611 research outputs found
Tidally Induced Oscillations and Orbital Decay in Compact Triple-Star Systems
We investigate the nature of tidal effects in compact triple-star systems.
The hierarchical structure of a triple system produces tidal forcing at high
frequencies unobtainable in binary systems, allowing for the tidal excitation
of high frequency p-modes in the stellar components. The tidal forcing exists
even for circular, aligned, and synchronized systems. We calculate the
magnitude and frequencies of three-body tidal forcing on the central primary
star for circular and coplanar orbits, and we estimate the amplitude of the
tidally excited oscillation modes. We also calculate the secular orbital
changes induced by the tidally excited modes, and show that they can cause
significant orbital decay. During certain phases of stellar evolution, the
tidal dissipation may be greatly enhanced by resonance locking. We then compare
our theory to observations of HD 181068, which is a hierarchical triply
eclipsing star system in the Kepler field of view. The observed oscillation
frequencies in HD 181068 can be naturally explained by three-body tidal
effects. We then compare the observed oscillation amplitudes and phases in HD
181068 to our predictions, finding mostly good agreement. Finally, we discuss
the past and future evolution of compact triple systems like HD 181068.Comment: 19 pages, 8 figures, published in MNRA
The shape distribution of asteroid families -- evidence for evolution driven by small impacts
A statistical analysis of brightness variability of asteroids reveals how
their shapes evolve from elongated to rough spheroidal forms, presumably driven
by impact-related phenomena. Based on the Sloan Digital Sky Survey Moving
Object Catalog, we determined the shape distribution of 11,735 asteroids, with
special emphasis on eight prominent asteroid families. In young families,
asteroids have a wide range of shape elongations, implying
fragmentation-formation. In older families we see an increasing number of rough
spheroids, in agreement with the predictions of an impact-driven evolution. Old
families also contain a group of moderately elongated members, which we suggest
correspond to higher-density, more impact-resistant cores of former fragmented
asteroids that have undergone slow shape erosion. A few percent of asteroids
have very elongated shapes, and can either be young fragments or tidally
reshaped bodies. Our results confirm that the majority of asteroids are
gravitationally bound ``rubble piles''.Comment: Accepted by Icarus. 22 pages, 1 table, 6 figures, 31 figure panel
Implication of two new paradigms for futures studies
The paper considers the emergence of two recent perspectives in futures work. One is evolutionary futures studies. The other is critical futures studies. After describing aspects of
each, the paper considers them as alternative rival paradigms in relation to criteria that include: the role of the human being as a subject, the role of interpretation and differences in methodological premises. It concludes that both have contributed to the development of futures methods but that a number of theoretical and methodological problems still remain unsolved
A Mesh-Free Solid-Mechanics Approach for Simulating the Friction Stir-Welding Process
In this chapter, we describe the development of a new approach to simulate the friction stir-welding (FSW) process using a solid-mechanics formulation of a mesh-free Lagrangian method called smoothed particle hydrodynamics (SPH). Although this type of a numerical model typically requires long calculation times, we have developed a very efficient parallelization strategy on the graphics processing unit (GPU). This simulation approach allows the determination of temperature evolution, elastic and plastic deformation, defect formation, residual stresses, and material flow all within the same model. More importantly, the large plastic deformation and material mixing common to FSW are well captured by the mesh-free method. The parallel strategy on the GPU provides a means to obtain meaningful simulation results within hours as opposed to many days or even weeks with conventional FSW simulation codes
High-resolution spectroscopic view of planet formation sites
Theories of planet formation predict the birth of giant planets in the inner,
dense, and gas-rich regions of the circumstellar disks around young stars.
These are the regions from which strong CO emission is expected. Observations
have so far been unable to confirm the presence of planets caught in formation.
We have developed a novel method to detect a giant planet still embedded in a
circumstellar disk by the distortions of the CO molecular line profiles
emerging from the protoplanetary disk's surface. The method is based on the
fact that a giant planet significantly perturbs the gas velocity flow in
addition to distorting the disk surface density. We have calculated the
emerging molecular line profiles by combining hydrodynamical models with
semianalytic radiative transfer calculations. Our results have shown that a
giant Jupiter-like planet can be detected using contemporary or future
high-resolution near-IR spectrographs such as VLT/CRIRES or ELT/METIS. We have
also studied the effects of binarity on disk perturbations. The most
interesting results have been found for eccentric circumprimary disks in
mid-separation binaries, for which the disk eccentricity - detectable from the
asymmetric line profiles - arises from the gravitational effects of the
companion star. Our detailed simulations shed new light on how to constrain the
disk kinematical state as well as its eccentricity profile. Recent findings by
independent groups have shown that core-accretion is severely affected by disk
eccentricity, hence detection of an eccentric protoplanetary disk in a young
binary system would further constrain planet formation theories.Comment: IAU Symposium 276 (contributed talk
Effect of Ketamine on Dendritic Arbor Development and Survival of Immature GABAergic Neurons In Vitro
Ketamine, a noncompetitive antagonist of the N-methyl-D-aspartate type of glutamate receptors, was reported to induce neuronal cell death when administered to produce anesthesia in young rodents and monkeys. Subanesthetic doses of ketamine, as adjuvant to postoperative sedation and pain control, are also frequently administered to young children. However, the effects of these low concentrations of ketamine on neuronal development remain unknown. The present study was designed to evaluate the effects of increasing concentrations (0.01-40 μg/ml) and durations (1-96 h) of ketamine exposure on the differentiation and survival of immature γ-aminobutyric acidergic (GABAergic) interneurons in culture. In line with previous studies (Scallet et al., 2004), we found that a 1-h-long exposure to ketamine at concentrations ≥ 10 μg/ml was sufficient to trigger cell death. At lower concentrations of ketamine, cell loss was only observed when this drug was chronically (> 48 h) present in the culture medium. Most importantly, we found that a single episode of 4-h-long treatment with 5 μg/ml ketamine induced long-term alterations in dendritic growth, including a significant (p 24 h) of neurons to ketamine at concentrations as low as 0.01 μg/ml also severely impaired dendritic arbor development. These results suggest that, in addition to its dose-dependent ability to induce cell death, even very low concentrations of ketamine could interfere with dendritic arbor development of immature GABAergic neurons and thus could potentially interfere with the development neural network
The Polysialylated Neural Cell Adhesion Molecule Promotes Neurogenesis in vitro
A characteristic feature of neurogenic sites in the postnatal brain is the expression of the polysialylated forms of the neural cell adhesion molecule (PSA-NCAM). To investigate the role of PSA-NCAM in generation of neuronal populations, we developed an in vitro model where neurogenesis occurs in primary cortical cultures following serum withdrawal. We show that removal or inactivation of the PSA tail of NCAM in these cultures leads to a significant decrease in the number of newly generated neurons. Similarly, cultures prepared from NCAM knock-out mice exhibit a significantly reduced neurogenesis. Pulse-chase experiments using the proliferation marker BrdU reveal that the lack of PSA does not affect the mitotic rate of neural progenitors but rather, it reduces the early survival of newly generated neurons. These results suggest that, in addition to its role in the migration of neuronal progenitors, PSA-NCAM is required for the adequate survival of these cell
VEGF is a chemoattractant for FGF-2–stimulated neural progenitors
Mmigration of undifferentiated neural progenitors is critical for the development and repair of the nervous system. However, the mechanisms and factors that regulate migration are not well understood. Here, we show that vascular endothelial growth factor (VEGF)-A, a major angiogenic factor, guides the directed migration of neural progenitors that do not display antigenic markers for neuron- or glia-restricted precursor cells. We demonstrate that progenitor cells express both VEGF receptor (VEGFR) 1 and VEGFR2, but signaling through VEGFR2 specifically mediates the chemotactic effect of VEGF. The expression of VEGFRs and the chemotaxis of progenitors in response to VEGF require the presence of fibroblast growth factor 2. These results demonstrate that VEGF is an attractive guidance cue for the migration of undifferentiated neural progenitors and offer a mechanistic link between neurogenesis and angiogenesis in the nervous system
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