2,342 research outputs found
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
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
Homogeneous abundance analysis of dwarf, subgiant and giant FGK stars with and without giant planets
We have analyzed high-resolution and high signal-to-noise ratio optical
spectra of nearby FGK stars with and without detected giant planets in order to
homogeneously measure their photospheric parameters, mass, age, and the
abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn,
Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar
neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29
are subgiants, and 140 are giants. The photospheric parameters are derived from
the equivalent widths of Fe I and Fe II lines. Masses and ages come from the
interpolation in evolutionary tracks and isochrones on the HR diagram. The
abundance determination is based on the equivalent widths of selected atomic
lines of the refractory elements and on the spectral synthesis of C_2, CN, C I,
O I, and Na I features. We apply a set of statistical methods to analyze the
abundances derived for the three subsamples. Our results show that: i) giant
stars systematically exhibit underabundance in [C/Fe] and overabundance in
[N/Fe] and [Na/Fe] in comparison with dwarfs, a result that is normally
attributed to evolution-induced mixing processes in the envelope of evolved
stars; ii) for solar analogs only, the abundance trends with the condensation
temperature of the elements are correlated with age and anticorrelated with the
surface gravity, which is in agreement with recent studies; iii) as in the case
of [Fe/H], dwarf stars with giant planets are systematically enriched in [X/H]
for all the analyzed elements, except for O and Ba (the former due to
limitations of statistics), confirming previous findings in the literature that
not only iron has an important relation with the planetary formation; and iv)
giant planet hosts are also significantly overabundant for the same metallicity
when the elements from Mg to Cu are combined together.Comment: 20 pages, 16 figures, 8 table
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
Modular termination verification for non-blocking concurrency
© Springer-Verlag Berlin Heidelberg 2016.We present Total-TaDA, a program logic for verifying the total correctness of concurrent programs: that such programs both terminate and produce the correct result. With Total-TaDA, we can specify constraints on a thread’s concurrent environment that are necessary to guarantee termination. This allows us to verify total correctness for nonblocking algorithms, e.g. a counter and a stack. Our specifications can express lock- and wait-freedom. More generally, they can express that one operation cannot impede the progress of another, a new non-blocking property we call non-impedance. Moreover, our approach is modular. We can verify the operations of a module independently, and build up modules on top of each other
Self-Regulation of Star Formation in Low Metallicity Clouds
We investigate the process of self-regulated star formation via
photodissociation of hydrogen molecules in low metallicity clouds. We evaluate
the influence region's scale of a massive star in low metallicity gas clouds
whose temperatures are between 100 and 10000 Kelvin. A single O star can
photodissociate hydrogen molecules in the whole of the host cloud. If
metallicity is smaller than about 10^{-2.5} of the solar metallicity, the
depletion of coolant of the the host cloud is very serious so that the cloud
cannot cool in a free-fall time, and subsequent star formation is almost
quenched. On the contrary, if metallicity is larger than about 10^{-1.5} of the
solar metallicity, star formation regulation via photodissociation is not
efficient. The typical metallicity when this transition occurs is about 1/100
of the solar metallicity. This indicates that stars do not form efficiently
before the metallicity becomes larger than about 1/100 of the solar metallicity
and we considered that this value becomes the lower limit of the metallicity of
luminous objects such as galaxies.Comment: 14 pages, including 5 figures, To appear in ApJ, Vol. 53
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