3,853 research outputs found
Improved asteroseismic inversions for red-giant surface rotation rates
Asteroseismic observations of internal stellar rotation have indicated a
substantial lack of angular momentum transport in theoretical models of
subgiant and red-giant stars. Accurate core and surface rotation rate
measurements are therefore needed to constrain internal transport processes
included in the models. We eliminate substantial systematic errors of
asteroseismic surface rotation rates found in previous studies. We propose a
new objective function for the Optimally Localized Averages method of
rotational inversions for red-giant stars, which results in more accurate
envelope rotation rate estimates obtained from the same data. We use synthetic
observations from stellar models across a range of evolutionary stages and
masses to demonstrate the improvement. We find that our new inversion technique
allows us to obtain estimates of the surface rotation rate that are independent
of the core rotation. For a star at the base of the red-giant branch, we reduce
the systematic error from about 20% to a value close to 0, assuming constant
envelope rotation. We also show the equivalence between this method and the
method of linearised rotational splittings. Our new rotational inversion method
substantially reduces the systematic errors of red-giant surface rotation
rates. In combination with independent measures of the surface rotation rate,
this will allow better constraints to be set on the internal rotation profile.
This will be a very important probe to further constrain the internal angular
momentum transport along the lower part of the red-giant branch.Comment: 17 pages, 22 figures, accepted for publication in Astronomy and
Astrophysic
Asteroseismic sensitivity to internal rotation along the red-giant branch
Transport of angular momentum in stellar interiors is currently not well
understood. Asteroseismology can provide us with estimates of internal rotation
of stars and thereby advances our understanding of angular momentum transport.
We can measure core-rotation rates in red-giant stars and we can place upper
bounds on surface-rotation rates using measurements of dipole () modes.
Here, we aim to determine the theoretical sensitivity of modes of different
spherical degree towards the surface rotation. Additionally, we aim to identify
modes that can potentially add sensitivity at intermediate radii. We used
asteroseismic rotational inversions to probe the internal stellar rotation
profiles in red-giant models from the base of the red-giant branch up to the
luminosity bump. We used the inversion method of multiplicative optimally
localised averages (MOLA) to assess how well internal and surface rotation
rates can be recovered from different mode sets and different synthetic
rotation profiles. We confirm that dipole mixed modes are sufficient to set
constraints on the average core-rotation rates in red giants. However,
surface-rotation rates estimated with only dipole mixed modes are contaminated
by the core rotation. We show that the sensitivity to the surface rotation
decreases from the base of the red-giant branch until it reaches a minimum at
0.6-0.8 due to a glitch in the buoyancy frequency. Thereafter a
narrow range of increased surface sensitivity just below the bump luminosity
exists. Quadrupole and octopole modes have more sensitivity in the outer parts
of the star. If observed, quadrupole and octopole modes enable us to
distinguish between differential and solid body rotation in the convection
zone. To obtain accurate estimates of rotation rates at intermediate radii,
acoustic oscillation modes with a spherical degree of are needed.Comment: accepted for publication in Astronomy and Astrophysics, revised
manuscript after language editin
Descending pathways from hypothalamus to dorsal motor vagus and ambiguus nuclei in the rat
The anatomical pathways between the hypothalamus and cell groups of the lower medulla that are involved in the neural control of endocrine pancreas activity were investigated. As part of this control system the descending pathways originating from lateral, dorsomedial and ventromedial hypothalamic nuclei towards the dorsal motor vagus and ambiguus nuclei, were studied by retrograde transport of horseradish peroxidase. Very small injections of the tracer, by means of the iontophoretic delivery method, were placed in the dorsal motor vagus, ambiguus and solitary tract nucleus as well as in the various nuclei of the medullary reticular formation. Subsequent retrograde labeling was studied in the hypothalamus and the brainstem. The appearance of considerable retrograde labeling in mesencephalic periventricular grey and rostral mesencephalic reticular formation indicated a possible role for these structures as intermediates in an indirect hypothalamo-medullary control circuitry. This led us to extend the peroxidase injections to these mesencephalic areas after which the hypothalamus was investigated for retrograde labeling. All data combined indicated the existence of three descending pathways, direct and indirect, between hypothalamus and the parasympathetic motor nuclei of the lower medulla.
Inverse analysis of asteroseismic data: a review
Asteroseismology has emerged as the best way to characterize the global and
internal properties of nearby stars. Often, this characterization is achieved
by fitting stellar evolution models to asteroseismic observations. The star
under investigation is then assumed to have the properties of the best-fitting
model, such as its age. However, the models do not fit the observations
perfectly. This is due to incorrect or missing physics in stellar evolution
calculations, resulting in predicted stellar structures that are discrepant
with reality. Through an inverse analysis of the asteroseismic data, it is
possible to go further than fitting stellar models, and instead infer details
about the actual internal structure of the star at some locations in its
interior. Comparing theoretical and observed stellar structures then enables
the determination of the locations where the stellar models have discrepant
structure, and illuminates a path for improvements to our understanding of
stellar evolution. In this invited review, we describe the methods of
asteroseismic inversions, and outline the progress that is being made towards
measuring the interiors of stars.Comment: 12 pages, 1 figure. Invited review, Dynamics of the Sun and Star
Solar Evolution Models with a Central Black Hole
Hawking proposed that the Sun may harbor a primordial black hole (BH) whose accretion supplies some of the solar luminosity. Such an object would have formed within the first 1 s after the Big Bang with the mass of a moon or an asteroid. These light BHs are a candidate solution to the dark matter problem, and could grow to become stellar-mass BHs if captured by stars. Here we compute the evolution of stars having such a BH at their center. We find that such objects can be surprisingly long-lived, with the lightest BHs having no influence over stellar evolution, while more massive ones consume the star over time to produce a range of observable consequences. Models of the Sun born about a BH whose mass has since grown to approximately 10−6M⊙ are compatible with current observations. In this scenario, the Sun would first dim to half its current luminosity over a span of 100 Myr as the accretion starts to generate enough energy to quench nuclear reactions. The Sun would then expand into a fully convective star, where it would shine luminously for potentially several gigayears with an enriched surface helium abundance, first as a sub-subgiant star, and later as a red straggler, before becoming a subsolar-mass BH. We also present results for a range of stellar masses and metallicities. The unique internal structures of stars harboring BHs may make it possible for asteroseismology to discover them, should they exist. We conclude with a list of open problems and predictions
A pilot randomized controlled trial to promote healthful fish consumption during pregnancy: The Food for Thought Study
Background: Nutritionists advise pregnant women to eat fish to obtain adequate docosahexaenoic acid (DHA), an essential nutrient important for optimal brain development. However, concern exists that this advice will lead to excess intake of methylmercury, a developmental neurotoxicant. Objective: Conduct a pilot intervention to increase consumption of high-DHA, low-mercury fish in pregnancy. Methods: In April-October 2010 we recruited 61 women in the greater Boston, MA area at 12–22 weeks gestation who consumed = 200mg/d of DHA from fish, compared with 33% in the Advice arm (p=0.005) and 53% in the Advice+GC arm (p=0.0002). We did not detect any differences in mercury intake or in biomarker levels of mercury and DHA between groups. Conclusions: An educational intervention increased consumption of fish and DHA but not mercury. Future studies are needed to determine intervention effects on pregnancy and childhood health outcomes. Trial registration Registered on clinicaltrials.gov as NCT0112676
Residual cognitive deficits 50 years after lead poisoning during childhood
The long term neurobehavioural consequences of childhood lead poisoning are not known. In this study adult subjects with a documented history of lead poisoning before age 4 and matched controls were examined with an abbreviated battery of neuropsychological tests including measures of attention, reasoning, memory, motor speed, and current mood. The subjects exposed to lead were inferior to controls on almost all of the cognitive tasks. This pattern of widespread deficits resembles that found in children evaluated at the time of acute exposure to lead rather than the more circumscribed pattern typically seen in adults exposed to lead. Despite having completed as many years of schooling as controls, the subjects exposed to lead were lower in lifetime occupational status. Within the exposed group, performance on the neuropsychological battery and occupational status were related, consistent with the presumed impact of limitations in neuropsychological functioning on everyday life. The results suggest that many subjects exposed to lead suffered acute encephalopathy in childhood which resolved into a chronic subclinical encephalopathy with associated cognitive dysfunction still evident in adulthood. These findings lend support to efforts to limit exposure to lead in childhood
Desynchronizing effect of high-frequency stimulation in a generic cortical network model
Transcranial Electrical Stimulation (TCES) and Deep Brain Stimulation (DBS)
are two different applications of electrical current to the brain used in
different areas of medicine. Both have a similar frequency dependence of their
efficiency, with the most pronounced effects around 100Hz. We apply
superthreshold electrical stimulation, specifically depolarizing DC current,
interrupted at different frequencies, to a simple model of a population of
cortical neurons which uses phenomenological descriptions of neurons by
Izhikevich and synaptic connections on a similar level of sophistication. With
this model, we are able to reproduce the optimal desynchronization around
100Hz, as well as to predict the full frequency dependence of the efficiency of
desynchronization, and thereby to give a possible explanation for the action
mechanism of TCES.Comment: 9 pages, figs included. Accepted for publication in Cognitive
Neurodynamic
Principles and Practices of Neurodevelopmental Assessment in Children: Lessons Learned from the Centers for Children’s Environmental Health and Disease Prevention Research
Principles and practices of pediatric neurotoxicology are reviewed here with the purpose of guiding the design and execution of the planned National Children’s Study. The developing human central nervous system is the target organ most vulnerable to environmental chemicals. An investigation of the effects of environmental exposures on child development is a complex endeavor that requires consideration of numerous critical factors pertinent to a study’s concept, design, and execution. These include the timing of neurodevelopmental assessment, matters of biologic plausibility, site, child and population factors, data quality assurance and control, the selection of appropriate domains and measures of neurobehavior, and data safety and monitoring. Here we summarize instruments for the assessment of the neonate, infant, and child that are being employed in the Centers for Children’s Environmental Health and Disease Prevention Research, sponsored by the National Institute of Environmental Health Sciences and the U.S. Environmental Protection Agency, discuss neural and neurobiologic measures of development, and consider the promises of gene–environment studies. The vulnerability of the human central nervous system to environmental chemicals has been well established, but the contribution these exposures may make to problems such as attention deficit disorder, conduct problems, pervasive developmental disorder, or autism spectrum disorder remain uncertain. Large-scale studies such as the National Children’s Study may provide some important clues. The human neurodevelopmental phenotype will be most clearly represented in models that include environmental chemical exposures, the social milieu, and complex human genetic characteristics that we are just beginning to understand
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