Red giant collisions in the galactic centre

We simulate collisions involving red-giant stars in the centre of our galaxy. Such encounters may explain the observed paucity of highly luminous red giants within the central 0.2pc. The masses of the missing stars are likely to be in the range 2 to 8 solar masses. Recent models of the galactic centre cluster's density and velocity distributions are used to calculate two-body collision rates. In particular we use stellar-evolution models to calculate the number of collisions a star will have during different evolutionary phases. We find that the number of two-body collisions per star is \lo 1 in the central 0.1 to 0.2 pc, depending strongly on the galactocentric radius. Using a 3D numerical hydrodynamics code (SPH) we simulate encounters involving cluster stars of various masses with 2 and 8 solar-mass red giants. The instantaneous mass loss in such collisions is rarely enough to destroy either giant. A fraction of the collisions do, however, lead to the formation of common envelope systems where the impactor and giant's core are enshrouded by the envelope of the giant. Such systems may evolve to expel the envelope, leaving a tight binary; the original giant is destroyed. The fraction of collisions that produce common envelope systems is sensitive to the local velocity dispersion and hence galactocentric radius. Using our collision-rate calculations we compute the time-scales for a giant star to suffer such a collision within the galactic centre. These time-scales are >10^{9-10}years and so are longer than the lifetimes of stars more-massive than 2 solar masses. Thus the observed paucity of luminous giants is unlikely to be due to the formation of common envelope systems as a result of two-body encounters involving giant stars.Comment: 10 pages, 11 figures, MNRAS (in press

Lorca, the tower of the Iglesia de San Patricio [Material gráfico]

en la lám.: 5

Lorca, the Iglesia de San Mateo [Material gráfico]

en verso de la lám.: 5

Earthquakes: from chemical alteration to mechanical rupture

In the standard rebound theory of earthquakes, elastic deformation energy is progressively stored in the crust until a threshold is reached at which it is suddenly released in an earthquake. We review three important paradoxes, the strain paradox, the stress paradox and the heat flow paradox, that are difficult to account for in this picture, either individually or when taken together. Resolutions of these paradoxes usually call for additional assumptions on the nature of the rupture process (such as novel modes of deformations and ruptures) prior to and/or during an earthquake, on the nature of the fault and on the effect of trapped fluids within the crust at seismogenic depths. We review the evidence for the essential importance of water and its interaction with the modes of deformations. Water is usually seen to have mainly the mechanical effect of decreasing the normal lithostatic stress in the fault core on one hand and to weaken rock materials via hydrolytic weakening and stress corrosion on the other hand. We also review the evidences that water plays a major role in the alteration of minerals subjected to finite strains into other structures in out-of-equilibrium conditions. This suggests novel exciting routes to understand what is an earthquake, that requires to develop a truly multidisciplinary approach involving mineral chemistry, geology, rupture mechanics and statistical physics.Comment: 44 pages, 1 figures, submitted to Physics Report

Changing Politics: Towards a New Democracy

In October 2014 the PSA joint-funded a Consultation event ‘Changing Politics – Towards a New Democracy’ with St. George’s House. The Chair of the PSA, Professor Matthew Flinders, chaired the event which brought together participants from a range of fields (including academics, think tankers and practitioners in several policy areas). Today, St. George’s House has published a report which highlights the main themes emerging from the discussion as well as some conclusions and recommendations. It identifies several areas where changes are urgently needed to reinvigorate democracy. The report concludes that to fully succeed in addressing the growth of political apathy and disengagement, parties and leaders must forget their differences and join citizens, academics, charities and others to address this problem with all available energy and resources

The outstanding scientist, R.A. Fisher:His views on eugenics and race

R.A. Fisher was one of the greatest scientists of the 20th century (Fig. 1). He was a man of extraordinary ability and originality whose scientific contributions ranged over a very wide area of science, from biology through statistics to ideas on continental drift, and whose work has had a huge positive impact on human welfare. Not surprisingly, some of his large volume of work is not widely used or accepted at the current time, but his scientific brilliance has never been challenged. He was from an early age a supporter of certain eugenic ideas, and it is for this reason that he has been accused of being a racist and an advocate of forced sterilisation (Evans 2020). His promotion of eugenics has recently caused various organisations to remove his name from awards and dedications of buildings (Tarran 2020; Rothamsted Research 2020; Society for the Study of Evolution 2020; Gonville and Caius College 2020). A primary aim of this paper is to conduct a careful analysis of his own writings in these areas. Our purpose is neither to defend nor attack Fisher’s work in eugenics and views on race, but to present a careful account of their substance and nature.Publisher PDFPeer reviewe

Teaching evidence-based management with a focus on producing local evidence

We present an approach to teaching evidence-based management (EBMgt) that trains future managers how to produce local evidence. Local evidence is causally interpretable data, collected on-site in companies to address a specific business problem. Our teaching method is a variant of problem-based learning, a method originally developed to teach evidence-based medicine. Following this method, students learn an evidence-based problem-solving cycle for addressing actual business cases. Executing this cycle, students use and produce scientific evidence through literature searches and the design of local, experimental tests of causal hypotheses. We argue the value of teaching EBMgt with a focus on producing local evidence, how it can be taught, and what can be taught. We conclude by outlining our contribution to the literature on teaching EBMgt and by discussing limitations of our approach