Population persistence under advection-diffusion in river networks

An integro-differential equation on a tree graph is used to model the evolution and spatial distribution of a population of organisms in a river network. Individual organisms become mobile at a constant rate, and disperse according to an advection-diffusion process with coefficients that are constant on the edges of the graph. Appropriate boundary conditions are imposed at the outlet and upstream nodes of the river network. The local rates of population growth/decay and that by which the organisms become mobile, are assumed constant in time and space. Imminent extinction of the population is understood as the situation whereby the zero solution to the integro-differential equation is stable. Lower and upper bounds for the eigenvalues of the dispersion operator, and related Sturm-Liouville problems are found, and therefore sufficient conditions for imminent extinction are given in terms of the physical variables of the problem

Expertise, motivation and teaching in learning companion systems

This paper describes work carried out to explore the role of a learning companion as a teachable student of the human student. A LCS for Binary Boolean Algebra has been developed to explore the hypothesis that a learning companion with less expertise than the human student would be beneficial if the student taught it. The system implemented two companions with different expertise and two types of motivational conditions. An empirical evaluation was conducted. Although significant differential learning gains between the experimental conditions were not observed, differences in learner behaviour between these conditions were. In particular students in the motivated condition with a weak companion taught it many more times than in the other experimental conditions and in general worked harder. Finally, the experiment also suggested that learning companions might be confusing for students if they try to resemble human behaviour, i.e. if they do not perform exactly as they are told

High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet

We study the stars of the binary system 16 Cygni to determine with high precision their chemical composition. Knowing that the component B has a detected planet of at least 1.5 Jupiter masses, we investigate if there are chemical peculiarities that could be attributed to planet formation around this star. We perform a differential abundance analysis using high resolution (R = 81,000) and high S/N (~700) CFHT/ESPaDOnS spectra of the 16 Cygni stars and the Sun; the latter was obtained from light reflected of asteroids. We determine differential abundances of the binary components relative to the Sun and between components A and B as well. We achieve a precision of about 0.005 dex and a total error ~0.01 dex for most elements. The effective temperatures and surface gravities found for 16 Cyg A and B are Teff = 5830+/-7 K, log g = 4.30+/-0.02 dex, and Teff = 5751+/-6 K, log g = 4.35+/-0.02 dex, respectively. The component 16 Cyg A has a metallicity ([Fe/H]) higher by 0.047+/-0.005 dex than 16 Cyg B, as well as a microturbulence velocity higher by 0.08 km/s. All elements show abundance differences between the binary components, but while the volatile difference is about 0.03 dex, the refractories differ by more and show a trend with condensation temperature, which could be interpreted as the signature of the rocky accretion core of the giant planet 16 Cyg Bb. We estimate a mass of about 1.5-6 M_Earth for this rocky core, in good agreement with estimates of Jupiter's core.Comment: ApJ Letters. Press release: http://cfht.hawaii.edu/en/news/16CygAB