1,643 research outputs found
Repeatable method of thermal stress fracture test of brittle materials
Method heats specimens slowly and with sufficient control so that the critical temperature gradient in the specimens cannot occur before temperature equilibrium is reached
Assessment of stochastic and deterministic models of 6304 quasar lightcurves from SDSS Stripe 82
The optical light curves of many quasars show variations of tenths of a
magnitude or more on time scales of months to years. This variation often
cannot be described well by a simple deterministic model. We perform a Bayesian
comparison of over 20 deterministic and stochastic models on 6304 QSO light
curves in SDSS Stripe 82. We include the damped random walk (or
Ornstein-Uhlenbeck [OU] process), a particular type of stochastic model which
recent studies have focused on. Further models we consider are single and
double sinusoids, multiple OU processes, higher order continuous autoregressive
processes, and composite models. We find that only 29 out of 6304 QSO
lightcurves are described significantly better by a deterministic model than a
stochastic one. The OU process is an adequate description of the vast majority
of cases (6023). Indeed, the OU process is the best single model for 3462 light
curves, with the composite OU process/sinusoid model being the best in 1706
cases. The latter model is the dominant one for brighter/bluer QSOs.
Furthermore, a non-negligible fraction of QSO lightcurves show evidence that
not only the mean is stochastic but the variance is stochastic, too. Our
results confirm earlier work that QSO light curves can be described with a
stochastic model, but place this on a firmer footing, and further show that the
OU process is preferred over several other stochastic and deterministic models.
Of course, there may well exist yet better (deterministic or stochastic) models
which have not been considered here.Comment: accepted by AA, 12 pages, 11 figures, 4 table
New stellar encounters discovered in the second Gaia data release
Passing stars may play an important role in the evolution of our solar
system. We search for close stellar encounters to the Sun among all 7.2 million
stars in Gaia-DR2 that have six-dimensional phase space data. We characterize
encounters by integrating their orbits through a Galactic potential and
propagating the correlated uncertainties via a Monte Carlo resampling. After
filtering to remove spurious data, we find 694 stars that have median (over
uncertainties) closest encounter distances within 5 pc, all occurring within 15
Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc
(and 7 within 0.5 pc), all but one of which are newly discovered here. We
further confirm some and refute several other previously-identified encounters,
confirming suspicions about their data. The closest encounter in the sample is
Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU).
Taking mass estimates from Gaia astrometry and multiband photometry for
essentially all encounters, we find that Gl 710 also has the largest impulse on
the Oort cloud. Using a Galaxy model, we compute the completeness of the
Gaia-DR2 encountering sample as a function of perihelion time and distance.
Only 15% of encounters within 5 pc occurring within +/- 5 Myr of now have been
identified, mostly due to the lack of radial velocities for faint and/or cool
stars. Accounting for the incompleteness, we infer the present rate of
encounters within 1 pc to be 19.7 +/- 2.2 per Myr, a quantity expected to scale
quadratically with the encounter distance out to at least several pc.
Spuriously large parallaxes in our sample from imperfect filtering would tend
to inflate both the number of encounters found and this inferred rate. The
magnitude of this effect is hard to quantify.Comment: 12 pages. Accepted to A&A. Added to this version: section 3.2 and
Fig. 8 (CMD) with discussion of astrometric quality metrics; full versions of
tables 2 and 3 as ancillary dat
Operational Model of the Alliance for Grassland Renewal
Novel endophyte tall fescue was developed over 20 years ago, and novel endophyte cultivars are among the most innovative cultivars developed in recent decades. Yet adoption of novel endophyte technology is slow. In order to expedite adoption, a diverse group of stakeholders self assembled as the Alliance for Grassland Renewal, a nonprofit organization that includes colleagues from the sectors of industry, university, government, and production. Since 2012, the Alliance has held workshops for producers, educators, and industry across the southeastern US, where tall fescue is the predominant pasture grass. Also since 2012, the Alliance has monitored and reported endophyte purity and viability in novel endophyte cultivars. Activities of the Alliance are self-funded according to a fee structure developed by its Board of Directors. At present, the Alliance has expanded to include seven land-grant universities and seven private companies, along with partners in the federal government, livestock production, and nonprofit organization
Evaluation of the Irritable Bowel Syndrome Quality of Life (IBS-QOL) questionnaire in diarrheal-predominant irritable bowel syndrome patients
Background Diarrhea-predominant irritable bowel syndrome (IBS-d) significantly diminishes the health-related quality of life (HRQOL) of patients. Psychological and social impacts are common with many IBS-d patients reporting comorbid depression, anxiety, decreased intimacy, and lost working days. The Irritable Bowel Syndrome Quality of Life (IBS-QOL) questionnaire is a 34-item instrument developed and validated for measurement of HRQOL in non-subtyped IBS patients. The current paper assesses this previously-validated instrument employing data collected from 754 patients who participated in a randomized clinical trial of a novel treatment, eluxadoline, for IBS-d. Methods Psychometric methods common to HRQOL research were employed to evaluate the IBS-QOL. Many of the historical analyses of the IBS-QOL validations were used. Other techniques that extended the original methods were applied where more appropriate for the current dataset. In IBS-d patients, we analyzed the items and substructure of the IBS-QOL via item reduction, factor structure, internal consistency, reproducibility, construct validity, and ability to detect change. Results This study supports the IBS-QOL as a psychometrically valid measure. Factor analyses suggested that IBS-specific QOL as measured by the IBS-QOL is a unidimensional construct. Construct validity was further buttressed by significant correlations between IBS-QOL total scores and related measures of IBS-d severity including the historically-relevant Irritable Bowel Syndrome Adequate Relief (IBS-AR) item and the FDAâs Clinical Responder definition. The IBS-QOL also showed a significant ability to detect change as evidenced by analysis of treatment effects. A minority of the items, unrelated to the IBS-d, performed less well by the standards set by the original authors. Conclusions We established that the IBS-QOL total score is a psychometrically valid measure of HRQOL in IBS-d patients enrolled in this study. Our analyses suggest that the IBS-QOL items demonstrate very good construct validity and ability to detect changes due to treatment effects. Furthermore, our analyses suggest that the IBS-QOL items measure a univariate construct and we believe further modeling of the IBS-QOL from an item response theory (IRT) approach under both non-treatment and treatment conditions would greatly further our understanding as item-based methods could be used to develop a short form
Heating and Trapping of Electrons in ECRIS from Scratch to Afterglow
Plasmas in Electron Cyclotron Resonance Ion Sources (ECRIS) are collisionless and can therefore be simulated by just following the motion of electrons in the confining static magnetic and oscillating microwave (MW) electric field of ECRIS. With a powerful algorithm the three-dimensional trajectories of 104 ECR-heated and confined electrons are calculated in a standard ECRIS with a deep minimum of |B| and a new ECRIS with a very flat minimum of |B|. The spatial electron (plasma) densities and electron energy densities deduced from these trajectories yield new and surprising insight in the performance of ECRIS. With computer animation we plan to present: The energy increase of certain electrons on extremely stable trajectories, the power dependence of the electron energy density up to the X-ray collapse, the time dependent build up of the electron density and energy density distributions, and the time evolution of these electron distributions under afterglow conditions
Time resolved fission in metal clusters
We explore from a theoretical point of view pump and probe (P&P) analysis for
fission of metal clusters where probe pulses are generalized to allow for
scanning various frequencies. We show that it is possible to measure the time
the system needs to develop to scission. This is achieved by a proper choice of
both delay and frequency of the probe pulse. A more detailed analysis even
allows to access the various intermediate stages of the fission process.Comment: 4 pages, 4 figure
Dynamics of metal clusters in rare gas clusters
We investigate the dynamics of Na clusters embedded in Ar matrices. We use a
hierarchical approach, accounting microscopically for the cluster's degrees of
freedom and more coarsely for the matrix. The dynamical polarizability of the
Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken
into account. We discuss the impact of the matrix on the cluster gross
properties and on its optical response. We then consider a realistic case of
irradiation by a moderately intense laser and discuss the impact of the matrix
on the hindrance of the explosion, as well as a possible pump probe scenario
for analyzing dynamical responses.Comment: Proceedings of the 30th International Workshop on Condensed Matter
Theories, Dresden, June 05 - 10, 2006, World Scientific. 3 figure
Nitrogen Transformations in Broiler Litter-Amended Soils
Nitrogen mineralization rates in ten surface soils amended with (200âÎŒg N gâ1 soil) or without broiler litter were investigated. The soil-broiler litter mixture was incubated at 25±1âC for 28 weeks. A nonlinear regression approach for N mineralization was used to estimate the readily mineralizable organic N pools (N0) and the first-order rate constant (k). The cumulative N mineralized in the nonamended soils did not exceed 80âmg N kgâ1 soil. However, in Decatur soil amended with broiler litter 2, it exceeded 320âmg N kgâ1 soil. The greatest calculated N0 of the native soils was observed in Sucarnoochee soil alone (123âmg NO3â kgâ1 soil) which when amended with broiler litter 1 reached 596âmg N kgâ1 soil. The added broiler litter mineralized initially at a fast rate (k1) followed by a slow rate (k2) of the most resistant fraction. Half-life of organic N remaining in the soils alone varied from 33 to 75 weeks and from 43 to 15 weeks in the amended soils. When N0 was regressed against soil organic N (=0.782ââ) and C (=0.884âââ), positive linear relationships were obtained. The N0 pools increased with sand but decreased with silt and clay contents
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