Astronomy Reports最新文献

We have investigated existence of Lagrangian points and analyzed stability in the Earth–Moon–Sun system including the effect of Earth’s equatorial ellipticity parameter (gamma ). First we have expressed the equations of motion of the Moon in the spherical coordinate system using the potential of the Earth. We observed that there exist collinear and non-collinear Lagrangian points for different values of Earth’s equatorial ellipticity (gamma ). We also analyzed the effect of (gamma ) on the stability of Lagrangian points including the effect of (gamma ). We observed that all the Lagrangian points are unstable for different values of (gamma ). Finally, we have drawn and analyzed zero velocity curves by taking different values of Jacobi constants.

Explaining the causes of angular momentum transfer in accretion stellar disks is an important astrophysical problem, since it is the process that determines the rate of accretion of matter onto the central gravitating body. Previously, within the framework of a two-dimensional approach, it was shown that the introduction of small perturbations into the flow of accretion disk leads to the appearance of shear instability. This process is accompanied by the development of large-scale vortex structures. Their movement and evolution lead to a redistribution of angular momentum in the accretion disk. The action of the described mechanism was previously studied numerically only within a two-dimensional approximation, so the goal of the current work is to carry out full-scale three-dimensional modeling. The processes under study are described within the framework of the system of equations of ideal gas dynamics. The article briefly describes the method for their numerical integration, which is based on a conservative finite-difference scheme and the solution of the Riemann problem. The initial data is a stationary toroidal gas state surrounded by a matter with low density and pressure. At the next step, small perturbations of one of the gas-dynamic variables are introduced. The modeling and analysis of the results of numerical calculations show the emergence of vortex structures in the shear flow of a three-dimensional accretion disk. Their movement is accompanied by a redistribution of matter and angular momentum in the volume of the disk, leading to accretion of matter onto the central body.

The paper contains an essay on the physical mechanisms of solar activity cycles.

The problem of calculating the electromagnetic field of a charged test particle moving in the field of a Schwarzschild black hole is considered. In the framework of the non-stationary problem, it is shown that the electromagnetic field produced by the particle moving along the radial geodesic tends to spherical symmetry as the particle approaches the event horizon.

We consider electron–positron plasma in strong electric field capable of creating electron–positron pairs. The dynamics of pair creation and plasma oscillations depends on pair distribution function. Assuming Fermi–Dirac distribution we found that for small temperatures pair creation is suppressed, while for small chemical potentials it is enhanced, with respect to the case when plasma degeneracy is neglected. Astrophysical settings where such conditions may occur are discussed.

The paper presents an overview describing the main types of solar activity: sunspots, faculae, flares, coronal mass ejections, coronal holes, solar wind.

Thanks to IXPE, the X-ray spectro-polarimeter launched at the end of 2021, X-ray polarimetry has finally become an extraordinary tool in investigating the physics of accretion in low mass X-ray binaries. Similarly to what happened with gravitational waves, X-ray polarimetry would play a new complementary but at the same time fundamental role in the high-energy astrophysical domain. We summarize here the first 1.5 yr results on accreting low-mass X-ray binaries obtained by a huge IXPE observation campaign coordinated with the principal X-ray and (gamma )-ray telescopes. Then we compare these results with the theoretical prediction highlighting the unexpected results.

FU Orionis type objects (fuors) are characterized by rapid (tens to hundreds years) episodic outbursts, during which the luminosity increases by orders of magnitude. One of the possible causes of such events is a close encounter between stars and protoplanetary disks. Numerical simulations show that the fuor-like outburst ignition requires a very close encounter ranging from a few to a few tens of astronomical units. In contrast, the observed stellar objects in fuor binaries are usually hundreds of astronomical units apart. Simple mathematical estimates show that if such a close approach took place, the binary stellar components would have an unrealistic relative velocity, at least an order of magnitude greater than the observed velocity dispersion in young stellar clusters. Thus, the bursts are either triggered with a certain delay after passage of the periastron or their ignition does not necessary require a close encounter and hence the outburst is not caused by the primordial gravitational perturbation of the protoplanetary disk. In this work, an encounter of a star surrounded by a protoplanetary disk with a diskless external stellar object was modeled using numerical hydrodynamics simulations. We showed that even fly-bys with a relatively large periastron (at least 500 AU) can result in fuor-like outbursts. Moreover, the delay between the periastron passage and the burst ignition can reach several kyr. It was shown for the first time by means of numerical modeling that the perturbation of the disk caused by the external object can trigger a cascade process, which includes the development of the thermal instability in the innermost disk followed by the magneto-rotational instability ignition. Because of the sequential development of these instabilities, the rapid increase in the accretion rate occurs, resulting in the luminosity increase by more than two orders of magnitude.

Accretion of matter onto black holes (BHs) is a prevalent phenomenon in the cosmos, resulting in consequential changes to both the mass and irreducible mass of the BH. These alterations significantly impact the rotational energy reservoir harbored within. This study investigates the relationship between the increase in a BH’s irreducible mass and the augmentation of its total mass due to the infall of matter (test particles) from the innermost stable circular orbit (ISCO) of a Kerr BH. Interestingly, the ratio of total mass growth to irreducible mass growth proves to be a non-monotonic function concerning the dimensionless spin parameter. It initially rises with spin, culminating near an extreme BH ((hat {a} simeq 0.998)), before declining. At the extreme BH position, the ratio is (frac{{dM}}{{d{{M}_{{{text{irr}}}}}}} = sqrt 2 ), indicating that accretion along the ISCO leads to the ultimate stabilization of the BH as an extreme one. Conversely, massless particles falling along unstable circular orbits exhibit a continuous increasing ratio of total mass growth to irreducible mass growth with respect to the dimensionless spin parameter.

We present the results of our photometric observations of the chromospherically active star FK Com (prototype of its group of stars) acquired during the recent 5 years (2018–2023) at the INASAN observatories in Zvenigorod and Simeiz and at the Russian–Cuban observatory in Habana (Republic of Cuba). In total, we obtained 9060 estimates of the star’s brightness in the V filter between 2018 and 2023. Our measurements, as well as data from the literature and from the Kamogata Wide-field Survey (KWS) archive, were combined into a unified set containing 17 653 measurements for a time interval about 57 years long. Based on the power spectrum derived from these data, we identified possible activity cycles ({{P}_{{{text{cycl}}}}}), estimated by us to be 2.4, 5.63, 8, 13.6, 30, and 49 years. The dominating ({{P}_{{{text{cycl}}}}}) is 5.63 years long. We show that the dominating cycle found by us from extensive data to be about 5.63 years (5.4–5.8 years from other sources in the literature) can also be noticed from the analysis of earlier studies. We compared our results on the activity cycles of FK Com to data on long-term variations of two other stars of the same type, HD 199178 (V1794 Cyg) and ET Dra. Data on ({{P}_{{{text{cycl}}}}}) of other chromospherically active stars (from the literature and from our measurements) were used to analyze the (log (1{text{/}}{{P}_{{{text{rot}}}}}){-} log ({{P}_{{{text{cycl}}}}}{text{/}}{{P}_{{{text{rot}}}}})) diagram. We make a conclusion on the compatibility of our determinations of activity cycles for stars of the FK Com type with data for RS CVn stars.