579 research outputs found
A Simple Calculation in Service of Constraining the Rate of FU Orionis Outburst Events from Photometric Monitoring Surveys
An enigmatic and rare type of young stellar object is the FU Orionis class.
The members are interpreted as "outbursting," that is, currently in a state of
enhanced accretion by several orders of magnitude relative to the more modest
disk-to-star accretion rates measured in typical T Tauri stars. They are key to
our understanding of the history of stellar mass assembly and pre-main sequence
evolution, as well as critical to consider in the chemical and physical
evolution of the circumstellar environment -- where planets form. A common
supposition is that *all* T Tauri stars undergo repeated such outbursts, more
frequently in their earlier evolutionary stages when the disks are more
massive, so as to build up the requisite amount of stellar mass on the required
time scale. However, the actual data supporting this traditional picture of
episodically enhanced disk accretion are limited, and the observational
properties of the known sample of FU Ori objects quite diverse. To improve our
understanding of these rare objects, we outline the logic for meaningfully
constraining the rate of FU Ori outbursts and present numbers to guide
parameter choices in the analysis of time domain surveys.Comment: accepted for publication in Ap
Binarity in Cool Asymptotic Giant Branch Stars: A Galex Search for Ultraviolet Excesse
The search for binarity in AGB stars is of critical importance for our
understanding of how planetary nebulae acquire the dazzling variety of
aspherical shapes which characterises this class. However, detecting binary
companions in such stars has been severely hampered due to their extreme
luminosities and pulsations. We have carried out a small imaging survey of AGB
stars in ultraviolet light (using GALEX) where these cool objects are very
faint, in order to search for hotter companions. We report the discovery of
significant far-ultraviolet excesses towards nine of these stars. The
far-ultraviolet excess most likely results either directly from the presence of
a hot binary companion, or indirectly from a hot accretion disk around the
companion.Comment: revised for Astrophysical Journa
Fault Diagnosis for Polynomial Hybrid Systems
Safety requirements of technological processes trigger an increased demand for elaborate fault diagnosis tools. However, abrupt changes in system behavior are hard to formulate with continuous models but easier to represent in terms of hybrid systems. Therefore, we propose a set-based approach for complete fault diagnosis of hybrid polynomial systems formulated as a feasibility problem. We employ mixed-integer linear program relaxation of this formulation to exploit the presence of discrete variables. We improve the relaxation with additional constraints for the discrete variables. The efficiency of the method is illustrated with a simple two-tank example subject to multiple faults
Fast, inexpensive, and reliable HPLC method to determine monomer fractions in poly(3-hydroxybutyrate-co-3-hydroxyvalerate)
The determination of the monomer fractions in polyhydroxyalkanoates is of great importance for research on microbial-produced plastic material. The development of new process designs, the validation of mathematical models, and intelligent control strategies for production depend enormously on the correctness of the analyzed monomer fractions. Most of the available detection methods focus on the determination of the monomer fractions of the homopolymer poly(3-hydroxybutyrate). Only a few can analyze the monomer content in copolymers such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate), which usually require expensive measuring devices, a high preparation time or the use of environmentally harmful halogenated solvents such as chloroform or dichloromethane. This work presents a fast, simple, and inexpensive method for the analysis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with high-performance liquid chromatography. Samples from a bioreactor experiment for the production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with Cupriavidus necator H16 were examined regarding their monomer content using the new method and gas chromatography analysis, one of the most frequently used methods in literature. The results from our new method were validated using gas chromatography measurements and show excellent agreement. Key points ∙ The presented HPLC method is an inexpensive, fast and environmentally friendly alternative to existing methods for quantification of monomeric composition of PHBV. ∙ Validation with state of the art GC measurement exhibits excellent agreement over a broad range of PHBV monomer fractions
A Sample Glossary of Systems Analysis
Every activity -- and systems analysis is no exception tends to develop its own vocabulary. Indeed, systems analysis, because of its interdisciplinary nature, has been more prone than most not only to invent new words for new concepts but also -- and more often -- to borrow established terms from the disciplines it employs and to change their meaning, sometimes slightly, sometimes grossly, sometimes inconsistently. The result of this can be confusion, misunderstanding, and failure of communication. This glossary is an attempt at resolving part of the ambiguity.
The glossary, as it stands now, is tentative. It has been prepared for the preliminary version of the Handbook of Applied Systems Analysis, and the terms included are those used in the Handbook
Control and Coordination in Hierarchical Systems
This book presents the applied theory of control and cooordination in hierarchical systems which are those where decision making has been divided in a certain way. It concentrates on various aspects of optimal control in large scale systems and covers a range of topics from multilevel methods for optimizing by interactive feedback procedures to methods for sequential, hierarchical control in large dynamic systems
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