5,372 research outputs found
The V471 Tauri System: A Multi-datatype Probe
V471 Tauri, a white dwarf--red dwarf eclipsing binary in the Hyades, is well
known for stimulating development of common envelope theory, whereby novae and
other cataclysmic variables form from much wider binaries by catastrophic orbit
shrinkage. Our evaluation of a recent imaging search that reported negative
results for a much postulated third body shows that the object could have
escaped detection or may have actually been seen. The balance of evidence
continues to favor a brown dwarf companion about 12 AU from the eclipsing
binary. A recently developed algorithm finds unified solutions from three
datatypes. New radial velocities (RVs) of the red dwarf and BV RCIC light
curves are solved simultaneously along with white dwarf and red dwarf RVs from
the literature, uvby data, the MOST mission light curve, and 40 years of
eclipse timings. Precision-based weighting is the key to proper information
balance among the various datasets. Timewise variation of modeled starspots
allows unified solution of multiple data eras. Light curve amplitudes strongly
suggest decreasing spottedness from 1976 to about 1980, followed by
approximately constant spot coverage from 1981 to 2005. An explanation is
proposed for lack of noticeable variation in 1981 light curves, in terms of
competition between spot and tidal variations. Photometric spectroscopic
distance is estimated. The red dwarf mass comes out larger than normal for a
K2V star, and even larger than adopted in several structure and evolution
papers. An identified cause for this result is that much improved red dwarf RVs
curves now exist
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COACHE Summary Tables: Selected Dimensions on Faculty Workplace Climate by Discipline, Race/Ethnicity, and Gender
Teacher Challenges to Implement Engineering Design in Secondary Technology Education (Third in a Three Part Series)
This descriptive study examined the current status of technology education teacher practices with respect to engineering design. This article is the third article in a three-part series presenting the results of this study. The first article in the series titled Examination of Engineering Design Curriculum Content highlighted the research findings regarding engineering design curriculum content delivered by technology education teachers. The second article in the series titled Examination of Assessment Practices for Engineering Design Projects in Secondary Technology Education reported technology education teachers’ assessment practices when implementing engineering design projects in the classroom. The sample for this study was drawn from the current International Technology Education Association (ITEA) membership database. This article will present the research findings that identified challenges faced by technology educators when seeking to implement engineering design
Examination of Assessment Practices for Engineering Design Projects in Secondary Education (Second in a Three Part Series)
Examination of Assessment Practices for Engineering Design Projects in Secondary Education (First in a Three Part Series)
The following descriptive study was designed to determine the national status of secondary technology education curriculum content and assessment practices as they relate to engineering design. The results of this study were divided into a three-part article series. Although this study focused on the larger construct of the national status of the infusion of engineering design into technology education, three separate sub-constructs emerged. The three sub-constructs were: a) status of engineering design curriculum content; b) the status of assessment practices of engineering design projects, and c) what selected challenges are identified by secondary technology educators in teaching engineering design
Fluorescence and Chromospheric Activity of V471 Tau
The red dwarf + white dwarf eclipsing binary V471 Tau shows a variable Hα feature that varies from absorption during eclipse to maximum emission during white dwarf transit. In 1998 we obtained simultaneous BVRI photometry and Hα spectroscopy, with thorough phase coverage of the 12.5 hour orbital period. A binary star model was used with our light curve, radial velocity, and Hα data to refine stellar and orbital parameters. Combined absorption-emission profiles were generated by the model and fit to the observations, yielding a red star radius of 0.94⊙. Orbital inclination 78° is required with this size and other known parameters. The model includes three spots 1,000 K cooler than the surrounding photosphere. The variable Hα profile was modeled as a chromospheric region (essentially on the surface of the red star) centered at the substellar point. Additional emission seen outside our modeled profiles may be large co-rotating prominences that complicate the picture
Synthesis of neutral nickel catalysts for ethylene polymerization – the influence of ligand size on catalyst stability
A facile synthesis of nickel salicylaldimine complexes with labile dissociating ligands is described. In addition to producing highly active ethylene polymerization catalysts, important insights into the effect of ligand size on catalyst stability and information on the mechanism of polymerization are provided
Gain Stabilization of a Submillimeter SIS Heterodyne Receiver
We have designed a system to stabilize the gain of a submillimeter heterodyne
receiver against thermal fluctuations of the mixing element. In the most
sensitive heterodyne receivers, the mixer is usually cooled to 4 K using a
closed-cycle cryocooler, which can introduce ~1% fluctuations in the physical
temperature of the receiver components. We compensate for the resulting mixer
conversion gain fluctuations by monitoring the physical temperature of the
mixer and adjusting the gain of the intermediate frequency (IF) amplifier that
immediately follows the mixer. This IF power stabilization scheme, developed
for use at the Submillimeter Array (SMA), a submillimeter interferometer
telescope on Mauna Kea in Hawaii, routinely achieves a receiver gain stability
of 1 part in 6,000 (rms to mean). This is an order of magnitude improvement
over the typical uncorrected stability of 1 part in a few hundred. Our gain
stabilization scheme is a useful addition to SIS heterodyne receivers that are
cooled using closed-cycle cryocoolers in which the 4 K temperature fluctuations
tend to be the leading cause of IF power fluctuations.Comment: 7 pages, 6 figures accepted to IEEE Transactions on Microwave Theory
and Technique
Variational Trajectory Optimization Tool Set: Technical description and user's manual
The algorithms that comprise the Variational Trajectory Optimization Tool Set (VTOTS) package are briefly described. The VTOTS is a software package for solving nonlinear constrained optimal control problems from a wide range of engineering and scientific disciplines. The VTOTS package was specifically designed to minimize the amount of user programming; in fact, for problems that may be expressed in terms of analytical functions, the user needs only to define the problem in terms of symbolic variables. This version of the VTOTS does not support tabular data; thus, problems must be expressed in terms of analytical functions. The VTOTS package consists of two methods for solving nonlinear optimal control problems: a time-domain finite-element algorithm and a multiple shooting algorithm. These two algorithms, under the VTOTS package, may be run independently or jointly. The finite-element algorithm generates approximate solutions, whereas the shooting algorithm provides a more accurate solution to the optimization problem. A user's manual, some examples with results, and a brief description of the individual subroutines are included
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