Hyperaccretion during tidal disruption events: weakly bound debris envelopes and jets

After the destruction of the star during a tidal disruption event (TDE), the cataclysmic encounter between a star and the supermassive black hole (SMBH) of a galaxy, approximately half of the original stellar debris falls back onto the hole at a rate that can initially exceed the Eddington limit by orders of magnitude. We argue that the angular momentum of this matter is too low to allow it to attain a disk-like configuration with accretion proceeding at a mildly super-Eddington rate, the excess energy being carried away by a combination of radiative losses and radially distributed winds. Instead, we propose that the infalling gas traps accretion energy until it inflates into a weakly-bound, quasi-spherical structure with gas extending nearly to the poles. We study the structure and evolution of such "Zero-Bernoulli accretion" flows (ZEBRAs) as a model for the super-Eddington phase of TDEs. We argue that such flows cannot stop extremely super-Eddington accretion from occurring, and that once the envelope is maximally inflated, any excess accretion energy escapes through the poles in the form of powerful jets. We compare the predictions of our model to Swift J1644+57, the putative super-Eddington TDE, and show that it can qualitatively reproduce some of its observed features. Similar models, including self-gravity, could be applicable to gamma-ray bursts from collapsars and the growth of supermassive black hole seeds inside quasi-stars.Comment: 19 pages, 14 figures. Accepted for publication in Ap

Financial and environmental behavior of the regulated firm: A case study of the United States nuclear power industry, 1974-1984

This dissertation is a study of the U.S. commercial nuclear power industry from 1974-1984 covering the operations of 87 power plants. It seeks to help explain the actions of a regulated firm faced with environmental constraints from the Nuclear Regulatory Commission and financial constraints from State regulatory bodies. Theoretical and applied conceptions of the regulated monopoly are reviewed in a historical and integrated format using both the neoclassical and institutional positions. For the neoclassical approach, I\u27ve attempted to find empirical support for the Averch-Johnson hypothesis by including profit maximizing and environmental constraints in my econometric model. For the institutionalist approach, I have tried to look into the institutional reasons for the behavior exhibited by the firms. The seminal hypothesis for this project was that because of the unique plant-specific characteristics of the U.S. nuclear industry that emissions were a function of specific plant characteristics, operational data, financial results and regulatory requirements. A data base which consists of financial, radioactive emissions, and technical operations information has been compiled in order to allow testing of various hypotheses drawn from theoretical and applied sources. For this longitudinal data base, a semi-log, fixed-effect model with a lagged dependent variable was estimated. The estimation requires the use of a two-stage least squares procedure which results in consistent estimates. The results of my analysis support five very clear conclusions. First, nuclear power plant emissions have dramatically trended downward since 1978/1979 across most of the elements examined. Second, there is little indication that variability in emissions is affected by variability in the firm\u27s financial results. Third, the statistics reveal the very clear individual nature of the nuclear power plants in the U.S. Fourth, in spite of these dramatic declines in emissions releases, evidence was presented that the environmental inventories of some isotopes have been increasing. Fifth, for this one example of environmental behavior by one group of regulated monopoly firms, the increased vigilance by the regulatory officials within the Nuclear Regulatory Commission and the Environmental Protection Agency has indeed had its intended effects

F/A-18A-D Flight Control Computer OFP Versions 10.6.1 and 10.7 Developmental Flight Testing: Out-of-Controlled Flight Test Program Yields Reduced Falling Leaf Departure Susceptibility and Enhanced Aircraft Maneuverability

This thesis analyzes the recent Version 10.7 Operational Flight Program (v10.7 OFP) Flight Control System upgrade to the F/A-18A-D (legacy) Hornet fighter aircraft. This developmental program endeavored to improve high angle-of-attack (AOA) maneuverability while vastly reducing the aircraft’s susceptibility to sustained out-of- controlled flight events. Although the original F/A-18 Hornet, designated F/A-18A through F/A-18D, has been acclaimed for its departure resistance as well as its exceptional maneuverability as a fighter aircraft, the model, in actuality, has suffered from significant losses due to out-of- controlled flight (OCF) mishaps. Since its development in the early 1980s, eighteen Hornets have been lost to a particular OCF mode called “Falling Leaf”, including eight aircraft crashed since 1999. With no improvements, 10 additional aircraft, at a cost of $40 million each, were forecast to be lost. Two-seat aircraft are lost at a higher rate per flight hour than the more common single-seat version. Analysis of flight test data indicates that more two-seat aircraft sustain Falling Leaf mode due to their increased departure susceptibility. Additionally, it is apparent that the increased sprung mass of the control system, due to the addition of the rear cockpit control stick, may delay or inhibit recovery from a sustained Falling Leaf departure. This may be caused by uncommanded Flight Control System inputs from lateral control stick inertial motion induced by high sideforces encountered during a Falling Leaf. The v10.7 OFP test effort conducted a complete out-of-control flight test program without the benefit of having an attached spin-recovery parachute during testing. The specific test method and risk mitigation techniques used during this test program are reviewed and documented in this thesis to provide a historical record for future testing. By using the lessons learned from the development of the F/A-18E/F Super Hornet testing conducted a few years earlier, the v10.7 Team was able to complete the test at a large cost and schedule savings. The author concludes that the test program is an exceptional success. The new low airspeed and high AOA maneuvering capabilities inherent with the v10.7 software revolutionize how pilot aircrew will fight the aircraft. Further, the extremely enhanced resistance to sustained departure modes during out-of-controlled flight events will substantially reduce the frequency of aircraft mishaps and the associated loss of training and assets

Evaluating and Prioritizing Stream Restoration in Mined Appalachian Watersheds.

The state of West Virginia requires stream mitigation to offset anthropogenic impacts to streams; consequently there is a high concentration of mitigation projects in the mountaintop mining/valley fill region. Projects are typically outside mine boundaries and sites are selected at the discretion of the West Virginia Department of Environmental Protection. This dissertation evaluated current in-stream, natural channel design (NCD) structures and prioritized future project locations. First, we assessed the Little Coal River, which is one of the largest physical restoration projects in the state. The Little Coal drains 994 km2 with extensive historic and contemporary coal mining. The objectives of our study were to quantify the effects of in-stream structure construction on stream channel morphology, fish habitat quality, sediment composition, bank stability, fish, and macroinvertebrate assemblages and to determine the extent to which benefits persisted over our study period (5 years). Our results indicate that the beneficial effects of structures include: improved fish habitat quality, increased bed complexity, and increased substrate diversity. Post-construction fish habitat quality and streambed complexity, although dynamic, appear to be relatively stable over time. We observed significant localized macroinvertebrate response to restoration that was mediated by shifts from sand dominated substrates to cobble and gravel. However, overall improvements to invertebrate assemblages at the river reach scale were not observed, because restoration did not affect substrate composition at the larger scale. In contrast, we did observe reach-scale effects of restoration on fish assemblages. However, the overall response was difficult to interpret as being ecologically beneficial or not. Total fish biomass, total abundance, and sucker abundance increased in response to NCD structure construction. Fish species richness and integrity measures remained unchanged, and gamefish abundance decreased on the Little Coal River. Second, we expanded our region and assessed the biological and physical responses to 14 restoration projects in an intensively mined region. Our results suggest that typical restoration practices consistently improve physical habitat quality regardless of drainage area. Restored reaches generally have higher habitat condition scores than adjacent reference reaches and tend to be higher than the average reach in the region. In contrast, macroinvertebrate assemblages demonstrated no measurable positive response to physical habitat restoration, regardless of drainage area, water quality, or the condition of neighboring streams. Fish assemblage response to restoration was strongly context dependent. Restoration projects on smaller streams (i.e. \u3c 50 km2 drainage area) with lower electrical conductivity (\u3c 1000 μs/cm) tended to result in a more positive response by fishes than projects on small, highly conductive streams. However, the most consistent response by fishes to restoration was an increased abundance and biomass of tolerant taxa