Turbulent Mixing in Stars: Theoretical Hurdles

A program is outlined, and first results described, in which fully three-dimensional, time dependent simulations of hydrodynamic turbulence are used as a basis for theoretical investigation of the physics of turbulence in stars. The inadequacy of the treatment of turbulent convection as a diffusive process is discussed. A generalization to rotation and magnetohydrodynamics is indicated, as are connection to simulations of 3D stellar atmospheres.Comment: 5 pages, 1 figure, IAU Symposium 265, 200

Turbulent Cells in Stars: I. Fluctuations in Kinetic Energy and Luminosity

Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning (Meakin and Arnett, 2007b) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a general instability of the convective cell, requiring only a localized source of heating or cooling. Such fluctuations are shown to be suppressed in simulations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to chaotic fluctuations in time of velocity and, as we show, luminosity. Study of simulations suggests that the behavior of a Lorenz convective roll may resemble that of a cell in convective flow. We examine some implications of this simplest approximation, and suggest paths for improvement. Using the Lorenz model as representative of a convective cell, a multiple-cell model of a convective layer gives total luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants (Schwarzschild 1975)), and of the long secondary period feature in semi-regular AGB variables (Stothers 2010, Wood, Olivier and Kawaler 2004). This "tau-mechanism" is a new source for stellar variability, which is inherently non-linear (unseen in linear stability analysis), and one closely related to intermittency in turbulence. It was already implicit in the 3D global simulations of Woodward, Porter and Jacobs (2003). This fluctuating behavior is seen in extended 2D simulations of CNeOSi burning shells (Arnett and Meakin 2011b), and may cause instability which leads to eruptions in progenitors of core collapse supernovae PRIOR to collapse.Comment: 30 pages, 13 figure

Towards Realistic Progenitors of Core-Collapse Supernovae

Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a 23 solar mass star, close to core collapse (about 1 hour, in 1D), with simultaneously active C, Ne, O, and Si burning shells, are presented and contrasted to existing 1D models (which are forced to be quasi-static). Pronounced asymmetries, and strong dynamical interactions between shells are seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to sufficiently large amplitudes that it couples to the nuclear burning. Dramatic growth of low order modes is seen, as well as large deviations from spherical symmetry in the burning shells. The vigorous dynamics is more violent than that seen in earlier burning stages in the 3D simulations of a single cell in the oxygen burning shell, or in 2D simulations not including an active Si shell. Linear perturbative analysis does not capture the chaotic behavior of turbulence (e.g., strange attractors such as that discovered by Lorenz), and therefore badly underestimates the vigor of the instability. The limitations of 1D and 2D models are discussed in detail. The 2D models, although flawed geometrically, represent a more realistic treatment of the relevant dynamics than existing 1D models, and present a dramatically different view of the stages of evolution prior to collapse. Implications for interpretation of SN1987A, abundances in young supernova remnants, pre-collapse outbursts, progenitor structure, neutron star kicks, and fallback are outlined. While 2D simulations provide new qualitative insight, fully 3D simulations are needed for a quantitative understanding of this stage of stellar evolution. The necessary properties of such simulations are delineated.Comment: 26 pages, 1 table, 4 figure

Turbulent Convection in Stellar Interiors. II. The Velocity Field

We analyze stellar convection with the aid of 3D hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations (we relate temperature and density fluctuations via coefficients); the procedure (CABS, Convection Algorithms Based on Simulations) is terrestrially testable and is amenable to systematic improvement. We develop a turbulent kinetic energy equation which contains both nonlocal and time dependent terms, and is appropriate if the convective transit time is shorter than the evolutionary time scale. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect; MLT forces the mixing length to be an imposed constant. Direct tests show that the damping associated with the flow is that suggested by Kolmogorov. The eddy size is approximately the depth of the convection zone, and this dissipation length corresponds to the "mixing length". New terms involving local heating by turbulent dissipation should appear in the stellar evolution equations. The enthalpy flux ("convective luminosity") is directly connected to the buoyant acceleration, and hence the velocity scale. MLT tends to systematically underestimate this velocity scale. Quantitative comparison with a variety of 3D simulations reveals a previously recognized consistency. Examples of application to stellar evolution will be presented in subsequent papers in this series.Comment: 47 pages, 7 figures, accepted by Ap

Janus monolayers of transition metal dichalcogenides.

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements

Presupernova Structure of Massive Stars

Issues concerning the structure and evolution of core collapse progenitor stars are discussed with an emphasis on interior evolution. We describe a program designed to investigate the transport and mixing processes associated with stellar turbulence, arguably the greatest source of uncertainty in progenitor structure, besides mass loss, at the time of core collapse. An effort to use precision observations of stellar parameters to constrain theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics and Space Science, submitted for peer review: 7 pages, 3 figure

Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence

We employ simulations of supersonic super-Alfvenic turbulence decay as a benchmark test problem to assess and compare the performance of nine astrophysical MHD methods actively used to model star formation. The set of nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER, and ZEUS. We present a comprehensive set of statistical measures designed to quantify the effects of numerical dissipation in these MHD solvers. We compare power spectra for basic fields to determine the effective spectral bandwidth of the methods and rank them based on their relative effective Reynolds numbers. We also compare numerical dissipation for solenoidal and dilatational velocity components to check for possible impacts of the numerics on small-scale density statistics. Finally, we discuss convergence of various characteristics for the turbulence decay test and impacts of various components of numerical schemes on the accuracy of solutions. We show that the best performing codes employ a consistently high order of accuracy for spatial reconstruction of the evolved fields, transverse gradient interpolation, conservation law update step, and Lorentz force computation. The best results are achieved with divergence-free evolution of the magnetic field using the constrained transport method, and using little to no explicit artificial viscosity. Codes which fall short in one or more of these areas are still useful, but they must compensate higher numerical dissipation with higher numerical resolution. This paper is the largest, most comprehensive MHD code comparison on an application-like test problem to date. We hope this work will help developers improve their numerical algorithms while helping users to make informed choices in picking optimal applications for their specific astrophysical problems.Comment: 17 pages, 5 color figures, revised version to appear in ApJ, 735, July 201

Local helioseismology and correlation tracking analysis of surface structures in realistic simulations of solar convection

We apply time-distance helioseismology, local correlation tracking and Fourier spatial-temporal filtering methods to realistic supergranule scale simulations of solar convection and compare the results with high-resolution observations from the SOHO Michelson Doppler Imager (MDI). Our objective is to investigate the surface and sub-surface convective structures and test helioseismic measurements. The size and grid of the computational domain are sufficient to resolve various convective scales from granulation to supergranulation. The spatial velocity spectrum is approximately a power law for scales larger than granules, with a continuous decrease in velocity amplitude with increasing size. Aside from granulation no special scales exist, although a small enhancement in power at supergranulation scales can be seen. We calculate the time-distance diagram for f- and p-modes and show that it is consistent with the SOHO/MDI observations. From the simulation data we calculate travel time maps for surface gravity waves (f-mode). We also apply correlation tracking to the simulated vertical velocity in the photosphere to calculate the corresponding horizontal flows. We compare both of these to the actual large-scale (filtered) simulation velocities. All three methods reveal similar large scale convective patterns and provide an initial test of time-distance methods.Comment: 15 pages, 9 figures (.ps format); accepted to ApJ (tentatively scheduled to appear in March 10, 2007 n2 issue); included files ms.bbl, aabib.bst, aabib.sty, aastex.cl

Proyecto técnico de nueva Instalación de una fábrica de yogur en el TM de Requena

[ES] Este Trabajo Final de Grado tiene como objetivo la inscripción en el Registro de Establecimientos Agroalimentarios (en adelante R.E.A.) de una nueva instalación de una fábrica de yogur. Se trata de un proyecto técnico en el que se detalla el objeto del proyecto, la legislación aplicable, el titular de la industria, la distribución de las superficies del terreno en el que se localiza la instalación, un programa productivo de materias primas, productos obtenidos, y un cuadro de capacidades anuales, entre otros. Además, se refleja en dicho proyecto la maquinaria que tiene la fábrica, el proceso industrial del producto (con el consiguiente diagrama), y un estudio económico-financiero de la actividad y los ingresos y gastos. El producto del proyecto es yogur, tanto líquido como firme. De la misma forma, se desarrollan dos instalaciones: la eléctrica y la de refrigeración. La inscripción de una instalación o industria en el R.E.A. es obligatoria para los titulares de empresas con actividades y establecimientos incluidos en el ámbito de aplicación del Reglamento del R.E.A. Dicha inscripción es importante al tratarse de un requisito indispensable para que las industrias agroalimentarias puedan acogerse a los auxilios económicos de todo tipo que se arbitren mediante programas de ayudas regulados por convocatorias de la Generalitat, y financiados por fondos comunitarios, nacionales o de ámbito autonómico (Decreto 97/2005, de 20 mayo).[EN] The aim of this project is the registration in the Registro de Establecimientos Agroalimentarios (hereinafter R.E.A.) of a new facility of a yogurt factory. This is a technical project in which the aim of the project is detailed, the applicable legislation, the owner of the industry, the distribution of the areas of land on which the facility is located, a production program of raw materials, obtained products, and a table of annual capacity, among others. In addition, the machinery that the factory has is reflected, the manufacturing process of the product (with the resulting diagram), and an economic and financial study of the activity and income and expenses. The product of the project is yogurt, firm and drinkable. Also, two installations are developed: electrical and cold installations. The registration of a facility or industry in the R.E.A. is mandatory for owners of companies with activities and facilities included in the scope of the R.E.A. Regulation. This registration is important as this is a prerequisite for food processing industries to be eligible for economic aids of all kinds that are arbitrated by aid programs regulated by official announcements of the Generalitat, and financed by EU, national or regional funds (Decree 97/2005 May 20th).Nordlund Sierra, DP. (2016). Proyecto técnico de nueva instalación de una fábrica de yogur en el TM de Requena. http://hdl.handle.net/10251/76539.TFG