948 research outputs found
Short Gamma-Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts
To test whether the short GRB rates, redshift distribution and host galaxies
are consistent with current theoretical predictions, we use avery large
database of population synthesis calculations to examine BH-NS and NS-NS merger
rates in the universe, factoring in (i) the star formation history of the
universe, (ii) a heterogeneous population of star-forming galaxies, including
spirals and ellipticals, and (iii) a simple flux-limited selection model for
short GRB detection. When we require our models reproduce the known short GRB
rates and redshift measurements (and, for NS-NS, the merger rates extrapolated
from binary pulsars in the Galaxy), a small fraction of models reproduce all
observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most
commonly models produce mergers preferentially in spiral galaxies if short GRBs
arise from NS-NS mergers alone. Model universes where present-day binary
mergers occur preferentially in elliptical galaxies necessarily include a
significant fraction of binaries with long delay times between birth and merger
(often ). Though long delays occur, almost all of our models
predict that a higher proportion of short GRBs should occur at moderate to high
redshift (e.g., ) than has presently been observed, in agreement with
recent observations which suggest a selection bias towards successful follow-up
of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have
the right combination of masses and spins to make GRBs, then at best only a
small fraction of BH-NS models could be consistent with all {\em current}
available data. (Abridged)Comment: 14 figures, using bitmapped fonts (via eps2eps) to fit in archive
space restrictions; better resolution figures are available from the author.
Accepted for publication in ApJ. v3 updates reference
Constraining population synthesis models via the binary neutron star population
The observed sample of double neutron-star (NS-NS) binaries presents a
challenge to population-synthesis models of compact object formation: the
parameters entering into these models must be carefully chosen so as to match
(i) the observed star formation rate and (ii) the formation rate of NS-NS
binaries, which can be estimated from the observed sample and the selection
effects related to the discoveries with radio-pulsar surveys. In this paper, we
select from an extremely broad family of possible population synthesis models
those few (2%) which are consistent with the observed sample of NS-NS binaries.
To further sharpen the constraints the observed NS-NS population places upon
our understanding of compact-object formation processes, we separate the
observed NS-NS population into two channels: (i) merging NS-NS binaries, which
will inspiral and merge through the action of gravitational waves within
Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset
of astrophysically consistent models, we explore the implications for the rates
at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS
binaries will merge through the emission of gravitational waves.Comment: (v1) Submitted to ApJ. Uses emulateapj.cls. 8 pages, 7 figures. (v2)
Minor textual changes in response to referee queries. Substantial additions
in appendicies, including a detailed discussion of sample multidimensional
population synthesis fit
Spermatogenesis and sertoli cell activity in mice lacking Sertoli cell receptors for follicle stimulating hormone and androgen
Spermatogenesis in the adult male depends on the action of FSH and androgen. Ablation of either hormone has deleterious effects on Sertoli cell function and the progression of germ cells through spermatogenesis. In this study we generated mice lacking both FSH receptors (FSHRKO) and androgen receptors on the Sertoli cell (SCARKO) to examine how FSH and androgen combine to regulate Sertoli cell function and spermatogenesis. Sertoli cell number in FSHRKO-SCARKO mice was reduced by about 50% but was not significantly different from FSHRKO mice. In contrast, total germ cell number in FSHRKO-SCARKO mice was reduced to 2% of control mice (and 20% of SCARKO mice) due to a failure to progress beyond early meiosis. Measurement of Sertoli cell-specific transcript levels showed that about a third were independent of hormonal action on the Sertoli cell, whereas others were predominantly androgen dependent or showed redundant control by FSH and androgen. Results show that FSH and androgen act through redundant, additive, and synergistic regulation of spermatogenesis and Sertoli cell activity. In addition, the Sertoli cell retains a significant capacity for activity, which is independent of direct hormonal regulation
Comparing compact binary parameter distributions I: Methods
Being able to measure each merger's sky location, distance, component masses,
and conceivably spins, ground-based gravitational-wave detectors will provide a
extensive and detailed sample of coalescing compact binaries (CCBs) in the
local and, with third-generation detectors, distant universe. These
measurements will distinguish between competing progenitor formation models. In
this paper we develop practical tools to characterize the amount of
experimentally accessible information available, to distinguish between two a
priori progenitor models. Using a simple time-independent model, we demonstrate
the information content scales strongly with the number of observations. The
exact scaling depends on how significantly mass distributions change between
similar models. We develop phenomenological diagnostics to estimate how many
models can be distinguished, using first-generation and future instruments.
Finally, we emphasize that multi-observable distributions can be fully
exploited only with very precisely calibrated detectors, search pipelines,
parameter estimation, and Bayesian model inference
Sunn Hemp as an Alternative Forage in the Texas High Plains Region
Forage production in the Texas High Plains, a semi-arid region, is critical to sustain the local cattle industry. However, the main source of water for irrigation is the highly depleted Ogallala Aquifer, making forage crop water productivity (CWP) of high importance. In this one-year study, three types of forages were cultivated under deficit irrigation treatments of 80% and 50% of full crop water use. The forages were: a non-brown midrib forage sorghum; a legume, sunn hemp; and a sorghum/sunn hemp mix. The experiment was conducted in Bushland, Texas in 2019. Treatment plots were arranged in a split plot design with four replications under a 6-span variable rate irrigation center pivot sprinkler. Above ground biomass samples were taken by hand from a 1 m2 quadrat in each of the 24 plots on Jul 23, Aug 6 for forage analysis, and on Aug 19 to assess biomass yield and CWP. Crude protein was significantly higher at the first and second cuttings in the sunn hemp monocrop, but not significantly different between irrigation levels. The final biomass yields and CWP were similar between the sorghum (23.6 Mg ha-1) and sorghum/sunn hemp (25.1 Mg ha-1) forages, irrespective of irrigation level. The sunn hemp monocrop produced significantly lower biomass (9.75 Mg ha-1), which reduced CWP
Irreversibility and Polymer Adsorption
Physisorption or chemisorption from dilute polymer solutions often entails
irreversible polymer-surface bonding. We present a theory of the
non-equilibrium layers which result. While the density profile and loop
distribution are the same as for equilibrium layers, the final layer comprises
a tightly bound inner part plus an outer part whose chains make only fN surface
contacts where N is chain length. The contact fractions f follow a broad
distribution, P(f) ~ f^{-4/5}, in rather close agreement with strong
physisorption experiments [H. M. Schneider et al, Langmuir v.12, p.994 (1996)].Comment: 4 pages, submitted to Phys. Rev. Let
Mapping inspiral rates on population synthesis parameters
Formation rates of compact-object binaries are often derived from population
synthesis calculations. However, such calculations depend sensitively on a
relatively large number of model input parameters. Given considerable
uncertainty in those model parameters, the predicted inspiral rates for double
compact objects relevant to gravitational-wave interferometric detectors have
been shown to be are uncertain by several orders of magnitude. Typically,
inspiral rates are estimated for only a small set of models with a remarkably
poor coverage of the highly multi-dimensional parameter space (primarily
because of limited computer resources). Here, using as an example seven
population-synthesis model parameters, we show that it is possible to derive
fits of double-compact-object inspiral rates dependent simultaneously on all
seven parameters. We find these fits to be accurate to 50% for binary black
holes and to 40% for binary neutron stars. The availability of such fits
implies that (i) depending on the problem of interest, it is not necessary to
complete large numbers of computationally demanding population synthesis
calculations; and (ii) for the first time, the sufficient exploration of the
relevant phase space and the assessment of the uncertainties involved is not
limited by computational resources and becomes feasible. Finally, we have also
produced a histogram of the (a priori likely) binary black hole inpsiral rate,
assuming our population synthesis models are equally likely. This histogram,
effectively an a priori probability distribution for the BH-BH inspiral rate,
suggests that merger rate is conservatively bounded below by
10^(-8)/yr/Milky-Way-galaxy.Comment: Accepted Oct. 10 2004 for publication in Astrophysical Journa
Non-Equilibrium in Adsorbed Polymer Layers
High molecular weight polymer solutions have a powerful tendency to deposit
adsorbed layers when exposed to even mildly attractive surfaces. The
equilibrium properties of these dense interfacial layers have been extensively
studied theoretically. A large body of experimental evidence, however,
indicates that non-equilibrium effects are dominant whenever monomer-surface
sticking energies are somewhat larger than kT, a common case. Polymer
relaxation kinetics within the layer are then severely retarded, leading to
non-equilibrium layers whose structure and dynamics depend on adsorption
kinetics and layer ageing. Here we review experimental and theoretical work
exploring these non-equilibrium effects, with emphasis on recent developments.
The discussion addresses the structure and dynamics in non-equilibrium polymer
layers adsorbed from dilute polymer solutions and from polymer melts and more
concentrated solutions. Two distinct classes of behaviour arise, depending on
whether physisorption or chemisorption is involved. A given adsorbed chain
belonging to the layer has a certain fraction of its monomers bound to the
surface, f, and the remainder belonging to loops making bulk excursions. A
natural classification scheme for layers adsorbed from solution is the
distribution of single chain f values, P(f), which may hold the key to
quantifying the degree of irreversibility in adsorbed polymer layers. Here we
calculate P(f) for equilibrium layers; we find its form is very different to
the theoretical P(f) for non-equilibrium layers which are predicted to have
infinitely many statistical classes of chain. Experimental measurements of P(f)
are compared to these theoretical predictions.Comment: 29 pages, Submitted to J. Phys.: Condens. Matte
Transition from inspiral to plunge for eccentric equatorial Kerr orbits
Ori and Thorne have discussed the duration and observability (with LISA) of
the transition from circular, equatorial inspiral to plunge for stellar-mass
objects into supermassive () Kerr black holes. We
extend their computation to eccentric Kerr equatorial orbits. Even with orbital
parameters near-exactly determined, we find that there is no universal length
for the transition; rather, the length of the transition depends sensitively --
essentially randomly -- on initial conditions. Still, Ori and Thorne's
zero-eccentricity results are essentially an upper bound on the length of
eccentric transitions involving similar bodies (e.g., fixed). Hence the
implications for observations are no better: if the massive body is
, the captured body has mass , and the process occurs at
distance from LISA, then , with the precise constant depending on
the black hole spin. For low-mass bodies () for which the
event rate is at least vaguely understood, we expect little chance (probably
[much] less than 10%, depending strongly on the astrophysical assumptions) of
LISA detecting a transition event with during its run; however, even a
small infusion of higher-mass bodies or a slight improvement in LISA's noise
curve could potentially produce transition events during LISA's
lifetime.Comment: Submitted to PR
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Endoplasmic Reticulum Lumenal Indicators in Drosophila Reveal Effects of HSP-Related Mutations on Endoplasmic Reticulum Calcium Dynamics
Genes for endoplasmic reticulum (ER)-shaping proteins are among the most commonly mutated in hereditary spastic paraplegia (HSP). Mutation of these genes in model organisms can lead to disruption of the ER network. To investigate how the physiological roles of the ER might be affected by such disruption, we developed tools to interrogate its Ca2+ signaling function. We generated GAL4-driven Ca2+ sensors targeted to the ER lumen, to record ER Ca2+ fluxes in identified Drosophila neurons. Using GAL4 lines specific for Type Ib or Type Is larval motor neurons, we compared the responses of different lumenal indicators to electrical stimulation, in axons and presynaptic terminals. The most effective sensor, ER-GCaMP6-210, had a Ca2+ affinity close to the expected ER lumenal concentration. Repetitive nerve stimulation generally showed a transient increase of lumenal Ca2+ in both the axon and presynaptic terminals. Mutants lacking neuronal reticulon and REEP proteins, homologs of human HSP proteins, showed a larger ER lumenal evoked response compared to wild type; we propose mechanisms by which this phenotype could lead to neuronal dysfunction or degeneration. Our lines are useful additions to a Drosophila Ca2+ imaging toolkit, to explore the physiological roles of ER, and its pathophysiological roles in HSP and in axon degeneration more broadly
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