88 research outputs found
Semi-automatic meteoroid fragmentation modeling using genetic algorithms
Meteoroids are pieces of asteroids and comets. They serve as unique probes to
the physical and chemical properties of their parent bodies. We can derive some
of these properties when meteoroids collide with the atmosphere of Earth and
become a meteor or a bolide. Even more information can be obtained when
meteoroids are mechanically strong and slow enough to drop meteorites. Through
physical modeling of bright meteors, we describe their fragmentation in the
atmosphere. We also derive their mechanical strength and the mass distribution
of the fragments, some of which may hit the ground as meteorites. We developed
a semi-automatic program for meteoroid fragmentation modeling using parallel
genetic algorithms. This allowed us to determine the most probable
fragmentation cascade of the meteoroid, and also to specify its initial mass
and velocity. These parameters can be used in turn to derive the heliocentric
orbit of the meteoroid and to place constraints on its likely age as a separate
object. The program offers plausible solutions for the majority of fireballs we
tested, and the quality of the solutions is comparable to that of manual
solutions. The two solutions are not the same in detail, but the derived
quantities, such as the fragment masses of the larger fragments and the proxy
for their mechanical strength, are very similar. With this method, we would
like to describe the mechanical properties and structure of both meteoroids
belonging to major meteor showers and those that cause exceptional fireballs.Comment: Accepted for publication in A&A, 17 pages, 27 figure
Ultrasound exfoliation of inorganic analogues of graphene
High-intensity ultrasound exfoliation of a bulk-layered material is an attractive route for large-scale preparation of monolayers. The monolayer slices could potentially be prepared with a high yield (up to 100%) in a few minutes. Exfoliation of natural minerals (such as tungstenite and molybdenite) or bulk synthetic materials (including hexagonal boron nitride (h-BN), hexagonal boron carbon nitride (h-BCN), and graphitic carbon nitride (g-C(3)N(4))) in liquids leads to the breakdown of the 3D graphitic structure into a 2D structure; the efficiency of this process is highly dependent upon the physical effects of the ultrasound. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were employed to verify the quality of the exfoliation. Herein, this new method of exfoliation with ultrasound assistance for application to mono- and bilayered materials in hydrophobic and hydrophilic environments is presented
Mechanical strength distribution in Geminid meteoroids derived via fireball modeling
Geminids are the most active annual meteor shower observed on Earth. Their
parent is an active asteroid, (3200) Phaethon, which is a target of the planned
DESTINY+ mission of the Japan Aerospace Exploration Agency (JAXA). The exact
physical nature of (3200) Phaethon and Geminids is still debated. This paper is
devoted to fragmentation modeling of bright Geminid fireballs, which should
reveal information about the structure of centimeter-sized Geminid meteoroids.
These fireballs were observed by the European Fireball Network (EN) over the
past few years. We aim to describe their disintegration cascade in the
atmosphere and their mechanical properties, and to derive their precise initial
masses and velocities. We used a semi-empirical fragmentation model that
employs an automatic procedure based on parallel genetic algorithms to
determine the aerodynamic pressures at which a meteoroid and its parts
fragment. This serves as a proxy for the mechanical strength of the body and
its subsequent fragments. It enabled us to derive the minimum, median, and
maximum mechanical strength and the strength distribution inside the meteoroid
and reveal its internal structure. We find that the Geminids begin to crumble
at pressures 1--100 kPa, with the strongest parts reaching pressures of between
0.4 and 1.55 MPa before fragmenting. Knowing the spectral type of (3200)
Phaethon (a B-type asteroid, part of the C complex), we conclude that the
Geminids are made of compact and coherent carbonaceous material. We also find
that the minimum aerodynamic pressure that causes the fragmentation of Geminids
increases with increasing entry mass of Geminids. In contrast, the median
aerodynamic pressure decreases as their entry mass increases. The spectra of
all the observed Geminid fireballs show normal content and little variation in
terms of sodium.Comment: Accepted for publication in A&A, 13 pages, 16 figure
Design and performance of novel self-cleaning g-C3N4/PMMA/PUR membranes
In the majority of photocatalytic applications, the photocatalyst is dispersed as a suspension of nanoparticles. The suspension provides a higher surface for the photocatalytic reaction in respect to immobilized photocatalysts. However, this implies that recovery of the particles by filtration or centrifugation is needed to collect and regenerate the photocatalyst. This complicates the regeneration process and, at the same time, leads to material loss and potential toxicity. In this work, a new nanofibrous membrane, g-C3N4/PMMA/PUR, was prepared by the fixation of exfoliated g-C3N4 to polyurethane nanofibers using thin layers of poly(methyl methacrylate) (PMMA). The optimal amount of PMMA was determined by measuring the adsorption and photocatalytic properties of g-C3N4/PMMA/PUR membranes (with a different PMMA content) in an aqueous solution of methylene blue. It was found that the prepared membranes were able to effectively adsorb and decompose methylene blue. On top of that, the membranes evinced a self-cleaning behavior, showing no coloration on their surfaces after contact with methylene blue, unlike in the case of unmodified fabric. After further treatment with H2O2, no decrease in photocatalytic activity was observed, indicating that the prepared membrane can also be easily regenerated. This study promises possibilities for the production of photocatalytic membranes and fabrics for both chemical and biological contaminant control.Web of Science124art. no. 85
Improvement of Orange II Photobleaching by Moderate Ga 3+
Highly photoactive Ga3+-doped anatase modification of titania was prepared by homogeneous hydrolysis of aqueous solutions mixture of titanium oxo-sulphate TiOSO4 and gallium(III) nitrate with urea. Incorporation of Ga3+ into the anatase lattice has a clear positive effect on the photocatalytic activity under UV and Vis light irradiation up to a certain extent of Ga. Ga3+ doping decreased the size of the crystallites, increased surface area, and affected texture of the samples. Higher amount of gallium leads to the formation of a nondiffractive phase, probably photocatalytically inactive. The titania sample with 2.18 wt.% Ge3+ had the highest activity during the photocatalysed degradation in the UV and visible light regions; the total bleaching of dye Orange II was achieved within 29 minutes. Ga concentration larger than 5% (up to 15%) significantly inhibited the growth of the anatase crystal domains which formed the nondiffractive phase content and led to remarkable worsening of the photobleaching efficiency
YETI observations of the young transiting planet candidate CVSO 30 b
CVSO 30 is a unique young low-mass system, because, for the first time, a
close-in transiting and a wide directly imaged planet candidates are found
around a common host star. The inner companion, CVSO 30 b, is the first
possible young transiting planet orbiting a previously known weak-lined T-Tauri
star. With five telescopes of the 'Young Exoplanet Transit Initiative' (YETI)
located in Asia, Europe and South America we monitored CVSO 30 over three years
in a total of 144 nights and detected 33 fading events. In two more seasons we
carried out follow-up observations with three telescopes. We can confirm that
there is a change in the shape of the fading event between different
observations and that the fading event even disappears and reappears. A total
of 38 fading event light curves were simultaneously modelled. We derived the
planetary, stellar, and geometrical properties of the system and found them
slightly smaller but in agreement with the values from the discovery paper. The
period of the fading event was found to be 1.36 s shorter and 100 times more
precise than the previous published value. If CVSO 30 b would be a giant planet
on a precessing orbit, which we cannot confirm, yet, the precession period may
be shorter than previously thought. But if confirmed as a planet it would be
the youngest transiting planet ever detected and will provide important
constraints on planet formation and migration time-scales.Comment: 14 pages (20 with appendix), 7 figures (16 with appendix), 6 tables
(7 with appendix
Recovery of Cerium Dioxide from Spent Glass-Polishing Slurry and Its Utilization as a Reactive Sorbent for Fast Degradation of Toxic Organophosphates
The recovery of cerium (and possibly other rare earth elements) from the spent glass-polishing slurries is rather difficult because of a high resistance of polishing-grade cerium oxide toward common digestion agents. It was shown that cerium may be extracted from the spent polishing slurries by leaching with strong mineral acids in the presence of reducing agents; the solution may be used directly for the preparation of a ceria-based reactive sorbent. A mixture of concentrated nitric acid and hydrogen peroxide was effective in the digestion of partially dewatered glass-polishing slurry. After the removal of undissolved particles, cerous carbonate was precipitated by gaseous NH3 and CO2. Cerium oxide was prepared by a thermal decomposition of the carbonate precursor in an open crucible and tested as reactive sorbent for the degradation of highly toxic organophosphate compounds. The samples annealed at the optimal temperature of approximately 400°C exhibited a good degradation efficiency toward the organophosphate pesticide fenchlorphos and the nerve agents soman and VX. The extraction/precipitation procedure recovers approximately 70% of cerium oxide from the spent polishing slurry. The presence of minor amounts of lanthanum does not disturb the degradation efficiency
Dust and Metal Column Densities in Gamma-Ray Burst Host Galaxies
In this paper we present the results from the analysis of a sample of 28
gamma-ray burst (GRB) afterglow spectral energy distributions, spanning the
X-ray through to near-infrared wavelengths. This is the largest sample of GRB
afterglow spectral energy distributions thus far studied, providing a strong
handle on the optical depth distribution of soft X-ray absorption and
dust-extinction systems in GRB host galaxies. We detect an absorption system
within the GRB host galaxy in 79% of the sample, and an extinction system in
71% of the sample, and find the Small Magellanic Cloud (SMC) extinction law to
provide an acceptable fit to the host galaxy extinction profile for the
majority of cases, consistent with previous findings. The range in the soft
X-ray absorption to dust-extinction ratio, N_{H,X}/Av, in GRB host galaxies
spans almost two orders of magnitude, and the typical ratios are significantly
larger than those of the Magellanic Clouds or Milky Way. Although dust
destruction could be a cause, at least in part, for the large N_{H,X}/Av
ratios, the good fit provided by the SMC extinction law for the majority of our
sample suggests that there is an abundance of small dust grains in the GRB
environment, which we would expect to have been destroyed if dust destruction
were responsible for the large N_{H,X}/Av ratios. Instead, our analysis
suggests that the distribution of N_{H,X}/Av in GRB host galaxies may be mostly
intrinsic to these galaxies, and this is further substantiated by evidence for
a strong negative correlation between N_{H,X}/Av and metallicity for a
subsample of GRB hosts with known metallicity. Furthermore, we find the
N_{H,X}/Av ratio and metallicity for this subsample of GRBs to be comparable to
the relation found in other more metal-rich galaxies.Comment: 23 pages, 10 figures, accepted for publication in MNRA
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