29,897 research outputs found
Lower Limits on Aperture Size for an ExoEarth-Detecting Coronagraphic Mission
The yield of Earth-like planets will likely be a primary science metric for
future space-based missions that will drive telescope aperture size. Maximizing
the exoEarth candidate yield is therefore critical to minimizing the required
aperture. Here we describe a method for exoEarth candidate yield maximization
that simultaneously optimizes, for the first time, the targets chosen for
observation, the number of visits to each target, the delay time between
visits, and the exposure time of every observation. This code calculates both
the detection time and multi-wavelength spectral characterization time required
for planets. We also refine the astrophysical assumptions used as inputs to
these calculations, relying on published estimates of planetary occurrence
rates as well as theoretical and observational constraints on terrestrial
planet sizes and classical habitable zones. Given these astrophysical
assumptions, optimistic telescope and instrument assumptions, and our new
completeness code that produces the highest yields to date, we suggest lower
limits on the aperture size required to detect and characterize a
statistically-motivated sample of exoEarths.Comment: Accepted for publication in ApJ; 38 pages, 16 Figures, 3 Table
Metastability-Containing Circuits
In digital circuits, metastability can cause deteriorated signals that
neither are logical 0 or logical 1, breaking the abstraction of Boolean logic.
Unfortunately, any way of reading a signal from an unsynchronized clock domain
or performing an analog-to-digital conversion incurs the risk of a metastable
upset; no digital circuit can deterministically avoid, resolve, or detect
metastability (Marino, 1981). Synchronizers, the only traditional
countermeasure, exponentially decrease the odds of maintained metastability
over time. Trading synchronization delay for an increased probability to
resolve metastability to logical 0 or 1, they do not guarantee success.
We propose a fundamentally different approach: It is possible to contain
metastability by fine-grained logical masking so that it cannot infect the
entire circuit. This technique guarantees a limited degree of metastability
in---and uncertainty about---the output.
At the heart of our approach lies a time- and value-discrete model for
metastability in synchronous clocked digital circuits. Metastability is
propagated in a worst-case fashion, allowing to derive deterministic
guarantees, without and unlike synchronizers. The proposed model permits
positive results and passes the test of reproducing Marino's impossibility
results. We fully classify which functions can be computed by circuits with
standard registers. Regarding masking registers, we show that they become
computationally strictly more powerful with each clock cycle, resulting in a
non-trivial hierarchy of computable functions
Photon number correlation for quantum enhanced imaging and sensing
In this review we present the potentialities and the achievements of the use
of non-classical photon number correlations in twin beams (TWB) states for many
applications, ranging from imaging to metrology. Photon number correlations in
the quantum regime are easy to be produced and are rather robust against
unavoidable experimental losses, and noise in some cases, if compared to the
entanglement, where loosing one photon can completely compromise the state and
its exploitable advantage. Here, we will focus on quantum enhanced protocols in
which only phase-insensitive intensity measurements (photon number counting)
are performed, which allow probing transmission/absorption properties of a
system, leading for example to innovative target detection schemes in a strong
background. In this framework, one of the advantages is that the sources
experimentally available emit a wide number of pairwise correlated modes, which
can be intercepted and exploited separately, for example by many pixels of a
camera, providing a parallelism, essential in several applications, like wide
field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally,
non-classical correlation enables new possibilities in quantum radiometry, e.g.
the possibility of absolute calibration of a spatial resolving detector from
the on-off- single photon regime to the linear regime, in the same setup
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Improved Log(gf) Values for Lines of Ti I and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937 (Accurate Transition Probabilities for Ti I)
New atomic transition probability measurements for 948 lines of Ti I are reported. Branching fractions from Fourier transform spectra and from spectra recorded using a 3 m echelle spectrometer are combined with published radiative lifetimes from laser-induced fluorescence measurements to determine these transition probabilities. Generally good agreement is found in comparisons to the NIST Atomic Spectra Database. The new Ti I data are applied to re-determine the Ti abundance in the photospheres of the Sun and metal-poor star HD 84937 using many lines covering a range of wavelength and excitation potential to explore possible non-local thermal equilibrium effects. The variation of relative Ti/Fe abundance with metallicity in metal-poor stars observed in earlier studies is supported in this study.NSF AST-1211055, AST-0908978, AST-1211585NSF REU grant AST-1004881ESO Science Archive Facility 073.D-0024, 266.D-5655NASA NAS 5-26555Astronom
Global soil moisture bimodality in satellite observations and climate models
A new diagnostic metric based on soil moisture bimodality is developed in order to examine and compare soil moisture from satellite observations and Earth System Models. The methodology to derive this diagnostic is based on maximum likelihood estimator encoded into an iterative algorithm, which is applied to the soil moisture probability density function. This metric is applied to satellite data from the Advanced Microwave Scanning Radiometer for the Earth Observing System and global climate models data from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results show high soil moisture bimodality in transitional climate areas and high latitudes, potentially associated with land-atmosphere feedback processes. When comparing satellite versus climate models, a clear difference in their soil moisture bimodality is observed, with systematically higher values in the case of CMIP5 models. These differences appear related to areas where land-atmospheric feedback may be overestimated in current climate models
Exploring the effect of sex on empirical fitness landscapes
The nature of epistasis has important consequences for the evolutionary significance of sex and recombination. Recent efforts to find negative epistasis as a source of negative linkage disequilibrium and associated long-term advantage to sex have yielded little support. Sign epistasis, where the sign of the fitness effects of alleles varies across genetic backgrounds, is responsible for the ruggedness of the fitness landscape, with several unexplored implications for the evolution of sex. Here, we describe fitness landscapes for two sets of strains of the asexual fungus Aspergillus niger involving all combinations of five mutations. We find that 30% of the single-mutation fitness effects are positive despite their negative effect in the wild-type strain and that several local fitness maxima and minima are present. We then compare adaptation of sexual and asexual populations on these empirical fitness landscapes by using simulations. The results show a general disadvantage of sex on these rugged landscapes, caused by the breakdown by recombination of genotypes on fitness peaks. Sex facilitates movement to the global peak only for some parameter values on one landscape, indicating its dependence on the landscape’s topography. We discuss possible reasons for the discrepancy between our results and the reports of faster adaptation of sexual population
Exoplanet atmospheres with GIANO. I. Water in the transmission spectrum of HD 189733b
High-resolution spectroscopy (R 20,000) at near-infrared wavelengths
can be used to investigate the composition, structure, and circulation patterns
of exoplanet atmospheres. However, up to now it has been the exclusive dominion
of the biggest telescope facilities on the ground, due to the large amount of
photons necessary to measure a signal in high-dispersion spectra. Here we show
that spectrographs with a novel design - in particular a large spectral range -
can open exoplanet characterisation to smaller telescope facilities too. We aim
to demonstrate the concept on a series of spectra of the exoplanet HD 189733 b
taken at the Telescopio Nazionale Galileo with the near-infrared spectrograph
GIANO during two transits of the planet. In contrast to absorption in the
Earth's atmosphere (telluric absorption), the planet transmission spectrum
shifts in radial velocity during transit due to the changing orbital motion of
the planet. This allows us to remove the telluric spectrum while preserving the
signal of the exoplanet. The latter is then extracted by cross-correlating the
residual spectra with template models of the planet atmosphere computed through
line-by-line radiative transfer calculations, and containing molecular
absorption lines from water and methane. By combining the signal of many
thousands of planet molecular lines, we confirm the presence of water vapour in
the atmosphere of HD 189733 b at the 5.5- level. This signal was
measured only in the first of the two observing nights. By injecting and
retrieving artificial signals, we show that the non-detection on the second
night is likely due to an inferior quality of the data. The measured strength
of the planet transmission spectrum is fully consistent with past CRIRES
observations at the VLT, excluding a strong variability in the depth of
molecular absorption lines.Comment: 10 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysics. v2 includes language editin
Metabolomics-based studies in the field of Leishmania/leishmaniasis
Leishmania is a neglected protozoan parasite which creates some problems for public health with different clinical infections in different countries around. Due to the lack of an effective drug without side effects and the emergence drug resistance, there is an urgent need to introduce the novel drug targets and new drugs and vaccines to control leishmaniasis during recent years, metabolomics and other ‘‘Omics” platforms has become an important approach to comprehensive knowledge of the Leishmania parasites biology. The study of metabolite profiles can open the insights for discovering novel therapeutic targets in this infection in both of the parasites and human host. In addition, specifying the metabolomics profile changes among promastigotes, amastigotes and during metacyclogenesis can pay the way for achieving parasite survival parameters and the host-parasite interaction. The previous studies in this field have been extracted from the databases, literature and their detailed major concepts. The present review highlights the role of metabolomics approach in the field of Leishmania research. Also, several important metabolite signatures introduced in various aspect of leishmania parasite such as drug resistance and parasite biology which would be useful in the field of biomarker and drug discovery process. Finally, metabolomics plays a potential role in introducing metabolic pathways related to Leishmania parasite and its treatment design.Keywords: Omics, System biology, Metabolomics, Leishmania, Parasite, Leishmaniasis, Biomarke
In search of isoglosses: continuous and discrete language embeddings in Slavic historical phonology
This paper investigates the ability of neural network architectures to
effectively learn diachronic phonological generalizations in a multilingual
setting. We employ models using three different types of language embedding
(dense, sigmoid, and straight-through). We find that the Straight-Through model
outperforms the other two in terms of accuracy, but the Sigmoid model's
language embeddings show the strongest agreement with the traditional
subgrouping of the Slavic languages. We find that the Straight-Through model
has learned coherent, semi-interpretable information about sound change, and
outline directions for future research
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