Abstract The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical Herbig Ae/Be star (HAeBeS) with fossil large-scale magnetic field is modeled taking into account the buoyant and drag forces, radiative heat exchange with the surrounding gas, and the magnetic field of the disk. The structure of the disk is simulated using our magnetohydrodynamic model, taking into account the heating of the surface layers of the disk with the stellar radiation. The simulations show that MFTs periodically rise from the innermost region of the disk with speeds up to 10–12 km s − 1 {{\rm{s}}}^{-1} . MFTs experience decaying magnetic oscillations under the action of the external magnetic field near the disk’s surface. The oscillation period increases with distance from the star and initial plasma beta of the MFT, ranging from several hours at r = 0.012 au r=0.012\hspace{0.33em}{\rm{au}} up to several months at r = 1 au r=1\hspace{0.33em}{\rm{au}} . The oscillations are characterized by pulsations of the MFT’s characteristics including its temperature. We argue that the oscillations can produce observed IR-variability of HAeBeSs, which would be more intense than in the case of T Tauri stars, since the disks of HAeBeSs are hotter, denser, and have stronger magnetic field.

The theory of the fossil magnetic field of young stars and their accretion disks has been verified by comparing the observational data with the results of numerical simulations of the collapse of protostellar clouds. A new model of dust evaporation has been proposed, in which the parameter is not the thickness of the mantles, but the initial ratio of the core radius to the mantle radius of a dust grain. A semi-analytical description of the evolution of the radius distribution of dust grains was constructed. On its basis, the variations in the relative number density of dust grains, as well as the average values of the radius, cross-sectional area, and mass of dust grains, were calculated. It was shown that at the stage of disappearance of dust cores, these averages reach their maxima, but this does not affect the interaction of dust with gas particles, since the dust becomes scarce. Using cloud models W3 (main), NGC 2024, and DR 21 OH1, it has been demonstrated that neglecting dust evaporation underestimates the fossil magnetic field by several times. The possibility of formation of a magnetic compaction at the outer boundary of the zone of strong magnetic field diffusion (dead zone) has been confirmed. It is concluded that a correct calculation of dust evolution, ionization of the medium, and collapse anisotropy makes it possible to match the theoretical and observed magnetic fields of young stars and their accretion disks.

When a space body enters Earth’s atmosphere, its surface is exposed to high pressure and temperatures. The airflow tears off small droplets from the meteoroid forming a cloud of meteorite dust. Can new materials be synthesized in these unique conditions (high temperature, pressure, gaseous atmosphere, catalysts)? As a rule, meteoritic dust dissipates in the atmosphere without a trace or is mixed with terrestrial soil. The Chelyabinsk superbolide, the biggest in the twenty-first century, which exploded on February 15, 2013 above snowy fields of the Southern Urals, was an exception. The unique carbon crystals with a size of several micrometers, which were not observed before, were found during an in-depth study of the meteoritic dust. In order to explain the experimental findings, a multiple twin growth mechanism for the formation of closed shell graphite microcrystals was proposed based on DFT and classical/ab initio MD simulations. It was found that among several possible embryo carbon nanoclusters, the C60 fullerene and polyhexacyclooctadecane –C18H12– may be the main suspects, responsible for the formation of the experimentally observed closed shell quasi-spherical and hexagonal rod graphite microcrystals.

We investigate the dynamics of the accretion disks of young stars with fossil large-scale magnetic field. The author’s magnetohydrodynamic (MHD) model of the accretion disks is generalized to take into account the dynamical influence of the magnetic field on gas rotation speed and vertical structure of the disks. With the help of the developed MHD model, the structure of an accretion disk of a solar mass T Tauri star is simulated for different accretion rates $$\dot {M}$$ and dust grain sizes $${{a}_{d}}$$ . The simulations of the radial structure of the disk show that the magnetic field in the disk is kinematic, and the electromagnetic force does not affect the rotation speed of the gas for typical values $$\dot {M} = {{10}^{{ - 8}}}$$ $${{M}_{ \odot }}$$ /yr and $${{a}_{d}} = 0.1$$ µm. In the case of large dust grains, $${{a}_{d}} \geqslant 1$$ mm, the magnetic field is frozen into the gas and a dynamically strong magnetic field is generated at radial distances from the star $$r \gtrsim 30$$ AU, the tensions of which slow down the rotation speed by $$ \lesssim {\kern 1pt} 1.5$$ % of the Keplerian velocity. This effect is comparable to the contribution of the radial gradient of gas pressure and can lead to the increase in the radial drift velocity of dust grains in the accretion disks. In the case of high accretion rate, $$\dot {M} \geqslant $$ 10–7 $${{M}_{ \odot }}$$ /yr, the magnetic field is also dynamically strong in the inner region of the disk, $$r < $$  0.2 AU. The simulations of the vertical structure of the disk show that, depending on the conditions on the surface of the disk, the vertical gradient of magnetic pressure can lead to both decrease and increase in the characteristic thickness of the disk as compared to the hydrostatic one by 5–20%. The change in the thickness of the disk occurs outside the region of low ionization fraction and effective magnetic diffusion (“dead” zone), which extends from $$r = 0.3$$ to 20 AU.

Abstract We numerically model the collapse of magnetic rotating protostellar clouds with mass of 10 M ⊙ {M}_{\odot } . The simulations are carried out with the help of 2D MHD code Enlil. The structure of the cloud at the isothermal stage of the collapse is investigated for the cases of weak, moderate, and strong initial magnetic field. Simulations reveal the universal hierarchical structure of collapsing protostellar clouds, consisting of the flattened envelope with the qausi-magnetostatc disk inside and the first core in its center. The size of the primary disk increases with the initial magnetic energy of the cloud. The magnetic braking efficiently transports the angular momentum from the primary disk into the envelope in the case, when the initial magnetic energy of the cloud is more than 20% of its gravitational energy. The intensity of the outflows launched from the region near the boundary of the first core increases with initial magnetic energy. The “dead” zone with small ionization fraction, x < 1 0 − 11 x\lt 1{0}^{-11} , forms inside the first hydrostatic core and at the base of the outflow. Ohmic dissipation and ambipolar diffusion determine conditions for further formation of the protostellar disk in this region.

The mechanical properties of Chelyabinsk LL5 chondrite (Chelyabinsk meteorite) were studied by uniaxial compression and diametral compression/indirect tension test. Twenty cylindrical samples, 10 for compression and 10 for tension, with the diameter 3.3 mm and 1.65 mm in height have been prepared for testing. It was shown that the strength of the tested samples under compression almost 45 times greater than it is at tension: 372 ± 10 MPa and 8.2 ± 0.7 MPa, respectively. Fracture behaviour under compression and tension was similar and can be characterised as brittle. The obtained compression strength of the Chelyabinsk meteorite lies close to the maximal values of strength for many other chondrites, whereas its tensile strength magnitude resides in the bottom quarter of the range of similar measurements. It may be caused by the small sizes of the investigated samples together with a large number of tiny cracks between the grains in the Chelyabinsk chondrite. Our estimations have shown that if one assumes that the initial shape of the Chelyabinsk fireball was spherical or ellipsoidal, then its fragmentation stress is close to the experimental tensile strength and much lower than the compression strength. Hence, a stress state equivalent to one appearing at the indirect tension test could occur in the Chelyabinsk fireball during its fall in the Earth atmosphere.

We perform numerical MHD simulations of the isothermal collapse of magnetic rotating protostellar clouds to investigate initial conditions for protostellar disks formation. The simulations show that a hierarchical structure of the cloud is formed during the isothermal collapse: a flattened cloud envelope with a primary magnetostatic disk inside. The first core forms inside the primary disk further. We investigate the dependence of the characteristics (size, mass, angular momentum) of the envelope and primary disk on the initial thermal, magnetic, and rotational energies. Based on the results of numerical simulation the average brightness temperature in the NH3 line (2,2) is under construction.

Работа Н. С. Каргальцевой выполнена при поддержке Российского научного фонда (проект 19-72-10012), работа А. Е. Дудорова — при поддержке Российского фонда фундаментальных исследований (проект 18-02-01067), работа С. А. Хайбрахманова — при поддержке Российского фонда фундаментальных исследований (проект 18-52-52006).

AbstractWe numerically investigate the gravitational collapse of rotating magnetic protostellar clouds. The simulations are performed using 2D MHD code ‘Enlil’. The code is based on TVD scheme of increased order of accuracy. We developed a model of the initially non-uniform cloud, which self-consistently treats gas density and large-scale magnetic field distribution. Simulation results for the typical parameters of a solar mass cloud are presented. In agreement with our previous results for the uniform cloud, the isothermal collapse of the non-uniform cloud results in formation of hierarchical structure of the cloud, consisting of flattened envelope and thin quasi-magnetostatic primary disk near its equatorial plane. The non-uniform cloud collapses longer than the uniform one, since the magnetic field is dynamically stronger at the periphery of the cloud in the former case.

We perform a statistical analysis of 926 confirmed falls of meteorites that occurred in the period 1860 to 2017. The distribution of meteorites over masses is approximated with a lognormal law. It has been shown that the mean interval between the detections of falls of Chelyabinsk-like meteorites is ~25 years. The time dependence of the annual number of meteorite falls is analyzed with the autocorrelation method. The (10–11)-year periodicity has been found for a group of H-chondrites and iron and iron-stone meteorites in the interval 1860 to 1960. The distributions of the number of recorded falls of meteorites over years, months, and time of day are compared to the respective distributions for bolides. The number of bolides is largest in 2005 and 2015, which suggests that there is a cycle 10−11 years long in the distribution of the bolide numbers over years. It has been noticed that more meteorites fall during the time interval from noon to midnight and in the spring-and-summer season. The number of recorded bolides does not depend on the time of day and season.

Работа С. А. Хайбрахманова выполнена при финансовой поддержке Российского научного фонда (проект 19-72-10012), работа А. Е. Дудорова выполнена при финансовой поддержке оссийского фонда фундаментальных исследований (проект 18-02-01067).

Some statistical distributions for meteorites that fell to the Earth for the last 100 years are calculated The histogram of the distribution falling by years is presented The mass distribution of the studied meteorites is described by log-normal law. The influx of meteoritic material is calculated: M = 4.3 • 10 4 tons/year with the help of the cumulative distribution of the number of meteorites to the masses. The frequency of meteorite falling on the Earth in dependence of the initial diameter of the meteorite within the range of 5 to 30 meters is evaluated The average interval between fallings for meteorites similar to Chelyabinsk one is equal to 30-40 years.