70 research outputs found
Numerical modelling of heat transfer during impact of a molten droplet on a surface
SPH-based numerical technique for modelling of impact of molten drops on a surface with heat transfer and phase transitions effects is proposed. Computational algorithm uses SPH with procedure of restoring of particle consistence and variational approach to calculation of acceleration field. Also, boundary algorithm for free and contact surfaces in 3D setting are develope
Characterization of Population III Stars with Stellar Atmosphere and Evolutionary Modeling and Predictions of their Observability with the James Webb Space Telescope
Population III stars were the first stars to form after the Big Bang, and are
believed to have made the earliest contribution to the metal content of the
universe beyond the products of the Big Bang Nucleosynthesis. These stars are
theorized to have had extremely short lifespans, and therefore would only be
observable at high redshifts () and faint apparent magnitudes
(). The direct detection of Population III stars therefore
remains elusive. However, the recently launched James Webb Space Telescope
(JWST) may be capable of detecting stars in the relevant magnitude range in the
event of favorable gravitational lensing. Theoretical models are required to
interpret these future observations. In this study, new evolutionary models and
non-equilibrium model atmospheres were used to characterize the observable
properties of zero-age main sequence Population III stars. The calculated
models cover a wide range of possible Population III stellar masses, from the
minimum mass predicted by star formation studies to the maximum mass capable of
maintaining hydrostatic equilibrium. Synthetic photometry and theoretical
color-magnitude diagrams were calculated for the bands of the Near-Infrared
Camera (NIRCam) on JWST. The final results are compared to the scales of known
lensing events and JWST magnitude limits. The purpose of this study is to
calculate the observable parameters of Population III stars in the most optimal
JWST bands in order to provide a theoretical foundation for anticipated future
observations of this stellar population
Bjorken Sum Rule and pQCD frontier on the move
The reasonableness of the use of perturbative QCD notions in the region close
to the scale of hadronization, i.e., below \lesssim 1 \GeV is under study.
First, the interplay between higher orders of pQCD expansion and higher twist
contributions in the analysis of recent Jefferson Lab (JLab) data on the
Generalized Bjorken Sum Rule function at is studied. It is shown that the inclusion of the higher-order
pQCD corrections could be absorbed, with good numerical accuracy, by change of
the normalization of the higher-twist terms. Second, to avoid the issue of
unphysical singularity (Landau pole at Q=\Lambda\sim 400 \MeV ), we deal with
the ghost-free Analytic Perturbation Theory (APT) that recently proved to be an
intriguing candidate for a quantitative description of light quarkonia spectra
within the Bethe-Salpeter approach. The values of the twist coefficients
extracted from the mentioned data by using the APT approach provide
a better convergence of the higher-twist series than with the common pQCD. As
the main result, a good quantitative description of the JLab data down to
350 MeV is achieved.Comment: 10 pages, 3 figures, minor change
Simulation of viscous melt drop crystallization by the SPH method
Heat and mass transfer during absorption on a film of lithium bromide water solution flowing by a cooled wall in the steam atmosphere is numerically investigated in this paper. The self-similar solutions are using as the initial conditions for solving the problem beyond the entrance region. The key criteria characterizing heat and mass transfer in the film absorption with uniform velocity profile and with a constant thickness have been determined
On zero-divisors in group rings of groups with torsion
Nontrivial pairs of zero-divisors in group rings are introduced and
discussed. A problem on the existence of nontrivial pairs of zero-divisors in
group rings of free Burnside groups of odd exponent is solved in the
affirmative. Nontrivial pairs of zero-divisors are also found in group rings of
free products of groups with torsion.Comment: 8 pages, to appear in Canadian Math. Bul
Intensive systematic surveys of palaeospits in the Narva-Luga klint bay area: Analysis of methodology and results
Non peer reviewe
A New Grid of Model Atmospheres for Metal-poor Ultracool Brown Dwarfs
We present a new grid of model atmospheres and synthetic spectra down to extremely metal-poor ([M/H] ≥ -3.0), ultracool (Teff ≥ 900 K) brown dwarfs based on the PHOENIX code (Hauschildt et al. 1997). The grid is publicly available and continuously updated as new models are computed. The models are aimed at characterizing metal-poor halo brown dwarfs identified in surveys such as Backyard Worlds: Planet 9 (Kuchner et al. 2017; Schneider et al. 2020) and brown dwarfs in globular clusters (Dieball et al. 2019). Our setup includes the formation and depletion by gravitational settling of condensate clouds as defined for the BT-Settl grid of Allard et al. (2012), using the version 15.5 branch of PHOENIX, which was split off from the main branch (Barman et al. 2011; Husser et al. 2013). Our calculations employ the Allard & Homeier cloud formation model (Helling et al. 2008; Allard et al. 2012)...
Exploring the Chemistry and Mass Function of the Globular Cluster 47 Tucanae with New Theoretical Color-Magnitude Diagrams
Despite their shared origin, members of globular clusters display
star-to-star variations in composition. The observed pattern of element
abundances is unique to these stellar environments, and cannot be fully
explained by any proposed mechanism. It remains unclear whether stars form with
chemical heterogeneity, or inherit it from interactions with other members.
These scenarios may be differentiated by the dependence of chemical spread on
stellar mass; however, obtaining a sufficiently large mass baseline requires
abundance measurements on the lower main sequence that is too faint for
spectroscopy even in the nearest globular clusters. We developed a stellar
modelling method to obtain precise chemical abundances for stars near the end
of the main sequence from multiband photometry, and applied it to the globular
cluster 47 Tucanae. The computational efficiency is attained by matching
chemical elements to the model components that are most sensitive to their
abundance. We determined [O/Fe] for ~5000 members below the main sequence knee
at the level of accuracy, comparable to the spectroscopic measurements of
evolved members in literature. The inferred distribution disfavors stellar
interactions as the origin of chemical spread; however, an accurate theory of
accretion is required to draw a more definitive conclusion. We anticipate that
future observations of 47 Tucanae with JWST will extend the mass baseline of
our analysis into the substellar regime. Therefore, we present predicted
color-magnitude diagrams and mass-magnitude relations for the brown dwarf
members of 47 Tucanae
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