607 research outputs found
Spectroscopy of Seven Cataclysmic Variables with Periods Above Five Hours
We present spectroscopy of seven cataclysmic variable stars with orbital
periods P(orb) greater than 5 hours, all but one of which are known to be dwarf
novae. Using radial velocity measurements we improve on previous orbital period
determinations, or derive periods for the first time. The stars and their
periods are
TT Crt, 0.2683522(5) d;
EZ Del, 0.2234(5) d;
LL Lyr, 0.249069(4) d;
UY Pup, 0.479269(7) d;
RY Ser, 0.3009(4) d;
CH UMa, 0.3431843(6) d; and
SDSS J081321+452809, 0.2890(4) d.
For each of the systems we detect the spectrum of the secondary star,
estimate its spectral type, and derive a distance based on the surface
brightness and Roche lobe constraints. In five systems we also measure the
radial velocity curve of the secondary star, estimate orbital inclinations, and
where possible estimate distances based on the MV(max) vs.P(orb) relation found
by Warner. In concordance with previous studies, we find that all the secondary
stars have, to varying degrees, cooler spectral types than would be expected if
they were on the main sequence at the measured orbital period.Comment: 25 pages, 2 figures, accepted for Publications of the Astronomical
Society of the Pacifi
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Testing of an impedance heating system for solar power tower applications
A non-conventional type of heating system is being tested at Sandia National Laboratories for solar thermal power tower applications. In this system, called impedance heating, electric current flows directly through the pipe to maintain the desired temperature. The pipe becomes the resistor where the heat is generated. Impedance heating has many advantages over previously used mineral insulated (MI) heat trace. An impedance heating system should be much more reliable than heat trace cable since delicate junctions and cabling are not used and the main component, a transformer, is inherently reliable. A big advantage of impedance heating is the system can be sized to rapidly heat up the piping to provide rapid response times necessary in cyclic power plants such as solar power towers. In this paper, experimental results from testing an impedance heating system are compared to MI cable heat trace. We found impedance heating was able to heat piping rapidly and effectively. There were not significant stray currents and impedance heating did not affect instrumentation
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Comparison of an impedance heating system to mineral insulated heat trace for power tower applications
A non-conventional type of heating system is being tested at Sandia National Laboratories for solar thermal power tower applications. In this system, called impedance heating, electric current flows directly through the pipe to maintain the desired temperature. The pipe becomes the resistor where the heat is generated. Impedance heating has many advantages over previously used mineral insulated (MI) heat trace. An impedance heating system should be much more reliable than heat trace cable since delicate junctions and cabling are not used and the main component, a transformer, is inherently reliable. A big advantage of impedance heating is the system can be sized to rapidly heat up the piping to provide rapid response times necessary in cyclic power plants such as solar power towers. In this paper, experimental results from testing an impedance heating system are compared to MI heat trace. The authors found impedance heating was able to heat piping rapidly and effectively. There were not significant stray currents and impedance heating did not affect instrumentation
Ribbons on the CBR Sky: A Powerful Test of a Baryon Symmetric Universe
If the Universe consists of domains of matter and antimatter, annihilations
at domain interfaces leave a distinctive imprint on the Cosmic Background
Radiation (CBR) sky. The signature is anisotropies in the form of long, thin
ribbons of width , separated by angle where L is the characteristic domain size, and
y-distortion parameter . Such a pattern could potentially be
detected by the high-resolution CBR anisotropy experiments planned for the next
decade, and such experiments may finally settle the question of whether or not
our Hubble volume is baryon symmetric.Comment: LaTeX, 10 pages, 4 figures in epsf. Revised version corrects a couple
of relevant mistake
The reflective learning continuum: reflecting on reflection
The importance of reflection to marketing educators is increasingly recognized. However, there is a lack of empirical research which considers reflection within the context of both the marketing and general business education literature. This paper describes the use of an instrument which can be used to measure four identified levels of a reflection hierarchy: habitual action, understanding, reflection and intensive reflection and two conditions for reflection: instructor to student interaction and student to student interaction. Further we demonstrate the importance of reflective learning in predicting graduatesâ perception of program quality. Although the focus was on assessment of MBA level curricula, the findings have great importance to marketing education and educators
A Hamilton-Jacobi approach to non-slow-roll inflation
I describe a general approach to characterizing cosmological inflation
outside the standard slow-roll approximation, based on the Hamilton-Jacobi
formulation of scalar field dynamics. The basic idea is to view the equation of
state of the scalar field matter as the fundamental dynamical variable, as
opposed to the field value or the expansion rate. I discuss how to formulate
the equations of motion for scalar and tensor fluctuations in situations where
the assumption of slow roll is not valid. I apply the general results to the
simple case of inflation from an ``inverted'' polynomial potential, and to the
more complicated case of hybrid inflation.Comment: 21 pages, RevTeX (minor revisions to match published version
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Woody plants optimise stomatal behaviour relative to hydraulic risk
Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbonâmaximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle.W.R.L.A. acknowledges funding for this research from NSF 1714972 and from the USDA National Institute of Food and Agriculture, Agricultural and Food Research Initiative Competitive Programme, Ecosystem Services and Agro-Ecosystem Management, grant no. 2017-05521. We thank T. Brodribb and one anonymous reviewer for their insightful reviews, B. Medlyn and Y.S. Lin for sharing data and R. Norby for providing Vcmax data for several species. We appreciate the assistance from Marion Feifel in collecting data of leaf photosynthetic parameters of five European tree species. S.L. acknowledges financial support from the China Scholarship Council (CSC). VRD acknowledges funding from a Ram on y Cajal fellowship (RYC-2012-10970). B.T.W. was supported by the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. DJC acknowledges funding from the National Science Centre, Poland (NN309 713340)
Detecting long-term changes in stomatal conductance: challenges and opportunities of tree-ring ÎŽ18 O proxy
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