6,805 research outputs found
Formation and evolution of disk galaxies
Global star formation is the key to understanding galaxy disk formation. This
in turn depends on gravitational instability of disks and continuing gas
accretion as well as minor merging. A key component is feedback from
supernovae. Primary observational constraints on disk galaxy formation and
evolution include the Schmidt-Kennicutt law, the Tully-Fisher relation and the
galaxy luminosity function. I will review how theory confronts phenomenology,
and discuss future prospects for refining our understanding of disk formation.Comment: to appear in The Galaxy Disk in Cosmological Context Proceedings IAU
Symposium No. 254, 2008, J. Andersen, J. Bland-Hawthorn & B. Nordstrm, ed
The Entropy Principle and the Influence of Sociological Pressures on SETI
We begin with the premise that the law of entropy could prove to be
fundamental for the evolution of intelligent life and the advent of
technological civilization. Building on recent theoretical results, we combine
a modern approach to evolutionary theory with Monte Carlo Realization
Techniques. A numerical test for a proposed significance of the law of entropy
within the evolution of intelligent species is performed and results are
compared with a neutral test hypothesis. Some clarifying aspects on the
emergence of intelligent species arise and are discussed in the framework of
contemporary astrobiology.Comment: 11 pages, 5 figures, accepted for publication in the International
Journal of Astrobiolog
Periodic orbits 1-5 of quadratic polynomials on a new coordinate plane
While iterating the quadratic polynomial f_{c}(x)=x^{2}+c the degree of the
iterates grows very rapidly, and therefore solving the equations corresponding
to periodic orbits becomes very difficult even for periodic orbits with a low
period. In this work we present a new iteration model by introducing a change
of variables into an (u,v)-plane, which changes situation drastically. As an
excellent example of this we can compare equations of orbits period four on
(x,c)- and (u,v)-planes. In the latter case, this equation is of degree two
with respect to u and it can be solved explicitly. In former case the
corresponding equation
((((x^{2}+c)^{2}+c)^{2}+c)^{2}+c-x)/((x^{2}+c)^{2}+c-x)=0 is of degree 12 and
it is thus much more difficult to solve
Numerical Testing of The Rare Earth Hypothesis using Monte Carlo Realisation Techniques
The Search for Extraterrestrial Intelligence (SETI) has thus far failed to
provide a convincing detection of intelligent life. In the wake of this null
signal, many "contact pessimistic" hypotheses have been formulated, the most
famous of which is the Rare Earth Hypothesis. It postulates that although
terrestrial planets may be common, the exact environmental conditions that
Earth enjoys are rare, perhaps unique. As a result, simple microbial life may
be common, but complex metazoans (and hence intelligence) will be rare. This
paper uses Monte Carlo Realisation Techniques to investigate the Rare Earth
Hypothesis, in particular the environmental criteria considered imperative to
the existence of intelligence on Earth.
By comparing with a less restrictive, more optimistic hypothesis, the data
indicates that if the Rare Earth hypothesis is correct, intelligent
civilisation will indeed be relatively rare. Studying the separations of pairs
of civilisations shows that most intelligent civilisation pairs (ICPs) are
unconnected: that is, they will not be able to exchange signals at lightspeed
in the limited time that both are extant. However, the few ICPs that are
connected are strongly connected, being able to participate in numerous
exchanges of signals. This may provide encouragement for SETI researchers:
although the Rare Earth Hypothesis is in general a contact-pessimistic
hypothesis, it may be a "soft" or "exclusive" hypothesis, i.e. it may contain
facets that are latently contact-optimistic.Comment: 13 pages, 10 figures, accepted for publication in the International
Journal of Astrobiolog
Steps in modular specifications for concurrent modules
© 2015 Published by Elsevier B.V.The specification of a concurrent program module is a difficult problem. The specifications must be strong enough to enable reasoning about the intended clients without reference to the underlying module implementation. We survey a range of verification techniques for specifying concurrent modules, in particular highlighting four key concepts: auxiliary state, interference abstraction, resource ownership and atomicity. We show how these concepts combine to provide powerful approaches to specifying concurrent modules
First Chemical Analysis of Stars in The Triangulum-Andromeda Star Cloud
We undertake the first high resolution spectroscopic study of the
Triangulum-Andromeda (TriAnd) star cloud --- an extended, mid-latitude Milky
Way halo substructure about 20 kpc away in the second Galactic quadrant ---
through six M giant star candidates selected to be both spatially and
dynamically associated with this system. The abundance patterns of [Ti/Fe],
[Y/Fe] and [La/Fe] as a function of [Fe/H] for these stars support TriAnd as
having an origin in a dwarf galaxy with a chemical enrichment history somewhat
similar to that of the Sagittarius dwarf spheroidal (dSph) galaxy. We also
investigate the previously proposed hypothesis that TriAnd is an outlying,
dynamically older piece of the Monoceros Stream (also known as the Galactic
Anticenter Stellar Structure, "GASS") under the assumption that both features
come from the tidal disruption of the same accreted Milky Way satellite and
find that net differences in the above abundance patterns between the TriAnd
and GASS stars studied suggest that these two systems are independent and
unrelated.Comment: Accepted for publication in ApJ Letter
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