6,091 research outputs found
Collisions and close encounters involving massive main-sequence stars
We study close encounters involving massive main sequence stars and the
evolution of the exotic products of these encounters as common--envelope
systems or possible hypernova progenitors. We show that parabolic encounters
between low-- and high--mass stars and between two high--mass stars with small
periastrons result in mergers on timescales of a few tens of stellar freefall
times (a few tens of hours). We show that such mergers of unevolved low--mass
stars with evolved high--mass stars result in little mass loss (
M) and can deliver sufficient fresh hydrogen to the core of the
collision product to allow the collision product to burn for several million
years. We find that grazing encounters enter a common--envelope phase which may
expel the envelope of the merger product. The deposition of energy in the
envelopes of our merger products causes them to swell by factors of .
If these remnants exist in very densely-populated environments
( pc), they will suffer further collisions which may
drive off their envelopes, leaving behind hard binaries. We show that the
products of collisions have cores rotating sufficiently rapidly to make them
candidate hypernova/gamma--ray burst progenitors and that of massive
stars may suffer collisions, sufficient for such events to contribute
significantly to the observed rates of hypernovae and gamma--ray bursts.Comment: 15 pages, 13 figures, LaTeX, to appear in MNRAS (in press
A new algorithm for modelling photoionising radiation in smoothed particle hydrodynamics
We present a new fast algorithm which allows the simulation of ionising
radiation emitted from point sources to be included in high-resolution
three-dimensional smoothed particle hydrodynamics simulations of star cluster
formation. We employ a Str\"omgren volume technique in which we use the
densities of particles near the line-of-sight between the source and a given
target particle to locate the ionisation front in the direction of the target.
Along with one--dimensional tests, we present fully three--dimensional
comparisons of our code with the three--dimensional Monte-Carlo radiative
transfer code, MOCASSIN, and show that we achieve good agreement, even in the
case of highly complex density fields.Comment: 10 pages, 7 figures, submitted to MNRA
A knowledge-based system design/information tool for aircraft flight control systems
Research aircraft have become increasingly dependent on advanced control systems to accomplish program goals. These aircraft are integrating multiple disciplines to improve performance and satisfy research objectives. This integration is being accomplished through electronic control systems. Because of the number of systems involved and the variety of engineering disciplines, systems design methods and information management have become essential to program success. The primary objective of the system design/information tool for aircraft flight control system is to help transfer flight control system design knowledge to the flight test community. By providing all of the design information and covering multiple disciplines in a structured, graphical manner, flight control systems can more easily be understood by the test engineers. This will provide the engineers with the information needed to thoroughly ground test the system and thereby reduce the likelihood of serious design errors surfacing in flight. The secondary objective is to apply structured design techniques to all of the design domains. By using the techniques in the top level system design down through the detailed hardware and software designs, it is hoped that fewer design anomalies will result. The flight test experiences of three highly complex, integrated aircraft programs are reviewed: the X-29 forward-swept wing, the advanced fighter technology integration (AFTI) F-16, and the highly maneuverable aircraft technology (HiMAT) program. Significant operating anomalies and the design errors which cause them, are examined to help identify what functions a system design/information tool should provide to assist designers in avoiding errors
The effect of the virial state of molecular clouds on the influence of feedback from massive stars
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society, following peer review. The version of record [James E. Dale, ‘The effect of the virial state of molecular clouds on the influence of feedback from massive stars’, Monthly Notices of the Royal Astronomical Society, Vol. 467 (1): 1067-1082, May 2017, first published online 10 January 2017] is available online at doi: https://doi.org/10.1093/mnras/stx028A set of Smoothed Particle Hydrodynamics simulations of the influence of photoionising radiation and stellar winds on a series of 10^4 solar-mass turbulent molecular clouds with initial virial ratios of 0.7, 1.1, 1.5, 1.9 and 2.3 and initial mean densities of 136, 1135 and 9096 cm^−3 are presented. Reductions in star formation efficiency rates are found to be modest, in the range 30% − 50% and to not vary greatly across the parameter space. In no case was star formation entirely terminated over the ≈ 3 Myr duration of the simulations. The fractions of material unbound by feedback are in the range 20 − 60%, clouds with the lowest escape velocities being the most strongly affected. Leakage of ionised gas leads to the HII regions rapidly becoming underpressured. The destructive effects of ionisation are thus largely not due to thermally–driven expansion of the HII regions, but to momentum transfer by photoevaporation of fresh material. Our simulations have similar global ionisation rates and we show that the effects of feedback upon them can be adequately modelled as a steady injection of momentum, resembling a momentum–conserving wind.Peer reviewedFinal Accepted Versio
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