We explore the hypothesis that, due to small van der Waals forces between constituent grains, small rubble pile asteroids have a small but nonzero cohesive strength. The nature of this model predicts ...that the cohesive strength should be constant independent of asteroid size, which creates a scale dependence with relative strength increasing as size decreases. This model counters classical theory that rubble pile asteroids should behave as scale‐independent cohesionless collections of rocks. We explore a simple model for asteroid strength that is based on these weak forces, validate it through granular mechanics simulations and comparisons with properties of lunar regolith, and then explore its implications and ability to explain and predict observed properties of small asteroids in the NEA and Main Belt populations, and in particular of asteroid 2008 TC3. One conclusion is that the population of rapidly rotating asteroids could consist of both distributions of smaller grains (i.e., rubble piles) and of monolithic boulders.
► Pyrolysis of lignocellulosic biomass and marine biomass was studied by TGA–MS. ► The main gases produced were identified in the temperature range 200–450°C. ► CO2, H2O and light hydrocarbons were ...the main products detected. ► H2 was also observed at high temperatures due to secondary reactions. ► A multiple-step model was used to predict the pyrolysis of biomass.
The pyrolysis characteristics of three lignocellulosic biomasses (fir wood, eucalyptus and pine bark) and a marine biomass (Nannochloropsis gaditana microalgae) were investigated by thermogravimetric analysis coupled with mass spectrometry (TGA–MS). Thermal degradation of lignocellulosic biomass was divided into four zones, corresponding to the decomposition of their main components (cellulose, hemicellulose and lignin) and a first step associated to water removal. Differences in volatile matter and cellulose content of lignocellulosic species resulted in different degradation rates. Microalgae pyrolysis occurred in three stages due to the main components of them (proteins), which are greatly different from lignocellulosic biomass. Heating rate effect was also studied. The main gaseous products formed were CO2, light hydrocarbons and H2O. H2 was detected at high temperatures, being associated to secondary reactions (char self-gasification). Pyrolysis kinetics were studied using a multiple-step model. The proposed model successfully predicted the pyrolytic behaviour of these samples resulting to be statistically meaningful.
Numerical Semigroups García-Sánchez, P. A; Rosales, J. C
2010, 2009, 20090821, 2009-09-01, Letnik:
20
eBook
Odprti dostop
"This monograph is the first devoted exclusively to the development of the theory of numerical semigroups. In this concise, self-contained text, graduate students and researchers will benefit from ...this broad exposition of the topic. Key features of ""Numerical Semigroups"" include: - Content ranging from the basics to open research problems and the latest advances in the field, - Exercises at the end of each chapter that expand upon and support the material, - Emphasis on the computational aspects of the theory, algorithms are presented to provide effective calculations, - Many examples that illustrate the concepts and algorithms, - Presentation of various connections between numerical semigroups and number theory, coding theory, algebraic geometry, linear programming, and commutative algebra would be of significant interest to researchers."
Abstract
We introduce the Automatic Learning for the Rapid Classification of Events (ALeRCE) broker, an astronomical alert broker designed to provide a rapid and self-consistent classification of ...large etendue telescope alert streams, such as that provided by the Zwicky Transient Facility (ZTF) and, in the future, the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). ALeRCE is a Chilean-led broker run by an interdisciplinary team of astronomers and engineers working to become intermediaries between survey and follow-up facilities. ALeRCE uses a pipeline that includes the real-time ingestion, aggregation, cross-matching, machine-learning (ML) classification, and visualization of the ZTF alert stream. We use two classifiers: a stamp-based classifier, designed for rapid classification, and a light curve–based classifier, which uses the multiband flux evolution to achieve a more refined classification. We describe in detail our pipeline, data products, tools, and services, which are made public for the community (see
https://alerce.science
). Since we began operating our real-time ML classification of the ZTF alert stream in early 2019, we have grown a large community of active users around the globe. We describe our results to date, including the real-time processing of 1.5 × 10
8
alerts, the stamp classification of 3.4 × 10
7
objects, the light-curve classification of 1.1 × 10
6
objects, the report of 6162 supernova candidates, and different experiments using LSST-like alert streams. Finally, we discuss the challenges ahead in going from a single stream of alerts such as ZTF to a multistream ecosystem dominated by LSST.
We measured the gas abundance profiles in a sample of 122 face-on spiral galaxies observed by the CALIFA survey and included all spaxels whose line emission was consistent with star formation. This ...type of analysis allowed us to improve the statistics with respect to previous studies, and to properly estimate the oxygen distribution across the entire disc to a distance of up to 3−4 disc effective radii (re). We confirm the results obtained from classical H ii region analysis. In addition to the general negative gradient, an outer flattening can be observed in the oxygen abundance radial profile. An inner drop is also found in some cases. There is a common abundance gradient between 0.5 and 2.0 re of αO / H = − 0.075 dex /re with a scatter of σ = 0.016 dex /re when normalising the distances to the disc effective radius. By performing a set of Kolmogorov-Smirnov tests, we determined that this slope is independent of other galaxy properties, such as morphology, absolute magnitude, and the presence or absence of bars. In particular, barred galaxies do not seem to display shallower gradients, as predicted by numerical simulations. Interestingly, we find that most of thegalaxies in the sample with reliable oxygen abundance values beyond ~2 effective radii (57 galaxies) present a flattening of the abundance gradient in these outer regions. This flattening is not associated with any morphological feature, which suggests that it is a common property of disc galaxies. Finally, we detect a drop or truncation of the abundance in the inner regions of 27 galaxies in the sample; this is only visible for the most massive galaxies.
We present the second data release (DR2) of the X-shooter Spectral Library (XSL), which contains all the spectra obtained over the six semesters of that program. This release supersedes our first ...data release from Chen et al. (2014, A&A, 565, A117), with a larger number of spectra (813 observations of 666 stars) and with a more extended wavelength coverage as the data from the near-infrared arm of the X-shooter spectrograph are now included. The DR2 spectra then consist of three segments that were observed simultaneously and, if combined, cover the range between ∼300 nm and ∼2.45
μ
m at a spectral resolving power close to
R
= 10 000. The spectra were corrected for instrument transmission and telluric absorption, and they were also corrected for wavelength-dependent flux-losses in 85% of the cases. On average, synthesized broad-band colors agree with those of the MILES library and of the combined IRTF and Extended IRTF libraries to within ∼1%. The scatter in these comparisons indicates typical errors on individual colors in the XSL of 2−4%. The comparison with 2MASS point source photometry shows systematics of up to 5% in some colors, which we attribute mostly to zero-point or transmission curve errors and a scatter that is consistent with the above uncertainty estimates. The final spectra were corrected for radial velocity and are provided in the rest-frame (with wavelengths in air). The spectra cover a large range of spectral types and chemical compositions (with an emphasis on the red giant branch), which makes this library an asset when creating stellar population synthesis models or for the validation of near-ultraviolet to near-infrared theoretical stellar spectra across the Hertzsprung-Russell diagram.
•Analysis of the heat transport achieved by thermocapillary effects during the melting of PCMs in microgravity.•For large containers, heat transport can be substantially enhanced by a factor of up to ...20, depending on Ma.•For short containers, the enhancement can be on the order of 5 when oscillatory thermocapillary flow is present.•Existence of an optimal curve Γ(Ma) that maximises the thermocapillary contribution.
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An extensive numerical investigation of the heat transport enhancement achieved by the thermocapillary effect during the melting of Phase Change Materials in microgravity is presented. The phase change transition is analysed for the high Prandtl number paraffin n-octadecane in a two-dimensional rectangular container subjected to isothermal conditions along the lateral boundaries. The thermocapillary effect, supported by a free surface created as the solid/liquid front progresses, acts driving convection and as an effective mechanism for heat transport. For reference, the melting is first analysed under purely diffusive transport (conduction) and the heat transfer rate is shown to depend only on the Stefan number. Thermocapillary effects are then examined in the limiting cases of large and short containers. Results are presented for the representative aspect ratios Γ=12,2.25, where the thermocapillary flow is characterised by the appearance of oscillatory convection through the hydrothermal travelling wave and oscillatory standing wave modes, respectively, at a critical Marangoni number (Ma). The contribution of the thermocapillary effect to the heat transfer rate is quantified by the ratio of melting times between reference and thermocapillary simulations. For large Γ, heat transport can be greatly enhanced by a factor of up to 20, depending on Ma. For small Γ, this factor can be on the order of 5 when oscillatory flow is present. The analysis of the melting over a wide range of the governing parameters discovers the existence of an optimal curve Γ(Ma) that maximises the thermocapillary contribution.
ABSTRACT The "main sequence of galaxies"-defined in terms of the total star formation rate versus the total stellar mass M*-is a well-studied tight relation that has been observed at several ...wavelengths and at different redshifts. All earlier studies have derived this relation from integrated properties of galaxies. We recover the same relation from an analysis of spatially resolved properties, with integral field spectroscopic (IFS) observations of 306 galaxies from the CALIFA survey. We consider the SFR surface density in units of log(M yr−1 Kpc−2) and the stellar mass surface density in units of log(M Kpc−2) in individual spaxels that probe spatial scales of 0.5-1.5 Kpc. This local relation exhibits a high degree of correlation with small scatter ( = 0.23 dex), irrespective of the dominant ionization source of the host galaxy or its integrated stellar mass. We highlight (i) the integrated star formation main sequence formed by galaxies whose dominant ionization process is related to star formation, for which we find a slope of 0.81 0.02; (ii) for the spatially resolved relation obtained with the spaxel analysis, we find a slope of 0.72 0.04; and (iii) for the integrated main sequence, we also identified a sequence formed by galaxies that are dominated by an old stellar population, which we have called the retired galaxies sequence.
Abstract Amyloid deposits, neurofibrillary tangles, and neuronal cell death in selectively vulnerable brain regions are the chief hallmarks in Alzheimer's (AD) brains. Glycogen synthase kinase-3 ...(GSK-3) is one of the key kinases required for AD-type abnormal hyperphosphorylation of tau, which is believed to be a critical event in neurofibrillary tangle formation. GSK-3 has also been recently implicated in amyloid precursor protein (APP) processing/Aβ production, apoptotic cell death, and learning and memory. Thus, GSK-3 inhibition represents a very attractive drug target in AD and other neurodegenerative disorders. To investigate whether GSK-3 inhibition can reduce amyloid and tau pathologies, neuronal cell death and memory deficits in vivo , double transgenic mice coexpressing human mutant APP and tau were treated with a novel non-ATP competitive GSK-3β inhibitor, NP12. Treatment with this thiadiazolidinone compound resulted in lower levels of tau phosphorylation, decreased amyloid deposition and plaque-associated astrocytic proliferation, protection of neurons in the entorhinal cortex and CA1 hippocampal subfield against cell death, and prevention of memory deficits in this transgenic mouse model. These results show that this novel GSK-3 inhibitor has a dual impact on amyloid and tau alterations and, perhaps even more important, on neuronal survival in vivo further suggesting that GSK-3 is a relevant therapeutic target in AD.
Radionuclides in the environment is an important issue, many techniques have been developed for the removal of radionuclides from the environment. One of those techniques is the adsorption and ...natural and synthesized materials have been used to remove different radionuclides from water. The adsorbents used for removal of radionuclides should have high retention capacity and they should be resistant to radiation. One of the natural materials used is the zeolites due to its high ion exchange capacities, adsorption efficiency, resistance to radiation and abundance. The present review describes the advances made on radioactive waste treatments using zeolites as adsorbents, the elements: cesium, strontium, cobalt, molybdenum, uranium, plutonium, americium, samarium, and europium were selected according to their nuclear importance and their presence in the environment. Firstly, a brief description of the zeolites is given and then a review on the separation of these radionuclides from water by using zeolites is presented.
•Adsorption is an efficient method to treat nuclear wastes.•Zeolite are versatile materials used as adsorbents to treat nuclear wastes.•Metals present in nuclear wastes can be removed by zeolites.
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