AMD-stability and the classification of planetary systems

We present here in full detail the evolution of the angular momentum deficit (AMD) during collisions as it was described in (Laskar, PRL,2000). Since then, the AMD has been revealed to be a key parameter for the understanding of the outcome of planetary formation models. We define here the AMD-stability criterion that can be easily verified on a newly discovered planetary system. We show how AMD-stability can be used to establish a classification of the multiplanet systems in order to exhibit the planetary systems that are long-term stable because they are AMD-stable, and those that are AMD-unstable which then require some additional dynamical studies to conclude on their stability. The AMD-stability classification is applied to the 131 multiplanet systems from The Extrasolar Planet Encyclopaedia database (exoplanet.eu) for which the orbital elements are sufficiently well known.Comment: 18 pages, 13 figures, A&A in pres

AMD-stability in presence of first order Mean Motion Resonances

The AMD-stability criterion allows to discriminate between a-priori stable planetary systems and systems for which the stability is not granted and needs further investigations. AMD-stability is based on the conservation of the Angular Momentum Deficit (AMD) in the averaged system at all orders of averaging. While the AMD criterion is rigorous, the conservation of the AMD is only granted in absence of mean-motion resonances (MMR). Here we extend the AMD-stability criterion to take into account mean-motion resonances, and more specifically the overlap of first order MMR. If the MMR islands overlap, the system will experience generalized chaos leading to instability. The Hamiltonian of two massive planets on coplanar quasi-circular orbits can be reduced to an integrable one degree of freedom problem for period ratios close to a first order MMR. We use the reduced Hamiltonian to derive a new overlap criterion for first order MMR. This stability criterion unifies the previous criteria proposed in the literature and admits the criteria obtained for initially circular and eccentric orbits as limit cases. We then improve the definition of AMD-stability to take into account the short term chaos generated by MMR overlap. We analyze the outcome of this improved definition of AMD-stability on selected multi-planet systems from the Extrasolar Planets Encyclopeadia.Comment: Accepted by A and A 07/10/1

Efficient ortho-para conversion of H2 on interstellar grain surfaces

Context: Fast surface conversion between ortho- and para-H2 has been observed in laboratory studies, and this mechanism has been proposed to play a role in the control of the ortho-para ratio in the interstellar medium. Observations of rotational lines of H2 in Photo-Dissociation Regions (PDRs) have indeed found significantly lower ortho-para ratios than expected at equilibrium. The mechanisms controlling the balance of the ortho-para ratio in the interstellar medium thus remain incompletely understood, while this ratio can affect the thermodynamical properties of the gas (equation of state, cooling function). Aims: We aim to build an accurate model of ortho-para conversion on dust surfaces based on the most recent experimental and theoretical results, and to validate it by comparison to observations of H2 rotational lines in PDRs. Methods: We propose a statistical model of ortho-para conversion on dust grains with fluctuating dust temperatures, based on a master equation approach. This computation is then coupled to full PDR models and compared to PDR observations. Results: We show that the observations of rotational H2 lines indicate a high conversion efficiency on dust grains, and that this high efficiency can be accounted for if taking dust temperature fluctuations into account with our statistical model of surface conversion. Simpler models neglecting the dust temperature fluctuations do not reach the high efficiency deduced from the observations. Moreover, this high efficiency induced by dust temperature fluctuations is quite insensitive to the values of microphysical parameters of the model. Conclusions: Ortho-para conversion on grains is thus an efficient mechanism in most astrophysical conditions that can play a significant role in controlling the ortho-para ratio.Comment: Accepted in Astronomy & Astrophysic

Surface chemistry in the Interstellar Medium II. H2\mathrm{H}_2 formation on dust with random temperature fluctuations

The $\mathrm{H}_2$ formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the $\mathrm{H}_2$ formation rate under the full range of astrophysically relevant UV intensities and gas conditions. The master equation approach is generalized to coupled fluctuations in both the grain's temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region (PDR) edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under most astrophysically relevant conditions. Typical differences are a factor of 2-3 on the intensities of the $\mathrm{H}_2$ $v=0$ lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.Comment: 21 pages, 28 figures, accepted in A&

Obscurité des Lumières : les prisons d’Europe, d’après John Howard, autour de 1780

John Howard's State of Prisons portrays British prisons toward 1780 as places of injustice and arbitrariness, where conditions of detention were anything but humane. Continental prisons hardly appear better, except for some that Howard presents as models. After some years of easy life in the gentry, Howard devoted himself to his Grand Tour of European prisons. A philanthropist of his times, he analyzed prisons from a point of view that remains just as relevant today as it was 200 years ago

Darwinisme et catholicisme au Québec au début du xxe siècle : Autour du Dr Albert Laurendeau

Au début du xxe siècle, en pleine crise moderniste, l’Église catholique du Québec est sur la défensive devant le développement des sciences biologiques, en particulier l’évolutionnisme darwinien qui explique de façon laïque la naissance et l’évolution de la vie. Tandis que la médecine francophone de la province s’affirme avec de grandes revues, l’organisation de congrès internationaux et la préparation de la nouvelle loi médicale de 1909, un médecin de Saint-Gabriel-de-Brandon s’efforce de vulgariser le darwinisme. Oncle d’André Laurendeau, ce médecin de campagne anticonformiste et très actif, respecté par ses pairs pour son engagement au service de la profession, publie en 1911 La vie. Considérations biologiques qui veut réaliser la synthèse entre les travaux de Lamarck, de Darwin et de Haeckel. Critiquant l’insuffisance de la formation scientifique des francophones du Québec, cet ouvrage propose une réforme radicale des études dans les collèges classiques. J.-A. Archambault, évêque du nouveau diocèse de Joliette, condamnera le livre et son auteur.At the beginning of the 20th century, in the midst of the crisis of modernity, Quebec’s Catholic Church was on the defensive as it confronted developments in the biological sciences, especially Darwinian evolution which gave a non-religious explanation for the origins and evolution of life. While the province’s francophone medical establishment asserted itself by publishing in prominent journals, by organizing of international conferences, and by helping to prepare the new medical legislation of 1909, a physician from Saint-Gabriel- de-Brandon set out to popularize Darwinism. An uncle of André Laurendeau and respected by his peers for his commitment and contributions to the profession, this very active and non-conformist country doctor published the resulting book in 1911. La vie. Considérations biologiques (Life. Biological Considerations), was intended as a synthesis of the works of Lamarck, Darwin and Haeckel. Criticizing the lack of scientific training among Quebec francophones, the book recommended a radical reform of the programs of study offered in the province’s classic colleges. J.-A. Archambault, bishop of the new diocese of Joliette, would condemn the book and its author

Structure and evolution of the Baikal rift: A synthesis

Active continental rifts are spectacular manifestations of the deformation of continents but are not very numerous at the surface of the Earth. Among them, the Baikal rift has been extensively studied during the last decades. Yet no simple scenario explains its origin and development because the style of rifting has changed throughout its ∼30 Myr history. In this paper, we use forward and inverse models of gravity data to map the Moho and lithosphere-asthenosphere boundary in three dimensions. We then integrate these new results with existing geophysical and geological data on the Baikal rift structure and dynamics, and propose a scenario of its evolution. Earthquake depths, mantle xenoliths, heat flow, and seismic and gravity models advocate for a normal to moderately thinned continental lithosphere and crust, except beneath the Siberian craton, which exhibits a >100-km-thick lithosphere. Relatively thin lithosphere (70–80 km) is found east and south of the rift system and is in spatial connection with the Hangai-Hövsgöl region of anomalous mantle in Mongolia. From top to bottom, the rift structure is asymmetric and appears strongly controlled by the geometry of the suture zone bounding the Siberian craton. Moreover, the mode of topography support changes significantly along the length of the rift: mountain ranges south and north of the rift are underlain by negative Bouguer anomalies, suggesting deep crustal roots and/or anomalous mantle; rift shoulders in the center of the rift seem to result from flexural uplift. The commonly assumed “two-stage” rift evolution is not corroborated by all stratigraphic and seismic data; however, it seems clear that during the Oligocene, an “early stage,” which might be dominated by strike-slip tectonics instead of pure extension, created primitive basins much different from the present ones. Most of the “true” rift basins seem to have initiated later, during the Late Miocene or Pliocene. This kinematic change from strike-slip to extensional tectonics in the Baikal rift is part of a more general kinematic reorganization of Asia and can be associated with the rapid growth of the Tibetan plateau and the end of marginal basins opening along the Pacific boundary

Effect of the over-ageing treatment on the mechanical properties of AA2024 aluminum alloy.

The evolution of the hardness of the over-ageing AA2024 alloy scale was followed by measurements of Vickers hardness. The nanoindentation is adapted to the determination of elastoplastic properties (hardness and Young’s modulus) of the matrix and also of coarse intermetallic precipitates. Influence of the artificial over-ageing time to hardness and to mechanical properties as the local scale was investigated