288 research outputs found
Secular Instability and Planetesimal Formation in the Dust Layer
Late in the gaseous phase of a protostellar disk, centimeter-sized bodies
probably settle into a thin ``dust layer'' at the midplane. A velocity
difference between the dust layer and the gas gives rise to turbulence, which
prevents further settling and direct gravitational instability of the layer.
The associated drag on the surface of the layer causes orbital decay in a few
thousand years---as opposed to a few hundred years for an isolated meter-sized
body. Within this widely-accepted theoretical framework, we show that the
turbulent drag causes radial instabilities even if the selfgravity of the layer
is negligible. We formulate axisymmetric, height-integrated dynamical equations
for the layer that incorporate turbulent diffusion of mass and momentum in
radius and height, vertical settling, selfgravity, and resistance to
compression due to gas entrained within the dust layer. In steady-state, the
equations describe the inward radial drift of a uniform dust layer. In
perturbation, overdense rings form on an orbital timescale with widths
comparable to the dust-layer thickness. Selfgravity is almost irrelevant to the
linear growth rate but will eventually fragment and collapse the rings into
planetesimals larger than a kilometer. We estimate that the drag instability is
most efficient at 1 AU when most of the ``dust'' mass lies in the size range
0.1-10 meters.Comment: 25 pp., 2 figures. Uses aastex version 5.0
Relacje polityczne pomiędzy Unią Europejską a Japonią
Rozdział porusza kwestie relacji pomiędzy Unią Europejską i Japonią w obszarze polityki oraz wymiany kulturowej i naukowej.Publikacja została dofinansowana przez Unię Europejską w ramach projektu Erasmus+ Jean Monnet Module „Asia as a Challenge for the European Union
General algorithm of computation of c-table and detection of valleys
We present a review of all interesting results concerning the c-table obtained by the authors for the last two decades. These results are not widely known because they were presented in publications of limited circulation. We discuss different computational aspects of software producing the c-tables in the presence of blocs and their evolution following the evolution of the computer environment: effects of the use of 32-bit arithmetic .≈8 digits), 64-bit arithmetic (double precision, ≈16 digits), and Bailey’s Fortran multiprecision package .32 or 64 digits), competition between the ascending and descending algorithms, relationship between the complexity of computation and precision, overflow and underflow problems, competition between different formulas allowing one to overcome the blocs in the c-table, practical simple criterion of detecting numerical zeros in the c-table allowing to identify the blocs, and automatic detection of valleys.Наведено огляд усіх цікавих результатів щодо c-таблиць, одержаних авторами протягом двох останніх десятиліть, які маловідомі з причини публікації у виданнях обмеженого поширення. Розглянуто різні обчислювальні аспекти програм, що продукують с-таблиці з наявністю блоків, а також їх еволюцію, обумовлену еволюцією комп'ютерного середовища, а саме: наслідки використання 32-бітової арифметики (кз 8 розрядів), 64-бітової арифметики (подвійна точність, ≈ 16 розрядів) та високоточного пакету Фортрана Бейлі (32 або 64 розряди), порівняння зростаючих та спадних алгоритмів, зв'язок між складністю обчислень і точністю, проблеми надпотоків та недостатніх потоків, порівняння різних формул, шо дозволяють уникнути блоків у c-таблицях, практичний простий критерій для визначення числових нулів у c-таблицях, що дозволяють ідентифікувати блоки, автоматичне визначення точок мінімуму
Study of systematics effects on the Cross Power Spectrum of 21 cm Line and Cosmic Microwave Background using Murchison Widefield Array Data
Observation of the 21cm line signal from neutral hydrogen during the Epoch of
Reionization is challenging due to extremely bright Galactic and extragalactic
foregrounds and complicated instrumental calibration. A reasonable approach for
mitigating these problems is the cross correlation with other observables. In
this work, we present the first results of the cross power spectrum (CPS)
between radio images observed by the Murchison Widefield Array and the cosmic
microwave background (CMB), measured by the Planck experiment. We study the
systematics due to the ionospheric activity, the dependence of CPS on group of
pointings, and frequency. The resulting CPS is consistent with zero because the
error is dominated by the foregrounds in the 21cm observation. Additionally,
the variance of the signal indicates the presence of unexpected systematics
error at small scales. Furthermore, we reduce the error by one order of
magnitude with application of a foreground removal using a polynomial fitting
method. Based on the results, we find that the detection of the 21cm-CMB CPS
with the MWA Phase I requires more than 99.95% of the foreground signal
removed, 2000 hours of deep observation and 50% of the sky fraction coverage.Comment: 15 pages, 16 figures, accepted to MNRA
On the relation between measures defining the Stieltjes and the inverted Stieltjes functions
A compact formula is found for the measure of the inverted Stieltjes function expressed by the measure of the original Stieltjes function.Встановлено формулу для міри оберненої функції Стільтьєса, що виражена через міру початкової функції Стільтьєса
Physical modelling of tundish slag entrainment under various technological conditions
This paper deals with the issue of physical modelling of vortexes creation and tundish slag entrainment over the mouth of the nozzle into the individual casting strands. Proper physical model is equivalent to the operational continuous casting machine No. 2 in TRINECKE ZELEZARN, a.s. Physical modelling methodology and simulated operational conditions are shortly described. Physical modelling was used for the evaluation of current conditions of steel casting at the application of different impact pads in the tundish. Further, laboratory measurement on the physical model aiming the determination of exact critical periods of vortexes creation and study of the slag entrainment as a consequence of changes in surface level during the tundish refilling to standard level were realised. The obtained results were analysed and discussed.Web of Science6231471146
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