7,715 research outputs found
Circumstellar Disk Evolution: Constraining Theories of Planet Formation
Observations of circumstellar disks around stars as a function of stellar
properties such as mass, metallicity, multiplicity, and age, provide
constraints on theories concerning the formation and evolution of planetary
systems. Utilizing ground- and space-based data from the far-UV to the
millimeter, astronomners can assess the amount, composition, and location of
circumstellar gas and dust as a function of time. We review primarily results
from the Spitzer Space Telescope, with reference to other ground- and
space-based observations. Comparing these results with those from exoplanet
search techniques, theoretical models, as well as the inferred history of our
solar system, helps us to assess whether planetary systems like our own, and
the potential for life that they represent, are common or rare in the Milky Way
galaxy.Comment: To appear in IAU Symposium No. 258, Eds. E. Mamajek, D.R. Soderblom,
and R.F.G. Wys
The synthesis of boron carbide filaments final report
Synthesis, strength, crystal structure, and composite material formation of boron carbide whisker
Sintered silicon nitrode recuperator fabrication
The preliminary design and a demonstration of the feasibility of fabricating submodules of an automotive Stirling engine recuperator for waste heat recovery at 370 C are described. Sinterable silicon nitride (Sialon) tubing and plates were fabricated by extrusion and hydrostatic pressing, respectively, suitable for demonstrating a potential method of constructing ceramic recuperator-type heat exchangers. These components were fired in nitrogen atmosphere to 1800 C without significant scale formation so that they can be used in the as-fired condition. A refractory glass composition (Al2O3 x 4.5 CaO.MgO x 11SiO2) was used to join and seal component parts by a brazing technique which formed strong recuperator submodules capable of withstanding repeated thermal cycling to 1370 C. The corrosion resistance of these materials to Na2SO4 + NaCl carbon mixtures was also assessed in atmospheres of air, hydrogen and CO2-N2-H2O mixtures at both 870 C and 1370 C for times to 1000 hours. No significant reaction was observed under any of these test conditions
Full dimensional (15D) quantum-dynamical simulation of the protonated water-dimer I: Hamiltonian setup and analysis of the ground vibrational state
Quantum-dynamical full-dimensional (15D) calculations are reported for the
protonated water dimer (H5O2+) using the multiconfiguration time-dependent
Hartree (MCTDH) method. The dynamics is described by curvilinear coordinates.
The expression of the kinetic energy operator in this set of coordinates is
given and its derivation, following the polyspherical method, is discussed. The
PES employed is that of Huang et al. [JCP, 122, 044308, (2005)]. A scheme for
the representation of the potential energy surface (PES) is discussed which is
based on a high dimensional model representation scheme (cut-HDMR), but
modified to take advantage of the mode-combination representation of the
vibrational wavefunction used in MCTDH. The convergence of the PES expansion
used is quantified and evidence is provided that it correctly reproduces the
reference PES at least for the range of energies of interest. The reported zero
point energy of the system is converged with respect to the MCTDH expansion and
in excellent agreement (16.7 cm-1 below) with the diffusion Monte Carlo result
on the PES of Huang et al. The highly fluxional nature of the cation is
accounted for through use of curvilinear coordinates. The system is found to
interconvert between equivalent minima through wagging and internal rotation
motions already when in the ground vibrational-state, i.e., T=0. It is shown
that a converged quantum-dynamical description of such a flexible, multi-minima
system is possible.Comment: 46 pages, 5 figures, submitted to J. Chem. Phy
vibration mitigation of a linear host structure using a passive neutralizer effect of nonlinearity in the neutralizer suspension
Abstract Motivated by some experimental results on a test-rig, this paper presents some observations on the frequency response of a primary linear oscillator when an auxiliary nonlinear oscillator is attached to it, acting as a vibration neutralizer. In the experiments, an electro-dynamic shaker is used as the linear one-degree-of-freedom primary oscillator, and it is excited by an harmonic force. The nonlinear neutralizer is attached to the moving head of the shaker, and it is assembled to achieve a cubic stiffness characteristics, due to geometrical arrangement of linear elastic elements. For very low vibration amplitudes, the whole system behaves predominantly as a two-degree-of-freedom linear oscillator, but when the force excitation to the shaker is increased the shape of the frequency response curve changes, and exhibits resonance peak bending, jump phenomena and instabilities of the harmonic response. A theoretical model of the system is presented, with the aim to capture the qualitative phenomena observed in the experiments
Did the Euro Foster Online Price Competition? Evidence from an International Price Comparison Site
We study the impact of the Euro on prices charged by online retailers within the EU. Our data spans the period before and after the Euro was introduced, covers a variety of products, and includes countries inside and outside of the Eurozone. After controlling for cost, demand, and market structure effects, we show that the pure Euro changeover effect is to raise average prices in the Eurozone by 3% and average minimum prices by 7%. Finally, we develop a model of online pricing in the context of currency unions, and show that these price patterns are broadly consistent with clearinghouse models.Price competition, internet
Clicks, Discontinuities, and Firm Demand Online
The market values of online platforms, such as Yahoo, stem from their ability to monetize the clicks they generate for firms advertising on their sites. We exploit a unique dataset on clicks from one of Yahoo's price comparison sites to estimate the determinants of clicks received by online retailers. We find that a firm enjoys a 60% jump in its clicks when it offers the lowest price at the site. This discontinuity is consistent with a variety of models that have been used to rationalize the price dispersion observed in online markets. We also show that one may use estimates of the determinants of a firm's clicks to obtain bounds on its underlying demand parameters, including own- and cross-price elasticities. Our results have potentially significant ramifications for online retailers, platforms, and policymakers: Failure to account for discontinuities distorts parameter estimates by 50 to 100 percent.
Estimating Firm-Level Demand at a Price Comparison Site: Accounting for Shoppers and the Number of Competitors
Clearinghouse models of online pricing---such as Varian (1980), Rosenthal (1980), Narasimhan (1988), and Baye-Morgan (2001)---view a price comparison site as an 'information clearinghouse' where shoppers and loyals obtain price and product information to make online purchases. These models predict that the responsiveness of a firm's demand to a change in its price depends on the number of sellers and whether the price change results in the firm charging the lowest price in the market. Using a unique firm-level dataset from Kelkoo.com (Yahoo!'s European price comparison site), we examine these predictions by providing estimates of the demand for PDAs. Our results indicate that the number of competing sellers and both the firm's location on the screen and relative ranking in the list of prices are important determinants of an online retailer's demand. We find that an online monopolist faces an elasticity of demand of about -2, while sellers competing against 10 other sellers face an elasticity of about -6. We also find empirical evidence of a discontinuous jump in a firm's demand as its price declines from the second-lowest to the lowest price. Our estimates suggest that about 13% of the consumers at Kelkoo are 'shoppers' who purchase from the seller offering the lowest price.Internet, Price Dispersion, Advertising
- …