Sensitivity to initial conditions in self-organized critical systems

We discuss sensitivity to initial conditions in a model for avalanches in granular media displaying self-organized criticality. We show that damage, due to a small perturbation in initial conditions, does not spread. The damage persists in a statistically time-invariant and scale-free form. We argue that the origin of this behavior is the Abelian nature of the model, which generalizes our results to all Abelian models, including the BTW model and the Manna model. An ensemble average of the damage leads to seemingly time dependent damage spreading. Scaling arguments show that this numerical result is due to the time lag before avalanches reach the initial perturbation.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

Universality Class of One-Dimensional Directed Sandpile Models

A general n-state directed `sandpile' model is introduced. The stationary properties of the n-state model are derived for n < infty, and analytical arguments based on a central limit theorem show that the model belongs to the universality class of the totally asymmetric Oslo model, with a crossover to uncorrelated branching process behavior for small system sizes. Hence, the central limit theorem allows us to identify the existence of a large universality class of one-dimensional directed sandpile models.Comment: 4 pages, 2 figure

Comparison of academic performance of twins and singletons in adolescence : follow-up study

Objectives To determine whether twins in recent cohorts show similar academic performance in adolescence to singletons and to test the effect of birth weight on academic performance in twins and singletons. Design Follow-up study. Setting Denmark. Participants All twins (n = 3411) and a 5% random sample of singletons (n = 7796) born in Denmark during 1986-8. Main outcome measures Test scores in ninth grade (age 15 or 16), birth weight, gestational age at birth, parents’ age, and parents’ education. Results Ninth grade test scores were normally distributed, with almost identical mean and standard deviations for twins and singletons (8.02 v 8.02 and 1.05 v 1.06) despite the twins weighing on average 908 g (95% confidence interval 886 to 930 g) less than the singletons at birth. Controlling for birth weight, gestational age at birth, age at test, and parents’ age and education confirmed the similarity of test scores for twins and singletons (difference 0.04, 95% confidence interval − 0.03 to 0.10). A significant, positive association between test score and birth weight was observed in both twins and singletons, but the size of the effect was small: 0.06-0.12 standard deviations for every kilogram increase in birth weight. Conclusions Although older cohorts of twins have been found to have lower mean IQ scores than singletons, twins in recent Danish cohorts show similar academic performance in adolescence to that of singletons. Birth weight has a minimal effect on academic performance in recent cohorts; for twins this effect is best judged relative to what is a normal birth weight for twins and not for singletons

Testing and Improving a UAV-Based System Designed for Wetland Methane Source Measurements

Wetlands are the single highest emitting methane source category, but the magnitude of wetland fluxes remains difficult to fully characterize due to their large spatial extent and heterogeneity. Fluxes can vary with land surface conditions, vegetation type, and seasonal changes in environmental conditions. Unmanned aerial vehicles (UAVs) are an emerging platform to better characterize spatial variability in these natural ecosystems. While presenting some advantages over traditional techniques like towers and flux chambers, in that they are mobile vertically and horizontally, their use is still challenging, requiring continued improvement in sensor technology and field measurement approaches. In this work, we employ a small, fast response laser spectrometer on a Matrice 600 hexacopter. The system was previously deployed successfully for 40 flights conducted in a four-day period in 2018 near Fairbanks, Alaska. These flights revealed several potential areas for improvement, including: vertical positioning accuracy, the need for sensor health indicators, and approaches to deal with low wind speeds. An additional set of flights was conducted this year near Antioch in California. Flights were conducted several meters above ground up to 15-25 m in a curtain pattern. These curtains were flown both upwind and downwind of a tower site, allowing us to calculate a mass balance methane flux estimate that can be compared to eddy covariance fluxes from the tower. Testing will better characterize the extent to which altitude drifts in-flight and how GPS values compare with measurements from the onboard LIDAR, as well as the agreement between two-dimensional wind speed and direction on the ground versus measured onboard the UAV. Hardware improvements to the sensor and GPS are being considered to help reduce these sources of uncertainty. Results of this testing and how system performance relates to needs for quantifying wetland fluxes, will be presented

A possible close supermassive black-hole binary in a quasar with optical periodicity

Quasars have long been known to be variable sources at all wavelengths. Their optical variability is stochastic, can be due to a variety of physical mechanisms, and is well-described statistically in terms of a damped random walk model. The recent availability of large collections of astronomical time series of flux measurements (light curves) offers new data sets for a systematic exploration of quasar variability. Here we report on the detection of a strong, smooth periodic signal in the optical variability of the quasar PG 1302-102 with a mean observed period of 1,884 $\pm$ 88 days. It was identified in a search for periodic variability in a data set of light curves for 247,000 known, spectroscopically confirmed quasars with a temporal baseline of $\sim9$ years. While the interpretation of this phenomenon is still uncertain, the most plausible mechanisms involve a binary system of two supermassive black holes with a subparsec separation. Such systems are an expected consequence of galaxy mergers and can provide important constraints on models of galaxy formation and evolution.Comment: 19 pages, 6 figures. Published online by Nature on 7 January 201

A Quarter-Century of Observations of Comet 10P/Tempel 2 at Lowell Observatory: Continued Spin-Down, Coma Morphology, Production Rates, and Numerical Modeling

We report on photometry and imaging of Comet 10P/Tempel 2 obtained at Lowell Observatory from 1983 through 2011. We measured a nucleus rotation period of 8.950 +/- 0.002 hr from 2010 September to 2011 January. This rotation period is longer than the period we previously measured in 1999, which was itself longer than the period measured in 1988. A nearly linear jet was observed which varied little during a rotation cycle in both R and CN images acquired during the 1999 and 2010 apparitions. We measured the projected direction of this jet throughout the two apparitions and, under the assumption that the source region of the jet was near the comet's pole, determined a rotational pole direction of RA/Dec = 151deg/+59deg from CN measurements and RA/Dec = 173deg/+57deg from dust measurements (we estimate a circular uncertainty of 3deg for CN and 4deg for dust). Different combinations of effects likely bias both gas and dust solutions and we elected to average these solutions for a final pole of RA/Dec = 162 +/- 11deg/+58 +/- 1deg. Photoelectric photometry was acquired in 1983, 1988, 1999/2000, and 2010/2011. The activity exhibited a steep turn-on ~3 months prior to perihelion (the exact timing of which varies) and a relatively smooth decline after perihelion. The activity during the 1999 and 2010 apparitions was similar; limited data in 1983 and 1988 were systematically higher and the difference cannot be explained entirely by the smaller perihelion distance. We measured a "typical" composition, in agreement with previous investigators. Monte Carlo numerical modeling with our pole solution best replicated the observed coma morphology for a source region located near a comet latitude of +80deg and having a radius of ~10deg. Our model reproduced the seasonal changes in activity, suggesting that the majority of Tempel 2's activity originates from a small active region located near the pole.Comment: Accepted by AJ; 29 pages of text (preprint style), 8 tables, 7 figure

Engagement and Disengagement in Art Interventions with Memory Impairment

Introduction. Studies have shown art intervention to be an effective therapy for patients with memory impairments, leading to overall positive behaviors, increased quality of life and decreased caregiver burden. We conducted a preliminary study to evaluate and compare the effect of participation in weekly art appreciation and painting sessions on the behavior of memory impaired residents in an assisted living facility. Methods. Residents’ behaviors were observed during art appreciation and active painting sessions over a six-week period. Each session consisted of either viewing and discussing artwork or actively painting in the style of the artist discussed. Positive and negative behaviors were recorded and tallied throughout the sessions. Over the course of 12 sessions, the 7 observers made a total of 1957 observations of a variable patient population. The total number of both positive and negative behaviors was compared between activities, over time within sessions, as well as over the six weeks. Each session was percent normalized to the time interval with the highest occurrence of select behaviors. Results. Upward trends for positive and negative behaviors were noticed in ap- preciation and painting sessions respectively. The negative to positive engagements ratios for each painting session showed an increase in negative behaviors. Disengagement increased as the appreciation sessions progressed and decreased as painting sessions progressed. Overall, positive engagement increased in both appreciation and painting sessions. Conclusions. Despite several confounding variables encountered in this study, we demonstrated art appreciation and active painting to be a viable non- pharmacological therapeutic approach for individuals with memory impairments.https://scholarworks.uvm.edu/comphp_gallery/1258/thumbnail.jp