Том 63, № 9 (2019)

The atmospheric parameters and abundances of Mg, Si, Ca, and Ti have been determined for 20 stars using the Gaia DR2 parallaxes, high-resolution spectra, and modeling of lines without assuming LTE (non-LTE modeling). A sample of stars with homogeneous data on the abundances of α-process elements has thus been increased to 94. It is shown that applying a non-LTE approach and classical ID atmospheric models with spectroscopically determined gravitational accelerations (log g) based on Fe I and Fe II lines yields reliable results. Analysis of the full sample confirms the conclusions of earlier studies indicating excesses of Mg, Si, Ca, and Ti relative to Fe for halo and thick-disk stars, and strong excesses of these elements for thick-disk stars relative to stars with similar metallicities in the thin disk. New results are also obtained. The ratios [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] in the thick disk remain constant and similar to each other at the level ~ 0.3 when [Fe/H] ≲ —0.4, and fall off when the metallicity becomes higher, suggesting the onset of the production of iron in Type la supernovae. Halo stars have the same [α/Fe] values independent of their distance (within ~ 8 kpc of the Sun), providing evidence for a universal character of the evolution of the abundances of α-process elements in different parts of the Galaxy. The excess abundances relative to iron for halo stars are, on average, the same and at the level ~ 0.3 dex for Mg, Si, Ca, and Ti. These data are important for refining nucleosynthesis models. The scatter of [α/Fe] increases for [Fe/H] ≲ —2.6, but the scatter of the ratios between the different α-process elements remains small, possibly indicating incomplete mixing of nucleosynthesis products during the formation of these stars.

The structure of the flows in the eclipsing polar V808 Aur is studied. Comparison of computations with observed light curves is used to identify and clarify a number of features in the system, such as the drift of the hot spot—the place where the stream from the inner Lagrange point approaches the surface of the white dwarf, changes in the brightness in the secondary minimum, a dip in the light curve in the high state before entering the eclipse, the asymmetrical eclipse profile in high state, and others. A three-dimensional numerical MHD model based on the approximation of modified magnetic hydrodynamics is used in these studies. Numerical computations were carried out for several mass-exchange rates corresponding to different states of activity of the V808 Aur system. The computations show that, as the mass-exchange rate increases, the length of the ballistic part of the accretion stream increases, leading to changes in the spatial configuration of the flow and an appreciable drift of the region of energy release on the surface of the white dwarf (by up to 30° in longitude). These changes in the flow structure lead to effects in the light curve that are in good agreement with the available observations.

One possible means of counteraction against a hazardous asteroid is discussed: destruction of the object with a nuclear device during an earlier encounter with the Earth. This is feasible, since virtually all hazardous asteroids appear in near-Earth space several times before they hit the Earth. Computations show that this method is effective and essentially harmless if certain conditions are satisfied. Two possibilities are acceptable. In the first, a spacecraft overtakes the asteroid in a heliocentric orbit. In the second, the asteroid overtakes the spacecraft, which demands a substantially lower characteristic geocentric velocity for the spacecraft. This method for eliminating this cosmic threat is reasonable in two cases: when it is not possible to achieve a soft departure of the object from the collision orbit, and when the object continuously returns to the Earth. A soft departure from a collision orbit can be required multiple times, whereas the destruction of such an object must occur only once.