Extrasolar comets : the origin of dust in exozodiacal disks?

Comets have been invoked in numerous studies as a potentially important source of dust and gas around stars, but none has studied the thermo-physical evolution, out-gassing rate, and dust ejection of these objects in such stellar systems. We investigate the thermo-physical evolution of comets in exo-planetary systems in order to provide valuable theoretical data required to interpret observations of gas and dust. We use a quasi 3D model of cometary nucleus to study the thermo-physical evolution of comets evolving around a single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to 70 solar luminosities. This paper provides mass ejection, lifetimes, and the rate of dust and water gas mass productions for comets as a function of the distance to the star and stellar luminosity. Results show significant physical changes to comets at high stellar luminosities. The models are presented in such a manner that they can be readily applied to any planetary system. By considering the examples of the Solar System, Vega and HD 69830, we show that dust grains released from sublimating comets have the potential to create the observed (exo)zodiacal emission. We show that observations can be reproduced by 1 to 2 massive comets or by a large number of comets whose orbits approach close to the star. Our conclusions depend on the stellar luminosity and the uncertain lifetime of the dust grains. We find, as in previous studies, that exozodiacal dust disks can only survive if replenished by a population of typically sized comets renewed from a large and cold reservoir of cometary bodies beyond the water ice line. These comets could reach the inner regions of the planetary system following scattering by a (giant) planet.Comment: 21 pages, 10 figure

More From the #Jury Box: The Latest on Juries and Social Media

This Article presents the results of a survey of jurors in federal and state court on their use of social media during their jury service. We began surveying federal jurors in 2011 and reported preliminary results in 2012; since then, we have surveyed several hundred more jurors, including state jurors, for a more complete picture of juror attitudes toward social media. Our results support the growing consensus that jury instructions are the most effective tool to mitigate the risk of juror misconduct through social media. We conclude with a set of recommended best practices for using a social-media instruction

More From the #Jury Box: The Latest on Juries and Social Media

This Article presents the results of a survey of jurors in federal and state court on their use of social media during their jury service. We began surveying federal jurors in 2011 and reported preliminary results in 2012; since then, we have surveyed several hundred more jurors, including state jurors, for a more complete picture of juror attitudes toward social media. Our results support the growing consensus that jury instructions are the most effective tool to mitigate the risk of juror misconduct through social media. We conclude with a set of recommended best practices for using a social-media instruction

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

Generalized Thermalization in an Integrable Lattice System

After a quench, observables in an integrable system may not relax to the standard thermal values, but can relax to the ones predicted by the generalized Gibbs ensemble (GGE) [M. Rigol et al., Phys. Rev. Lett. 98, 050405 (2007)]. The GGE has been shown to accurately describe observables in various one-dimensional integrable systems, but the origin of its success is not fully understood. Here we introduce a microcanonical version of the GGE and provide a justification of the GGE based on a generalized interpretation of the eigenstate thermalization hypothesis, which was previously introduced to explain thermalization of nonintegrable systems. We study relaxation after a quench of one-dimensional hard-core bosons in an optical lattice. Exact numerical calculations for up to 10 particles on 50 lattice sites (~10^10 eigenstates) validate our approach.Comment: 8 pages, 9 figures, as publishe

Knowledge Summary 27: Death reviews: maternal, perinatal and child

Many maternal, perinatal and child deaths are preventable and progress towards Millennium Development Goals 4&5, to reduce child mortality and improve maternal health, has been insufficient in many parts of the world. Well-implemented death reviews provide opportunities to examine the circumstances surrounding a woman’s or child’s death, and improve the delivery of health services to prevent such deaths in the future. Several types of review processes exist to evaluate deaths in diverse settings, given different data availability and levels of service delivery. Both consistent surveillance and effective response are needed to ensure that maternal, perinatal and child deaths are identified and reviewed, so that recommendations can be made, and action can be taken to prevent further deaths