537 research outputs found
Binary Disruption by Massive Black Holes in Globular Clusters
A massive (~10^2-10^4 Msun) black hole at the center of a globular cluster
will tidally disrupt binaries that pass sufficiently close. Such an encounter
results in the capture of one binary component by the black hole and ejection
of the other star. A 10^3 Msun black hole disrupts one binary every 1-10 Myr;
captured stars orbit with initial periods of days to decades. Although >~100
stars are captured in only 1 Gyr, the number with periods of <10 yr at any one
time is probably less than 10, regulated by stellar interactions and tidal
disruption by the black hole. Binary disruption and stellar capture should be
critically examined with detailed numerical simulations, in order to
corroborate the analytic results given here and to weigh possible observational
consequences.Comment: 4 pages, 1 figure, Accepted by ApJ, minor correction
Cluster Origin of Triple Star HD 188753 and its Planet
The recent discovery by M. Konacki of a ``hot Jupiter'' in the hierarchical
triple star system HD 188753 challenges established theories of giant-planet
formation. If the orbital geometry of the triple has not changed since the
birth of the planet, then a disk around the planetary host star would probably
have been too compact and too hot for a Jovian planet to form by the
core-accretion model or gravitational collapse. This paradox is resolved if the
star was initially either single or had a much more distant companion. It is
suggested here that a close multi-star dynamical encounter transformed this
initial state into the observed triple, an idea that follows naturally if HD
188753 formed in a moderately dense stellar system--perhaps an open
cluster--that has since dissolved. Three distinct types of encounters are
investigated. The most robust scenario involves an initially single planetary
host star that changes places with the outlying member of a pre-existing
hierarchical triple.Comment: Accepted by ApJL; minor changes from origina
Racing with a purpose:Sustainability in Formula E
This study examined the seasonal sustainability reporting of Formula E for its content, variation, and linearity. Formula E was chosen since it was built as a sustainable sport enterprise rather than one which integrated sustainability into existing operations and for the accessibility of its annual sustainability reports. Using an exploratory approach for content and variation, and the Green Waves of sport sustainability for linearity, eight seasons of sustainability reports from Formula E were collected and examined via content analysis. Findings revealed the major content areas for Formula E concerned event management, car design, and community engagement with these initiatives classified as internal or external efforts. Reports were inconsistent in their structure, language, scope, and focus. The Green Waves suggest that while some efforts progress, others may regress according to strategy or the initiative’s life cycle. Formula E is progressing overall in their sustainability efforts, but exemplifies the lack of an end point to sustainability efforts. It is suggested that better coordination could yield higher strategic success and recognition
Cost studies for commercial fuselage crown designs
Studies were conducted to evaluate the cost and weight potential of advanced composite design concepts in the crown region of a commercial transport. Two designs from each of three design families were developed using an integrated design-build team. A range of design concepts and manufacturing processes were included to allow isolation and comparison of cost centers. Detailed manufacturing/assembly plans were developed as the basis for cost estimates. Each of the six designs was found to have advantages over the 1995 aluminum benchmark in cost and weight trade studies. Large quadrant panels and cobonded frames were found to save significant assembly labor costs. Comparisons of high- and intermediate-performance fiber systems were made for skin and stringer applications. Advanced tow placement was found to be an efficient process for skin lay up. Further analysis revealed attractive processes for stringers and frames. Optimized designs were informally developed for each design family, combining the most attractive concepts and processes within that family. A single optimized design was selected as the most promising, and the potential for further optimization was estimated. Technical issues and barriers were identified
Exploring the Nature of Weak Chandra Sources near the Galactic Centre
We present results from the first near-IR imaging of the weak X-ray sources
discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic
Centre (GC). These ~800 discrete sources, which contribute significantly to the
GC X-ray emission, represent an important and previously unknown population
within the Galaxy. From our VLT observations we will identify likely IR
counterparts to a sample of the hardest sources, which are most likely X-ray
binaries. With these data we can place constraints on the nature of the
discrete weak X-ray source population of the GC.Comment: In Proc. of ``Interacting Binaries: Accretion, Evolution, and
Outcomes'', eds. L. A. Antonelli et al., AIP, Cefalu, Sicily, 200
All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities
We demonstrate a new technique for detecting radio transients based on
interferometric closure quantities. The technique uses the bispectrum, the
product of visibilities around a closed-loop of baselines of an interferometer.
The bispectrum is calibration independent, resistant to interference, and
computationally efficient, so it can be built into correlators for real-time
transient detection. Our technique could find celestial transients anywhere in
the field of view and localize them to arcsecond precision. At the Karl G.
Jansky Very Large Array (VLA), such a system would have a high survey speed and
a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth.
The ability to localize dispersed millisecond pulses to arcsecond precision in
large volumes of interferometer data has several unique science applications.
Localizing individual pulses from Galactic pulsars will help find X-ray
counterparts that define their physical properties, while finding host galaxies
of extragalactic transients will measure the electron density of the
intergalactic medium with a single dispersed pulse. Exoplanets and active stars
have distinct millisecond variability that can be used to identify them and
probe their magnetospheres. We use millisecond time scale visibilities from the
Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect
dispersed pulses and reject local interference. The computational and data
efficiency of the bispectrum will help find transients on a range of time
scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include
discussion of non-Gaussian statistics of techniqu
Flammability limits, ignition energy, and flame speeds in H₂–CH₄–NH₃–N₂O–O₂–N₂ mixtures
Experiments on flammability limits, ignition energies, and flame speeds were carried out in a 11.25- and a 400-liter combustion vessel at initial pressures and temperatures of 100 kPa and 295 K, respectively. Flammability maps of hydrogen–nitrous oxide–nitrogen, methane–nitrous oxide–nitrogen, ammonia–nitrous oxide–nitrogen, and ammonia–nitrous oxide–air, as well as lean flammability limits of various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures were determined. Ignition energy bounds of methane–nitrous oxide, ammonia–nitrous oxide, and ammonia–nitrous oxide–nitrogen mixtures have been determined and the influence of small amounts of oxygen on the flammability of methane–nitrous oxide–nitrogen mixtures has been investigated. Flame speeds have been measured and laminar burning velocities have been determined for ammonia–air–nitrous oxide and various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures. Lower and upper flammability limits (mixing fan on, turbulent conditions) for ignition energies of 8 J are: H₂–N₂O: 4.5 ∼ 5.0% H₂(LFL), 76 ∼ 80% H₂(UFL); CH₄–N₂O: 2.5 ∼ 3.0% CH₄(LFL), 43 ∼ 50% CH₄(UFL); NH₃–N₂O: 5.0 ∼ 5.2% NH₃(LFL), 67.5 ∼ 68% NH₃(UFL). Inerting concentrations are: H₂–N₂O–N₂: 76% N₂; CH₄–N₂O–N₂: 70.5% N₂; NH₃–N₂O–N₂: 61% N₂; NH₃–N₂O–air: 85% air. Flammability limits of methane–nitrous oxide–nitrogen mixtures show no pronounced dependence on small amounts of oxygen (<5%). Generally speaking, flammable gases with large initial amounts of nitrous oxide or ammonia show a strong dependence of flammability limits on ignition energy
- …