The Dyer-Roeder relation in a universe with particle production

We have obtained analytical exact solutions of the Dyer-Roeder equation in a cosmological model where creation of particles occurs at the expenses of the gravitational field. We discussed the influences of inhomogeneities in the path of a light beam on the apparent diameter of astrophysical objects and consider both redshift independent as redshift dependent distributions of the inhomogeneities.Comment: 7 pages, 4 figures. Accepted to be published in the Astronomy and Astrophysics Journa

Vortex configurations and metastability in mesoscopic superconductors

The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clem's solution for the vortex core. The Bean-Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.Comment: 7 pages, 4 figures. Presented on the III European Conference on Vortex Matter in Superconductors, Crete, 2003. To appear in Physica

Differential rotation in early type stars

Using 2D models of rotating stars, the interferometric measurements of alpha Eri and its fundamental parameters corrected for gravitational darkening effects we infer that the star might have a core rotating 2.7 times faster than the surface. We explore the consequences on spectral lines produced by surface differential rotation combined with the effects due to a kind of internal differential rotation with rotational energies higher than allowed for rigid rotation which induce geometrical deformations that do not distinguish strongly from those carried by the rigid rotation.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A. Okazak

Strategies and economics of farming systems with coffee in the Atlantic Rainforest Biome

In the Zona da Mata of Minas Gerais State, Brazil, family farmers are adjusting to agroecological principles to reconcile sustainable agriculture, livelihood improvements and biodiversity conservation. Starting in 1993, experimentation with coffee agroforestry was gradually initiated on an increasing number of farms (37 in total), resulting in the simultaneous management of sun coffee (SC) and agroforestry coffee (AF) plots. We aimed (1) to identify factors that determine the farmers’ selection of trees used in AF; (2) to describe the agroecological farms in transition; and (3) to perform an economic comparison between AF and SC. These objectives were addressed by combining data from botanical surveys in 1993/1994 and 2007, by interviews with farmers and by detailed data on the production value and costs of labour and material inputs. The results showed considerable diversity in farming strategies and management among the farmers. Early adopters of AF had diversified towards production of different marketable products. The use of native trees in AF for this purpose, and for restoration of soil fertility (e.g., leguminous trees), had increased since the start of the experiments, while exotic tree species were eliminated. Over a period of 12 years AF was more profitable than SC due to the production of a diversity of agricultural goods, despite somewhat higher establishment costs. Other ecosystem services delivered by AF, such as biodiversity and cultural services are currently not valorized. Payment schemes for environmental services could further improve the economic benefits of AF for family farmers and alleviate establishment and learning cost