27 research outputs found
Mass extinctions and supernova explosions
A nearby supernova (SN) explosion could have negatively influenced life on
Earth, maybe even been responsible for mass extinctions. Mass extinction poses
a significant extinction of numerous species on Earth, as recorded in the
paleontologic, paleoclimatic, and geological record of our planet. Depending on
the distance between the Sun and the SN, different types of threats have to be
considered, such as ozone depletion on Earth, causing increased exposure to the
Sun's ultraviolet radiation, or the direct exposure of lethal x-rays. Another
indirect effect is cloud formation, induced by cosmic rays in the atmosphere
which result in a drop in the Earth's temperature, causing major glaciations of
the Earth. The discovery of highly intensive gamma ray bursts (GRBs), which
could be connected to SNe, initiated further discussions on possible
life-threatening events in Earth's history. The probability that GRBs hit the
Earth is very low. Nevertheless, a past interaction of Earth with GRBs and/or
SNe cannot be excluded and might even have been responsible for past extinction
events.Comment: Chapter for forthcoming book: Handbook of Supernovae, P. Murdin and
A. Alsabeti (eds.), Springer International Publishing (in press
Proper weak-coupling approach to the periodic s-d(f) exchange model
The periodic s-d(f) exchange model is characterized by a wide variety of
interesting applications, a simple mathematical structure and a limited number
of reliable approximations which take care of the quantum nature of the
participating spins. We suggest the use of a projection-operator method for
getting information perturbationally, which are not accessible via diagrammatic
approaches. In this paper we present in particular results beyond perturbation
theory, which are obtained such that almost all exactly known limiting cases
are incorporated correctly. We discuss a variety of possible methods and
evaluate their consequences for one-particle properties. These considerations
serve as a guideline for a more effective approach to the model.Comment: 11 pages, 6 figures; accepted by Phys. Rev.