4,810 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
Data Mining with Newton\u27s Method.
Capable and well-organized data mining algorithms are essential and fundamental to helpful, useful, and successful knowledge discovery in databases. We discuss several data mining algorithms including genetic algorithms (GAs). In addition, we propose a modified multivariate Newton\u27s method (NM) approach to data mining of technical data. Several strategies are employed to stabilize Newton\u27s method to pathological function behavior. NM is compared to GAs and to the simplex evolutionary operation algorithm (EVOP). We find that GAs, NM, and EVOP all perform efficiently for well-behaved global optimization functions with NM providing an exponential improvement in convergence rate. For local optimization problems, we find that GAs and EVOP do not provide the desired convergence rate, accuracy, or precision compared to NM for technical data. We find that GAs are favored for their simplicity while NM would be favored for its performance
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