9,643 research outputs found

    Insights into thermonuclear supernovae from the incomplete silicon burning process

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    Type Ia supernova (SNIa) explosions synthesize a few tenths to several tenths of a solar mass, whose composition is the result of incomplete silicon burning that reaches peak temperatures of 4 GK to 5 GK. The elemental abundances are sensitive to the physical conditions in the explosion, making their measurement a promising clue to uncovering the properties of the progenitor star and of the explosion itself. Using a parameterized description of the thermodynamic history of matter undergoing incomplete silicon burning, we computed the final composition for a range of parameters wide enough to encompass current models of SNIa. Then, we searched for combinations of elemental abundances that trace the parameters values and are potentially measurable. For this purpose, we divide the present study into two epochs of SNIa, namely the optical epoch, from a few weeks to several months after the explosion, and the X-ray epoch, which refers to the time period in which the supernova remnant is young, starting one or two hundred years age and ending a thousand years after the event. During the optical epoch, the only SNIa property that can be extracted from the detection of incomplete silicon burning elements is the neutron excess of the progenitor white dwarf at thermal runaway, which can be determined through measuring the ratio of the abundance of manganese to that of titanium, chromium, or vanadium. Conversely, in the X-ray epoch, any abundance ratio built using a couple of elements from titanium, vanadium, chromium, or manganese may constrain the initial neutron excess. Furthermore, measuring the ratio of the abundances of vanadium to manganese in the X-ray might shed light on the timescale of the thermonuclear explosion.Comment: Accepted for Astronomy and Astrophysics (16 pages, 3 tables, 15 figures

    Thermal X-ray emission from shocked ejecta in Type Ia Supernova Remnants. Prospects for explosion mechanism identification

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    The explosion mechanism behind Type Ia supernovae is a matter of continuing debate. The diverse attempts to identify or at least constrain the physical processes involved in the explosion have been only partially successful so far. In this paper we propose to use the thermal X-ray emission from young supernova remnants originated in Type Ia events to extract relevant information concerning the explosions themselves. We have produced a grid of thermonuclear supernova models representative of the paradigms currently under debate: pure deflagrations, delayed detonations, pulsating delayed detonations and sub-Chandrasekhar explosions, using their density and chemical composition profiles to simulate the interaction with the surrounding ambient medium and the ensuing plasma heating, non-equilibrium ionization and thermal X-ray emission of the ejecta. Key observational parameters such as electron temperatures, emission measures and ionization time scales are presented and discussed. We find that not only is it possible to identify the explosion mechanism from the spectra of young Type Ia Supernova Remnants, it is in fact necessary to take the detailed ejecta structure into account if such spectra are to be modeled in a self-consistent way. Neither element line flux ratios nor element emission measures are good estimates of the true ratios of ejected masses, with differences of as much as two or three orders of magnitude for a given model. Comparison with observations of the Tycho SNR suggests a delayed detonation as the most probable explosion mechanism. Line strengths, line ratios, and the centroid of the Fe Kalpha line are reasonably well reproduced by a model of this kind.Comment: 11 pages, 8 figures (5 of them color), accepted for publication by the Ap

    Research, Extension, and Information: Key Inputs in Agricultural Productivity Growth

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    The objective of this paper is to examine how economists have perceived the contributions of agriculture to the economic development process and then to present the case for the critical role that research, extension, and information can play in agricultural productivity growth and thus in economic development, particularly in low income countries. After a brief presentation of the framework commonly used to examine productivity growth, a distinction is made between technological change and technical efficiency. This distinction is crucial for policy purposes because the major impetus behind technological change are research and development, while education and experience are critical to improving managerial capabilities to make efficient use of a given technology. Empirical findings concerning the returns on agricultural research, with special attention to studies that have focused on Pakistan, are discussed. The paper then offers an overview of alternative methodologies available to measure technical efficiency, summarises the empirical literature, and finally focuses on studies dealing with Pakistani agriculture. Once it is established that improvements in technical efficiency could contribute significantly to increases in farm output and income, the discussion moves to some issues that have implications for the measurement and potential improvement of farm efficiency. An overview of a model of privatised extension services, currently being applied in some Latin American countries and which could have some relevance to conditions in Pakistan and elsewhere, is provided. The paper ends with the contention that significant improvements are needed in the collection and organisation of farm production data if we are to advance our understanding of the drivers of productivity growth at the farm level

    Insights on the physics of SNIa obtained from their gamma-ray emission

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    Type Ia supernovae are thought to be the outcome of the thermonuclear explosion of a carbon/oxygen white dwarf in a close binary system. Their optical light curve is powered by thermalized gamma-rays produced by the radioactive decay of 56^{56}Ni, the most abundant isotope present in the debris. Gamma-rays escaping the ejecta can be used as a diagnostic tool for studying the structure of the exploding star and the characteristics of the explosion. The fluxes of the 56^{56}Ni lines and the continuum obtained by INTEGRAL from SN2014J in M82, the first ever gamma-detected SNIa, around the time of the maximum of the optical light curve strongly suggest the presence of a plume of 56^{56}Ni in the outermost layers moving at high velocities. If this interpretation was correct, it could have important consequences on our current understanding of the physics of the explosion and on the nature of the systems that explode.Comment: Proceedings of the 11th INTEGRAL Conference Gamma-Ray AStrophysics in Multi-Wavelength Perspectiv

    RATES OF RETURN TO PRIVATE AGRICULTURAL EXTENSION: EVIDENCE FROM TWO FARM MANAGEMENT CENTERS IN EL SALVADOR

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    This paper evaluates the economic and the financial viability of implementing private farm management centers (FMC) in El Salvador. In doing so, an ex ante cost-benefit analysis is performed. The results of this analysis suggest that a combination of better farm prices (paid and received), reallocation of resources, and crop diversification that would be promoted by a FMC can lead to an increase in farm level profits that is sufficient to cover the operation of the center and to still generate net gains in household income.Farm Management,

    The Persistence of Memory, or How the X-Ray Spectrum of SNR 0509-67.5 Reveals the Brightness of its Parent Type Ia Supernova

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    We examine the dynamics and X-ray spectrum of the young Type Ia supernova remnant 0509-67.5 in the context of the recent results obtained from the optical spectroscopy of its light echo. Our goal is to estimate the kinetic energy of the supernova explosion using Chandra and XMM-Newton observations of the supernova remnant, thus placing the birth event of 0509-67.5 in the sequence of dim to bright Type Ia supernovae. We base our analysis on a standard grid of one-dimensional delayed detonation explosion models, together with hydrodynamic and X-ray spectral calculations of the supernova remnant evolution. From the remnant dynamics and the properties of the O, Si, S, and Fe emission in its X-ray spectrum we conclude that 0509-67.5 was originated ~400 years ago by a bright, highly energetic Type Ia explosion similar to SN 1991T. Our best model has a kinetic energy of 1.4x10E51 erg and synthesizes 0.97 Msun of 56Ni. These results are in excellent agreement with the age estimate and spectroscopy from the light echo. We have thus established the first connection between a Type Ia supernova and its supernova remnant based on a detailed quantitative analysis of both objects.Comment: 10 pages, 9 figures, plus an exclusive astro-ph-only Appendix; ApJ in press, companion paper to Rest et al. 0

    THE EFFECT OF SOIL CONSERVATION ON TECHNICAL EFFICIENCY: EVIDENCE FROM CENTRAL AMERICA

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    This study evaluates technical efficiency (TE) levels for rural households under high and low levels of investments in soil conservation in El Salvador and Honduras. To correct for potential self-selectivity bias a household-level switching regression framework is implemented to estimate separate stochastic production frontiers for the two groups of households under analysis. The main results indicate that a systematic difference exists between the two studied groups. Specifically, households with higher levels of investments in soil conservation show higher average TE than those with a lower level of investments. Constrains in the rural land and credit markets appear to be the reason behind these differences. Our estimations indicate that for farms with lower levels of investments in soil conservation access to credit is a significant factor explaining the sources of inefficiency. Conversely, households with higher levels of investments in soil conservation present the highest partial output elasticity for land, the highest levels of TE and the smallest farms. This result could suggest the presence of a market failure in the land market which is denying access to land to the more efficient producers.Land Economics/Use,
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