202 research outputs found
Enrichment of the HR 8799 planets by minor bodies and dust
In the Solar System, minor bodies and dust deliver various materials to
planetary surfaces. Several exoplanetary systems are known to host inner and
outer belts, analogues of the main asteroid belt and the Kuiper belt. We study
the possibility that exominor bodies and exodust deliver volatiles and
refractories to the exoplanets in the system HR8799 by performing N-body
simulations. The model consists of the host star, four giant planets (HR8799 e,
d, c, and b), 650000 test particles representing the inner belt, and 1450000
test particles representing the outer belt. Moreover we modelled dust
populations that originate from both belts. Within a million years, the two
belts evolve towards the expected dynamical structure (also derived in other
works), where mean-motion resonances with the planets carve the analogues of
Kirkwood gaps. We find that, after this point, the planets suffer impacts by
objects from the inner and outer belt at rates that are essentially constant
with time, while dust populations do not contribute significantly to the
delivery process. We convert the impact rates to volatile and refractory
delivery rates using our best estimates of the total mass contained in the
belts and their volatile and refractory content. Over their lifetime, the four
giant planets receive between and 10^{-3}M_\bigoplus of material
from both belts. The total amount of delivered volatiles and refractories,
{5\times10^{-3}\textrm{M}_\bigoplus}, is small compared to the total mass of
the planets, 11\times10^{3}\textrm{M}_\bigoplus. However, if the planets were
formed to be volatile-rich, their exogenous enrichment in refractory material
may well be significant and observable, for example with JWST-MIRI. If
terrestrial planets exist within the snow line of the system, volatile delivery
would be an important astrobiological mechanism and may be observable as
atmospheric trace gases.Comment: 11 pages, 8 figures, accepted for publication in
Astronomy&Astrophysic
Global regularity criterion for the 3D Navier-Stokes equations involving one entry of the velocity gradient tensor
In this paper we provide a sufficient condition, in terms of only one of the
nine entries of the gradient tensor, i.e., the Jacobian matrix of the velocity
vector field, for the global regularity of strong solutions to the
three-dimensional Navier-Stokes equations in the whole space, as well as for
the case of periodic boundary conditions
Role of air humidity in residual fatigue lifetime of railway axle
This research was funded by grant No. CK03000060 “Advanced design methodology of
railway axles for safe and efficient operation” of The Technology Agency of the Czech
Republic
The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth
The annual flux of extraterrestrial material on Earth is largely dominated by sub-millimetre particles. The mass distribution and absolute value of this cosmic dust flux at the Earth’s surface is however still uncertain due to the difficulty in monitoring both the collection efficiency and the exposure parameter (i.e. the area-time product in m2.yr). In this paper, we present results from micrometeorite collections originating from the vicinity of the CONCORDIA Station located at Dome C (Antarctica), where we performed several independent melts of large volumes of ultra-clean snow. The regular precipitation rate and the exceptional cleanliness of the snow from central Antarctica allow a unique control on both the exposure parameter and the collection efficiency. A total of 1280 unmelted micrometeorites (uMMs) and 808 cosmic spherules (CSs) with diameters ranging from 30 to 350 μm were identified. Within that size range, we measured mass fluxes of 3.0 μg.m−2.yr−1 for uMMs and 5.6 μg.m−2.yr−1 for CSs. Extrapolated to the global flux of particles in the 12-700 μm diameter range, the mass flux of dust at Earth’s surface is 5, 200 ± 1500 1200 tons.yr−1 (1, 600 ± 500 and 3, 600 ± 1000 700 tons.yr−1 of uMMs and CSs, respectively). We indicate the statistical uncertainties expected for collections with exposure parameters in the range of 0.1 up to 105 m2.yr. In addition, we estimated the flux of altered and unaltered carbon carried by heated and un-heated particles at Earth’s surface. The mass distributions of CSs and uMMs larger than 100 μm are fairly well reproduced by the CABMOD-ZoDy model that includes melting and evaporation during atmospheric entry of the interplanetary dust flux. These numerical simulations suggest that most of the uMMs and CSs originate from Jupiter family comets and a minor part from the main asteroid belt. The total dust mass input before atmospheric entry is estimated at 15,000 tons.yr−1. The existing discrepancy between the flux data and the model for uMMs below 100 μm suggests that small fragile uMMs may evade present day collections, and/or that the amount of small interplanetary particles at 1 AU may be smaller than expected
The Shaqadud Archaeological Project (Sudan): exploring prehistoric cultural adaptations in the Sahelian hinterlands
The authors present preliminary results from a new research project based in Jebel Shaqadud, Sudan. Their findings highlight the potential for this region's archaeological record to expand our understanding of the adaptation strategies used by human groups in arid north-east African environments away from rivers and lakes during the Holocene. Furthermore, they present exceptionally early radiocarbon dates that push postglacial human occupation in the eastern Sahel back to the twelfth millennium BP
Aligned nanofibres made of poly(3-hydroxybutyrate) grafted to hyaluronan for potential healthcare applications
In this work, a hybrid copolymer consisting of poly(3-hydroxybutyrate) grafted to hyaluronic acid (HA) was synthesised and characterised. Once formed, the P(3HB)-g-HA copolymer was soluble in water allowing a green electrospinning process. The diameters of nanofibres can be tailored by simply varying the Mw of polymer. The optimization of the process allowed to produce fibres of average diameter in the range of 100-150 nm and low polydispersity. The hydrophobic modification has not only increased the fibre diameter, but also the obtained layers were homogenous. At the nanoscale, the hybrid copolymer exhibited an unusual hairy topography. Moreover, the hardness and tensile properties of the hybrid were found to be superior compared to fibres made of unmodified HA. Particularly, this reinforcement was achieved at the longitudinal direction. Additionally, this work reports the use in the composition of a water-soluble copolymer containing photo cross-linkable moieties to produce insoluble materials post-electrospinning. The derivatives as well as their nanofibrous mats retain the biocompatibility of the natural polymers used for the fabrication
They are young, and they are many: dating freshwater lineages in unicellular dinophytes
Dinophytes are one of few protist groups that have an extensive fossil record and are therefore appropriate for time estimations. However, insufficient sequence data and strong rate heterogeneity have been hindering to put dinophyte evolution into a time frame until now. Marine‐to‐freshwater transitions within this group are considered geologically old and evolutionarily exceptional due to strong physiological constraints that prevent such processes. Phylogenies based on concatenated rRNA sequences (including 19 new GenBank entries) of two major dinophyte lineages, Gymnodiniaceae and Peridiniales, were carried out using an uncorrelated molecular clock and five calibration points based on fossils. Contrarily to previous assumptions, marine‐to‐freshwater transitions are more frequent in dinophytes (i.e. five marine‐freshwater transitions in Gymnodiniaceae, up to ten but seven strongly supported transitions in Peridiniales), and none of them occurred as early as 140 MYA. Furthermore, most marine‐to‐freshwater transitions, and the followed diversification, took place after the Cretaceous–Paleogene boundary. Not older than 40 MYA, the youngest transitions within Gymnodiniaceae and Peridiniales occurred under the influence of the Eocene climate shift. Our evolutionary scenario indicates a gradual diversification of dinophytes without noticeable impact of catastrophic events, and their freshwater lineages have originated several times independently at different points in time
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