Ionization Front Instabilities in Primordial H II Regions

Radiative cooling by metals in shocked gas mediates the formation of ionization front instabilities in the galaxy today that are responsible for a variety of phenomena in the interstellar medium, from the morphologies of nebulae to triggered star formation in molecular clouds. An important question in early reionization and chemical enrichment of the intergalactic medium is whether such instabilities arose in the H II regions of the first stars and primeval galaxies, which were devoid of metals. We present three-dimensional numerical simulations that reveal both shadow and thin-shell instabilities readily formed in primordial gas. We find that the hard UV spectra of Population III stars broadened primordial ionization fronts, causing H2 formation capable of inciting violent thin- shell instabilities in D-type fronts, even in the presence of intense Lyman-Werner flux. The high post- front gas temperatures associated with He ionization sustained and exacerbated shadow instabilities, unaided by molecular hydrogen cooling. Our models indicate that metals eclipsed H2 cooling in I-front instabilities at modest concentrations, from 0.001- 0.01 solar. We conclude that ionization front instabilities were prominent in the H II regions of the first stars and galaxies, influencing the escape of ionizing radiation and metals into the early universe.Comment: 13 pages, 11 figures, accepted by ApJ with minor revision

Virtual-to-Real-World Transfer Learning for Robots on Wilderness Trails

Robots hold promise in many scenarios involving outdoor use, such as search-and-rescue, wildlife management, and collecting data to improve environment, climate, and weather forecasting. However, autonomous navigation of outdoor trails remains a challenging problem. Recent work has sought to address this issue using deep learning. Although this approach has achieved state-of-the-art results, the deep learning paradigm may be limited due to a reliance on large amounts of annotated training data. Collecting and curating training datasets may not be feasible or practical in many situations, especially as trail conditions may change due to seasonal weather variations, storms, and natural erosion. In this paper, we explore an approach to address this issue through virtual-to-real-world transfer learning using a variety of deep learning models trained to classify the direction of a trail in an image. Our approach utilizes synthetic data gathered from virtual environments for model training, bypassing the need to collect a large amount of real images of the outdoors. We validate our approach in three main ways. First, we demonstrate that our models achieve classification accuracies upwards of 95% on our synthetic data set. Next, we utilize our classification models in the control system of a simulated robot to demonstrate feasibility. Finally, we evaluate our models on real-world trail data and demonstrate the potential of virtual-to-real-world transfer learning.Comment: iROS 201

Asymmetry in the prime rate and firms' preference for internal finance

This article tests for asymmetry in thebehavior of bank lending rates by testing the hypothesis that the prime rate responds more fully and quickly to increase than decreases in market interest rates. The econometric methodology used is better suited to the discreteness and rigidity of the prime rate than that of previous studies. Our results suggest that banks adjust the prime rate asymmetrically in response to change in the discount rate, the commercial paper rate, and the spread between the prime and commercial paper rates. Asymmetry in bank lending rates is implied by several explanations for the preference among small firms for internal finance. Asymmetry in bank lending rates may result from the fact that individual banks have acquired costly information which prevents their customers from responding quickly to changes in loan terms, or it may stem from a cyclical "lemons" premium resulting from informational asymmetries [Oliner and Rudebusch (1992)]. Either way, asymmetric behavior of bank lending rates, such as the prime rate, may be part of a more complete explanation of small firms' preference for internal finance.Corporations - Finance ; Prime rate

Do bank loan rates exhibit a countercyclical mark-up?

Based on a switching-cost model, we examine empirically the hypotheses that bank loan mark-ups are countercyclical and asymmetric in their responsiveness to recessionary and expansionary impulses. The first econometric model treats changes in the mark-up as a continuous variable. The second treats them as an ordered categorical variable due to the discrete nature of prime rate changes. By allowing the variance to switch over time as a Markov process, we present the first conditionally heteroskedastic discrete choice (ordered probit) model for time-series applications. This feature yields a remarkable improvement in the likelihood function. Specifications that do not account for conditional heteroskedasticity find evidence of both countercyclical and asymmetric mark-up behavior. In contrast, the heteroskedastic ordered probit finds the mark-up to be countercyclical but not significantly asymmetric. We explain why controlling for conditional heteroskedasticity may be important when testing for downward stickiness in loan rates.Bank loans ; Interest rates

Dynamic Web File Format Transformations with Grace

Web accessible content stored in obscure, unpopular or obsolete formats represents a significant problem for digital preservation. The file formats that encode web content represent the implicit and explicit choices of web site maintainers at a particular point in time. Older file formats that have fallen out of favor are obviously a problem, but so are new file formats that have not yet been fully supported by browsers. Often browsers use plug-in software for displaying old and new formats, but plug-ins can be difficult to find, install and replicate across all environments that one may use. We introduce Grace, an http proxy server that transparently converts browser-incompatible and obsolete web content into web content that a browser is able to display without the use of plug-ins. Grace is configurable on a per user basis and can be expanded to provide an array of conversion services. We illustrate how the Grace prototype transforms several image formats (XBM, PNG with various alpha channels, and JPEG 2000) so they are viewable in Internet Explorer.Comment: 12 pages, 9 figure

Frequency Conversion in a High Q-factor Sapphire Whispering Gallery Mode Resonator due to Paramagnetic Nonlinearity

Nonlinear frequency conversion is a well known and widely exploited family of effects in optics, often arising from a Kerr nonlinearity in a crystal medium. Here, we report high stability frequency conversion in the microwave regime due to a $\chi^{(3)}$ nonlinearity in sapphire introduced by a dilute concentration of paramagnetic spins. First, we produce a high stability comb from two microwave fields at 12.029 and 12.037 GHz corresponding to two high $Q$-factor Whispering Gallery (WG) modes within the Electron Spin Resonance (ESR) bandwidth of the Fe$^{3+}$ ion. The resulting comb is generated by a cascaded four-wave mixing effect with a 7.7 MHz repetition rate. Then, by suppressing four-wave mixing by increasing the threshold power, third harmonic generation is achieved in a variety of WG modes coupled to various species of paramagnetic ion within the sapphire

AUSSAT mobile satellite services

An overview of AUSSAT's planned mobile satellite system is given. The development program which is being undertaken to achieve the 1992 service date is described. Both business and technical aspects of the development program are addressed

"Patent Activity and Technical Change"

This paper presents an aggregate production function of the generalized Fechner-Thurstone (GFT) form to analyze the impact of an important component of intellectual industrial property, namely patent activity, on technical change in the USA for the period 1947-1981. We define a technology-changer as a variable that has an impact on the elasticity of the marginal rate of technical substitution (mrts) between inputs of the GFT production function over time. Various types of US patent grant activity, specifically total, domestic, foreign, successful and unsuccessful patents, are used as instruments for the technology-changer. Using the GFT specification, the impacts of various technology-changers on the elasticity of the mrts between inputs are estimated directly. It is found that granted (or successful) patents, patents granted to foreign companies and individuals, total patent applications, and even unsuccessful patent applications, have significant impacts on the rates at which inputs are substituted for each other over time in production.