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Microlensing near macro-caustics is a complex phenomenon in which swarms of micro-images produced by micro-caustics form on both sides of a macro-critical curve. Recent discoveries of highly magnified images of individual stars in massive galaxy cluster lenses, predicted to be formed by these micro-image swarms, have stimulated studies on this topic. In this article, we explore microlensing near macro-caustics using both simulations and analytic calculations. We show that the mean total magnification of the micro-image swarms follows that of an extended source in the absence of microlensing. Micro-caustics join into a connected network in a region around the macro-critical line of a width proportional to the surface density of microlenses; within this region, the increase of the mean magnification toward the macro-caustic is driven by the increase of the number of micro-images rather than individual magnifications of micro-images. The maximum achievable magnification in micro-caustic crossings decreases with the mass fraction in microlenses. We conclude with a review of applications of this microlensing phenomenon, including limits to the fraction of dark matter in compact objects, and searches of Population III stars and dark matter subhalos. We argue that the discovered highly magnified stars at cosmological distances already imply that less than ∼ 10% of the dark matter may be in the form of compact objects with mass above ∼ 10 − 6 M ⊙ . [ABSTRACT FROM AUTHOR]

The kinematics of the ϵ Cha young stellar association close to the Sun has been studied based on a list of candidate stars from the Dickson-Vandervelde et al. work. The working sample consists of 26 stars with parallaxes, proper motions from the Gaia DR3 catalog and radial velocities taken from literary sources. The orbits of the stars back to the past were constructed, and the moment when the association had a minimum spatial size was determined, as well as an analysis of the dependencies of the velocities on the coordinates was carried out. It is shown that the initial sample is divided into two parts with different kinematic properties. The first sample included 9 stars. Based on the construction of the orbits of these 9 stars, an age estimate of million years was obtained. An expansion coefficient in the plane with the value km/s/kpc was also found for them, on the basis of which another age estimate Ma was obtained. The second sample included 17 stars. The construction of their orbits gave an estimate of age Ma, and based on the gradient 248 km/s/kpc, a second estimate of their age Ma was obtained. This suggests that the Cha association either consists of two groupings of different ages, or a younger one arose as a result of a recent outbreak of star formation within a common star system. The question of the gravitational connection of the groupings has not been considered in the framework of this work. [ABSTRACT FROM AUTHOR]

We measured the equivalent widths ( s) of interstellar Ti II absorption lines at 3383.759 for about 250 reddened objects and found a good correlation of the s with distances to the background stars, estimated using Gaia DR3 parallaxes. Hipparcos trigonometric parallaxes were used for very bright objects which were not observed by Gaia. The Ti II based distance estimation procedure is similar to the well known Ca II-method (Megier et al., 2005, 2009). However, there are at least 3 advantages of the Ti II method: in contrast to interstellar Ca II H and K lines, blending of Ti II with stellar lines is not an issue even for late B stars; Ti II is to a much less extent influenced by the saturation effect; only a single line must be measured, i.e. there is only a single measurement error to be taken into account. The relation of between Ti II and distance is given by simple equation , where is the distance in pc and is the equivalent width of the Ti II 3383.759 line in mÅ. [ABSTRACT FROM AUTHOR]

Based on Cepheids located near the solar circle, we have determined the Galactocentric distance of the Sun and the Galactic rotation velocity at the solar distance . For our analysis we used a sample of 200 classical Cepheids from the catalogue by Skowron et al. (2019), where the distances to them were determined from the period–luminosity relation. For these stars the proper motions and line-of-sight velocities were taken from the Gaia DR3 catalogue. The values of found lie within the range 7.8–8.3 kpc, depending on the heliocentric distance of the sample stars, on the adopted solar velocity relative to the local standard of rest, and on whether or not the perturbations caused by the Galactic spiral density wave are taken into account. The dispersion of the estimates is 2 kpc. Similarly, the values of lie within the range 240–270 km s with a dispersion of the estimates of 70–90 km s . We consider the following estimates to be the final ones: kpc and km s found by taking into account the perturbations from the Galactic spiral density wave. [ABSTRACT FROM AUTHOR]

We present the results from our deep optical photometric observations of Bochum 2 (Boc2) star cluster obtained using the 1.3-m Devasthal Fast Optical Telescope along with archival photometric data from Pan-STARRS2/2MASS/UKIDSS surveys. We also used high-quality parallax and proper motion data from the Gaia Data Release 3. We found that the Boc2 cluster has a small size (∼ 1.1 pc) and circular morphology. Using Gaia parallax of member stars and isochrone fitting method, the distance of this cluster is estimated as 3.8 ± 0.4 kpc. We have found that this cluster holds young ( ∼ 5 Myr) and massive (O7–O9) stars as well as an older population of low mass stars. We found that the massive stars were formed in the inner region of the Boc2 cluster in a recent epoch of star formation. We have derived mass function slope (Γ ) in the cluster region as - 2.42 ± 0.13 in the mass range of ∼ 0.72 < M / M ⊙ < 2.8 . The tidal radius of the Boc2 cluster (∼ 7–9) is much more than its observed radius ( ∼ 1.1 pc). This suggests that most of the low-mass stars in this cluster are the remains of an older population of stars formed via an earlier epoch of star formation. [ABSTRACT FROM AUTHOR]

We have solved the Ogorodnikov–Milne stellar-kinematics equations in the Galactic rectangular coordinate system based on the total velocities for a special sample of stars with radial velocities from the final Gaia Data Release 3 catalogue. We have found the region of applicability of the linear model and the regions that it describes poorly. We have constructed a second-order model that takes into account the peculiarities of stellar kinematics more accurately and showed its applicability for stars at distances up to 5 kpc. [ABSTRACT FROM AUTHOR]

Actively accreting young stellar objects show H α emission line in their spectra. We present the results of survey for H α emission-line stars in the star-forming region IC 5070 taken with 2-m Himalyan Chandra Telescope. Based on the H α slitless spectroscopy data, we identified 131 emission-line stars in ∼ 0.29 square degrees area of the IC 5070 region. Using Gaia early data release 3, we estimated the mean proper motion and parallax of the emission-line stars. We also estimated the mean distance and reddening toward the region using the emission-line stars, which are ∼ 833 pc and ∼ 2 mag, respectively. By examining the locations of these stars in the color–magnitude diagrams constructed using Gaia and PanSTARRS1 data, we found that a majority of the H α emitters are young low-mass ( < 1.5 M ⊙ ) stars. We also compared our catalog of emission-line stars with the available young stellar catalogs and found that most of them are class ii/flat spectrum sources with the spectral type ranging from K to M. Based on the proper motion/parallax values and locations on the color–magnitude diagrams, about 20 emission-line stars are flagged as non-members. The relative proper motion of the emission-line stars with respect to the ionizing source suggest the possibility of the 'rocket effect' scenario in the remnant cloud (BRC 31). [ABSTRACT FROM AUTHOR]

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens. [ABSTRACT FROM AUTHOR]

Based on published data, we have assembled a sample of 126 radio stars with the trigonometric parallaxes and proper motions measured by VLBI and available in the Gaia DR3 catalogue (in fact, Gaia EDR3). Our analysis of the Gaia–VLBI proper motion differences for 84 radio stars based on the model of solid-body mutual rotation has revealed no rotation components differing significantly from zero, mas yr . Based on the trigonometric parallax differences for 90 stars, we have obtained a new estimate of the systematic offset between the optical and radio frames, mas, and showed that the parallax scale factor is close to unity, . [ABSTRACT FROM AUTHOR]

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 and dust grain sizes . 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 /yr and µm. In the case of large dust grains, mm, the magnetic field is frozen into the gas and a dynamically strong magnetic field is generated at radial distances from the star AU, the tensions of which slow down the rotation speed by % 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, 10–7 /yr, the magnetic field is also dynamically strong in the inner region of the disk, 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 to 20 AU. [ABSTRACT FROM AUTHOR]

Using Beltrami–Poincaré models in the Euclidean semiplane and semispace for two-dimensional and three-dimensional Lobachevsky spaces, we give a simple deduction of some equations and inequalities by Lobachevsky from his first published work "On the foundations of geometry." Lobachevsky had applied the equations and inequalities together with the stellar parallaxes then available to him to the question whether his "theory of parallels holds or fails in nature." We also list some applications of Lobachevsky geometry. [ABSTRACT FROM AUTHOR]

The optical spectra of the B-supergiant LS III +52°24 (IRAS 22023+5249) obtained at the 6-m BTA telescope with a resolution in 2010–2021 revealed signs of wind variability and velocity stratification in the extended atmosphere. The neutral hydrogen lines H and H have a P Cyg profile; their wind absorption component changes position in the range from to km/s. The H emission intensity reaches record-high values with respect to the local continuum: . The stationary radial velocity according to the positions of symmetric forbidden emissions and pemitted metal emissions is taken as the systemic velocity km/s. For the first time for this star, a time variability of the radial velocity in the range from to km/s was found based on the positions of absorptions of NII and OII ions, which indicates the possible presence of a companion and/or atmospheric pulsations. The variability of the oxygen triplet profile (OI 7774) due to the appearance of unstable emission was recorded. The set of interstellar absorptions of the NaI D-line profile in the range from to km/s is formed in the Local Arm and subsequent arms of the Galaxy. The distance to the star, kpc, combined with the high velocity indicates that the star is located in the interarm space of the Galaxy beyond the Scutum–Crux arm. [ABSTRACT FROM AUTHOR]

An analysis is presented of the scientific research accomplished by Ukrainian astronomer Mykola Evdokymov, a specialist in the field of astrometry. The astronomer's main works, carried out using a Repsold meridian circle, are dedicated to determining stellar parallaxes, the positions of zodiacal and faint circumpolar stars, and the positions of large planets. At Kharkiv Astronomical Observatory, Evdokymov conducted systematic observations of the following objects and phenomena: solar and lunar eclipses, including as a member of the observatory's expeditions during the total solar eclipses of 1914 and 1936; comets (Halley, Delavan, Stearns, Pons–Winnecke); and meteor showers. He participated in determining the positions of reference stars for the asteroid (433) Eros. He conducted systematic studies of the meridian circle, developed new astronomical instruments, organized the functioning of a time service at the observatory, and carried out the determination of star declinations by measuring the sums and differences of the zenith distances of star pairs by the Sanders–Raymond method (using a meridian circle and a transit instrument). [ABSTRACT FROM AUTHOR]

We have analyzed the kinematics of 9750 OB2 stars with proper motions and parallaxes selected by Xu et al. from the Gaia EDR3 catalogue. The relative parallax errors for these stars do not exceed 10 . Based on the entire sample of stars, we have found the velocities km s and the components of the angular velocity of Galactic rotation: km s kpc , km s kpc , and km s kpc , where the linear rotation velocity of the Galaxy at the solar distance is km s for the adopted kpc. There are 1812 OB2 stars with measured line-of-sight velocities, and the space velocities and have been calculated from them. Based on a spectral analysis independently for the radial and residual tangential velocities, we have obtained the following estimates: km s , km s , kpc, kpc, , and for the adopted four-armed () spiral pattern. Thus, both velocity perturbation amplitudes are nonzero at a high significance level. [ABSTRACT FROM AUTHOR]

An analytical expression for the boresight rotation accuracy of a star sensor is derived. This error is computed as a function of the distance of the star measurement on the detector to its centre. The note also obtains the ratio between the cross-axis error and the boresight rotation error. The analytical findings are seen to be in agreement with the experimental findings in the literature. Numerical experiments illustrate the rotation accuracies for star sensors with different fields of view. [ABSTRACT FROM AUTHOR]

The distribution of visual interstellar extinction A V has been mapped in selected areas over the Northern sky, using available LAMOST DR5 and Gaia DR2/EDR3 data. A V was modelled as a barometric function of galactic latitude and distance. The function parameters were then approximated by spherical harmonics. The resulting analytical tridimensional model of the interstellar extinction can be used to predict A V values for stars with known parallaxes, as well as the total Galactic extinction in a given location in the sky. [ABSTRACT FROM AUTHOR]

Direct exoplanet imaging with a stellar coronagraph increases the contrast of the image in the diffraction vicinity of the star. Insufficient reduction of the stellar background is due to the required quality of the wavefront, which exceeds the capabilities of modern optical and astronomical systems. This study evaluates the achievable coronagraphic contrasts for ground-based and extra-atmospheric telescopes without the use of adaptive optics (AO); further, it is planned to use AO devices: tip-tilt mirror and pixel-based AO for the correction of the zonal error. For practical testing of a prospective multicircuit telescope system with active wavefront-correcting optics, we constructed a coronagraph with a small telescope aperture, which makes it possible to observe the effect of coronagraphic attenuation of a star. The latter is important for the testing of precision guidance of the star by the automated parallax drive of the telescope to construct a practical sequence of the operation of adaptive optics devices. The errors of the wavefront inside the interference coronagraph were estimated, experiments were commenced, and line of sight retention was obtained. [ABSTRACT FROM AUTHOR]

Abstract—Positional and kinematic parameters of hypergiants HD 168607 B8.5 Ia-0, HD 168625 B5.0 Ia-0, Schulte 12 B4.0 Ia-0, and 6 Cas A A2.5 Ia-0 belonging to OB associations do not indicate that they are ejected from their formation regions in molecular clouds, and the field hypergiant HD 183143 B7 Ia-0 located in the gap between spiral arms does not show anomalously high velocity relative to the stellar and gas neighborhood. We give two examples of the refinement of the distance to stars from the interstellar line profiles in their spectra. [ABSTRACT FROM AUTHOR]

In this present paper, a slowly rotating stat is investigated in shift symmetric scalar torsion theory framework using a nondiagonal tetrad that gives an axially symmetric spacetime. We present the general equations for a general Lagrangian in a spherical symmetric space time and then in an axially symmetric spacetime. The obtained equations will allow us to study the behaviour of a specific model at the center of the star and at large distance. We find that this particular model affects the behaviour at the center but it is not case for large value of the radial coordinate r. The integration of the equations of motion, for different realistic equations of state (EoS), confirms that the mass, the radius as well as the moment of inertia are effected by varying the parameters of the model. Finally, we examine the universal relation of normalized moment of inertia and the stellar compactness of neutron star in slow rotation approximation. We showed that for all values of parameters present in the model leads to a deviation from GR for all EoS with a relative deviation below 10 % . [ABSTRACT FROM AUTHOR]

We have performed Monte Carlo simulations of the trajectories of several runaway stars using their parallaxes and proper motions from the Gaia EDR3 catalogue. We have confirmed the hypothesis that the stars AE Aur and Col are a product of the multiple system breakup Myr ago and the Orion Trapezium may be the parent cluster for this pair of stars. We show that the data from the Gaia EDR3 catalogue for the star Ori, mainly the parallax, do not allow us to talk about the breakup of the multiple system of AE Aur, Col, and Ori. The existence of close pair encounters between the stars HD 30112 and HD 43112 Myr ago has been confirmed. Close triple encounters confirm the hypothesis that the stars HD 30112 and HD 43112 escaped from the parent cluster Col 69. We show that the stars HIP 28133 and TYC 5368-1541-1 have a nonzero probability of escape from the region within 10 pc of the center of the Orion Trapezium cluster and a fairly high probability (about 8 ) that they were both at distances less than 20 pc from the center of the Orion Trapezium Myr ago. It has been established for the first time that the stars Gaia EDR3 3021115184676332288 and Gaia EDR3 2983790269606043648 have a probability of about 0.5 that they broke up as a binary system Myr ago. The star Gaia EDR3 3021115184676332288 has a probability of about 16 that it escaped from the region within 10 pc of the center of the Orion Trapezium cluster Myr ago. [ABSTRACT FROM AUTHOR]

The effects of total ionizing dose (TID) radiation from 60Co gamma-rays on an 8-transistor global shutter exposure complementary metal-oxide semiconductor image sensor (CIS) within a star sensor is presented to analyze the sources of star sensor performance degradation and the decrease of attitude measurement accuracy. The dark current, dark signal non-uniformity, and photon response non-uniformity versus the TID are investigated. The signal-to-noise ratio, star diagonal distance accuracy, and star point centroid positioning accuracy of the star sensor versus the TID are also analyzed. By establishing the correlation between space radiation, CIS noise, and star sensor performance parameters, the transfer mechanism of CIS parameter degradation to star sensor parameter degradation is revealed. [ABSTRACT FROM AUTHOR]

We have studied the kinematic properties of young pre-main-sequence stars. We have selected these stars based on data from the Gaia DR2 catalogue by invoking a number of photometric infrared surveys. Using 4564 stars with parallax errors less than 20%, we have found the following parameters of the angular velocity of Galactic rotation: km s–1 kpc–1, km s–1 kpc–2, and km s–1 kpc–3, where the Oort constants are km s–1 kpc–1 and km s–1 kpc–1. The circular rotation velocity of the solar neighborhood around the Galactic center is km s–1 for the adopted Galactocentric distance of the Sun kpc. The residual velocity dispersion for the stars considered is shown to be low, suggesting that they are extremely young. The residual velocity dispersion averaged over three coordinates is ~11 km s–1 for Herbig Ae/Be stars and ~7 km s–1 for T Tauri stars. [ABSTRACT FROM AUTHOR]

Evidence of wind variability and velocity stratification in the extended atmosphere has been found in the spectra of the supergiant V340 Ser ( IRAS 17279 1119) taken at the 6-m BTA telescope with a resolution . The H line has a P Cyg profile whose absorption component ( km s ) is formed in the upper layers of the expanding atmosphere close the circumstellar medium. For four dates the mean velocity has been derived from the positions of 300–550 symmetric metal absorptions with an accuracy better than km s : , , , and km s . A lot of low-excitation metal lines have an inverse P Cyg profile. The mean positions of their emission components, km s , differ systematically from the velocity inferred from symmetric absorptions, suggesting the presence of a velocity gradient in the supergiant's extended atmosphere. The multicomponent profile of the Na I D lines contains the interstellar, km s , and circumstellar, km s , components and the component forming in the upper atmospheric layers, km s . The mean velocity from 20–30 diffuse interstellar bands (DIBs) identified in the spectra, km s , agrees with the velocity from interstellar Na I and K I components. The equivalent width of the oxygen triplet Å corresponds to an absolute magnitude of the star , which, given the total (interstellar circumstellar) extinction, leads to a distance to the star kpc. [ABSTRACT FROM AUTHOR]

We compare the spatial stellar color variations with our three-dimensional analytical model of the spatial dust distribution to refine the properties of the dust layer in Galactic solar neighborhoods. We use a complete sample of 93 992 clump giants with a small admixture of branch giants from the Gaia DR2 catalogue in a spatial cylinder with a radius of 700 pc around the Sun extending to |Z| = 1800 pc along the Galactic Z axis. Accurate parallaxes and photometry of these stars in the Gaia DR2 GRP and WISE W3 bands have allowed the spatial GRP - W3 color variations to be used to calculate the model parameters and two characteristics of the sample, the mode of the dereddened color (GRP - W3)0 of the giant clump and the linear change of this mode with coordinate |Z|. As a result, an improved version of the three-dimensional model first proposed by Gontcharov (2009b) has been obtained. As in the previous version, the model suggests two dust layers, along the Galactic equator and in the Gould Belt, that intersect near the Sun at an angle of 18° ± 2°. In contrast to the previous version of the model with a midplane of the Gould Belt dust layer in the form of a circle with the center at the Sun, in the new version this midplane is an ellipse decentered relative to the Sun. A scale height of 170 ± 40 pc has been found for both dust layers. A rather large reddening E(GRP - W3) = 0.16 ± 0.02 through half of the Galactic dust layer above or below the Sun has been found for giants far from the Galactic plane (|Z| > 600 pc). This can be explained by a possible difference between the extinction law far from the Galactic plane and the commonly adopted law by Cardelli et al. (1989) with RV = 3.1. The modes of the absolute magnitude MW3 = -1.70 ± 0.02 and the dereddened color (GRP - W3)0 = (1.43 ± 0.01) - (0.020 ± 0.007)|Z|, where Z is expressed in kpc, have been calculated for the giant clump near the Sun. These estimates are consistent with the estimates from the theoretical PARSEC and MIST isochrones for a sample dominated by giants with an age of 2 Gyr and metallicity [Fe/H] = -0.1 in agreement with the TRILEGAL stellar population model. The dispersions of the quantities under consideration have allowed the natural small-scale density fluctuations of the dust medium relative to the mean reddening calculated from the model to be characterized. These fluctuations make a major contribution to the uncertainty in the reddening. Because of them, the reddening of a specific star can differ from the model reddening by a random value that decreases from 80 to <20% of the model reddening when passing from low latitudes far from the Sun to the remaining space. [ABSTRACT FROM AUTHOR]

The paper is devoted to determining the parameters of the three-dimensional Ogorodnikov—Milne kinematic model based on the Gaia DR2 star catalogue (the subset of seven million stars containing radial velocities). The behavior of the parameters as a function of the distance to stars is studied. The parameters derived from the stellar proper motions and radial velocities agree well with one another. It is noted that the angular velocity of rotation retains its stability up to distances of ~3 kpc, while the parameter responsible for the translational motion along the Y axis abruptly begins to change from distances starting from 1 kpc. [ABSTRACT FROM AUTHOR]

V CVn is a red semiregular variable star with an amplitude of its V-band brightness variations of ≈2m. An unusually high amplitude of its polarization variability, up to 6%, a noticeable inverse correlation between polarization and total flux, and relative constancy of the angle of polarization distinguish this star from other semiregular variables. To clarify the nature of these peculiarities, we have observed the object with the 2.5-m telescope at the Caucasian Observatory of the Sternberg Astronomical Institute of the Moscow State University using differential speckle polarimetry at wavelengths of 550, 625, and 880 nm. The observations were performed on 20 dates distributed over three pulsation periods. We have detected an asymmetric reflection nebula around the star at a distance of ≈35 mas. Three regions that change their brightness with the same characteristic time scale as the star, but with different phase shifts are identified in the nebula. We consider several hypotheses that could explain this behavior. [ABSTRACT FROM AUTHOR]

Under certain conditions, stars close to intermediate-mass black holes (IMBHs) can form close binary systems with these objects, in which the Roche lobe can be filled by the star and intense accretion of the star's matter onto the IMBH is possible. Recently, accreting IMBHs have been associated with hyperluminous X-ray sources (HLXs), whose X-ray luminosities can exceed 1041 erg/s. In this paper, the evolution of star—IMBH binary systems is investigated assuming that the IMBH mainly accretes the matter of its companion star, and that the presence of gas in the vicinity of the IMBH does not appreciably affect changes in the orbit of the star. The computations take into account all processes determining the evolution of ordinary binary systems, as well as the irradiation of a star by hard radiation during the accretion of its matter onto the IMBH. The absorption of external radiation in the stellar envelope was calculated applying the same formalism that is used to calculate the opacity of the stellar matter. The computations also assumed that, if the characteristic time for the mass transfer is less than the thermal time scale of the star, there is no exchange betwween the orbital angular momentum of the system and the angular momentum of the matter flowing onto the IMBH. Numerical simulations have shown that, under these assumptions, three types of evolution are possible for such a binary system, depending on the mass of the IMBH and the star, as well as on the star's initial distance from the IMBH. The first type ends with the destruction of the star. For low-mass main sequence (MS) stars, only this option is realized, even in the case of large initial distances from IMBH. For massive MS stars, the star is also destroyed if the mass of the IMBH is high and the initial distance of the star from the IMBH is sufficiently small. The second type of evolution can occur for massive MS stars, which are initially located farther from the IMBH than in the first type of evolution. In this case, the massive star fills its Roche lobe during its evolutionary expansion, after which a stage of intense mass transfer begins. It is in this phase of the evolution that the star- IMBH system can manifest itself as a HLX, when its X-ray luminosity LX exceeds 1041 erg/s for a fairly long time. Numerical simulations show that the initial mass of the donor star in systems with MBH = (103−105)M⊙ must be close to ∼10 M⊙ in this case. The characteristic duration of the HLX stage is 30 000–70 000 years. For smaller initial donor masses close to ∼5M⊙, LX does not reach 1041 erg/s in the stage of intense mass transfer, but can exceed 1040 erg/s. The duration of this stage of evolution is 300 000–800 000 years. A characteristic feature of this second type of evolution is an increase in the orbital period of the system over time. As a result, after a period of intense mass loss, the star "retreats" inside the Roche lobe. A remnant of the star in the form of a white dwarf is left behind, and can end up fairly far from the IMBH. The third type of evolution can occur for massive MS stars that are initially even farther from the IMBH, as well as for massive stars that are already evolved at the initial time. In this case, conservative mass exchange in the presence of intense stellar wind leads to the star moving away from the IMBH, without filling its Roche lobe at all. For massive stars with sufficiently strong stellar winds (for example, stars with masses ∼50M⊙), the accretion rate of matter onto the IMBH in this case can reach values that are characteristic of HLXs. As in the case of the second type of evolution, the stellar remnant can remain at a fairly large distance from the IMBH. [ABSTRACT FROM AUTHOR]

The systematic differences between the trigonometric parallaxes of the Hipparcos and TGAS catalogues have been investigated using spherical harmonics. The most significant harmonics in the expansion have been determined. The distribution of the parallax difference dispersion in various regions of the celestial sphere has also been studied. The distribution of the rms deviation has the simplest form in the ecliptic coordinate system. [ABSTRACT FROM AUTHOR]

We take a fresh look at the determination of distances and velocities of neutron stars. The conversion of a parallax measurement into a distance, or distance probability distribution, has led to a debate quite similar to the one involving Cepheids, centering on the question whether priors can be used when discussing a single system. With the example of PSR J0218+4232, we show that a prior is necessary to determine the probability distribution for the distance. The distance of this pulsar implies a gamma-ray luminosity larger than 10% of its spindown luminosity. For velocities, the debate is whether a single Maxwellian describes the distribution for young pulsars. By limiting our discussion to accurate (VLBI) measurements we argue that a description with two Maxwellians, with distribution parameters $$\sigma _1=77$$ and $$\sigma _2=320$$ km/s, is significantly better. Corrections for galactic rotation, to derive velocities with respect to the local standards of rest, are insignificant. [ABSTRACT FROM AUTHOR]

Based on archival Hubble Space Telescope (HST) ACS/WFC images, we have performed stellar photometry for eight fields of the spiral galaxy NGC 1313 and its satellite, the low-mass Sph/Irr galaxy AM0319-662. Stars of various ages have been identified on the constructed Hertzsprung-Russell diagrams: young supergiants, middle-aged stars, and old stars (red giants); their apparent distributions over the body of the galaxy are presented. The red supergiants and giants have been divided into groups with larger and smaller color indices, corresponding to a difference in stellar metallicity. These groups of stars are shown to have different spatial distributions and to belong to two galaxies, NGC1313 itself and the disrupted satellite. We have determined the distance to NGC 1313, D = 3.88 ± 0.07 Mpc, by the TRGB method from six fields. Our photometry of 2014 HST images has revealed an emerged charge transfer inefficiency on the ACS/WFC CCDs, which manifests itself as a dependence of the photometry of stars on their coordinates on the CCD. [ABSTRACT FROM AUTHOR]

Photometry from the Tycho-2, 2MASS, andWISE catalogues for clump and branch giants at a distance up to 25 kpc toward the Galactic poles has allowed the variations of various characteristics of the infrared interstellar extinction law with distance to be analyzed. The results obtained by the extinction law extrapolation method are consistent for different classes of stars and different characteristics as well as with previous studies. The conventional extinction law with a low infrared extinction is characteristic of only a thin layer no farther than 100 pc from the Galactic plane and of two thin layers near Z = −600 and +500 pc. Far from the Galactic plane, in the Galactic halo, the infrared extinction law is different: the extinction in the Ks, W1, W2, W3, and W4 bands is, respectively, 0.17, 0.16, 0.16, 0.07, and 0.03 of the extinction in the V band. The accuracy of these coefficients is 0.03. If the extinction law reflects primarily the grain size distribution, then the fraction of large dust grains far from the Galactic plane is greater than that in the circumsolar interstellar medium. [ABSTRACT FROM AUTHOR]

Super Li-rich stars form a very small and enigmatic group whose existence cannot be explained in terms of the standard stellar evolution theory. The goal of our study is to check the reality of this group of cool giants based on an independent technique. We have carried out such a check using the K giant HD 77361 (HR 3597), which has previously been assigned to this rare type, as an example. We have redetermined the effective temperature T and surface gravity log g for this star. We have applied two different methods, photometric and spectroscopic, to estimate T (the accuracy of the Li-abundance determination depends significantly on this parameter). The value of log g has been found from the highly accurate parallax of this nearby star. To apply the photometric method of determining T, we have performed U BV observations of the star, which yielded V = 6.18 ± 0.03, B − V = 1.13 ± 0.01, and U − B = 1.18±0.05. The following parameters of the star have been found: effective temperature T = 4370± 100 K, surface gravity log g = 2.30 ± 0.10, iron abundance log ϵ(Fe) = 7.49 ± 0.14, microturbulence V = 1.1 ± 0.2 km s, rotational velocity V sin i = 4.5 km s, and mass M = 1.3 ± 0.2 M The lithium abundance has been determined from a non-LTE analysis of three Li I lines: the resonance line at 6707.8 Å and the subordinate lines at 6103.6 and 8126.4 Å (the latter in a blend with a CN molecular line). We have found a high lithium abundance, log ϵ(Li) = 3.75 ± 0.11, which exceeds considerably the initial abundance log ϵ(Li) = 3.2 ± 0.1 for young stars in the solar neighborhood. Thus, we have confirmed that the K giant HD 77361 actually belongs to the type of super Li-rich stars. It is noted that a high lithium abundance in such cool giants is inconsistent with predictions of the standard stellar evolution theory and may suggest a recent synthesis of lithium in these stars. [ABSTRACT FROM AUTHOR]

Based on observations performed with the Pulkovo normal astrograph in 2008-2015 and data from sky surveys (DSS, 2MASS, SDSS DR12, WISE), we have investigated the motions of 1308 stars with proper motions larger than 300 mas yr down to magnitude 17. The main idea of our search for binary stars based on this material is reduced to comparing the quasi-mean (POSS2-POSS1; an epoch difference of ≈50 yr) and quasi-instantaneous (2МASS, SDSS, WISE, Pulkovo; an epoch difference of ≈10 yr) proper motions. If the difference is statistically significant compared to the proper motion errors, then the object may be considered as a Δ μ-binary candidate. One hundred and twenty one stars from among those included in the observational program satisfy this requirement. Additional confirmations of binarity for a number of stars have been obtained by comparing the calculated proper motions with the data from several programs of stellar trigonometric parallax determinations and by analyzing the asymmetry of stellar images on sky-survey CCD frames. Analysis of the highly accurate SDSS photometric data for four stars (J0656+3827, J0838+3940, J1229+5332, J2330+4639) allows us to reach a conclusion about the probability that these Δ μ binaries are white dwarf +Mdwarf pairs. [ABSTRACT FROM AUTHOR]

The radial velocities of the 21-cm absorption lines in the spectra of the pulsars B1937+21, J0332+5434, J0738-4042, J0835-4510, J1559-4438, J1645-0317, J1752-2806, J1825-0935, and J2157+4017 have been measured. Distance estimates have been obtained for six of these objects. The observations were conducted on the 34-m radio telescope of the Kashima Space TechnologyCenter (Japan) and the Giant Metrewave Radio Telescope (GMRT) in India. The data are in a good agreement with the results of other studies. [ABSTRACT FROM AUTHOR]

Based on archival Hubble Space Telescope images, we have performed stellar photometry of several fields in the isolated spiral galaxies NGC 6503 and NGC 6946 with high peculiar velocities. Based on the TRGB method, we have determined the distances to the galaxies: D = 6.30 ± 0.10 Mpc for NGC 6503 and D = 6.72 ± 0.15 Mpc for NGC 6946. The current stellar content of the galaxies does not differ from that of other similar galaxies. The metallicity for young stars in NGC 6503 is Z = 0.02 (corresponding to the solar metallicity), while the metallicity for stars in NGC 6946 reaches Z = 0.05. Very few old globular clusters have been found in NGC 6946, while they have not been found at all in NGC 6503. The number density distribution of stars with different ages in NGC 6503 does not differ from the analogous distributions in other galaxies. The large sizes of the thick disk in NGC 6503, which is clearly seen up to 6 kpc from the galactic disk plane and whose possible extension is noticeable up to 8.6 kpc from the plane, are a difference. The sizes of the region occupied by red giants of the disk in NGC 6503 are 51 × 17 kpc, which are not much larger than the sizes of this galaxy from H I radio observations. [ABSTRACT FROM AUTHOR]

From the XHIP catalogue, we have selected 1872 F-G-K stars with relative parallax measurement errors <20% and absolute values of their space velocities relative to the Sun <15 km s. For all these stars, we have constructed their Galactic orbits for 4.5 Gyr into the past using an axisymmetric Galactic potential model with allowance made for the perturbations from the spiral density wave. Parameters of the encounter with the solar orbit have been calculated for each orbit. We have detected three new stars whose Galactic orbits were close to the solar one during a long time interval in the past. These stars are HIP 43852, HIP 104047, and HIP 112158. The spectroscopic binary HIP 112158 is poorly suited for the role of a kinematic sibling of the Sun by its age and spectroscopic characteristics. For the single star HIP 43852 and the multiple system HIP 104047, this role is quite possible. We have also confirmed the status of our previously found candidates for close encounters, HIP 47399 and HIP 87382. The star HIP 87382 with a chemical composition very close to the solar one is currently the most likely candidate, because it persistently shows close encounters with the Sun on time scales of more than 3 Gyr when using various Galactic potential models both without and with allowance made for the influence of the spiral density wave. [ABSTRACT FROM AUTHOR]

A method for determining the velocity field parameters free from the distortions due to the systematic variations of stellar parallaxes over the celestial sphere is proposed. The method is based on the approximation of parallaxes as a function of coordinates on the sphere using spherical harmonics and can be applied in those cases where the solar motion cannot be eliminated from the stellar proper motions. Numerical experiments have shown that our method is able to obtain accurate coordinates of the solar apex and to calculate the kinematic parameters of the Ogorodnikov-Milne model to within three coefficients of the decomposition of parallaxes into first-order spherical harmonics. Examples of applying the method to the stellar proper motions of the Hipparcos catalogue, which admits checking the results using trigonometric parallaxes, are provided. Such a check has been found to yield a positive result only for nearby stars at heliocentric distances that do not exceed 400 pc and for which the parallaxes were determined with a relative error of at least 30%. An interesting feature of this method is the possibility to construct the shape of the figure which is formed by the deviations of the parallaxes from the sphere corresponding to the average parallaxes of the stars under consideration. It should be specially emphasized that all of this is done in the complete absence of information about the stellar parallaxes. The 'solar terms' of the stellar proper motions that are formed by the products of the parallaxes by the solar motion components relative to the centroid of stars are the main source of information about the parallaxes here. [ABSTRACT FROM AUTHOR]

The Galactic orbits of 27 440 stars of all classes with accurate coordinates and parallaxes of more than 3 mas from the Hipparcos catalogue, proper motions from the Tycho-2 catalogue, and radial velocities from the Pulkovo Compilation of Radial Velocities (PCRV) are analyzed. The sample obtained is much more representative than the Geneva-Copenhagen survey and other studies of Galactic orbits in the solar neighborhood. An estimation of the influence of systematic errors in the velocities on orbital parameters shows that the errors of the proper motions due to the duplicity of stars are tangible only in the statistics of orbital parameters for very small samples, while the errors of the radial velocities are noticeable in the statistics of orbital parameters for halo stars. Therefore, previous studies of halo orbits may be erroneous. The distribution of stars in selection-free regions of the multidimensional space of orbital parameters, dereddened colors, and absolute magnitudes is considered. Owing to the large number of stars and the high accuracy of PCRV radial velocities, nonuniformities of this distribution (apart from the well-known dynamical streams) have been found. Stars with their peri- and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the halo, and perigalacticons in the disk and apogalacticons in the halo have been identified. Thus, the bulge and the halo are inhomogeneous structures, each consisting of at least two populations. The radius of the bulge has been determined: 2 kpc. [ABSTRACT FROM AUTHOR]

We combined the ( K, J− K) data in Laney et al. (Mon. Not. R. Astron. Soc. 419:1637, ) with the V apparent magnitudes and trigonometric parallaxes taken from the Hipparcos catalogue and used them to fit the $M_{K_{s}}$ absolute magnitude to a linear polynomial in terms of V− K colour. The mean and standard deviation of the absolute magnitude residuals, −0.001 and 0.195 mag, respectively, estimated for 224 red clump stars in Laney et al. () are (absolutely) smaller than the corresponding ones estimated by the procedure which adopts a mean $M_{K_{s}}=-1.613~\mbox{mag}$ absolute magnitude for all red clump stars, −0.053 and 0.218 mag, respectively. The statistics estimated by applying the linear equation to the data of 282 red clump stars in Alves (Astrophys. J. 539:732, ) are larger, $\Delta M_{K_{s}}=0.209$ and σ=0.524 mag, which can be explained by a different absolute magnitude trend, i.e. condensation along a horizontal distribution. [ABSTRACT FROM AUTHOR]

The Ultraviolet Imaging Telescope (UVIT) is scheduled to be launched as a part of the ASTROSAT satellite. As part of the mission planning for the instrument we have studied the efficacy of UVIT observations for interstellar extinction measurements. We find that in the best case scenario, the UVIT can measure the reddening to an accuracy of about 0.02 magnitudes, which combined with the derived distances to the stars, will enable us to model the three-dimensional distribution of extinction in our Galaxy. The knowledge of the distribution of the ISM will then be used to study distant objects, affected by it. This work points the way to further refining the UVIT mission plan to best satisfy different science studies. [ABSTRACT FROM AUTHOR]

Outward flows of galaxies are observed around groups of galaxies on spatial scales of about 1 Mpc, and around galaxy clusters on scales of 10 Mpc. Using recent data from the Hubble Space Telescope (HST), we have constructed two synthetic velocity-distance phase diagrams: one for four flows on galaxy-group scales and the other for two flows on cluster scales. It has been shown that, in both cases, the antigravity produced by the cosmic dark-energy background is stronger than the gravity produced by the matter in the outflow volume. The antigravity accelerates the flows and introduces a phase attractor that is common to all scales, corresponding to a linear velocity-distance relation (the local Hubble law). As a result, the bundle of outflow trajectories mostly follow the trajectory of the attractor. A comparison of the two diagrams reveals the universal self-similar nature of the outflows: their gross phase structure in dimensionless variables is essentially independent of their physical spatial scales, which differ by approximately a factor of 10 in the two diagrams. [ABSTRACT FROM AUTHOR]

It is shown that the observed width of the emission profile W and the maximum derivative C of the polarization position angle for the mean profile of a pulsar can be used to calculate the ratio n of the emission-cone radius θ to the minimum distance between the line of sight and the center of this cone fairly accurately. Estimates of n obtained earlier by eye based on the shape of the emission profiles are close to these more accurate values for pulsars from a catalog at a frequency near 1 GHz. Values of n are calculated for several dozen pulsars using data at 10 and 20 cm. In the standard model, the ratio of n at two frequencies is equal to the ratio of the squares of the distances from the center of the neutron star to the emission levels at the two frequencies. Statistical dependences of the profile width on the pulsar period for these wavelengths and a model assuming emission at the local plasma frequency are used to determine the absolute values of these distances. These estimates display good consistency and yield distances to the emission levels of the order of several tens of neutron-star radii. The calculations take into account possible variation of the dimensions of the polar cap associated with the inclination of the emission cone to the rotational axis of the pulsar; i.e., the influence of the angle β between the magnetic moment and rotational axis of the neutron star. Values of β calculated earlier for the pulsar sample considered are used for this analysis. [ABSTRACT FROM AUTHOR]

Amethod for determining the most probable spectral types, color excesses E, and distances of stars from multicolor photometry is described. The main idea of the method is modeling the photometric data using various models for the stellar spectra and the interstellar extinction law, and applying the maximum likelihood method. The reliability of the method is estimated using stars with known spectral types and WBVR photometry, based on the empirical library of stellar spectra of Pickles and the model for the interstellar extinction law developed by Fluks et al. [ABSTRACT FROM AUTHOR]

The analysis of deep imaging data of bright elliptical galaxies is capable of providing fundamental information on the stellar content and the distance of the target, via the Surface Brightness Fluctuation method (SBF hereafter). Here, I present the study of the properties of two ellipticals in the Virgo cluster, NGC 4621 and NGC 4374, based on deep BVR imaging data obtained with the FORS2 camera at the VLT. The V and R SBF measures are used to get accurate distances of the targets. In addition, since both galaxies hosted type Ia Supernova events, I also compare the new SBF distances with the ones based on SNe Ia lightcurve analysis. In all cases SBF distances agree well with distances obtained by other distance indicators. For what concerns stellar population analysis, I present a detailed comparison between SBF data and models to constrain the properties of the dominant stellar components. Among the other results, the measures presented here seem to suggest that the B-band SBF can be reconciled to model predictions only through the presence of a percentage of hot-HB stars 'polluting' the old and metal rich dominant stellar population. [ABSTRACT FROM AUTHOR]

RR Lyrae variables play an important role in several fields of stellar evolution and cosmology, moreover, they are the most popular primary standard candles for Population II stellar systems. In this paper, their properties are examined within the framework of modern computations of stellar evolution and pulsation models in order to discuss their use as distance indicators. The comparison between selected observed data and predicted relations, for the determination of both apparent and intrinsic distance moduli, is also presented. [ABSTRACT FROM AUTHOR]

AGB variables, particularly the large amplitude Mira type, are a vital step on the distance scale ladder. They will prove particularly important in the era of space telescopes and extremely large ground-based telescopes with adaptive optics, which will be optimized for infrared observing. Our current understanding of the distances to these stars is reviewed with particular emphasis on improvements that came from Hipparcos as well as on recent work on Local Group galaxies. In addition to providing the essential calibration for extragalactic distances Gaia may also provide unprecedented insight into the poorly understood mass-loss process itself. [ABSTRACT FROM AUTHOR]

Factors contributing to the scatter around the ridge-line period-luminosity relationship are listed, followed by a discussion how to eliminate the adverse effects of these factors (mode of pulsation, crossing number, temperature range, reddening, binarity, metallicity, non-linearity of the relationship, blending), in order to reduce the dispersion of the P- L relationship. [ABSTRACT FROM AUTHOR]

The ESA Gaia mission will bring a new era to the domain of standard candles. Progresses in this domain will be achieved thanks to unprecedented astrometric precision, whole-sky coverage and the combination of photometric, spectrophotometric and spectroscopic measurements. The fundamental outcome of the mission will be the Gaia catalogue produced by the Gaia Data Analysis and Processing Consortium (DPAC), which will contain a variable source classification and specific properties for stars of specific variability types. We review what will be produced for Cepheids, RR Lyrae, Long Period Variable stars and eclipsing binaries. [ABSTRACT FROM AUTHOR]

The article focuses on astronomical discoveries particularly on the measures of interstellar distance and the origin of the Universe. It highlights the contributions of several notable people in the field of astronomy including Edwin Powell Hubble, Friederich Bessel and Henry Norris Russell. It also says that the use of parallax in measuring the distances of stars and other heavenly bodies is a breakthrough in the field. Moreover, it provides details regarding the Big Bang Theory.

In the last decade or so, there have been numerous searches for hot subdwarfs in close binaries. There has been little to no attention paid to wide binaries, however. The advantages of understanding these systems can be many. The stars can be assumed to be coeval, which means they have common properties. The distance and metallicity, for example, are both unknown for the subdwarf component, but may be determinable for the secondary, allowing other properties of the subdwarf to be estimated. With this in mind, we have started a search for common proper motion pairs containing a hot subdwarf component. We have uncovered several promising candidate systems, which are presented here. [ABSTRACT FROM AUTHOR]