Scattering of small bodies by planets: a potential origin for exozodiacal dust ?

High levels of exozodiacal dust are observed around a growing number of main sequence stars. The origin of such dust is not clear, given that it has a short lifetime against both collisions and radiative forces. Even a collisional cascade with km-sized parent bodies, as suggested to explain outer debris discs, cannot survive sufficiently long. In this work we investigate whether the observed exozodiacal dust could originate from an outer planetesimal belt. We investigate the scattering processes in stable planetary systems in order to determine whether sufficient material could be scattered inwards in order to retain the exozodiacal dust at its currently observed levels. We use N-body simulations to investigate the efficiency of this scattering and its dependence on the architecture of the planetary system. The results of these simulations can be used to assess the ability of hypothetical chains of planets to produce exozodi in observed systems. We find that for older (>100Myr) stars with exozodiacal dust, a massive, large radii (>20AU) outer belt and a chain of tightly packed, low-mass planets would be required in order to retain the dust at its currently observed levels. This brings into question how many, if any, real systems possess such a contrived architecture and are therefore capable of scattering at sufficiently high rates to retain exozodi dust on long timescales

Grain Size segregation in debris discs

In most debris discs, dust grain dynamics is strongly affected by stellar radiation pressure. As this mechanism is size-dependent, we expect dust grains to be spatially segregated according to their sizes. However, because of the complex interplay between radiation pressure, collisions and dynamical perturbations, this spatial segregation of the particle size distribution (PSD) has proven difficult to investigate with numerical models. We propose to explore this issue using a new-generation code that can handle some of the coupling between dynamical and collisional effects. We investigate how PSDs behave in both unperturbed discs "at rest" and in discs pertubed by planetary objects. We use the DyCoSS code of Thebault(2012) to investigate the coupled effect of collisions, radiation pressure and dynamical perturbations in systems having reached a steady state. We consider 2 setups: a narrow ring perturbed by an exterior planet, and an extended disc into which a planet is embedded. For both setups we consider an additional unperturbed case with no planet. We also investigate how possible spatial size segregation affect disc images at different wavelengths. We find that PSDs are always strongly spatially segregated. The only case for which they follow a standard dn/dr = C.r**(-3.5) law is for an unperturbed narrow ring, but only within the parent body ring itself. For all other configurations, the PSD can strongly depart from such power laws and have strong spatial gradients. As an example, the geometrical cross section of the disc is rarely dominated by the smallest grains on bound orbits, as it is expected to be in standard PSDs in s**q with q<-3. Although the exact profiles and spatial variations of PSDs are a complex function of the considered set-up, we are however able to derive some robust results that should be useful for image-or-SED-fitting models of observed discs.Comment: Accepted in A&A // Figure quality has been downgraded. A high-res version of the paper can be found at http://lesia.obspm.fr/perso/philippe-thebault/sizepap_rev.pdf /V2: typos correcte

Should you believe in the Shanghai ranking?

This paper proposes a critical analysis of the "Academic Ranking of World Universities", published every year by the Institute of Higher Education of the Jiao Tong University in Shanghai and more commonly known as the Shanghai ranking. After having recalled how the ranking is built, we first discuss the relevance of the criteria and then analyze the proposed aggregation method. Our analysis uses tools and concepts from Multiple Criteria Decision Making (MCDM). Our main conclusions are that the criteria that are used are not relevant, that the aggregation methodology is plagued by a number of major problems and that the whole exercise suffers from an insufficient attention paid to fundamental structuring issues. Hence, our view is that the Shanghai ranking, in spite of the media coverage it receives, does not qualify as a useful and pertinent tool to discuss the "quality" of academic institutions, let alone to guide the choice of students and family or to promote reforms of higher education systems. We outline the type of work that should be undertaken to oer sound alternatives to the Shanghai ranking.Shanghai ranking; multiple criteria decision analysis; evaluation models; higher education.

Acculturative Stress and Adaptability Levels Between Documented versus Undocumented Hispanic College Students

Hispanic students often suffer from acculturative stress as they adapt to U.S. college environments; however, few scholars have examined the acculturative stress relationship among undocumented versus documented Hispanic college students. In this quantitative, correlational study design, adaptation levels related to acculturative stress between both statuses were examined. The theoretical foundations of this study are based on the social cognitive career theory. This investigation focused on determining how adaptation levels predict Hispanic college students\u27 acculturative stress and whether this realtionship differ between documented and undocumented college students. The I-Adapt measure was used to measure participants\u27 level of adaptability and the social, attitudinal, familial and educational or the Social, Attitudinal, Familial and Educational (S.A.F.E) measurement was used to measure their acculturative stress levels. The sample consisted of 165 Hispanic college students recruited from a private northeastern university. Contrarily to the main hypothesis, Regression analysis revealed that higher levels of cultural and crisis adaptability predicted lower levels of acculturative stress while higher levels of work stress adaptability predicted higher levels of acculturative stress. Future research should focus on further examination differences in adaptation toward acculturative stress and the aftermath of acculturative stress adaptation methods between documented and undocumented college students. The findings of this study can contribute to social change by informing immigration laws to adopt in order to protect college educated, skilled and productive immigrants

Hand2Face: Automatic Synthesis and Recognition of Hand Over Face Occlusions

A person's face discloses important information about their affective state. Although there has been extensive research on recognition of facial expressions, the performance of existing approaches is challenged by facial occlusions. Facial occlusions are often treated as noise and discarded in recognition of affective states. However, hand over face occlusions can provide additional information for recognition of some affective states such as curiosity, frustration and boredom. One of the reasons that this problem has not gained attention is the lack of naturalistic occluded faces that contain hand over face occlusions as well as other types of occlusions. Traditional approaches for obtaining affective data are time demanding and expensive, which limits researchers in affective computing to work on small datasets. This limitation affects the generalizability of models and deprives researchers from taking advantage of recent advances in deep learning that have shown great success in many fields but require large volumes of data. In this paper, we first introduce a novel framework for synthesizing naturalistic facial occlusions from an initial dataset of non-occluded faces and separate images of hands, reducing the costly process of data collection and annotation. We then propose a model for facial occlusion type recognition to differentiate between hand over face occlusions and other types of occlusions such as scarves, hair, glasses and objects. Finally, we present a model to localize hand over face occlusions and identify the occluded regions of the face.Comment: Accepted to International Conference on Affective Computing and Intelligent Interaction (ACII), 201

Inviolable energy conditions from entanglement inequalities

Via the AdS/CFT correspondence, fundamental constraints on the entanglement structure of quantum systems translate to constraints on spacetime geometries that must be satisfied in any consistent theory of quantum gravity. In this paper, we investigate such constraints arising from strong subadditivity and from the positivity and monotonicity of relative entropy in examples with highly-symmetric spacetimes. Our results may be interpreted as a set of energy conditions restricting the possible form of the stress-energy tensor in consistent theories of Einstein gravity coupled to matter.Comment: 25 pages, 3 figures, v2: refs added, expanded discussion of strong subadditivity constraints in sections 2.1 and 4.