We quantitatively investigate how collisional avalanches may develop in debris discs as the result of the initial breakup of a planetesimal or comet-like object, triggering a collisional chain reaction due to outward escaping small dust grains. We use a specifically developed numerical code that follows both the spatial distribution of the dust grains and the evolution of their size-frequency distribution due to collisions. We investigate how strongly avalanche propagation depends on different parameters (e.g., amount of dust released in the initial breakup, collisional properties of dust grains, and their distribution in the disc). Our simulations show that avalanches evolve on timescales of ∼1000 years, propagating outwards following a spiral-like pattern, and that their amplitude exponentially depends on the number density of dust grains in the system. We estimate the probability of witnessing an avalanche event as a function of disc densities, for a gas-free case around an A-type star, and find that features created by avalanche propagation can lead to observable asymmetries for dusty systems with a β Pictoris-like dust content or higher. Characteristic observable features include: (i) a brightness asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open spiral or a lumpy structure in the case of face-on orientation. A possible system in which avalanche-induced structures might have been observed is the edge-on seen debris disc around HD 32297, which displays a strong luminosity difference between its two sides.

Aims. We put theoretical constraints on the presence and survival of icy grains in debris discs. Particular attention is paid to UV sputtering of water ice, which has so far not been studied in detail in this context. Methods. We present a photosputtering model based on available experimental and theoretical studies. We quantitatively estimate the erosion rate of icy and ice-silicate grains, under the influence of both sublimation and photosputtering, as a function of grain size, composition and distance from the star. The effect of erosion on the grain's location is investigated through numerical simulations coupling the grain size to its dynamical evolution. Results. Our model predicts that photodesorption efficiently destroy ice in optically thin discs, even far beyond the sublimation snow line. For the reference case of β Pictoris, we find that only > ∼ 5 mm grains can keep their icy component for the age of the system in the 50-150 AU region. When taking into account the collisional reprocessing of grains, we show that the water ice survival on grains improves (grains down to 20 µm might be partially icy). However, estimates of the amount of gas photosputtering would produce on such a hypothetical population of big icy grains lead to values for the O I column density that strongly exceed observational constraints for β Pic, thus ruling out the presence of a significant amount of icy grains in this system. Erosion rates and icy grains survival timescales are also given for a set of 11 other debris disc systems. We show that, with the possible exception of M stars, photosputtering cannot be neglected in calculations of icy grain lifetimes.

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The infant mortality included in the structure of the mortality of children population is considered to be one of the major demographic factors most clearly reflecting the country’s level of the development and on-going economic and social changes. The infant mortality rate is a key index of the development of the health system. The significance of the infant mortality rate is determined by its high contribution to the child mortality rate in general. So the share of deaths among children aged 0-14 years in infants of the first year of life amounts of 55% to 65%. 40% of newborns died in the early neonatal period and 30% babies - in the post-neonatal period. In accordance with the Concept of the demographic policy of the Russian Federation for the period up to 2025 approved by the decree of the President of the Russian Federation dated 09.10.07 No. 315, one of the most important tasks of the demographic policy in the country is the reduction of the infant mortality rate at least by two times. The correct and timely analysis of the infant mortality rate allows developing a set of specific measures to reduce the morbidity and mortality rate of children, to assess the effectiveness of measures to describe and to plan the work for the protection of motherhood and childhood in general. In the article there is presented the analysis of the underlying causes and dynamics of the infant mortality in the Republic of Sakha (Yakutia) over the period of the implementation of major projects in the field of health. The development and implementation of a three-level system of medical care for pregnant women and mothers in the Republic of Sakha (Yakutia) was the key to the reducing this index.