Numerical models for stationary superfluid neutron stars in general relativity with realistic equations of state

We present a numerical model for uniformly rotating superfluid neutron stars, for the first time with realistic microphysics including entrainment, in a fully general relativistic framework. We compute stationary and axisymmetric configurations of neutron stars composed of two fluids, namely superfluid neutrons and charged particles (protons and electrons), rotating with different rates around a common axis. Both fluids are coupled by entrainment, a non-dissipative interaction which in case of a non-vanishing relative velocity between the fluids, causes the fluid momenta being not aligned with the respective fluid velocities. We extend the formalism by Comer and Joynt (2003) in order to calculate the equation of state (EoS) and entrainment parameters for an arbitrary relative velocity. The resulting entrainment matrix fulfills all necessary sum rules and in the limit of small relative velocity our results agree with Fermi liquid theory ones, derived to lowest order in the velocity. This formalism is applied to two new nuclear equations of state which are implemented in the numerical model. We are able to obtain precise equilibrium configurations. Resulting density profiles and moments of inertia are discussed employing both EoSs, showing the impact of entrainment and the dependence on the EoS.Comment: 18 pages, 10 figures, minor changes to match published version in PRD, a typo present in Eq.A1 in the published version has been correcte

Questions of Costs About the French Bio-Fuel Sector by Year 2010

By the end of year 2010, each member state of the European Union (the EU) ought to incorporate 5.75% of bio-fuels in the total of fuels used for transportation purposes. In order to achieve such a target, tax incentives are implemented by the French government given that the production of bio-fuels still remains unprofitable, even if oil prices are about $60/barel. After a brief introduction (1), we will first demonstrate the importance borne by the cost of agricultural raw material in the total cost of biofuels (2). For this purpose a sequential multi annual LP model is used (3). Emphasis must be placed on the possible competition between food and energy crops, should the production of energy crops require land exceeding the mandatory 10% set-aside (4). An assessment of the profitability of the different types of bio-fuels is then carried out (5).bio-fuels, Common Agricultural Policy, opportunity cost, energy crops, Kyoto Protocol, Resource /Energy Economics and Policy, C61, Q18, Q42,

Economic impact of biofuel chains in France

Given the current situation with the high price of oil (an average of $53 per barrel in 2005) and the risk of global warming, the European Union (EU) is reinforcing its objectives related to the production of biofuels: they should account for 5.75% of the overall fuel consumption by 2010 in France, as opposed to 1% in 2005. In keeping with the objective set for 2010, the biodiesel derived from rapeseed is still the preferred biofuel (27.5 million hectolitres projected), compared to the ethanol derived from wheat or sugarbeet (9.3 million hectolitres projected). Our model makes it possible to foresee that there will be a competition between food and energy crops by 2006 and that this will occur even before all of the fallow areas are requisitioned. Our paper stresses the fact that the energy and economic advantages of first-generation biofuels are not sufficient to replace large quantities of petroleum resources.Resource /Energy Economics and Policy,

Min-max regret versus gross margin maximization in arable sector modeling

"A sector model presented in this article, uses about 200 representative French cereal-oriented farms to estimate policy impacts by means of mathematical modeling. Usually, such models suppose that farmers intend to maximize expected gross margin. This rationality hypothesis however seems hardly justifiable, especially these days, when gross margin variability due to European Common Agricultural Policy changes may become significant. Increasing uncertainty introduces bounded rationality to the decision problem so that crop gross margins may be better approximated by interval rather than by expected (precise) values. The initial LP problem is specified as an “Interval Linear Programming (ILP)”. We assume that farmers tend to decide upon their surface allocation prudently in order to get through with minimum loss, which is precisely the rationale underlying the minimization of maximum regret decision criterion. Recent advances in operations research, namely Mausser and Laguna algorithms, are exploited to implement the min-max regret criterion to arable agriculture ILP. The validation against observed crop mix proved that as uncertainty increases about 40% of the farmers adopt the min-max regret decision rule instead of the gross margin maximization."Interval Linear Programming, Min-Max Regret, Common Agricultural Policy, Arable cropping, France

Global numerical simulations of the rise of vortex-mediated pulsar glitches in full general relativity

In this paper, we study in detail the role of general relativity on the global dynamics of giant pulsar glitches as exemplified by Vela. For this purpose, we carry out numerical simulations of the spin up triggered by the sudden unpinning of superfluid vortices. In particular, we compute the exchange of angular momentum between the core neutron superfluid and the rest of the star within a two-fluid model including both (non-dissipative) entrainment effects and (dissipative) mutual friction forces. Our simulations are based on a quasi-stationary approach using realistic equations of state (EoSs). We show that the evolution of the angular velocities of both fluids can be accurately described by an exponential law. The associated characteristic rise time $\tau_{\text{r}}$, which can be precisely computed from stationary configurations only, has a form similar to that obtained in the Newtonian limit. However, general relativity changes the structure of the star and leads to additional couplings between the fluids due to frame-dragging effects. As a consequence, general relativity can have a large impact on the actual value of $\tau_{\text{r}}$: the errors incurred by using Newtonian gravity are thus found to be as large as $\sim 40 \%$ for the models considered. Values of the rise time are calculated for Vela and compared with current observational limits. Finally, we study the amount of gravitational waves emitted during a glitch. Simple expressions are obtained for the corresponding characteristic amplitudes and frequencies. The detectability of glitches through gravitational wave observatories is briefly discussed.Comment: 19 pages, 12 figures, minor changes to match version to be published in MNRA

L'ambivalence des filières biocarburants

D'une situation de faible importance dans la sole agricole française (324 000 hectares en 2004 dont 300 000 hectares de colza), les cultures énergétiques devraient connaître un rapide accroissement de leur surface afin de permettre d'atteindre un niveau d'incorporation de 5,75% de biocarburants dans les carburants fossiles à l'horizon 2010 (valeur recommandée par la Directive 2003/30/CE, dite "promotion des biocarburants"). Le gouvernement français a d'ailleurs récemment augmenté les autorisations de production. Dans le contexte actuel où le prix du pétrole côtoie les 70$ par baril et où la lutte contre le réchauffement climatique devient une priorité, les biocarburants sont présentés sous un angle très favorable. Cependant, compte tenu de leur rendement énergétique par hectare de terre dans l'ensemble très moyen et de leurs coûts élevés, on est amené à tempérer les analyses très optimistes qui les entourent. Présentés essentiellement comme des filières énergétiques, on oublie de souligner que les biocarburants sont également un moyen indirect de soutien de l'agro-industrie et de l'agriculture, sous la responsabilité de chaque pays. Les principaux résultats résumés ici concernent la France. Ils sont obtenus en utilisant un modèle d'équilibre partiel et dynamique, OSCAR, mis au point par l'INRA (encadré 1). Les points forts de ce modèle consistent en une formalisation détaillée des offres agricoles, alimentaire et non alimentaire, compte tenu des évolutions de la PAC et des impacts des biocarburants sur les revenus ainsi que sur les emplois agricoles.

La révision à mi-parcours de la PAC (juin 2003) et les exploitations céréalières des régions intermédiaires

A l'aide d'un modèle régional d'offre optimisant le revenu agricole des exploitations sous un ensemble de contraintes et simulant parallèlement les évolutions des structures agricoles sur la période 2002-2012, les impacts du compromis de Luxembourg sur les revenus et sur les structures agricoles, sur les assolements et sur la déprise (non culture) ont été analysés, dans les régions céréalières intermédiaires. Les simulations montrent un léger risque de déprise, 8% des exploitations, si les prix des céréales diminuent nettement, 81€ à l'horizon 2012. Un recouplage de 25% des aides fait disparaître la déprise dans ce scénario de prix qui suppose une baisse du prix d'intervention. Suite au compromis de Luxembourg, le maintien du prix d'intervention devrait normalement empêcher une baisse aussi importante des prix et freiner donc naturellement la déprise. Alors que les ultimes propositions "Fischler" (scénario 1) auraient entraîné des baisses importantes de revenu agricole par hectare et par exploitation, le compromis de Luxembourg (scénario 2) devrait permettre un maintien du revenu par exploitation en euros constants (taux d'inflation 1,4%), sachant que 12% des exploitations disparaissent à l'horizon 2012, selon le modèle. Le découplage fait peu évoluer l'équilibre entre les céréales et les oléagineux, ce qui n'est pas surprenant puisque les aides couplées par culture sont identiques depuis l'application de l'Agenda 2000.

De l’économie de l’exploitation agricole à la politique scientifique régionale de l’INRA. L’itinéraire d’un homme du « service public »

Jean-Claude Tirel nous a quittés le 20 novembre 2007 Ses compétences d’économiste, son engagement pour l’INRA marqué par un fort esprit de « service public », ses qualités d’ouverture : « Toujours faire en sorte que la diversité des opinions et des approches soit mieux respectée », de diplomate pour résoudre les difficultés et réduire les « guêpiers », sa totale disponibilité, son humour, rétif à toute bureaucratie, complété d’une gentillesse exquise ont marqué l’institution et toute une géné..