Modeling of Spectroscopic and Interferometric Observations of the Herbig Star VV Ser with Hybrid Models

Modeling of hydrogen emission lines is a powerful tool to study physical processes in the nearest vicinity of young stars because spectral lines carry information on the kinematics and physical conditions of the gas. One of the lines that probe emitting regions closest to the star is the Br$$\gamma $$ line. We consider different types of hybrid models to reproduce both interferometric VLTI-AMBER observations and LBT-LUCIFER spectroscopic observations of the single-peak profile of the Br$$\gamma $$ line of the Herbig AeBe star (HAEBE) VV Ser, a member of the UX Ori type subclass. We computed models of a magneto-centrifugal disk wind, a magnetospheric accretion region (magnetosphere), Cranmer’s polar wind, and scattered light from circumstellar polar dust. Furthermore, we calculated hybrid two-component models consisting of a disk wind and one of the aforementioned models. We computed visibilities and line profiles for all types of models and compared them with the available interferometric observations to constrain model parameters. We conclude that for the inclinations reported for this star (60°–70°), the disk wind alone cannot explain the Br$$\gamma $$ line profile although it may be a dominant contributor to the hydrogen line radiation. However, magneto-centrifugal disk wind in combination with aforementioned emitting regions (magnetosphere, polar wind, or scattered light from polar dust) may be able to reproduce the observations.     

Structure of the circumbinary envelope around a young binary system

The structure of a circumstellar envelope around a young binary T Tauri star is considered. The supersonic orbital motion of the system components in the envelope gas leads to the formation of bow shocks around the star. Two- and three-dimensional numerical modeling indicates an important role of these shocks in the formation of the structure of the circumbinary envelope. In particular, for systems with circular orbits, the size of the central region of the envelope that is not filled with matter (the “gap”) is essentially determined by the parameters of the bow shocks. These modeling results are supported by comparisons of the obtained estimates for the gap parameters with observations.     

Proper motions of stars in the region of the Great Nebula in Orion

Six Tautenburg astrograph plates have been used to derive proper motions for 12 740 stars in the region of the Great Nebula in Orion. The Oricat catalog of proper motions and B and R photometry has been compiled, incorporating as well data from other published catalogs. The proper motions presented in different catalogs are compared. The Oricat catalog is useful for studies of features of the structure and kinematics of star clusters and groups.     

Structure and kinematics of the ISM near WR 139

The structure and kinematics of the ISM in an extended vicinity of the star WR 139 is analyzed using the results of original Hα interferometric observations together with radio and infrared data. A CO cavity with a size of up to 40′ has been detected around the star at velocities of V _LSR ∼ 2.5–10 km/s; the cavity is bounded to the North by a shell radiating in the optical. Ionized hydrogen emits at the systematic velocities V _LSR ∼ 6–14 km/s toward the CO cavity, and at V _LSR ≃ 4–11 km/s toward the shell. High-velocity motions of ionized hydrogen inside the cavity testify to the probable expansion of gas that has been swept out by the stellar wind of WR 139 at velocities of up to 60–80 km/s.     

Gamma-Ray Emission During the Accretion of Matter from a Supernova Envelope onto a Compact Remnant

The aim of this study is to investigate the accretion of matter onto a compact gravitating remnant (neutron star) in the central region of the expanding shell of a Type II supernova. Computations of an explosion with the energetics of a Type II supernova have been performed to derive the structure of matter in the vicinity of the neutron star. The energy of the expanding shell and the parameters of the presupernova correspond to the known values for SN 1987A. This accretion leads to the formation of a layer of fairly dense and hot gas at the surface of the compact remnant, providing the conditions for nucleosynthesis reactions. Thus, one result of the study is to demonstrate the importance of the r and rbc processes, or explosive nucleosynthesis, in the compact envelope of a neutron star. A second result is the production of emission lines from unstable elements formed in the central part of the neutron-star envelope.

     

Photometric and polarimetric activity of RZ Psc

We present the results of synchronous photometric and polarimetric U BV RI observations of the irregular variable star RZ Psc, acquired at the Crimean Astrophysical Observatory in 1989–2002. The star’s photometric behavior is characterized by short, sporadic Algol-like dimmings. We observed only one deep minimum, with a V amplitude of about 1.5^m, during the entire observation time. During this minimum, the star’s linear polarization reached 3.5%. Comparisons with polarization observations of RZ Psc during another deep minimum in 1989 show that the two minima can be described by the same polarization-brightness relation, testifying to an eclipsing nature for the minima. This provides evidence that the optical characteristics of the flattened circumstellar dust envelope that gives rise to the star’s intrinsic polarization have remained virtually unchanged over the last 13 years. We argue that the origin of this stability is the presence of a large dust-free cavity in the central region of the circumstellar dust disk of RZ Psc. The cavity could be associated with binarity of the star or the formation of a planetary system, with most of the dust in the central region of the disk being transformed into large bodies—planetesimals and planets.     

Infrared emission and the destruction of dust in HII regions

The generation of infrared (IR) radiation and the observed IR-intensity distribution at wavelengths of 8, 24, and 100 µm in the ionized hydrogen region around a young, massive star is investigated. The evolution of the HII region is treated using a self-consistent chemical-dynamical model in which three dust populations are included—large silicate grains, small graphite grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer model taking into account stochastic heating of small grains and macromolecules is used to model the IR spectral energy distribution. The computational results are compared with Spitzer and Herschel observations of the RCW 120 nebula. The contributions of collisions with gas particles and the radiation field of the star to stochastic heating of small grains are investigated. It is shown that a model with a homogeneous PAH content cannot reproduce the ring-like IR-intensity distribution at 8 µm. A model in which PAHs are destroyed by ultraviolet radiation of the star, generating region HII, provides a means to explain this intensity distribution. This model is in agreement with observations for realistic characteristic destruction times for the PAHs.     

Structure and kinematics of the interstellar medium near WR 137 and supernova remnants CTB 87 and G73.9+0.9

Using the results of our Hα interferometric observations and observational data on the 21 cm and CO lines, we have analyzed the structure and kinematics of the interstellar medium in the extended vicinity of the star WR 137 and the supernova remnants CTB 87 and G73.9+0.9. A shell structure with a radius of up to 40′ observable in optical lines has been discovered around WR 137. The high-velocity motions of ionized hydrogen inside this shell can be interpreted as expansion of the gas swept out by the wind of WR 137 at velocities of up to 60 km/s. The ionized hydrogen near WR 137 emits at the systematic velocity V _LSR ∼ 6–18 km/s. The expansion ofG73.9+0.9 at a velocity of up to 55 km/s has been confirmed. The systematic velocities of the ionized hydrogen toward this supernova remnant are V _LSR ≃ −14…+14 km/s. An HI shell around G73.9+0.9 has been detected at velocities V _LSR≃−14…−8 km/s. A very faint optical shell of CTB 87 with a size of about 20′ has also been detected. Evidence that CTB 87 is located in the Cygnus Arm is presented.     

A study of the region of massive star formation L379IRS1 in radio lines of methanol and other molecules

The results of spectral observations of the region of massive star formation L379IRS1 (IRAS18265–1517) are presented. The observations were carried out with the 30-m Pico Veleta radio telescope (Spain) at seven frequencies in the 1-mm, 2-mm, and 3-mm wavelength bands. Lines of 24 molecules were detected, from simple diatomic or triatomic species to complex eight- or nine-atom compounds such as CH₃OCHO or CH₃OCH₃. Rotation diagrams constructed from methanol andmethyl cyanide lines were used to determine the temperature of the quiescent gas in this region, which is about 40–50 K. In addition to this warm gas, there is a hot component that is revealed through high-energy lines of methanol and methyl cyanide, molecular lines arising in hot regions, and the presence of H₂O masers and Class II methanol masers at 6.7 GHz, which are also related to hot gas. One of the hot regions is probably a compact hot core, which is located near the southern submillimeter peak and is related to a group of methanol masers at 6.7 GHz. High-excitation lines at other positions may be associated with other hot cores or hot post-shock gas in the lobes of bipolar outflows. The rotation diagrams can be use to determine the column densities and abundances of methanol (10^?9) and methyl cyanide (about 10^?11) in the quiescent gas. The column densities of A- and E-methanol in L379IRS1 are essentually the same. The column densities of other observedmolecules were calculated assuming that the ratios of the molecular level abundances correspond to a temperature of 40 K. The molecular composition of the quiescent gas is close to that in another region of massive star formation, DR21(OH). The only appreciable difference is that the column density of SO₂ in L379IRS1 is at least a factor of 20 lower than the value in DR21(OH). The SO₂/CS and SO₂/OCS abundance ratios, which can be used as chemical clocks, are lower in L379IRS1 than in DR21(OH), suggesting that L379IRS1 is probably younger than DR21(OH).     

Stellar groups and clusters in the region of Orion’s Sword

The region of Orion’s Sword with coordinates α = 83.79°, δ = −5.20° and a size of 1.0° × 2.5° is analyzed. We compiled a master catalog of stars observed in the optical, containing positions, proper motions, and UBV photometry for 1634 stars. Using the nearest-neighbor-distance technique, we subdivided the region into stellar groups with different numbers of members. The positions of five groups coincide with known clusters, and two groups coincide with aggregates of stars with Hα emission. We have identified groups with low membership that are moving away from the system. We also considered the kinematic structure of the groups using the AD-diagram method we developed earlier. Most of the stellar clusters and groups display similar kinematics, with the exception of the group OMC-2, which is moving toward its own apex. We also confirmed the existence of the kinematic star group 189 discovered earlier; its position is close to the cluster NGC 1977, and it is probably a component of its corona.     

VLA observations of class I methanol masers in the region of low-mass star formation L1157

We present the results of VLA observations of a maser candidate in the low-mass star formation region L1157 in the 7₀-6₁ A ⁺ transition at 44 GHz. The line is emitted by a compact, undoubtedly maser source associated with clump B0a, which is seen in maps of L1157 in thermal lines of methanol and other molecules. A much weaker compact source is associated with clump B1a, which is brighter than B0a in thermal methanol lines. The newly detected masers may form in thin layers of turbulent post-shock gas. In this case, the maser emission may be beamed, so that only an observer located in or near the planes of the layers can observe strong masers. On the other hand, the maser lines are double with a “red” asymmetry, indicating that the masers may form in collapsing clumps. A detailed analysis of collapsing-cloud maser models and their applicability to the masers in L1157 will be developed in subsequent papers.     

Observations of stellar objects at a shell boundary in the star-forming complex in the galaxy IC 1613

The single region of ongoing star formation in the galaxy IC 1613 has been observed in order to reveal the nature of compact emission-line objects at the edges of two shells in the complex, identified earlier in Hα line images. The continuum images show these compact objects to be stars. Detailed spectroscopic observations of these stars and the surrounding nebulae were carried out with an MPFS spectrograph mounted on the 6-m telescope of the Special Astrophysical Observatory. The resulting stellar spectra were used to determine the spectral types and luminosity classes of the objects. Of star we identified the only object of this spectral type in IC 1613. The results of optical observations of the multishell complex are compared to 21-cm radio observations. The shells harboring the stars at their boundaries constitute the most active part of the star-forming region. There is evidence that shocks have played an important role in the formation of the shells.     

Gamma-ray bursts—tracers of the history of star formation in the Universe

The rate of gamma-ray bursts (GRBs) in the Galaxy is estimated assuming that these events result from the formation of rapidly rotating Kerr black holes during the core collapse of massive, helium, Wolf-Rayet secondary components in very close binary systems. This process brings about rapid rotation of the cores of such Wolf-Rayet stars, inevitably resulting in the formation of Kerr black holes during type Ib,c supernovae. The current rate of formation of Kerr black holes (GRBs) in the Galaxy is about 3×10^?5/year. Collimation of the gamma-ray radiation into a small solid angle (about 0.1–0.01 sr) brings this rate into consistency with the observed rate of GRBs, estimated to be 10^?6–10^?7/year. Possible immediate progenitors of GRBs are massive X-ray binaries with X-ray luminosities of 10³⁸–10⁴⁰ erg/s. Due to the short lifetimes of the progenitors and the very high brightnesses of GRBs, the GRB rate can provide information about the history of star formation in the Universe on the Hubble time scale. A model in which the star-formation rate is determined by the conditions for ionization of the interstellar gas, whose density and volume are determined by supernovae, yields a Galactic star-formation history that can be viewed as representing the history of star formation in the Universe. The theoretical history of star formation is in satisfactory agreement with the history reconstructed from observations. The theoretical model for the history of star formation in the Galaxy can also be used to assess the influence of dust on optical observations of supernovae and GRBs in galaxies of various ages.     

Chemodynamical evolution of gas near an expanding HII region

A self-consistent model for the chemical-dynamical evolution of a region of ionized hydrogen around a massive young star and of the surrounding molecular gas is presented. The model includes all main chemical and physical processes, namely the photoionization of atomic hydrogen, photodissociation of molecular hydrogen and other molecules, and the evaporation of molecules from the mantles of dust particles. Heating and cooling processes are taken into account in the temperature calculations, including cooling in molecular and atomic lines. The hydrodynamical equations were solved using the Zeus2D hydrodynamical software package. This model is used to analyze the expansion of a region of ionized hydrogen around massive stars (effective temperature of 30 000 and 40 000 K) in a medium with various initial density distributions. The competition between evaporation from dust mantles and the photodissociation of molecules results in the formation of a transition layer between the hot HII region and cool quiescent medium, characterized by high abundances of molecules in the gas phase. The thickness of the transition layer is different for different molecules. Since there is a velocity gradient along the transition layer, and the maxima in the distributions of different molecules are at different distances from the star, observations of molecular emission lines should reveal distinction in shifts of lines of different molecules relative to the velocity of the quiescent gas. Such shifts have indeed been detected during molecular observations of the region of ionized hydrogen Sh2-235. For an initial gas density of 10³ cm^?3, the increase in the abundances of H₂O and H₂CO in the transition layer after desorption from dust occurs gradually rather than in a jump-like fashion; therefore, the concept of a “evaporation front” can be used only formally. In addition, the distances between the evaporation fronts for different molecules are significant. At higher initial gas densities (10⁴ cm^?3), sharp evaporation fronts are formed for the different molecules, which are close to each other and to the shock front. In this case, it is possible to speak of a single evaporation front for CO, H₂O, and H₂CO.     

Flares of H2O maser emission from SGR B2 in 2005–2012

We present observations of H₂O maser emission from the complex region of active star formation Sgr B2 performed in 2005–2012. The observations were carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. Seven flares with flux densities higher than 1000 Jy were detected. The flares occurred in all three main sites of star formation in Sgr B2, N,M, and S. The highest peak flux densities were 3200 Jy (60.9 km/s), 2350 Jy (69.4 km/s), and 7300 Jy (69.3 km/s) in N, M, and S, respectively. This last flare was the strongest during our monitoring campaign from 1982 to 2012, both in S and in the entire Sgr B2 complex. Possible associations of the flares were determined. High-velocity, short-lived emission was detected at 124–128 km/s. Emission at 127 km/s with a flux density of 23 Jy is associated with region M. Emission at 80.6 and 84.6 km/s, at radial velocities higher than those observed previously, was detected in region S.     

Stellar activity from observations with the KEPLER space telescope: The M dwarf GJ 1243

We used continuous observations with the KEPLER space telescope during 44.45 days to study the activity of the fully convective low-mass M dwarf GJ 1243. The star’s rotational period is 0.593 days. Our precision photometry of GJ 1243 made it possible to study its surface temperature inhomogeneities and follow their evolution. We find evidence for two active longitudes on the surface of the star, separated by 203° in longitude or 0.56 in phase. The position of spots on the surface was found to be very stable during the analyzed 74 rotation periods. Assuming that the rotational axis is inclined to the line of sight by i = 60°, the total spotted area S is, on average, 3.2% of the total visible stellar surface, and increased by 0.7% in 100 days; if i = 30°, the area is S = 5.6%. The change in S is due to an increase in the area of the smaller active region. We compare the magnetic-activity features of GJ 1243 and another fully convective star, V374 Peg.     

The Spatial–Kinematic Structure of the Region of Massive Star Formation S255N on Various Scales

The results of a detailed analysis of SMA, VLA, and IRAM observations of the region of massive star formation S255N in CO(2–1), N₂H⁺(3–2), NH₃(1, 1), C¹⁸O(2–1) and some other lines is presented. Combining interferometer and single-dish data has enabled a more detailed investigation of the gas kinematics in the moleclar core on various spatial scales. There are no signs of rotation or isotropic compression on the scale of the region as whole. The largest fragments of gas (≈0.3 pc) are located near the boundary of the regions of ionized hydrogen S255 and S257. Some smaller-scale fragments are associated with protostellar clumps. The kinetic temperatures of these fragments lie in the range 10–80 K. A circumstellar torus with inner radius R_in ≈ 8000 AU and outer radius R_out ≈ 12 000 AU has been detected around the clump SMA1. The rotation profile indicates the existence of a central object with mass ≈8.5/ sin²(i) M_⊙. SMA1 is resolved into two clumps, SMA1–NE and SMA1–SE, whose temperatures are≈150Kand≈25 K, respectively. To all appearances, the torus is involved in the accretion of surrounding gas onto the two protostellar clumps.     

The photopolarimetric activity and circumstellar environment of the T Tauri star CO Ori

We present the results of simultaneous UBVRI photometry and polarimetry of the classical T Tauri star CO Ori carried out at the Crimean Astrophysical Observatory during the 18 years between 1986 and 2004. We show that the variations of linear polarization accompanying the star’s brightness variations follow the law characteristic of UX Ori stars. This suggests that the brightness variations of the star are mainly due to changes of the circumstellar extinction due to non-uniform structure of the circumstellar environment, and to an “optimal” orientation of the circumstellar gas and dust disk relative to the observer, whose line of sight crosses the gas and dust atmosphere of the disk. We determine the star’s intrinsic polarization due to scattering of light in the circumstellar disk. The polarization position angle indicates the orientation of the disk’s symmetry axis in the plane of the sky. Our analysis of an archival light curve for CO Ori confirms the existence of a many-year cycle of photometric activity, suspected by us earlier. The refined period of this cycle is 12.4 years. The existence of such activity cycles of UX Ori stars testifies to considerable deviations of their circumstellar disks from axial symmetry, a reflection of either stellar binarity or the commencement of the process of planetary formation.     

The formation and evolution of the most massive stars

We consider the formation of massive stars under the assumption that a young star accretes material from the protostellar cloud through its accretion disk while losing gas in the polar directions via its stellar wind. The mass of the star reaches its maximum when the intensity of the gradually strengthening stellar wind of the young star becomes equal to the accretion rate. We show that the maximum mass of the forming stars increases with the temperature of gas in the protostellar cloud T ₀, since the rate at which the protostellar matter is accreted increases with T ₀. Numerical modeling indicates that the maximum mass of the forming stars increases to ～900 M _⊙ for T ₀ ～ 300 K. Such high temperatures of the protostellar gas can be reached either in dense star-formation regions or in the vicinity of bright active galactic nuclei. It is also shown that, the lower the abundance of heavy elements in the initial stellar material Z, the larger the maximum mass of the star, since the mass-loss rate due to the stellar wind decreases with decreasing Z. This suggests that supermassive stars with masses up to 10⁶ M _⊙ could be formed at early stages in the evolution of the Universe, in young galaxies that are almost devoid of heavy elements. Under the current conditions, for T ₀ = (30–100) K, the maximum mass of a star can reach ～100M _⊙, as is confirmed by observations. Another opportunity for the most massive stars to increase their masses emerges in connection with the formation and early stages of evolution of the most massive close binary systems: the most massive stars can be produced either by coalescence of the binary components or via mass transfer in such systems.     

Activity observed by the Kepler space telescope: The M dwarf LHS 6351 (KIC 2164791)

Continuous 123.87-day observations with the “Kepler” space telescope are used to study the activity of the fully convective, low-mass M dwarf LHS 6351. The axial-rotation period of the star is 3.36 day. High-precision photometric observations of LHS 6351 enabled studies of its surfacetemperature inhomogeneities and their evolution. The difference in the longitudes of active regions increased from 120° at the beginning to 207° at the end of the observations, for i = 60° (and from 156° to 198° for i = 30°). This variation of the locations of the spots on the stellar surface provides evidence for differential rotation of the star. According to our estimates, the rate of displacement of the active regions is (0.006–0.014) ± 0.002 rad/day. Assuming i = 60°, the total area of spots S decreased, on average, from 1.2% to 0.92% of the total visible surface of the star; if i = 30°, this area decreased from 1.8% to 1.0%. We compared manifestations of the magnetic activity of LHS 6351 with the properties of the fully convective M dwarfs V374 Peg and GJ 1243, studied earlier. We derived the dependence of ΔΩ on the Rossby number for these M dwarfs, and identified two groups of stars with differing mass and differential rotation.