Periodic instabilities in protostellar disks with azimuthal magnetic fields

The stability of magnetohydrodynamic oscillations in a protostellar disk with a toroidal magnetic field is analyzed. It is shown that, apart from the aperiodic magnetorotational instability, two other types of periodic instabilities of non-axisymmetric perturbations can exist. The simultaneous presence of azimuthal and vertical components of the wave vector are necessary for these to exist. One instability is due to the inductive winding-up of the azimuthal magnetic field of the wave, and the other arises when the field amplitude is increased by a comoving Hall wave, transferring magnetic field into a region of enhanced field intensity. The bandwidths of the unstable wave numbers are analyzed as a function of the Hall current, the β parameter of a plasma, and the angle between the direction of wave propagation and the plane of the disk. Regions in the accretion disks typical of T Tauri stars are indentified where these instabilities could be most active.     

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.     

The Initial Mass Spectra of Astronomical Objects

The fundamental premises concerning the existence of a universal basic distribution describing the initial mass functions (IMFs) of various astronomical objects on scales from brown dwarfs to clusters of galaxies are considered. This distribution has the form dN ∝ M^?2dM, where M is the mass of an object and N the number of objects with a given mass. It is shown that, at least for objects forming as a result of fragmentation (e.g., stars, star clusters), the basis of this distribution may correspond to a white-noise model. The classical problem of the formation of the IMF for stars is discussed in this context, incuding the relationship between the mass function of protostellar clouds and the stellar IMF. The main factors determining the mass functions of galaxies and galaxy clusters are also considered.     

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.     

Analysis of density fluctuations in models of open clusters

The frequencies for homologous and non-homologous density flucutations in unisolated, ellipsoidal models of open clusters are determined. These fluctuations are stable. In the cases considered, the frequencies of non-homologous fluctuations are higher than those for homologous fluctuations of the ellipsoidal models. The instability of the proper fluctuations of the phase density at the center of a homogeneous ellipsoid whose central characteristics correspond to the core parameters of a dynamical, numericalmodel cluster is demonstrated. The development of instability of the phase-density flucutations at the center of such an ellipsoid leads to the domination of fluctuations with the frequency γ ₁, corresponding to the growth increment γ ₂ for this instability. Estimates of γ ₁ and γ ₂ agree with estimates obtained for the core of the corresponding dynamical, numerical model cluster. Astrophysical applications of the results are discussed.     

Ballooning instability and oscillations of coronal loops

The excitation of the ballooning instability by the eigenoscillations of coronal loops is analyzed using the energy method. The second variation of the potential energy for the case of a plasma—plasma boundary is obtained via the linearized ideal MHD equations. It is shown that the eigenmodes of a magnetic tube and of a toroidal coronal loop coincide in a first approximation. The bending oscillations of the loops are able to excite the ballooning instability when β ? 1. The effects of the instability in solar coronal loops are discussed.     

Composite dust grains: Modeling of infrared absorption bands

We investigate the influence of small-scale asphericity of the surfaces of dust grains on the characteristics of the two deepest absorption bands observed in the spectra of protostellar objects and stars (the 3.1 μm water-ice and 9.7 μm silicate bands). The model used has composite particles in the form of radially inhomogeneous spheres with intermediate layers in which the index of refraction changes. The observed band widths and the ratios of the optical depths at the band centers can be explained if the grains are composed of small particles consisting of silicate cores with thin ice mantles and rough surfaces. The grain surface roughness considerably broadens the profile of the silicate band.     

Search for class I methanol maser emission in various types of objects in the interstellar medium

Observations of various types of objects in the northern sky were obtained at 44 GHz in the 7₀-6₁ A ⁺ methanol line on the 20-m Onsala radio telescope (Sweden), in order to search for Class I methanol maser emission in the interstellar medium: regions of formation of high-mass stars, dust rings around HII regions, and protostellar candidates associated with powerful molecular outflows and Galactic HII regions. Seven new Class Imethanolmasers have been discovered toward regions of formation of highmass stars, and the existence of two previously observed masers confirmed. The following conclusions are drawn: (1) neither the association of a bipolar outflow manifest in the wings of CO lines with a highmass protostellar object (HMPO) nor the presence of thermal emission in lines of complex molecules are sufficient conditions for the detection of Class I methanol emission; no association with HMPOs radiating at 44 GHz was found for EGOs (a new class of object tracing bipolar outflows); (2) the existence of H₂O masers and Class II methanol masers in the region of aHMPOenhances the probability of detecting Class I methanol emission toward the HMPO; Class II methanol masers with stronger line fluxes are associated with Class I methanol masers.     

Microstructure characterization of mechanically activated hematite using XRD line broadening

The effect of dry milling in a vibratory mill on the structural changes and microstructural characteristics of hematite using different methods was investigated. We have described the line profile analysis (LPA) to extract the size of coherently diffracting domains and the lattice strain of activated hematite in a vibratory mill. The Warren–Averbach and Williamson–Hall methods were used as the main tools for characterization. The changes in the particle size, surface area and new phase formation of hematite concentrate were also investigated. It was concluded that the breakage and agglomeration of particles take place mainly at lower and higher levels of specific energy input, respectively. The pores in agglomerates remain accessible for the nitrogen gas. Milling of hematite increased specific surface area up to 18.4 m²/g. The hematite milled under various levels of specific energy input did not undergo a significant reaction or phase transformation during milling. The Williamson–Hall method confirms its merit for a rapid overview of the line broadening effects and possible understanding of the main causes. The anisotropic character of line broadening for deformed hematite as a function of specific energy input was revealed. Higher level of specific energy input favors the generation of small crystallite size, higher microstrain, BET surface area, amorphization and line breadth. The Warren–Averbach method suggested that the nanocrystalline hematite with grain sizes of 73.5–12.2 nm was formed by mechanical treatment using different milling intensities in the vibratory mill. The root mean square strain (RMSS) at L = 10 nm varies between 1.7 × 10^− 3 and 4.0 × 10^− 3 depending on the level of energy input. Limits in the applicability of Williamson–Hall method and reliability of the results are discussed in detail.     

Effect of the initial density and angular-velocity profiles of pre-stellar cores on the properties of young stellar objects

The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent densities Σ ∝ r ⁻¹ and Ω ∝ r ⁻¹. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial core configuration in all three evolutionary stages.     

Roamerella,a new name for the genus Roemerella Akopyan in (Mollusca,Gastropoda; Upper Cretaceous), preoccupied by Roemerella (Brachiopoda,Lingulata; Devonian)

The Late Cretaceous (Turonian–Campanian) turritellid gastropod Roemerella Akopyan in Akopyan et al., 1990, is a junior homonym of the Devonian inarticulate brachiopod Roemerella Hall and Clarke, 1890. The generic name Roamerella nom. nov. is proposed for the homonymous gastropod.     

Modeling of protostellar clouds and their observational properties

A physical model and two-dimensional numerical method for computing the evolution and spectra of protostellar clouds are described. The physical model is based on a system of magneto-gas-dynamical equations, including ohmic and ambipolar diffusion, and a scheme for calculating the thermal and ionization structure of a cloud. The dust and gas temperatures are determined when calculating the thermal structure of the cloud. The results of computing the dynamical and thermal structure of the cloud are used to model the transfer of continuum and molecular-line radiation in the cloud. Results are presented for clouds in hydrostatic and thermal equilibrium. The evolution of a rotating magnetic protostellar cloud that is compressed starting from a quasi-static equilibrium state is also considered. Spectral maps for optically thick lines of linear molecules are analyzed. The influence of the magnetic field and rotation can lead to a redistribution of angular momentum in the cloud and the formation of a characteristic rotational-velocity structure. As a result, the distribution of the velocity centroid of the molecular lines can acquire an hourglass shape. It is planned in future to use the developed program package and a model for the chemical evolution of clouds to interpret and model in detail observed starless and protostellar cores.     

A coupled,dynamical and chemical model for the prestellar core L1544: Comparison of modeled and observed C<Superscript>18</Superscript>O,HCO<Superscript>+</Superscript>, and CS emission spectra

We consider radiative transfer in C¹⁸O, HCO⁺, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure.     

Stability of the multiple star system ι UMa (ADS 7114)

The physical and orbital parameters of the quadruple star system ι UMa (HD 76644 = ADS 7114) were determined earlier, when it was concluded based on modeling the system’s dynamics and applying theoretical stability criteria that the system was probably unstable. Here the stability of the ι UMa system is studied by calculating the Lyapunov characteristic exponents for representative sets of parameters and initial conditions. The conclusions on the system’s stability (or instability) based on various stability criteria and the calculated Lyapunov exponents are compared. The instability of the system as a whole is confirmed rigorously based on massive computations of the Lyapunov exponents. This system appears to be the only known multiple system whose instability has been rigorously established. The Lyapunov time-disruption time statistical relations are constructed, which show that the Hamiltonian intermittency of the second kind dominates. Typical disruption times are shorter than 1000 years, and the Lyapunov times are shorter than 100 years.     

Timescales for mechanisms for the dynamical evolution of open star clusters

We have obtained the stellar velocity dispersion in three mutually perpendicular directions in the halos and cores of clusters as a function of time for several non-stationary open-cluster models. During the dynamical evolution of the open-cluster models, the velocity dispersions undergo oscillations that do not decay during 5–10 violent-relaxation timescales, τ _vr . We estimated the time for synchronization of the rotation of the open-cluster models and their motion around the center of the Galaxy, t _s , which, depending on the model parameters, is t _s ? (5–27)τ _vr . Synchronization mechanisms for the models are discussed. The disruption of the systems in the force field of the Galaxy is strongly affected by tidal friction. We have also estimated the time for the formation of a spherical stellar-velocity distribution in the cluster models, t _σ ? (6 ? 25)τ _vr . The impact of instability in the stellar motions in a cluster on the formation of a spherical velocity distribution in the open-cluster models is discussed. We have noted a tendency for a weakening of the dependence of the coarse phase density of the cluster on small initial perturbations of the stellar phase coordinates in the model cluster cores for times about five times longer than the violent-relaxation time.     

A multicomponent model for computing the thermal structure of collapsing protostellar clouds

A model for simulating the thermal and dynamical evolution of protostellar clouds is presented. In the model, the dust and gas temperatures are treated separately, making it possible to more precisely describe the initial stages of the cloud’s gravitational contraction and collapse. The model is fast enough to be applied in hydrodynamical computations, and has a high enough accuracy for the results to be used to compute emission spectra and comparing them with observational data. Two problems are considered as test examples and simple applications: calculation of the structure of clouds in thermal and hydrostatic equilibrium, and modeling the evolution of a protostellar cloud in a spherically symmetric approximation, including the formation of the first hydrostatic core.     

A Search for long-period brightness variations of the T Tauri stars T Tau,DI Cep,and SU Aur

We have searched for protostellar or protoplanetary structures in the vicinity of young T Tauri stars using a technique that is able to distinguish long-period components of the observed light curves. We perform a statistical spectral analysis of the one-year mean light curves of three selected stars (T Tau, DI Cep, and SU Aur) and plot synthetic light curves for the most reliable period of each star. In a first approximation, the results show a good coincidence between our synthetic light curves and the original light curves, supporting the hypothesis that protostellar or protoplanetary structures are present in the studied systems. An analysis of the 0.36–20 µm spectral energy distributions of the program stars also leads to the conclusion that infrared anomalies for young stars are most likely due to thermal radiation from not-yet-formed companions in the vicinity of the star.     

Long-term monitoring of the W44C (G 34.3+0.15) maser in the 1.35 cm water vapor and 18 cm hydroxyl lines

Results of monitoring the H₂O and OH masers in W44C, located near the cometary HII region G34.3+0.15, are reported. Observations in the water-vapor line at λ = 1.35 cm were carried out on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from November 1979 to March 2011, and in the hydroxyl lines at λ = 18 cm on the large Nançay radio telescope (France). Activity maxima and minima of the water maser alternated. The average period of the activity is ～ 14 years, consistent with results obtained earlier for a number of other sources associated with regions of active star formation. In periods of enhanced maser activity, two series of strong H₂O maser flares were observed, which were related to two different clusters of maser spots located at the front of a shock at the periphery of the ultracompact region UH II. These series of flares may be associated with cyclic activity of the protostellar object in UH II. In the remaining time intervals, there were mainly short-lived flares of single features. The Stokes parameters for the observations in the hydroxyl lines were determined. Zeeman splitting was observed in the profile of the 1667 MHz OH main line at a velocity of 58.5 km/s, and was used to estimate the intensity of the line-of-sight component of the magnetic field (1.2 mG).     

Variations in the accretion rate and luminosity in gravitationally unstable protostellar disks

Self-consistent modeling of a protostar and protostellar disk is carried out for early stages of their evolution. The accretion rate at distances of sevral astronomical units from the protostar is appreciably variable, which is reflected in the protostar’s luminosity. The amplitude of the variations in the accretion rate and luminosity grows together with the sampling period, as a consequence of the nature of gravitationally unstable protostellar disks. A comparison of model luminosity variations with those derived from observations of nearby sites of star formation shows that the model variations are appreciably lower than the observed values for sampling periods of less than 10 years, indicating the presence of additional sources of variability on small dynamical distances from the protostar.     

Ar40 diffusion in homogenous orthoclase and an interpretation of Ar diffusion in K-feldspars

Ar⁴⁰ diffusion in a natural, non-perthitic orthoclase has been studied isothermal heating experiments between 500° and 800°C under both vacuum and hydrothermal (2 kbar) conditions. The sample is a one-phase K-feldspar without detectable chemical of structural inhomogeneities as verified by heating experiments, chemical and microprobe analyses, and cell refinements. The orthoclase does not disorder detectably and is stable for the duration of the heating interval. Diffusion coefficients were calculated using an isotropic model for spherical grains. Agreement of diffusion coefficients obtained on grain-sizes which differed by a factor of four indicate that the effective dimension for Ar⁴⁰ diffusion is the actual particle size. A series of experiments at 700°C show that Ar⁴⁰ loss may be described by the ideal spherical model and that the diffusion coefficient does not change with time. The Arrhenius relation is obeyed with a single activation energy and the diffusion coefficients are described by: D = (0·00982) exp — (43800/RT). Agreement of experiments conducted under vacuum and hydrothermally (up to 2 kbar) indicate that pressure and H₂O do not significantly affect Ar⁴⁰ loss. Relatively small amounts of alkali exchange between the feldspar and hydrothermal salt solutions do not affect the loss behavior.The simple behavior obtained for this orthoclase is attributed of the use of a simple technique within the region of sample stability and to the homogeneous nature of the feldspar. Effects due to sample instability and to the use of perthites are discussed. The new data are compared to those for homogeneous feldspars showing that the orthoclase gives diffusion coefficients which are as low as those sanidine. It is suggested that perthitization of feldspars in nature may reduce the effective grain size for diffusion and thereby allow diffusional loss of Ar⁴⁰ at relatively low temperatures.