6,043 research outputs found
New constraints on the major neutron source in low-mass AGB stars
We compare updated Torino postprocessing asymptotic giant branch (AGB)
nucleosynthesis model calculations with isotopic compositions of mainstream SiC
dust grains from low-mass AGB stars. Based on the data-model comparison, we
provide new constraints on the major neutron source, 13C({\alpha},n)16O in the
He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba
grain data can only be consistently explained by the Torino model calculations
that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This
observation provides strong support to the suggestion of deep mixing of H into
the He-intershell at low 13C concentrations as a result of efficient transport
of H through magnetic tubes.Comment: ApJ, accepte
Superconducting Cosmc Strings and Primordial Magnetic Fields
We consider grand unified theories with superconducting cosmic strings and
which admit the mechanism for generating primordial magnetic fields recently
discussed by Vachaspati. We show that these models are severely constrained by
cosmological arguments. Quite generically, either stable springs or vortons
will form. Provided the mass per unit length of the strings is sufficiently
large, these stable configurations will overclose the Universe.Comment: BROWN-HET-830, 14 pages, use phyzz
Alpha1 -adrenergic stimulation selectively enhances endothelium-mediated vasodilation in rat cremaster arteries.
We have systematically investigated how vascular smooth muscle α1 -adrenoceptor activation impacts endothelium-mediated vasodilation in isolated, myogenically active, rat cremaster muscle 1A arteries. Cannulated cremaster arteries were pressurized intraluminally to 70 mmHg to induce myogenic tone, and exposed to vasoactive agents via bath superfusion at 34°C. Smooth muscle membrane potential was measured via sharp microelectrode recordings in pressurized, myogenic arteries. The α1 -adrenergic agonist phenylephrine (25-100 nmol/L) produced further constriction of myogenic arteries, but did not alter the vasorelaxant responses to acetylcholine (0.3 μmol/L), SKA-31 (an activator of endothelial Ca2+ -dependent K+ channels) (3 μmol/L) or sodium nitroprusside (10 μmol/L). Exposure to 0.25-1 μmol/L phenylephrine or 1 μmol/L norepinephrine generated more robust constrictions, and also enhanced the vasodilations evoked by acetylcholine and SKA-31, but not by sodium nitroprusside. In contrast, the thromboxane receptor agonist U46619 (250 nmol/L) dampened responses to all three vasodilators. Phenylephrine exposure depolarized myogenic arteries, and mimicking this effect with 4-aminopyridine (1 mmol/L) was sufficient to augment the SKA-31-evoked vasodilation. Inhibition of L-type Ca2+ channels by 1 μmol/L nifedipine decreased myogenic tone, phenylephrine-induced constriction and prevented α1 -adrenergic enhancement of endothelium-evoked vasodilation; these latter deficits were overcome by exposure to 3 and 10 μmol/L phenylephrine. Mechanistically, augmentation of ACh-evoked dilation by phenylephrine was dampened by eNOS inhibition and abolished by blockade of endothelial KCa channels. Collectively, these data suggest that increasing α1 -adrenoceptor activation beyond a threshold level augments endothelium-evoked vasodilation, likely by triggering transcellular signaling between smooth muscle and the endothelium. Physiologically, this negative feedback process may serve as a "brake" to limit the extent of vasoconstriction in the skeletal microcirculation evoked by the elevated sympathetic tone
Excitation spectrum of bosons in a finite one-dimensional circular waveguide via the Bethe ansatz
The exactly solvable Lieb-Liniger model of interacting bosons in
one-dimension has attracted renewed interest as current experiments with
ultra-cold atoms begin to probe this regime. Here we numerically solve the
equations arising from the Bethe ansatz solution for the exact many-body wave
function in a finite-size system of up to twenty particles for attractive
interactions. We discuss the novel features of the solutions, and how they
deviate from the well-known string solutions [H. B. Thacker, Rev. Mod. Phys.\
\textbf{53}, 253 (1981)] at finite densities. We present excited state string
solutions in the limit of strong interactions and discuss their physical
interpretation, as well as the characteristics of the quantum phase transition
that occurs as a function of interaction strength in the mean-field limit.
Finally we compare our results to those of exact diagonalization of the
many-body Hamiltonian in a truncated basis. We also present excited state
solutions and the excitation spectrum for the repulsive 1D Bose gas on a ring.Comment: 13 pages, 12 figure
Meta-Learning Strategies through Value Maximization in Neural Networks
Biological and artificial learning agents face numerous choices about how to
learn, ranging from hyperparameter selection to aspects of task distributions
like curricula. Understanding how to make these meta-learning choices could
offer normative accounts of cognitive control functions in biological learners
and improve engineered systems. Yet optimal strategies remain challenging to
compute in modern deep networks due to the complexity of optimizing through the
entire learning process. Here we theoretically investigate optimal strategies
in a tractable setting. We present a learning effort framework capable of
efficiently optimizing control signals on a fully normative objective:
discounted cumulative performance throughout learning. We obtain computational
tractability by using average dynamical equations for gradient descent,
available for simple neural network architectures. Our framework accommodates a
range of meta-learning and automatic curriculum learning methods in a unified
normative setting. We apply this framework to investigate the effect of
approximations in common meta-learning algorithms; infer aspects of optimal
curricula; and compute optimal neuronal resource allocation in a continual
learning setting. Across settings, we find that control effort is most
beneficial when applied to easier aspects of a task early in learning; followed
by sustained effort on harder aspects. Overall, the learning effort framework
provides a tractable theoretical test bed to study normative benefits of
interventions in a variety of learning systems, as well as a formal account of
optimal cognitive control strategies over learning trajectories posited by
established theories in cognitive neuroscience.Comment: Under Revie
FLOWERING LOCUS C -dependent and -independent regulation of the circadian clock by the autonomous and vernalization pathways
Background
The circadian system drives pervasive biological rhythms in plants. Circadian clocks integrate endogenous timing information with environmental signals, in order to match rhythmic outputs to the local day/night cycle. Multiple signaling pathways affect the circadian system, in ways that are likely to be adaptively significant. Our previous studies of natural genetic variation in Arabidopsis thaliana accessions implicated FLOWERING LOCUS C (FLC) as a circadian-clock regulator. The MADS-box transcription factor FLC is best known as a regulator of flowering time. Its activity is regulated by many regulatory genes in the "autonomous" and vernalization-dependent flowering pathways. We tested whether these same pathways affect the circadian system.
Results
Genes in the autonomous flowering pathway, including FLC, were found to regulate circadian period in Arabidopsis. The mechanisms involved are similar, but not identical, to the control of flowering time. By mutant analyses, we demonstrate a graded effect of FLC expression upon circadian period. Related MADS-box genes had less effect on clock function. We also reveal an unexpected vernalization-dependent alteration of periodicity.
Conclusion
This study has aided in the understanding of FLC's role in the clock, as it reveals that the network affecting circadian timing is partially overlapping with the floral-regulatory network. We also show a link between vernalization and circadian period. This finding may be of ecological relevance for developmental programing in other plant species
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