 breeding sites during 24 years and related these to age-specific survival rate and reproductive senescence to test predictions of the life history theory of senescence. We found evidence for actuarial senescence for the first time in this species. Survival rate increased until reaching a plateau in midlife and then decreased later. We also found that survival rate was higher for males than females. Local breeding density or predation risk did not affect survival as predicted by theory. Barn Swallows with short lives did not invest more in reproduction in early life, inconsistent with expectations for trade-offs between reproduction and survival as theory suggests. However, we found that the rate of reproductive decline during senescence was steeper for individuals exposed to intense competition, and predation pressure accelerated the rate of reproductive senescence, but only in sites with many breeding pairs. These latter results are in accordance with one of the predictions suggested by the life history theory of aging. These results emphasize the importance of considering intraspecific competition and interspecific interactions such as predation when analyzing reproductive and actuarial senescence
High-energy magnon dispersion and multi-magnon continuum in the two-dimensional Heisenberg antiferromagnet
We use quantum Monte Carlo simulations and numerical analytic continuation to
study high-energy spin excitations in the two-dimensional S=1/2 Heisenberg
antiferromagnet at low temperature. We present results for both the transverse
and longitudinal dynamic spin structure factor S(q,w) at q=(pi,0) and
(pi/2,pi/2). Linear spin-wave theory predicts no dispersion on the line
connecting these momenta. Our calculations show that in fact the magnon energy
at (pi,0) is 10% lower than at (pi/2,pi/2). We also discuss the transverse and
longitudinal multi-magnon continua and their relevance to neutron scattering
experiments.Comment: 4 page
An Enhanced Perturbational Study on Spectral Properties of the Anderson Model
The infinite- single impurity Anderson model for rare earth alloys is
examined with a new set of self-consistent coupled integral equations, which
can be embedded in the large expansion scheme ( is the local spin
degeneracy). The finite temperature impurity density of states (DOS) and the
spin-fluctuation spectra are calculated exactly up to the order . The
presented conserving approximation goes well beyond the -approximation
({\em NCA}) and maintains local Fermi-liquid properties down to very low
temperatures. The position of the low lying Abrikosov-Suhl resonance (ASR) in
the impurity DOS is in accordance with Friedel's sum rule. For its shift
toward the chemical potential, compared to the {\em NCA}, can be traced back to
the influence of the vertex corrections. The width and height of the ASR is
governed by the universal low temperature energy scale . Temperature and
degeneracy -dependence of the static magnetic susceptibility is found in
excellent agreement with the Bethe-Ansatz results. Threshold exponents of the
local propagators are discussed. Resonant level regime () and intermediate
valence regime () of the model are thoroughly
investigated as a critical test of the quality of the approximation. Some
applications to the Anderson lattice model are pointed out.Comment: 19 pages, ReVTeX, no figures. 17 Postscript figures available on the
WWW at http://spy.fkp.physik.th-darmstadt.de/~frithjof
Modulated phases in a three-dimensional Maier-Saupe model with competing interactions
This work is dedicated to the study of the discrete version of the Maier-Saupe model in the presence of competing interactions. The competition between interactions favoring different orientational ordering produces a rich phase diagram including modulated phases. Using a mean-field approach and Monte Carlo simulations, we show that the proposed model exhibits isotropic and nematic phases and also a series of modulated phases that meet at a multicritical point, a Lifshitz point. Though the Monte Carlo and mean-field phase diagrams show some quantitative disagreements, the Monte Carlo simulations corroborate the general behavior found within the mean-field approximation.We thank P. Gomes, R. Kaul, G. Landi, M. Oliveira, R. Oliveira, and S. Salinas for useful discussions and suggestions. P.F.B. was supported by Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) and the Condensed Matter Theory Visitors Program at Boston University. N.X. and A.W.S. were funded in part by the NSF under Grant No. DMR-1410126. Some of the calculations were carried out on Boston University's Shared Computing Cluster. (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Condensed Matter Theory Visitors Program at Boston University; DMR-1410126 - NSF)Accepted manuscrip
Measurement-dependent corrections to work distributions arising from quantum coherences
For a quantum system undergoing a unitary process work is commonly defined
based on the Two Projective Measurement (TPM) protocol which measures the
energies of the system before and after the process. However, it is well known
that projective measurements disregard quantum coherences of the system with
respect to the energy basis, thus removing potential quantum signatures in the
work distribution. Here we consider weak measurements of the system's energy
difference and establish corrections to work averages arising from initial
system coherences. We discuss two weak measurement protocols that couple the
system to a detector, prepared and measured either in the momentum or the
position eigenstates. Work averages are derived for when the system starts in
the proper thermal state versus when the initial system state is a pure state
with thermal diagonal elements and coherences characterised by a set of phases.
We show that by controlling only the phase differences between the energy
eigenstate contributions in the system's initial pure state, the average work
done during the same unitary process can be controlled. By changing the phases
alone one can toggle from regimes where the systems absorbs energy, i.e. a work
cost, to the ones where it emits energy, i.e. work can be drawn. This suggests
that the coherences are additional resources that can be used to manipulate or
store energy in a quantum system.Comment: 9 pages, 3 figure
Galactic consequences of clustered star formation
If all stars form in clusters and both the stars and the clusters follow a
power law distribution which favours the creation of low mass objects, then the
numerous low mass clusters will be deficient in high mass stars. Therefore, the
mass function of stars, integrated over the whole galaxy (the Integrated
Galactic Initial Mass Function, IGIMF) will be steeper at the high mass end
than the underlying IMF of the stars. We show how the steepness of the IGIMF
depends on the sampling method and on the assumptions made for the star cluster
mass function. We also investigate the O-star content, integrated photometry
and chemical enrichment of galaxies that result from several IGIMFs, as
compared to more standard IMFs.Comment: 4 pages, 2 figures, to appear in online version of Proceedings of IAU
S266, a two page version will appear in the Proceedings of IAU S26
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Neutrino fluence after r-process freeze-out and abundances of Te isotopes in presolar diamonds
Using the data of Richter et al. (1998) on Te isotopes in diamond grains from
a meteorite, we derive bounds on the neutrino fluence and the decay timescale
of the neutrino flux relevant for the supernova r-process. Our new bound on the
neutrino fluence F after freeze-out of the r-process peak at mass number A =
130 is more stringent than the previous bound F < 0.045 (in units of 10**37
erg/cm**2) of Qian et al. (1997) and Haxton et al. (1997) if the neutrino flux
decays on a timescale tau > 0.65 s. In particular, it requires that a fluence
of F = 0.031 be provided by a neutrino flux with tau < 0.84 s. Such a fluence
may be responsible for the production of the solar r-process abundances at A =
124-126 (Qian et al. 1997; Haxton et al. 1997). Our results are based on the
assumption that only the stable nuclei implanted into the diamonds are retained
while the radioactive ones are lost from the diamonds upon decay after
implantation (Ott 1996). We consider that the nanodiamonds are condensed in an
environment with C/O > 1 in the expanding supernova debris or from the exterior
H envelope. The implantation of nuclei would have occurred 10**4-10**6 s after
r-process freeze-out. This time interval may be marginally sufficient to permit
adequate cooling upon expansion for the formation of diamond grains. The
mechanisms of preferential retention/loss of the implanted nuclei are not well
understood.Comment: AASTeX, 11 pages, 3 Postscript figure
Time Diary Measures of Investment in Young Children
There is a rising importance of human capital in the total capital stock of an economy. In this paper we discuss the nature of investments in children and sketch out the diverse kinds of inputs to the developmental process across different countries at selected time points. The observable inputs are time and goods and they can arise both in the home and in formal care settings. Estimates are given for Sweden and the U.S. The mere size of the total of these investments in early human capital, more than 20 percent of GDP for Sweden, raises a number of interesting questions.
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