679 research outputs found
Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks
Planetesimals in gaseous protoplanetary disks may grow by collecting dust
particles. Hydrodynamical studies show that small particles generally avoid
collisions with the planetesimals because they are entrained by the flow around
them. This occurs when , the Stokes number, defined as the ratio of the
dust stopping time to the planetesimal crossing time, becomes much smaller than
unity. However, these studies have been limited to the laminar case, whereas
these disks are believed to be turbulent. We want to estimate the influence of
gas turbulence on the dust-planetesimal collision rate and on the impact
speeds. We used three-dimensional direct numerical simulations of a fixed
sphere (planetesimal) facing a laminar and turbulent flow seeded with small
inertial particles (dust) subject to a Stokes drag. A no-slip boundary
condition on the planetesimal surface is modeled via a penalty method. We find
that turbulence can significantly increase the collision rate of dust particles
with planetesimals. For a high turbulence case (when the amplitude of turbulent
fluctuations is similar to the headwind velocity), we find that the collision
probability remains equal to the geometrical rate or even higher for , i.e., for dust sizes an order of magnitude smaller than in the laminar
case. We derive expressions to calculate impact probabilities as a function of
dust and planetesimal size and turbulent intensity
A new view on exoplanet transits: Transit of Venus described using three-dimensional solar atmosphere Stagger-grid simulations
Stellar activity and, in particular, convection-related surface structures,
potentially cause fluctuations that can affect the transit light curves.
Surface convection simulations can help the interpretation of ToV. We used
realistic three-dimensional radiative hydrodynamical simulation of the Sun from
the Stagger-grid and synthetic images computed with the radiative transfer code
Optim3D to provide predictions for the transit of Venus in 2004 observed by the
satellite ACRIMSAT. We computed intensity maps from RHD simulation of the Sun
and produced synthetic stellar disk image. We computed the light curve and
compared it to the ACRIMSAT observations and also to the light curves obtained
with solar surface representations carried out using radial profiles with
different limb-darkening laws. We also applied the same spherical tile imaging
method to the observations of center-to-limb Sun granulation with HINODE. We
managed to explain ACRIMSAT observations of 2004 ToV and showed that the
granulation pattern causes fluctuations in the transit light curve. We
evaluated the contribution of the granulation to the ToV. We showed that the
granulation pattern can partially explain the observed discrepancies between
models and data. This confirms that the limb-darkening and the granulation
pattern simulated in 3D RHD Sun represent well what is imaged by HINODE. In the
end, we found that the Venus's aureole contribution during ToV is less intense
than the solar photosphere, and thus negligible. Being able to explain
consistently the data of 2004 ToV is a new step forward for 3D RHD simulations
that are becoming essential for the detection and characterization of
exoplanets. They show that the granulation have to be considered as an
intrinsic incertitude, due to the stellar variability, on precise measurements
of exoplanet transits of, most likely, planets with small diameters.Comment: Accepted for publication in Astronomy and Astrophysic
Gravitational instability and clustering in a disk of planetesimals
For a long time, gravitational instability in the disk of planetesimals has
been suspected to be the main engine responsible for the beginning of dust
growth, its advantage being that it provides for rapid growth. Its real
importance in planetary formation is still debated, mainly because the
potential presence of turbulence can prevent the settling of particles into a
gravitationally unstable layer. However, several mechanisms could yield
strongly inhomogeneous distributions of solids in the disk: radial drift,
trapping in vortices, perturbations by other massive bodies, etc. In this paper
we present a numerical study of a gravitationally unstable layer. This allows
us to go beyond the classical analytical study of linear perturbations,
exploring a highly non-linear regime. A hierarchical growth of structure in the
presence of dissipation (gas drag) can yield large, virialized clusters of
planetesimals, the first time such clusters have been observed in the context
of planetesimal disks.Comment: 12 pages, 12 figures (for Fig 1 and 7 at full resolution see
http://www.obs-nice.fr/tanga/) Accepted by A&
Mony a mickle maks a muckle: minor body observations with optical telescopes of all sizes
I review the current capabilities of small, medium and large telescopes in the study of minor bodies of the Solar System (MBOSS), with the goal of identifying those areas where the next generation of Extremely Large Telescopes (ELTs) are required to progress. This also leads to a discussion of the synergies between large and small telescopes. It is clear that the new facilities that will become available in the next decades will allow us to discover smaller and more distant objects (completing size distributions) and to characterise and even resolve larger individual bodies and multiple systems, however we must also recognise that there is still much to be learned from wide surveys that require more time on more telescopes than can ever be available on ELTs. Smaller telescopes are still required to discover and characterise large samples of MBOSS
Formes d'astéroïdes et formation de satellites : rÎle de la réaccumulation gravitationnelle
International audienceAsteroid shapes and satellites: role of gravitational reaccumulation. Following current evidences, it is widely accepted that many asteroids would be "gravitational aggregates", i.e. bodies lacking internal cohesion. They could mainly be originated during the catastrophic disruption of some parent bodies, through the gravitational reaccumulation of the resulting fragments. The same events produced the dynamical families that we observe. In this work we address the problem of the origin of shapes of gravitational aggregates, that could contain signatures of their origin. We use a N-body code to simulate the collapse of a cloud of fragments, with a variety of initial velocity distributions and total angular momentum. The fragments are treated as inhelastic spheres, that rapidly accumulate to form rotating aggregates. The resulting shapes and rotational properties are compared with theoretical predictions. The results show that only a precise category of shapes (flattened spheroids) are created via this mechanism. This may provide interesting constraints on the evolution of asteroid shapes, in particular for those with one or more satellites
Crossing Rural-Urban Spaces
Traverser les espaces ruraux-urbains. Les Takumbeng et lâactivisme dans la croisade dĂ©mocratique du Cameroun. â Cet article sâintĂ©resse au pouvoir exercĂ© par les femmes au sein des sociĂ©tĂ©s secrĂštes et autres « enclaves fĂ©minines » au Cameroun et ailleurs. Il montre comment ce pouvoir est soumis Ă une constante transformation dans le paysage politique moderne. Les soulĂšvements, protestations et manifestations instiguĂ©s par les femmes ont dĂ©jĂ Ă©tĂ© observĂ©s dans certaines sociĂ©tĂ©s camerounaises et dans dâautres rĂ©gions dâAfrique, mais cet article sâintĂ©resse tout particuliĂšrement Ă la sociĂ©tĂ© fĂ©minine traditionnelle des Takumbeng et Ă sa politisation en cette pĂ©riode de dĂ©mocratisation et de multipartisme. Nous examinerons la transformation de ce groupe dans le contexte de lâagitation collective et analyserons quel impact il subit sur la scĂšne politique actuelle qui est caractĂ©risĂ©e par une immense dĂ©ception face au dĂ©dain et aux politiques non dĂ©mocratiques du gouvernement national en place. Cette Ă©tude rĂ©vĂšle le symbolisme et les mĂ©taphores, les interactions et raisons dâĂȘtre culturelles qui se cachent derriĂšre lâactivisme de ce groupe en se plaçant du point de vue des acteurs en milieu urbain. Lâarticle conclut que le groupe a accentuĂ© son combat continu pour la dĂ©mocratie en sâappuyant sur son expĂ©rience de rĂ©sistance, en ravivant la flamme de la dĂ©mocratie sociale et en Ă©levant la conscience politique dans la province Nord-Ouest du Cameroun, bastion du parti dâopposition, le Front social-dĂ©mocrate.The article focuses on the power women wielded within secret societies and other womenâs enclaves in Cameroon and beyond. It shows how this power is continuously being transformed in the modern political landscape. Female riots, protests and demonstrations have been recorded in certain societies in Cameroon and elsewhere in Africa but this paper looks at the Takumbeng traditional female society and its politicisation in the context of multiparty politics and democratisation. In this paper, the transformation of the group is examined against the backdrop of collective agitation and how it impacted on the political scene marked by an atmosphere of disenchantment with the highhandedness and undemocratic policies of the national government in place. The study captures the symbolism and metaphors, the cultural interplay and raison dâĂȘtre behind the groupâs activism from the lenses of the actors as played-out in an urban milieu. It concludes that the group enhanced the continuing struggle for democratic deepening through a legacy of resistance, restoring the flame of social democracy, raising political consciousness in the Northwest Province of Cameroon, a bastion of the Social Democratic Front opposition party
Planetesimal clusters in a Keplerian disk I. gravitational evolution
It was recently demonstrated by numerical simulations that a turbulent flow in a rotating system is capable of
e#14;ciently concentrating passively advected particles â having a density larger than the fluid â inside anti-cyclonic vortices. This
process has important consequences on the distribution of solid particles in protoplanetary disks, since dust surface densities
#24;1â2 orders of magnitude higher than the background are rapidly reached in vortex cores. However, until now, the role of
self-gravitation of captured solids has been neglected. In this work we study the action of mutual gravitational interactions
â after the gas has dissipated â over the dynamics of planetesimals inside clusters similar to those created in vortex cores.
A comparison is made between the behavior of idealized clusters of planetesimals characterized by ad-hoc velocity profiles,
and more complex initial conditions such as those obtained in previous hydrodynamical simulations. We show here that, within
the explored interval of parameters, mutual scattering of particles can quickly disperse the cluster. Our results are demonstrated
to be not dependent on the resolution employed. It can be concluded that if large planetesimals were formed inside vortex cores,
they would be ejected by mutual perturbations
Ground-based astrometry calibrated by Gaia DR1: new perspectives in asteroid orbit determination
Context. The Gaia Data Release 1 (GDR1) is a first, important step on the path of evolution of astrometric accuracy towards a much improved situation. Although asteroids are not present in GDR1, this intermediate release already impacts asteroid astrometry.
Aims. Our goal is to investigate how the GDR1 can change the approach to a few typical problems, including the determination of orbits from short-arc astrometry, the exploitation of stellar occultations, and the impact risk assessment.
Methods.We employ optimised asteroid orbit determination tools, and study the resulting orbit accuracy and post-fit residuals. For this goal, we use selected ground-based asteroid astrometry, and occultation events observed in the past. All measurements are calibrated by using GDR1 stars.
Results. We show that, by adopting GDR1, very short measurement arcs can already provide interesting orbital solutions, capable of correctly identifying near-Earth asteroids (NEAs) and providing a much more accurate risk rating. We also demonstrate that occultations, previously used to derive asteroid size and shapes, now reach a new level of accuracy at which they can be fruitfully used to obtain astrometry at the level of accuracy of Gaia star positions
The nutritive value of black soldier fly larvae reared on common organic waste streams in Kenya
Open Access JournalIn Africa, livestock production currently accounts for about 30% of the gross value of agricultural production. However, production is struggling to keep up with the demands of expanding human populations, the rise in urbanization and the associated shifts in diet habits. High costs of feed prevent the livestock sector from thriving and to meet the rising demand. Insects have been identified as potential alternatives to the conventionally used protein sources in livestock feed due to their rich nutrients content and the fact that they can be reared on organic side streams. Substrates derived from organic by-products are suitable for industrial large-scale production of insect meal. Thus, a holistic comparison of the nutritive value of Black Soldier Fly larvae (BSFL) reared on three different organic substrates, i.e. chicken manure (CM), brewersâ spent grain (SG) and kitchen waste (KW), was conducted. BSFL samples reared on every substrate were collected for chemical analysis after the feeding process. Five-hundred (500) neonatal BSFL were placed in 23âĂâ15âcm metallic trays on the respective substrates for a period of 3â4 weeks at 28â±â2â°C and 65â±â5% relative humidity. The larvae were harvested when the prepupal stage was reached using a 5âmm mesh size sieve. A sample of 200 grams prepupae was taken from each replicate and pooled for every substrate and then frozen at â20â°C for chemical analysis. Samples of BSFL and substrates were analyzed for dry matter (DM), crude protein (CP), ether extracts (EE), ash, acid detergent fibre (ADF), neutral detergent fibre (NDF), amino acids (AA), fatty acids (FA), vitamins, flavonoids, minerals and aflatoxins. The data were then subjected to analysis of variance (ANOVA) using general linear model procedure. BSFL differed in terms of nutrient composition depending on the organic substrates they were reared on. CP, EE, minerals, amino acids, ADF and NDF but not vitamins were affected by the different rearing substrates. BSFL fed on different substrates exhibited different accumulation patterns of minerals, with CM resulting in the largest turnover of minerals. Low concentrations of heavy metals (cadmium and lead) were detected in the BSFL, but no traces of aflatoxins were found. In conclusion, it is possible to take advantage of the readily available organic waste streams in Kenya to produce nutrient-rich BSFL-derived feed
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