644 research outputs found
A New Model for the Spiral Structure of the Galaxy. Superposition of 2+4-armed patterns
We investigate the possibility of describing the spiral pattern of the Milky
Way in terms of a model of superposition 2- and 4-armed wave harmonics (the
simplest description, besides pure modes). Two complementary methods are used:
a study of stellar kinematics, and direct tracing of positions of spiral arms.
In the first method, the parameters of the galactic rotation curve and the free
parameters of the spiral density waves were obtained from Cepheid kinematics,
under different assumptions. To turn visible the structure corresponding to
these models, we computed the evolution of an ensemble of N-particles,
simulating the ISM clouds, in the perturbed galactic gravitational field. In
the second method, we present a new analysis of the longitude-velocity (l-v)
diagram of the sample of galactic HII regions, converting positions of spiral
arms in the galactic plane into locii of these arms in the l-v diagram. Both
methods indicate that the ``self-sustained'' model, in which the 2-armed and
4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively)
is a good description of the disk structure. An important conclusion is that
the Sun happens to be practically at the corotation circle. As an additional
result of our study, we propose an independent test for localization of the
corotation circle in a spiral galaxy: a gap in the radial distribution of
interstellar gas has to be observed in the corotation region.Comment: 17 pages, 9 figures, Latex, uses aas2pp4.st
On the difference between type E and A OH/IR stars
The observed SEDs of a sample of 60 OH/IR stars are fitted using a radiative
transfer model of a dusty envelope. Among the whole sample, 21 stars have
reliable phase-lag distances while the others have less accurate distances.
L*-P,Mlr-P and Mlr-L* relations have been plotted for these stars. It is found
that type E (with emission feature at 10um and type A (with absorption feature
at 10um) OH/IR stars have different L*-P and Mlr-L* relations while both of
them follow a single Mlr-P relation. The type E stars are proven to be located
in the area without large scale dense interstellar medium while the type A
stars are located probably in dense interstellar medium. It is argued here that
this may indicate the two types of OH/IR stars have different chemical
composition or zero age main sequence mass and so evolve in different ways.
This conclusion has reinforced the argument by Chen et al.(2001) who reached a
similar conclusion from the galactic distribution of about 1000 OH/IR stars
with the IRAS low-resolution spectra (LRS).Comment: 6 pages, 9 figures, 2 table
Does the momentum flux generated by gravitational contraction drive AGB mass-loss?
Gravitational contraction always generates a radially directed momentum flux.
A particularly simple example occurs in the electron-degenerate cores of AGB
stars, which contract steadily under the addition of helium ashes from shell
hydrogen burning. The resulting momentum flux is quantified here. And since the
cores of AGB stars lack efficient momentum cancellation mechanisms, they can
maintain equilibrium by exporting their excess momentum flux to the stellar
envelope, which disposes of much of it in a low velocity wind. Gravitational
contraction easily accounts for the momentum flux in the solar wind, as well as
the flux required to lift mass into the dust formation zone of every AGB star,
whereon radiation pressure continues its ejection as a low velocity wind. This
mechanism explains the dependence of the AGB mass-loss rate on core mass; its
generalization to objects with angular momentum and/or strong magnetic fields
suggests a novel explanation of why most planetary nebulae and proto planetary
nebulae exhibit axial symmetry.
Quasistatic contraction is inherently biased to the generation of the maximum
possible momentum flux. Its formalism is therefore readily adapted to providing
an upper limit to the momentum flux needed to sustain mass loss when this
begins from a semi-continuous rather than impulsive process.Comment: 35 pages, including 1 fig and 2 tables, to appear in Astrophysical
Journal -- ps documen
What Is Causing the Reduced Drug-Placebo Difference in Recent Schizophrenia Clinical Trials and What Can be Done About It?
On September 18, 2007, a collaborative session between the International Society for CNS Clinical Trials and Methodology and the International Society for CNS Drug Development was held in Brussels, Belgium. Both groups, with membership from industry, academia, and governmental and nongovernmental agencies, have been formed to address scientific, clinical, regulatory, and methodological challenges in the development of central nervous system therapeutic agents. The focus of this joint session was the apparent diminution of drug-placebo differences in recent multicenter trials of antipsychotic medications for schizophrenia. To characterize the nature of the problem, some presenters reported data from several recent trials that indicated higher rates of placebo response and lower rates of drug response (even to previously established, comparator drugs), when compared with earlier trials. As a means to identify the possible causes of the problem, discussions covered a range of methodological factors such as participant characteristics, trial designs, site characteristics, clinical setting (inpatient vs outpatient), inclusion/exclusion criteria, and diagnostic specificity. Finally, possible solutions were discussed, such as improving precision of participant selection criteria, improving assessment instruments and/or assessment methodology to increase reliability of outcome measures, innovative methods to encourage greater subject adherence and investigator involvement, improved rater training and accountability metrics at clinical sites to increase quality assurance, and advanced methods of pharmacokinetic/pharmacodynamic modeling to optimize dosing prior to initiating large phase 3 trials. The session closed with a roundtable discussion and recommendations for data sharing to further explore potential causes and viable solutions to be applied in future trials
Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates
We present JHK' photometric measurements of 78 objects mostly consisting of
proto-planetary nebula candidates. Photometric magnitudes are determined by
means of imaging and aperture photometry. Unlike the observations with a
photometer with a fixed-sized beam, the method of imaging photometry permits
accurate derivation of photometric values because the target sources can be
correctly identified and confusion with neighboring sources can be easily
avoided. Of the 78 sources observed, we report 10 cases in which the source
seems to have been misidentified or confused by nearby bright sources. We also
present nearly two dozen cases in which the source seems to have indicated a
variability which prompts a follow-up monitoring. There are also a few sources
that show previously unreported extendedness. In addition, we present H band
finding charts of the target sources.Comment: 3 tables, 1 figur
Using self-definition to predict the influence of procedural justice on organizational, interpersonal, and job/task-oriented citizenship behaviors
An integrative self-definition model is proposed to improve our understanding of how procedural justice affects different outcome modalities in organizational behavior. Specifically, it is examined whether the strength of different levels of self-definition (collective, relational, and individual) each uniquely interact with procedural justice to predict organizational, interpersonal, and job/task-oriented citizenship behaviors, respectively. Results from experimental and (both single and multisource) field data consistently revealed stronger procedural justice effects (1) on organizational-oriented citizenship behavior among those who define themselves strongly in terms of organizational characteristics, (2) on interpersonal-oriented citizenship behavior among those who define themselves strongly in terms of their interpersonal relationships, and (3) on job/task-oriented citizenship behavior among those who define themselves weakly in terms of their distinctiveness or uniqueness. We discuss the relevance of these results with respect to how employees can be motivated most effectively in organizational settings
The Eucalyptus spectrograph
As part of the Brazilian contribution to the 4.2 m SOAR telescope project we are building the Integral Field Unit spectrograph, "SIFUS." With the aim of testing the performance of optical fibers with 50 microns core size on IFUs, we constructed a prototype of the IFU and a spectrograph that were installed at the 1.6 m telescope of the Observatorio do Pico dos Dias (OPD), managed by Laboratorio Nacional de Astrofisica (LNA) in Brazil. The IFU has 512 fibers coupled to a LIMO microlens array (16 x 32) covering a 15" x 30" field on the sky. The spectrograph is a medium resolution instrument, operating in a quasi-Littrow mode. It was based on the design of the SPIRAL spectrograph built by the Anglo-Australian Observatory. The name Eucalyptus was given following the name of the native Australian tree that adapted very well in Brazil and it was given in recognition to the collaboration with the colleagues of the Anglo-Australian Observatory. The instrument first light occurred in the first semester of 2001. The results confirmed the possibility of using the adopted fibers and construction techniques for the SIFUS. We present the features of the instrument, some examples of the scientific data obtained, and the status of the commissioning, calibration and automation plans. The efficiency of this IFU was determined to be 53% during telescope commissioning tests
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