Flares of the H<Subscript>2</Subscript>O Maser Emission in the Young Stellar Object GH<Subscript>2</Subscript>O 092.67+03.07 (IRAS 21078+5211)

The results of monitoring the H₂O maser observed toward the region GH₂O 092.67+03.07 (IRAS 21078+5211) located in the Giant Molecular Cloud Cygnus OB7 are presented. The observations were carried out with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 2006–2017. Strong flares of the H₂O maser emission with flux densities up to 19 800 Jy were detected. The flares exhibited both global (over the source) and local characters. All the flares were accompanied by strong variations in the H₂O spectra within the corresponding radial-velocity ranges. Individual H₂O components form both compact clusters and chains 1–2-AU long. Analysis of the variations of the fluxes, radial velocities, and line shapes of features during the flares showed that the medium may be strongly fragmented, with small-scale turbulent motions taking place in the H₂Omaser region.     

Spots and the Activity of Stars in the Hyades Cluster from Observations with the Kepler Space Telescope (K2)

Observations of the K2 mission (continuing the program of the Kepler Space Telescope) are used to estimate the spot coverage S (the fractional area of spots on the surface of an active star) for stars of the Hyades cluster. The analysis is based on data on the photometric variations of 47 confirmed single cluster members, together with their atmospheric parameters, masses, and rotation periods. The resulting values of S for these Hyades objects are lower than those stars of the Pleiades cluster (on average, by ΔS ~ 0.05?0.06). A comparison of the results of studies of cool, low-mass dwarfs in the Hyades and Pleiades clusters, as well as the results of a study of 1570 M stars from the main field observed in the Kepler SpaceMission, indicates that the Hyades stars are more evolved than the Pleiades stars, and demonstrate lower activity. The activity of seven solar-type Hyades stars (S = 0.013 ± 0.006) almost approaches the activity level of the present-day Sun, and is lower than the activity of solar-mass stars in the Pleiades (S = 0.031 ± 0.003). Solar-type stars in the Hyades rotate faster than the Sun (〈P〉 = 8.6 ^d ), but slower than similar Pleiades stars.     

Evolution and Spectrum of the Radio Emission of Tycho’s Supernova Remnant

The radio spectrum of Tycho’s Supernova Remnant is constructed at frequencies 12.6–143 000 MHz for epoch 2010.3, taking into account the secular decrease in the radio flux density of the remnant at the rate d = ?(0.46 ± 0.03)%/year:

$$S_\nu ^{3C10} (t = 2010.3) = (43.1 \pm 1.8 Jy)(\nu [GHz])^{ - (0.592 \pm 0.019) + (0.041 \pm 0.012)\log (\nu [GHz])} .$$

The spectrum has positive curvature. The presence of a low-frequency turnover in the spectrum of the radio source 3C10 with its maximum at 7.7 MHz is predicted, due to absorption in the interstellar medium in the direction toward the source.

     

Monotone nonlinear finite-volume method for challenging grids

This article presents a new positivity-preserving finite-volume scheme with a nonlinear two-point flux approximation, which uses optimization techniques for the face stencil calculation. The gradient is reconstructed using harmonic averaging points with the constraint that the sum of the coefficients included in the face stencils must be positive. We compare the proposed scheme to a nonlinear two-point scheme available in literature and a few linear schemes. Using two test cases, taken from the FVCA6 benchmarks, the accuracy of the scheme is investigated. Furthermore, it is shown that the scheme is linearity-preserving on highly complex corner-point grids. Moreover, a two-phase flow problem on the Norne formation, a geological formation in the Norwegian Sea, is simulated. It is demonstrated that the proposed scheme is consistent in contrast to the linear Two-Point Flux Approximation scheme, which is industry standard for simulating subsurface flow on corner-point grids.     

Adjoint analysis of Buckley-Leverett and two-phase flow equations

This paper analyzes the adjoint equations and boundary conditions for porous media flow models, specifically the Buckley-Leverett equation, and the compressible two-phase flow equations in mass conservation form. An adjoint analysis of a general scalar hyperbolic conservation law whose primal solutions include a shock jump is initially presented, and the results are later specialized to the Buckley-Leverett equation. The non-convexity of the Buckley-Leverett flux function results in adjoint characteristics that are parallel to the shock front upstream of the shock and emerge from the shock front downstream of the shock. Thus, in contrast to the behavior of Burgers’ equation where the adjoint is continuous at a shock, the Buckley-Leverett adjoint, in general, contains a discontinuous jump across the shock. Discrete adjoint solutions from space-time discontinuous Galerkin finite element approximations of the Buckley-Leverett equation are shown to be consistent with the derived closed-form analytical solutions. Furthermore, a general result relating the adjoint equations for different (though equivalent) primal equations is used to relate the two-phase flow adjoints to the Buckley-Leverett adjoint. Adjoint solutions from space-time discontinuous Galerkin finite element approximations of the two-phase flow equations are observed to obey this relationship.     

Experimental Study of the SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O–KF–KCl–NaF System at 700–800°C and 1–2 kbar Based on Synthetic Fluid Inclusions in Quartz

The phase state of the fluid in the H₂O–KF ± KCl ± NaF system is studied in the presence of quartz for an experimental assay of the mutual influence of various salts of the fluid-forming mixture on heterogeneous fluid equilibria. The fluid inclusions were synthesized in quartz by the fracture healing method from solutions with KF + KCl and KF + NaF mixtures at 1 or 2 kbar and 700, 750, or 800°C. The results of the fluid inclusion study indicate a heterogeneous state of the fluid and variation in the fluid composition during experiments as a result of its interaction with quartz. The increase in temperature and pressure, as well as variation in the proportions of the salt contents in the fluid-forming mixture, changed the course of chemical reactions. After all the experiments, a glassy phase was observed in some types of inclusions. It is known that aqueous KF or KCl solutions, the solubility of which increases during heating, are characterized by phase equilibria of systems of the first type (Valyashko, 1990), when liquid and vapor are equilibrated for a heterogeneous state of the fluid. In this case, some inclusions should homogenize to vapor. However, no similar inclusions were observed in contrast to denser fluid phases (liquids), which are typical of the upper heterogeneous area of systems of the second (P–Q) type. Some inclusions host solid phases, the solubility of which decreases as the temperature increases. The results of experiments in the presence of KF + NaF solutions showed that the amount of inclusions of heterogeneous entrapment increases at higher temperatures simultaneously with a decrease in the H₂O content of the glassy phase.     

Fahlore and Sphalerite from the Darasun Gold Deposit in the Eastern Transbaikal Region,Russia: I. Mineral Assemblages and Intergrowths,Chemical Composition,and Its Evolution

The mineral assemblages, mode of occurrence, and chemical compositions of coexisting fahlore and sphalerite from the Darasun gold deposit have been described. Three generations of fahlore and three generations of sphalerite have been recognized. The FeS content in sphalerite coexisting with fahlore ranges from 0.8 to 9.4 mol %. The complete solid solution series Fe-tetrahedrite–Zn-tetrahedrite–Fe-tennantite–Zn-tennantite reflected in Sb/(Sb + As) and Fe/(Fe + Zn) ratios ranging from 0 to 0.97 and from 0.07 to 1.00, respectively, with a predominant negative relationship between these ratios has been identified for the first time at the deposit. Stepped, oscillatory, and combined stepped-oscillatory growth zonings within fahlore grains and heterogeneous aggregates of fahlore have been found. Fahlore is enriched in As with respect to Sb, and Zn-tetrahedrite is followed by Fe- and Zn-tennantite from early to late generation; Zn-tetrahedrite is followed by Fe-tennantite in zoned grains and overgrown rims; sphalerite crystallized at decreased temperature and sulfur fugacity. The evolution of the chemical composition of fahlores was caused by the evolving temperature, fluid salinity, and conditions of metal migration.     

Interior boundary-aligned unstructured grid generation and cell-centered versus vertex-centered CVD-MPFA performance

Grid generation for reservoir simulation must honor classical key constraints and be boundary aligned such that control-volume boundaries are aligned with geological features such as layers, shale barriers, fractures, faults, pinch-outs, and multilateral wells. An unstructured grid generation procedure is proposed that automates control-volume and/or control point boundary alignment and yields a PEBI-mesh both with respect to primal and dual (essentially PEBI) cells. In order to honor geological features in the primal configuration, we introduce the idea of protection circles, and to generate a dual-cell feature based grid, we construct halos around key geological features. The grids generated are employed to study comparative performance of cell-centred versus cell-vertex control-volume distributed multi-point flux approximation (CVD-MPFA) finite-volume formulations using equivalent degrees of freedom. The formulation of CVD-MPFA schemes in cell-centred and cell-vertex modes is analogous and requires switching control volume from primal to dual or vice versa together with appropriate data structures and boundary conditions. The relative benefits of both types of approximation, i.e., cell-centred versus vertex-centred, are made clear in terms of flow resolution and degrees of freedom required.     

Comparison between cell-centered and nodal-based discretization schemes for linear elasticity

In this paper, we study newly developed methods for linear elasticity on polyhedral meshes. Our emphasis is on applications of the methods to geological models. Models of subsurface, and in particular sedimentary rocks, naturally lead to general polyhedral meshes. Numerical methods which can directly handle such representation are highly desirable. Many of the numerical challenges in simulation of subsurface applications come from the lack of robustness and accuracy of numerical methods in the case of highly distorted grids. In this paper, we investigate and compare the Multi-Point Stress Approximation (MPSA) and the Virtual Element Method (VEM) with regard to grid features that are frequently seen in geological models and likely to lead to a lack of accuracy of the methods. In particular, we look at how the methods perform near the incompressible limit. This work shows that both methods are promising for flexible modeling of subsurface mechanics.