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A near infrared and optical photometric study of Herbig star V517 Cyg has been carried out. The infrared data were obtained in 2003–2017 at the Campo Imperatore Observatory (Italy) with the Pulkovo AZT-24 telescope in the Johnson JHK bands. The optical light curves in the Johnson UBVR bands were obtained at the Maidanak observatory. Additional optical photometry from different sources (ASAS, WISE, and AAVSO) was also used. It has been shown that the behavior of V517 Cyg in the near infrared is typical for UX Ori stars. A considerable contribution to the near infrared variability is made by changes in the extinction along the line of sight; however, in the H and K bands, the influence of the disk is strong: there is a significant correlation between V and J magnitudes (r ∼ 0.84), while H and K magnitudes poorly correlate with V magnitude. The amplitude of variability in the J band is quite large (∼1.8m). In the deepest minimum in V band ( ∼ 3.6m) the star demonstrates quasi-periodic variations with an amplitude of ∼0.8m and a period of ∼19d (plain).]; and their origin is still unclear. It is possible that these oscillations are caused by the presence of a companion, a cold T Tau star, and related to its rotation period. The spectrum of V517 Cyg reveals typical for UX Ori stars the double-peaked emission line Hα. The NaI D lines have the inverse P Cyg profiles, which is indicative of intensive gas accretion onto the star. The accretion rate determined from the equivalent width of the Hα line is = 3.6 × 10–8 per year. [ABSTRACT FROM AUTHOR]

Data were collected on 83 young double-lined spectroscopic binary stars, among them—Herbig Ae/Be stars, T Tauri stars and red dwarfs. Distributions of young spectroscopic binary stars by component mass ratios and eccentricities, an eccentricity–period diagram have been constructed, and their analysis has been carried out. The vast majority of short-period systems with have eccentricities close to zero. In this group, there are practically no young stars with an age million years. The mass ratio distribution of systems with has a pronounced maximum in the interval . Young spectroscopic binary stars with have a flatter distribution by mass ratio, about 12% of them have . The article is based on a talk presented at the astrophysical memorial seminar "Novelties in Understanding the Evolution of Binary Stars", dedicated to the 90th anniversary of Professor M.A. Svechnikov. [ABSTRACT FROM AUTHOR]

Two new Herbig-Haro flows were found in a study of the isolated Dobashi 5006 dark cloud (l = 216°.7, b = –13°.9): one certain (HH 1179) and one presumable, associated with the infrared sources 2MASS 06082284-0936139 and 2MASS 06081525-0933490, correspondingly. Judging from their spectral energy distributions, these sources may be Class 1 objects with luminosities of order 23 L⨀ and 3.6 L⨀, respectively. They are part of the poor star cluster MWSC 0739, study of which based on data from the Gaia DR3 survey has made it possible to detect 17 stars which are probably members of it. A list of them and their main parameters is given. The distance of the cluster is estimated to be 820 pc and the color excess on the path to the cluster is E(BP - RP) ≈ 1.05 mag. All of these stars are PMS-objects and most of them are optically variable. It is concluded that the newly discovered compact star-formation zone in the Dobashi 5006 cloud has an age of no more than a few million years and this process continues up to the present time. [ABSTRACT FROM AUTHOR]

In 2011 and 2015–2018 the classical T Tauri star RW Aur A dimmed by 2– . Previously, no such dimmings had been observed in historical observations since 1895: the variability amplitude did not exceed . During these eclipses the star showed evidence of UX Ori variability: a decrease in color index and an increase in polarization. To clarify the nature of the eclipses and the high polarization associated with them, we observed the object in the visible bands by the method of differential speckle polarimetry with the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University. The observations were carried out on October 23, 2016, at the end of a long and deep eclipse. As a simple geometric model shows, the polarized flux was reliably resolved. Using the MC3D radiative transfer simulation code, we have considered several three-dimensional dust envelope models. The two-component model that consists of a protoplanetary disk and a cone explains quantitatively the dimming and the shape of the resolved envelope. The typical size of the dust cone is 5–10 AU; this cone may be a dusty disk wind. However, our model does not reproduce the source of polarized radiation that coincides with the star and gives the bulk of the polarized flux from the object. The scattering envelope near the star unresolvable in our observations or the star itself, if its direct radiation is absorbed by aligned dust grains, can be this source. [ABSTRACT FROM AUTHOR]

This paper presents the results of the study of long-term photometric activity in the near infrared spectral region of the young Herbig Ae star VX Cas belonging to the family of irregular UX Ori variables. The infrared data were obtained in 2003–2017 at the Campo Imperatore Observatory (Italy) using the Pulkovo telescope AZT-24 in Johnson's photometric bands . Additional optical photometry from various sources was used as well. It was shown that the observed photometric activity of the star in the band and partly in the band was caused mainly by variations in circumstellar extinction in the protoplanetary disk. In a number of episodes, anticorrelation was observed between brightness variations in the optical band and and bands: the brightness in the and bands increased with weakening of the brightness in the band. This indicates that the gas and dust clouds that screened the star from the observer consisted of hot dust and were located near the sublimation zone. [ABSTRACT FROM AUTHOR]

The results of new photometric () observations of the young star V719 Per from the IC 348 cluster are presented. Up until 2014, this star demonstrated brightness variability typical for UX Ori stars, i.e., caused by strong fluctuations in circumstellar extinction. In 2014, the star dimmed by approximately 3 magnitudes in the band and remained in that state until 2017. There is reason to believe that the eclipse was caused by a disturbance in the innermost regions of the star's protoplanetary disk. The dust raised above the disk caused the star's radiation to be blocked for approximately three years. The role of various processes in the generation of such disturbances is discussed. [ABSTRACT FROM AUTHOR]

We have studied a sample of more than 25 000 young stars with proper motions and trigonometric parallaxes from the Gaia DR2 catalogue. The relative errors of their parallaxes do not exceed 10 . The selection of stars belonging to active star-forming regions was made by Marton et al. based on data from the Gaia DR2 catalogue by invoking infrared measurements from the WISE and Planck catalogues. Low-mass T Tauri stars constitute the majority of sample stars. The following parameters of the angular velocity of Galactic rotation have been found from them: km s kpc , km s kpc , and km s kpc . The Oort constants are km s kpc and km s kpc , while the circular rotation velocity of the solar neighborhood around the Galactic center is km s for the adopted Galactocentric distance of the Sun kpc. [ABSTRACT FROM AUTHOR]

The conditions for formation of helium lines in the disk and magnetospheric winds of Herbig Ae stars are studied. The program package CLOUDY is used to solve this problem. It is shown that the optical thickness of the gas in the HeI 10830 Å line needed to explain the observed P Cyg profiles is reached in the disk or magnetospheric winds of Herbig Ae stars for Te ≥ 10000 K. Here the x-ray emission produced at the surface and in the vicinity of a young star is the main ionization source for helium atoms. [ABSTRACT FROM AUTHOR]

The behavior of the linear polarization parameters of UX Ori stars during their eclipses by circumstellar dust clouds is studied. A circumstellar disk with a disk wind creating a puffing in the dust sublimation zone is considered. We show that the disk puffing can strongly affect the degree of polarization and color index of the star during its eclipse. A strong wind can change the orientation of the plane of linear polarization. The scattered radiation from a thin disk is polarized perpendicularly to its plane, but the radiation from a disk with a strong wind can be polarized along the disk plane. A situation where the disk-scattered radiation is not polarized in a certain spectral band is possible owing to the disk puffing. There can be different orientations of the linear polarization of the disk radiation in different spectral bands. [ABSTRACT FROM AUTHOR]

We have obtained new estimates of the Sun's distance from the symmetry plane Z and the vertical disk scale height h using currently available data on stellar OB associations, Wolf-Rayet stars, HII regions, and Cepheids. Based on individual determinations, we have calculated the mean Z = −16 ± 2 pc. Based on the model of a self-gravitating isothermal disk for the density distribution, we have found the following vertical disk scale heights: h = 40.2 ± 2.1 pc from OB associations, h = 47.8 ± 3.9 pc from Wolf-Rayet stars, h = 48.4 ± 2.5 pc from HII regions, and h = 66.2 ± 1.6 pc from Cepheids. We have estimated the surface, Σ = 6 kpc, and volume, D( Z) = 50.6 kpc, densities from a sample of OB associations. We have found that there could be ∼5000 OB associations in the Galaxy. [ABSTRACT FROM AUTHOR]

Theory of fossil magnetic field is based on the observations, analytical estimations and numerical simulations of magnetic flux evolution during star formation in the magnetized cores of molecular clouds. Basic goals, main features of the theory and manifestations of MHD effects in young stellar objects are discussed. [ABSTRACT FROM AUTHOR]

The model of a protoplanetary disk around a star with a low-mass companion ( M: M ≤ 0.1) moving in a circular orbit inclined at a small angle to the disk plane (≤10°) is considered. The SPH method is used to calculate the hydrodynamic flows. The orbital motion of the companion leads to a nonuniform distribution of matter in the disk: a matter-free gap, density waves, and gas flows are formed in it. As a result of perturbations, the inner part of the disk is inclined relative to its periphery and does not coincide with the orbital plane of the companion either. This leads to an anisotropic illumination of the disk by the star and, as a consequence, to the appearance of a large-scale inhomogeneity in the disk image: it has a bright horseshoe-shaped region and a small shadow zone located asymmetrically relative to the line of nodes. An asymmetry of the disk image is clearly seen even when it is viewed pole-on. The orbital motion of the companion does not lead to any synchronous motion of the dark (shadow) and bright regions: they only execute small oscillations relative to some preferential direction. The asymmetric image of the disk around the star LkH α 101 seen nearly pole-on can be reproduced rather accurately within the proposed model. A study of such asymmetric disks opens up new opportunities for the search of massive bodies in the neighborhoods of young stars. [ABSTRACT FROM AUTHOR]

Pre-main sequence stars are bright UV (UV) sources compared with their main sequence analogues. The source of this excess is the high energy processes associated with the physics of accretion/outflow during early stellar evolution. In this review, the main sources of UV excess are described as well as the most significant 'unknowns' in the field. Special emphasis is made on the results from the last observations carried out with the Hubble Space Telescope and on the relevance of future dedicated monitoring programs with the World Space Observatory-UV. [ABSTRACT FROM AUTHOR]

To understand how planetary systems form in the dusty disks around pre-main-sequence stars, a detailed knowledge of the structure and evolution of these disks is required. Although this is reasonably well understood for the regions of the disk beyond about 1 AU, the structure of these disks inward of 1 AU remains a puzzle. This is partly because it is very difficult to spatially resolve these regions with current telescopes. But it is also because the physics of this region, where the disk becomes so hot that the dust starts to evaporate, is poorly understood. With infrared interferometry it has become possible in recent years to directly spatially resolve the inner 1 AU of protoplanetary disks, albeit in a somewhat limited way. These observations have partly confirmed current models of these regions, but also posed new questions and puzzles. Moreover, it has turned out that the numerical modeling of these regions is extremely challenging. In this review, we give a rough overview of the history and recent developments in this exciting field of astrophysics. [ABSTRACT FROM AUTHOR]

We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ∼104 K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate Ṁ a = 10−8-10−6 M ⊙yr−1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind ( r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10−6 M ⊙yr−1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images. [ABSTRACT FROM AUTHOR]

We have determined the atomic hydrogen column density N HI toward all of the young stars from the Taurus-Auriga-Perseus star-forming complex for which the corresponding spectra are available in the Hubble Space Telescope archive (nine stars) by analyzing the Ly α line profile. We show that the stars studied, except DR Tau, lie not far from the edge of the gaseous cloud of the star-forming region closest to us or, more precisely, inside the outer H I shell of the cloud. This shell with a column density of N HI ≃ 6 × 1020 cm−2 surrounds the molecular gas of the cloud composed of a diffuse component (the so-called diffuse screen) in which dense, compact TMC-1 cores are embedded. The properties of the dust grains toward the stars that lie at the front edge of the cloud most likely differ only slightly from those of the interstellar dust outside star-forming regions. This casts doubt on the validity of the hypothesis that the extinction curve toward young stars has an anomalously low amplitude of the 2175 Å bump—such an extinction curve is observed for the field stars HD 29647 and HD 283809 toward which the line of sight passes through the TMC-1 core. [ABSTRACT FROM AUTHOR]

CCD photometry data of the T Tauri star H 187 are presented. They show that a new eclipse of this star began at the end of 2004. Since only one eclipse of this star has been observed previously with a duration of 3.5 years, our data indicate that the eclipses of this object are periodic with a period P=4.9 years between eclipses. Thus, in terms of the absolute duration of the eclipses and in terms of their relative length as a fraction of the period, H 187 is one of the most exotic objects in the sky. [ABSTRACT FROM AUTHOR]

I examine the question of purely accreting protostars, and set limits to the breakout time of a protostellar wind within the accretion flow forming the new star. Hypothesizing a wind launched from the protostellar surface, three temporal phases are derived: a crushed wind, a trapped wind, and an escaping wind. In the current model, evolution from one phase to the next is a consequence of the growing anisotropy of the infalling flow, a natural outcome of the collapse of a rotating cloud core. During the crushed wind phase, infall overcomes the wind at all solid angles, and the accretion directly strikes the protostellar surface. The trapped phase consists of a wind sufficiently strong to push material back from the stellar surface, but too weak to carry the heavy, shocked and swept-up infall out of the star's gravitational potential. Unless the wind turns on impulsively, a significant fraction of the pre-breakout life of the protostar may be spent in this trapped wind phase in which gas is launched from the protostar but is pulled back, crashing onto the protostellar and disk surfaces. It may be that some `starless cores' contain as-yet undetected, very young accreting protostars, and that episodic luminosity fluctuations associated with this trapped wind could be observed. [ABSTRACT FROM AUTHOR]

Preliminary results of broad-band and narrow-band charge-coupled device imaging of the young star cluster NGC 6823 and the associated bright nebula NGC 6820 are reported. The observations were made in August 2001 at Maidanak Observatory (Uzbekistan) as a collaborative initiative between the Graduate Institute of Astronomy, National Central University, Taiwan, and Ulugh Beg Astronomical Institute, Uzbek Academy of Sciences. The two institutes, together with colleagues from Lithuania and Latvia, are working together to modernize and instrument the existing telescope facilities at Maidanak to make use of the excellent astroclimate of the site. As a pilot project, we selected candidate young stars in NGC 6823, based on their 2MASS near-infrared colours and obtained their photometry with Maidanak telescopes. The youth natures of some of these sources were later confirmed by spectra taken by the 2.16 m telescope at Beijing Observatory. This demonstrates that combination of small telescopes and existing databases, such as 2MASS, ROSAT or Chandra, can yield fruitful science. Other Uzbek-Taiwan collaborations are being planned. [ABSTRACT FROM AUTHOR]

OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964a) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies. [ABSTRACT FROM AUTHOR]