60,544 research outputs found
Searching for jet rotation in Class 0/I sources observed with GEMINI/GNIRS.
Original article can be found at: http://www.aanda.org/
Copyright The European Southern ObservatoryContext: In recent years, there has been a number of detections of gradients in the radial velocity profile across jets from young stars. The significance of these results is considerable. They may be interpreted as a signature of jet rotation about its symmetry axis, thereby representing the only existing observational indications supporting the theory that jets extract angular momentum from star-disk systems. However, the possibility that we are indeed observing jet rotation in pre-main sequence systems is undergoing active debate.
Aims: To test the validity of a rotation argument, we must extend the survey to a larger sample, including younger sources.
Methods: We present the latest results of a radial velocity analysis on jets from Class 0 and I sources, using high resolution data from the infrared spectrograph GNIRS on GEMINI South. We obtained infrared spectra of protostellar jets HH 34, HH 111-H, HH 212 NK1 and SK1.
Results: The [Fe II] emission was unresolved in all cases and so Doppler shifts across the jet width could not be accessed. The H2 emission was resolved in all cases except HH 34. Doppler profiles across the molecular emission were obtained, and gradients in radial velocity of typically 3 km s-1 identified.
Conclusions: Agreement with previous studies implies they may be interpreted as jet rotation, leading to toroidal velocity and angular momentum flux estimates of 1.5 km s-1 and 1 × 10-5 yr-1 AU km s-1 respectively. However, caution is needed. For example, emission is asymmetric across the jets from HH 212 suggesting a more complex interpretation is warranted. Furthermore, observations for HH 212 and HH 111-H are conducted far from the source implying external influences are more likely to confuse the intrinsic flow kinematics. These observations demonstrate the difficulty of conducting this study from the ground, and highlight the necessity for high angular resolution via adaptive optics or space-based facilities
Housing flexibility effects on rotor stability
Preliminary rotordynamic evaluations are performed with a housing stiffness assumption that is typically determined only after the hardware is built. In addressing rotor stability, a rigid housing assumption was shown to predict an instability at a lower spin speed than a comparable flexible housing analysis. This rigid housing assumption therefore provides a conservative estimate of the stability threshold speed. A flexible housing appears to act as an energy absorber and dissipated some of the destabilizing force. The fact that a flexible housing is usually asymmetric and considerably heavier than the rotor was related to this apparent increase in rotor stability. Rigid housing analysis is proposed as a valuable screening criteria and may save time and money in construction of elaborate housing finite element models for linear stability analyses
Rotator and extender ferroelectrics: Importance of the shear coefficient to the piezoelectric properties of domain-engineered crystals and ceramics
The importance of a high shear coefficient d15 (or d24) to the piezoelectric
properties of domain-engineered and polycrystalline ferroelectrics is
discussed. The extent of polarization rotation, as a mechanism of piezoelectric
response, is directly correlated to the shear coefficient. The terms "rotator"
and "extender" are introduced to distinguish the contrasting behaviors of
crystals such as 4mm BaTiO3 and PbTiO3. In "rotator" ferroelectrics, where d15
is high relative to the longitudinal coefficient d33, polarization rotation is
the dominant mechanism of piezoelectric response; the maximum longitudinal
piezoelectric response is found away from the polar axis. In "extender"
ferroelectrics, d15 is low and the collinear effect dominates; the maximum
piezoelectric response is found along the polar axis. A variety of 3m, mm2 and
4mm ferroelectrics, with various crystal structures based on oxygen octahedra,
are classified in this way. It is shown that the largest piezoelectric
anisotropies d15/d33 are always found in 3m crystals; this is a result of the
intrinsic electrostrictive anisotropy of the constituent oxygen octahedra.
Finally, for a given symmetry, the piezoelectric anisotropy increases close to
ferroelectric-ferroelectric phase transitions; this includes morphotropic phase
boundaries and temperature induced polymorphic transitions.Comment: accepted in J. Appl. Phy
Self-stabilised fractality of sea-coasts through damped erosion
Erosion of rocky coasts spontaneously creates irregular seashores. But the
geometrical irregularity, in turn, damps the sea-waves, decreasing the average
wave amplitude. There may then exist a mutual self-stabilisation of the waves
amplitude together with the irregular morphology of the coast. A simple model
of such stabilisation is studied. It leads, through a complex dynamics of the
earth-sea interface, to the appearance of a stationary fractal seacoast with
dimension close to 4/3. Fractal geometry plays here the role of a morphological
attractor directly related to percolation geometry.Comment: 4 pages, 5 figure
A new numerical method to construct binary neutron star initial data
We present a new numerical method for the generation of binary neutron star
initial data using a method along the lines of the the Wilson-Mathews or the
closely related conformal thin sandwich approach. Our method uses six different
computational domains, which include spatial infinity. Each domain has its own
coordinates which are chosen such that the star surfaces always coincide with
domain boundaries. These properties facilitate the imposition of boundary
conditions. Since all our fields are smooth inside each domain, we are able to
use an efficient pseudospectral method to solve the elliptic equations
associated with the conformal thin sandwich approach. Currently we have
implemented corotating configurations with arbitrary mass ratios, but an
extension to arbitrary spins is possible. The main purpose of this paper is to
introduce our new method and to test our code for several different
configurations.Comment: 18 pages, 8 figures, 1 tabl
Dynamics of pairwise motions
We derive a simple closed-form expression, relating \vs(r) -- the mean
relative velocity of pairs of galaxies at fixed separation -- to the
two-point correlation function of mass density fluctuations, . We
compare our analytic model for \vs(r) with N-body simulations, and find
excellent agreement in the entire dynamical range probed by the simulations
(0.1 \lsim \xi \lsim 1000). Our results can be used to estimate the
cosmological density parameter, \Om, directly from redshift-distance surveys,
like Mark III.Comment: 10 pages 2 Figs., submitted to ApJ Let
State of the Art: Why do the lungs of patients with cystic fibrosis become infected and why can't they clear the infection?
Cystic Fibrosis (CF) lung disease, which is characterized by airway obstruction, chronic bacterial infection, and an excessive inflammatory response, is responsible for most of the morbidity and mortality. Early in life, CF patients become infected with a limited spectrum of bacteria, especially P. aeruginosa. New data now indicate that decreased depth of periciliary fluid and abnormal hydration of mucus, which impede mucociliary clearance, contribute to initial infection. Diminished production of the antibacterial molecule nitric oxide, increased bacterial binding sites (e.g., asialo GM-1) on CF airway epithelial cells, and adaptations made by the bacteria to the airway microenvironment, including the production of virulence factors and the ability to organize into a biofilm, contribute to susceptibility to initial bacterial infection. Once the patient is infected, an overzealous inflammatory response in the CF lung likely contributes to the host's inability to eradicate infection. In response to increased IL-8 and leukotriene B(4 )production, neutrophils infiltrate the lung where they release mediators, such as elastase, that further inhibit host defenses, cripple opsonophagocytosis, impair mucociliary clearance, and damage airway wall architecture. The combination of these events favors the persistence of bacteria in the airway. Until a cure is discovered, further investigations into therapies that relieve obstruction, control infection, and attenuate inflammation offer the best hope of limiting damage to host tissues and prolonging survival
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