6,416 research outputs found
Parametric Constructive Kripke-Semantics for Standard Multi-Agent Belief and Knowledge (Knowledge As Unbiased Belief)
We propose parametric constructive Kripke-semantics for multi-agent
KD45-belief and S5-knowledge in terms of elementary set-theoretic constructions
of two basic functional building blocks, namely bias (or viewpoint) and
visibility, functioning also as the parameters of the doxastic and epistemic
accessibility relation. The doxastic accessibility relates two possible worlds
whenever the application of the composition of bias with visibility to the
first world is equal to the application of visibility to the second world. The
epistemic accessibility is the transitive closure of the union of our doxastic
accessibility and its converse. Therefrom, accessibility relations for common
and distributed belief and knowledge can be constructed in a standard way. As a
result, we obtain a general definition of knowledge in terms of belief that
enables us to view S5-knowledge as accurate (unbiased and thus true)
KD45-belief, negation-complete belief and knowledge as exact KD45-belief and
S5-knowledge, respectively, and perfect S5-knowledge as precise (exact and
accurate) KD45-belief, and all this generically for arbitrary functions of bias
and visibility. Our results can be seen as a semantic complement to previous
foundational results by Halpern et al. about the (un)definability and
(non-)reducibility of knowledge in terms of and to belief, respectively
Segue 1: An Unevolved Fossil Galaxy from the Early Universe
We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red
giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star
observed by Norris et al. (2010), high-resolution spectra have now been
obtained for every red giant in Segue 1. Remarkably, three of these seven stars
have metallicities below [Fe/H] = -3.5, suggesting that Segue 1 is the least
chemically evolved galaxy known. We confirm previous medium-resolution analyses
demonstrating that Segue 1 stars span a metallicity range of more than 2 dex,
from [Fe/H] = -1.4 to [Fe/H] = -3.8. All of the Segue 1 stars are
alpha-enhanced, with [alpha/Fe] ~ 0.5. High alpha-element abundances are
typical for metal-poor stars, but in every previously studied galaxy [alpha/Fe]
declines for more metal-rich stars, which is typically interpreted as iron
enrichment from supernova Ia. The absence of this signature in Segue 1
indicates that it was enriched exclusively by massive stars. Other light
element abundance ratios in Segue 1, including carbon-enhancement in the three
most metal-poor stars, closely resemble those of metal-poor halo stars.
Finally, we classify the most metal-rich star as a CH star given its large
overabundances of carbon and s-process elements. The other six stars show
remarkably low neutron-capture element abundances of [Sr/H] < -4.9 and [Ba/H] <
-4.2, which are comparable to the lowest levels ever detected in halo stars.
This suggests minimal neutron-capture enrichment, perhaps limited to a single
r-process or weak s-process synthesizing event. Altogether, the chemical
abundances of Segue 1 indicate no substantial chemical evolution, supporting
the idea that it may be a surviving first galaxy that experienced only one
burst of star formation.Comment: ApJ, accepted, 20 pages (emulateapj), 9 figure
Orbital Circularization of Hot and Cool Kepler Eclipsing Binaries
The rate of tidal circularization is predicted to be faster for relatively
cool stars with convective outer layers, compared to hotter stars with
radiative outer layers. Observing this effect is challenging, because it
requires large and well-characterized samples including both hot and cool
stars. Here we seek evidence for the predicted dependence of circularization
upon stellar type, using a sample of 945 eclipsing binaries observed by Kepler.
This sample complements earlier studies of this effect, which employed smaller
samples of better-characterized stars. For each Kepler binary we measure
based on the relative timing of the primary and secondary
eclipses. We examine the distribution of as a function of period
for binaries composed of hot stars, cool stars, and mixtures of the two types.
At the shortest periods, hot-hot binaries are most likely to be eccentric; for
periods shorter than 4 days, significant eccentricities occur frequently for
hot-hot binaries, but not for hot-cool or cool-cool binaries. This is in
qualitative agreement with theoretical expectations based on the slower
dissipation rates of hot stars. However, the interpretation of our results is
complicated by the largely unknown ages and evolutionary states of the stars in
our sample.Comment: Accepted for publication in Ap
The geometry of the generic line complex
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1995.Includes bibliographical references (leaves 27-28).by Joshua S. Sher.Ph.D
Execution Models for Choreographies and Cryptoprotocols
A choreography describes a transaction in which several principals interact.
Since choreographies frequently describe business processes affecting
substantial assets, we need a security infrastructure in order to implement
them safely. As part of a line of work devoted to generating cryptoprotocols
from choreographies, we focus here on the execution models suited to the two
levels.
We give a strand-style semantics for choreographies, and propose a special
execution model in which choreography-level messages are faithfully delivered
exactly once. We adapt this model to handle multiparty protocols in which some
participants may be compromised.
At level of cryptoprotocols, we use the standard Dolev-Yao execution model,
with one alteration. Since many implementations use a "nonce cache" to discard
multiply delivered messages, we provide a semantics for at-most-once delivery
Spectroscopic Confirmation of the Dwarf Galaxies Hydra II and Pisces II and the Globular Cluster Laevens 1
We present Keck/DEIMOS spectroscopy of stars in the recently discovered Milky
Way satellites Hydra II, Pisces II, and Laevens 1. We measured a velocity
dispersion of 5.4 (+3.6 -2.4) km/s for Pisces II, but we did not resolve the
velocity dispersions of Hydra II or Laevens 1. We marginally resolved the
metallicity dispersions of Hydra II and Pisces II but not Laevens 1.
Furthermore, Hydra II and Pisces II obey the luminosity-metallicity relation
for Milky Way dwarf galaxies ( = -2.02 +/- 0.08 and -2.45 +/- 0.07,
respectively), whereas Laevens 1 does not ( = -1.68 +/- 0.05). The
kinematic and chemical properties suggest that Hydra II and Pisces II are dwarf
galaxies, and Laevens 1 is a globular cluster. We determined that two of the
previously observed blue stars near the center of Laevens 1 are not members of
the cluster. A third blue star has ambiguous membership. Hydra II has a radial
velocity = 303.1 +/- 1.4 km/s, similar to the leading arm of the
Magellanic stream. The mass-to-light ratio for Pisces II is 370 (+310 -240)
M_sun/L_sun. It is not among the most dark matter-dominated dwarf galaxies, but
it is still worthy of inclusion in the search for gamma rays from dark matter
self-annihilation.Comment: Accepted to ApJ. v2 has been revised in response to the referee's
repor
High-resolution spectroscopy of extremely metal-poor stars in the least evolved galaxies: Bootes II
We present high-resolution Magellan/MIKE spectra of the four brightest
confirmed red giant stars in the ultra-faint dwarf galaxy Bootes II (Boo II).
These stars all inhabit the metal-poor tail of the Boo II metallicity
distribution function. The chemical abundance pattern of all detectable
elements in these stars is consistent with that of the Galactic halo. However,
all four stars have undetectable amounts of neutron-capture elements Sr and Ba,
with upper limits comparable to the lowest ever detected in the halo or in
other dwarf galaxies. One star exhibits significant radial velocity variations
over time, suggesting it to be in a binary system. Its variable velocity has
likely increased past determinations of the Boo II velocity dispersion. Our
four stars span a limited metallicity range, but their enhanced
{\alpha}-abundances and low neutron-capture abundances are consistent with the
interpretation that Boo II has been enriched by very few generations of stars.
The chemical abundance pattern in Boo II confirms the emerging trend that the
faintest dwarf galaxies have neutron-capture abundances distinct from the halo,
suggesting the dominant source of neutron-capture elements in halo stars may be
different than in ultra-faint dwarfs.Comment: 10 pages, 5 figures, 4 tables. Updated to match ApJ accepted versio
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