Astrophysical Bulletin最新文献

The structure of a solar coronal filament with plasma parameters typical for quiescent solar prominences (particle concentration (10^{10}{-}10^{12}) cm({}^{-3}) and temperature in the coldest part ranging from 4000 to 20 000 K), located horizontally in the corona at heights of several tens of thousands of kilometers has been calculated. The filament is considered as the upper part of a slightly curved twisted magnetic loop, the legs of which are anchored in the photosphere. The magnetic field of the filament is helical (twisted). To maintain transverse equilibrium, an external longitudinal field is necessary. There is also a weak transverse magnetic field, which has a significant effect on the distribution of pressure and gas density in the filament. A characteristic feature of the model, reproduced theoretically for the first time, is the presence of a rarefaction (cavity) outside the filament. The fine filamentary structure of the prominence, consisting of dense and cold fibrils immersed in a weak uniform horizontal magnetic field, has been modeled. It is shown that the observed vertical movements of plasma elements in the prominence, usually interpreted as manifestations of ‘‘thermal convection’’ in its body, can be due to vertical mechanical displacements of individual magnetic fibrils that make up the body of the prominence. Shear (slip) plasma movements on the photosphere can change the sign of the weak transverse field and thus reduce the plasma density on the filament axis, i.e., bring the density of the electric current on its axis closer to the critical value at which the drift velocity of electrons equals the ion sound speed.

This paper is devoted to studying the variability of the (gamma) Cas variable star SAO 49725. We analyze both the optical and X-ray spectra of the star. The line profile variability in the spectrum of SAO 49725 was discovered on short (70–223 minutes) scales. Regular variations of the SAO 49725 light curves with a period of 1.1989 days are detected using the photometric light curves obtained with the TESS satellite, identified with the rotation period of the star. The pattern of the photometric variability of SAO 49725 according to the TESS data significantly varies at different observational epochs. The TESS components of the SAO 49725 light curves with the periods of about (3{-}21) days may be instrumental. The presence of variations with the period (P=230pm 178) minutes in the SAO 49725 X-ray light curves, obtained by the XMM-Newton telescope, is suspected.

We have selected candidate massive stars in the galaxy IC 342 based on archival images from the Hubble Space Telescope and images from the 2-m telescope at the National Astronomical Observatory, Rozhen, Bulgaria. Spectral observations of 24 out of 27 selected stars are carried out with the 6-m BTA telescope of SAO RAS and with the 3.5-m Apache Point Observatory telescope (USA) as part of the program for searching for bright massive stars in galaxies outside the Local Group. Our analysis reveals that 12 objects have spectra lacking prominent features, except for the emission lines of the surrounding nebulae and are identified as single supergiants of classes O9 to F5 or spatially unresolved young compact clusters. One source with an absorption spectrum probably belongs to our Galaxy. The spectra of seven other objects show features typical of Wolf–Rayet stars or compact clusters containing Wolf–Rayet stars. Another source is a compact supernova remnant. Two other objects are tentatively classified as cold LBV candidates, and one object is classified as a B[e]-supergiant candidate.

We present a refined speckle-interferometric orbit of a binary system (mu) Cet, with the main component studied based on the analysis of photometric and spectroscopic data, obtained with the SAO RAS 6-m telescope. The object was initially classified as a giant with chemical composition anomalies. As a result of our analysis, we conclude that the star belongs to the Main Sequence, to the class of non-peculiar stars. Analysis of photometric data from the TESS mission indicates that the main component of the system belongs to the (gamma) Dor pulsators.

The paper presents and analyzes a summary of magnetometric studies of single white dwarfs (WD) conducted at SAO RAS jointly with a number of other observatories. The goals of the studies are to search for regular large-scale magnetic fields in these stars and to construct a probability distribution of WD occurrence depending on the strength of these fields. Based on the analysis, the WDs were determined whose magnetospheres do not change their configurations for a long time, from several tens to hundreds of years at least, and possibly more. These stars can be used as standards of circular polarization. Using data from modern surveys and SAO RAS observations, versions of the observed probability distributions of WDs by their surface magnetic fields in the range of (10^{3}{-}10^{9}) G were constructed. The distribution of WDs with fields from (10^{6}) G to (10^{9}) G is well described by a power-law dependence. In the region of smaller fields the dependence is violated, demonstrating a selection ‘‘dip’’. The selection is caused by the technical complexity of magnetometric studies of WDs. The paper also discusses the physical nature of the selection associated with the complication of the interaction of global magnetic fields of low intensity (less than 10 kG) with dynamic processes in the surface layers of WDs. In particular, convection is capable of destroying the global symmetry of the surface magnetic field with an intensity of several kilogauss. Based on the analysis of the obtained distributions, it is concluded that all WDs with surface fields above several tens of kilogauss are carriers of regular global fields damped on times of (10^{10}) years. Magnetic fields with intensities of several kilogauss and lower lose their global structure, fragmenting into spots, which complicates and even makes impossible their detection by standard spectropolarimetric methods. Confirming examples and recommendations for improving the methodology of magnetometric observations of degenerate stars with weak magnetic fields are given.

We continue to study the ultra-luminous X-ray source NGC 7793 P13 in the optical range. In this work, we are testing the model of a spherically symmetric wind atmosphere of the donor star, previously identified as a B9 Ia supergiant. The model spectrum has shown good agreement with the observed one at a relatively high mass loss rate of (dot{M}approx 6times 10^{-6}M_{odot}text{ yr}^{-1}); other parameters turned out to be close to those expected for late B-supergiants. The increased mass loss rate can be explained by the high rotation velocity of the star. In addition, we have qualitatively demonstrated the effect of X-ray irradiation on the observed spectrum and discuss the fundamental possibility of wind acceleration under conditions of powerful irradiation.

The spectroscopic material obtained from 2015 to 2021 for the young Herbig Ae star HD 179218 is described and analyzed. The profiles of the H(beta) and He I (lambda 5876) lines and their variability on different time scales are studied. Analysis of the observation results showed that the profiles of the H(beta) line in the spectrum of HD 179218 have the shape of a single emission component with local absorption features superimposed on it, the radial velocities of which vary from the blue to the red edge of the emission. On certain dates, one can simultaneously observe several components persisting for at least three days. Over 17 observation dates, the H(beta) and He I (lambda 5876) line profiles looked like the inverse profile of P Cyg, the red boundary of which (v_{textrm{red}}) varied from date to date from (+235) to (+390) km s({}^{-1}). Analysis of variations in the parameter (v_{textrm{red}}) for these lines by the Lafleur–Kinman method showed the presence of a regular component with the period (P=1.341pm 0.002) days corresponding to the expected rotation period of the star. The detected variability of HD 179218 may be a sign of magnetospheric accretion. The object’s rotation axis inclination is determined, (i=23^{circ}pm 3^{circ}). The pattern of the observed variability of HD 179218 can be explained by the peculiarities of its magnetism.

Using a previously developed technique, a study of moderate-resolution optical spectra was carried out, and sets of parameters for the dwarf novae V355 UMa, V521 Peg, VY Aqr, and PQ And were obtained. Observations of the systems in late relaxation state after a burst or in a quiescent state were carried out in 2020 and 2021 with the 6-m BTA telescope of SAO RAS. Their analysis took into account the requirement for an optimum description of the observed absorption profiles of the Balmer series lines with the model spectra of white dwarfs with radiative and convective transfer. As a result, the atmospheric parameters of the primaries were determined for all the systems, and when they were subsequently compared with models of the internal structure of white dwarf and main-sequence stars, sets of fundamental parameters of the objects were also determined. It has been established that analysis of the spectra of dwarf novae containing moderately cool white dwarfs with (T_{textrm{eff}}=12000{-}15000) K provides better accuracy of their parameters than systems with hotter primaries. A simultaneous condition to be fulfilled for increasing accuracy is to carry out observations after the objects relax to a quiescent state, since the emission lines of the accretion disk significantly narrow the region of the analyzed spectrum. It has been found that the primary components of V355 UMa, V521 Peg, VY Aqr, and PQ And have masses within the range of (M_{1}=0.59{-}0.87) (M_{odot}), and their cool companions are red dwarfs with (M_{2}=0.11{-}0.13) (M_{odot}).

Cas 1 is a Local Volume (LV) dIr. In 2019 Tikhonov, using HST images and the Tip of Red Giant Branch (TRGB) method, derived its distance (D=1.61pm 0.1) Mpc and determined the stellar metallicity of Cas 1, (z=0.0004) (or (Z_{textrm{stars}}=Z_{odot}/50)). Such an eXtremely Metal-Poor (XMP) dwarf would be a nearby analogue of I Zw 18 and a valuable object for the follow-up detailed studies. Meanwhile, the distance to Cas 1 remains debatable. Its TRGB distance of (4.5pm 0.2) Mpc, based on the deeper HST images, was presented in the Extragalactic Distance Database (EDD). It places Cas 1 in the field between groups of IC 342 ((D=3.3) Mpc) and Maffei 1/Maffei 2 ((D=5.7) Mpc). To test the suggested XMP nature of Cas 1, we conduct the 6-m telescope (BTA, SAO RAS) spectroscopy to estimate O/H in its two H II regions. We also use these spectra to derive, via the observed Balmer decrements, the upper limit of the MW extinction in Cas 1. The derived values of (12+log(textrm{O/H})) are (7.83pm 0.1) and (7.58pm 0.1) dex. The measured Balmer decrements result in the upper limit MW extinction in this direction of (A_{B}=3overset{textrm{m}}{.}06pm 0overset{textrm{m}}{.}14). This is smaller by (0overset{textrm{m}}{.}63) than (A_{B}), used in the previous estimates of the distance to Cas 1. This reduces the original EDD distance to (4.1pm 0.36) Mpc. We use the LV galaxies relationship between their (12+log(textrm{O/H})) and blue absolute magnitude, (M_{B}), published by Berg et al. in 2012, to restrict (M_{B}) for Cas 1. The related distance estimate to Cas 1 is 1.64 Mpc with the 1 (sigma) uncertainty of a factor of 2.17. From obtained here (Z({textrm{gas}})sim Z_{odot}/10), we conclude that Cas 1 is not an XMP dwarf. Its newly derived distances are consistent with each other within 1 (sigma) errors, with the likely distance of about 3.73 Mpc. This distance favors Cas 1 to reside in the environs of IC 342, but not in the field.

In this paper, we study the fractal properties of the stellar medium in the solar vicinity based on the Gaia DR2 data for 200 000 stars of all spectral types at distances from 1 to 100 pc from the Sun. We consider the main stages of the development of ideas about the fractal structure in the stellar medium of galaxies. Analysis of the spatial distribution of stars is performed using by the ‘‘mass–radius’’ method. Numerical calculations show that the mean conditional local stellar density in spheres with the increasing radius (r) with the center in the (i)-th star is approximated by power laws, which confirms the de Vaucouleurs–Mandelbrot conclusions for fractal structures in gravitating media and indicates the presence of fractal structures in the stellar medium in the solar neighborhood with the fractal dimension (Dsimeq 2.41). The characteristics of the fractal properties of the medium, determined from a sample of 200 000 stars, are discussed and compared with an earlier result obtained for 13 000 F and G dwarfs in the solar neighborhood according to the Geneva–Copenhagen survey.