RoSSBi3D: a 3D and bi-fluid code for protoplanetary discs

The diversity of the structures recently observed in protoplanetary discs (PPDs) with the new generation of high-resolution instruments have made more acute the challenging questions that planet-formation models must answer. The challenge is in the theoretical side but also in the numerical one with the need to significantly improve the performances of the codes and to stretch the limit of PPD simulations. Multi-physics, fast, accurate, high-resolution, modular, and reliable 3D codes are needed to explore the mechanisms at work in PPDs and to try explaining the observed features. We present RoSSBi3D the 3D extension of the 2D code Rotating-System Simulations for Bi-fluids (RoSSBi) which was specifically developed to study the evolution of PPDs. This is a new code, even if based on the 2D version, that we describe in detail explaining its architecture and specificity but also its performances against test cases. This FORTRAN code solves the fully compressible inviscid continuity, Euler, and energy conservation equations for an ideal gas in non-homentropic conditions and for pressureless particles in a fluid approximation. It is a finite volume code which is second order in time and accounts for discontinuities thanks to an exact Riemann solver. The spatial scheme accounts for the equilibrium solution and is improved thanks to parabolic interpolation which permits to reach third order in space. The code is developed in 3D and structured for high-performance parallelism. The optimised version of the code works on high performance computers with a very good scalability. We checked its reliability against a 2D analogue of sod shock tube test and tests specific to PPDs that includes Rossby wave instability (RWI), streaming instability (SI), dust capture by a vortex and dust settling. We release this code under the terms of the CeCILL2 Licence and make it publicly available.Comment: 17 pages, 14 figure

NAS technical summaries. Numerical aerodynamic simulation program, March 1992 - February 1993

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1992-93 operational year concluded with 399 high-speed processor projects and 91 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

NAS Technical Summaries, March 1993 - February 1994

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1993-94 operational year concluded with 448 high-speed processor projects and 95 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

Planet formation and migration

We review the observations of extrasolar planets, ongoing developments in theories of planet formation, orbital migration, and the evolution of multiplanet systems.Comment: Revised version. Typos corrected in equations (36)-(48). Article available free at www.iop.org/journals/thismonth until December

Magnetic Fields in Proto-Neutron Stars and in Accretion Discs Around Neutron Stars

Themain characters of this thesis are themagnetic field, the plasma velocity field, the turbulentmagnetic resistivity and the numerical codes. They act on two different stages and on two different levels and occasionaly there are other bit players, e.g. the \u3b1-effect, the quenching, the differential rotation, themagnetic streamfunction, themagnetic Reynolds number, the Interactive Data Language and even ZEUS. All of them are led by the same invisible hand with the purpose of understanding better the intricate topic of the magnetic field - plasma relation. The two stages of the scene could not be more different, in one case everything is done in less than a minute inside a proto-neutron star soon after a supernova explosion, in the other case there is no time evolution at all and an equilibrium configuration is looked for inside a disc ofmatter spiraling around a neutron star. Nevertheless the same set of equations can describe the behaviour of the characters on both stages, this set is composed of the equations of the electromagnetic field plus the fluid equations. However knowing that the answers to all of your questions are written inside only one book, does not mean that you are able to read that book ... It is at this moment that the numerical codes come into the scene, offering you a way of translating the book in a language that you know. Unfortunately they like playing tricks and you cannot trust their translations unless you take many precautions every time. Eventually, after the equations have been solved, comes the art of interpreting the results; a task that might seem quite simple in comparison with the difficulties overcome on the path to get there, but that requires a deep knowledge of what has already been done and a good intuition about what can possibly happen later on. We do not presume to have made big leaps forward in the process of understanding the behaviour of the magnetic field in the cases considered here, nonetheless thanks to our simplified models we were able to grasp the fundamental aspects of the phenomena being considered, to gain some insights and to propose new falsifiable ideas. At the same time we have also developed new tools for making our models more elaborate and realistic. Therefore we expect to find even more characters in the future Chapters of this analysis, but that is another story, and will be told another time

Examining the effects of magnetic fields in neutron star mergers through numerical simulations

In this thesis, we present simulations of merging binary neutron stars, carried out using the publicly available FLASH code framework. These are 3D Newtonian magnetohydrodynamic simulations, in which we have included gravitational wave effects through the use of a source term. We trial different implementations of this source term and discuss the results. We then use this model to investigate the role of magnetic fields in binary neutron star mergers. We endow each neutron star with a dipolar magnetic field and examine how the orientation of the dipole affects the strength and structure of the magnetic field during the merger. This has important implications for the ability of the merger remnant to produce a short gamma ray burst jet. In a second project, we simulate a magnetized accretion torus surrounding a black hole. We implement a model black hole in the FLASH code framework using a Pseudo-Newtonian potential to reproduce features, such as the innermost stable circular orbit, which are important to accretion disc studies. We compare the results of our magnetized accretion disc simulations with similar studies, finding broad agreement with accretion rates and the general structure of the magnetic field

Research and Technology, 1995

This report presents some of the challenging research and technology accomplished at NASA Ames Research Center during FY95. The accomplishments address almost all goals of NASA's four Strategic Enterprises: Aeronautics and Space Transportation Technology, Space Sciences, Human Exploration and Development of Space, and Mission to Planet Earth. The report's primary purpose is to inform stakeholders, customers, partners, colleagues, contractors, employees, and the American people in general about the scope and diversity of the research and technology activities. Additionally, the report will enable the reader to know how these goals are being addressed

The Numerical Modelling Of Scenarios For The Herbig-Haro Object HH30

The classical T-Tauri star HH30 in Taurus-Auriga exhibits a well-collimated plume of hot, optically-emitting atomic and partially ionised Hydrogen, and also a colder, dense, wide-angle molecular Hydrogen ouflow. Observations suggest HH30 is a binary system system, surrounded by a circumbinary accretion disc. We investigated the propagation and interaction of dual atomic and molecular outflows from HH30, using a series of numerical simulations with parameters informed by observational campaigns. These 3-dimensional models were computed using the established Eulerian astrophysics code ZEUS-MP, with in-house modifications and an enhanced chemistry and cooling module. These simulations assumed off-domain launch and tracked the evolution of the jets over spatial scale of ~ 100 AU, and with a timescale ~ 100 - 200 years. The propagation in this region is of special interest, as this is where the greatest difference between the two scenarios is likely to emerge. Our work here differs from "classical" simulations of jet propagation by virtue of one or both outflow sources moving in an orbit. Two competing scenarios were investigated, in which the morphology of the light-year scale outflow from HH30 is explained by different kinds of motion of the atomic outflow source, and in which the launch site of the molecular outflow differs. In both cases a velocity-pulsed atomic jet emerges from the more massive binary object. In the Orbital scenario, the orbital motion of the primary explains the morphology seen at large scale, while the molecular flow is launched from the secondary partner; in the Precessional scenario, precession of the primary dominates the morphology, while launch of the molecular flow is from the inner edge of the circumbinary disc. The binary orbit and inner depletion zone of the circumbinary disc differs between the scenarios, with the Precessional scenario having a much smaller orbit and correspondingly reduced inner depletion zone. Clearly identifiable structural differences emerge between the simulated models. We compared the effects of the two different kinds of perturbing molecular outflow on the faster atomic jet; position, velocity, line mass per unit length, temperature and other variables, as a function of distance x (AU) from the binary source. Linear and quadratic fit functions were determined to facilitate comparison with observation. These quantify the expected behaviours of the atomic jet in the presence of the two different kinds of molecular flow. Where the fit function domains overlap direct comparisons may be drawn; where 26 < x < 42 AU, the average velocity as a function of distance is Vx(x) = (1.39×10^?1 ±2.15×10^?3)x + (246.82±1.29) km s^?1 in the Precessional model, while in the Orbital model we find Vx(x) = (?3.26 ± 0.26)x + (269.57 ± 6.75) km s^?1. In the region 10 < x < 60 AU, the Precessional model has temperature dependence T(x) = (64.53 ± 12.54)x + (3535 ± 330) K. Whilst in the same region of the Orbital model, T(x) = (401.99 ± 333.19)x + (4258.4 ± 1340.3) K. Synthetic Mass-Velocity Spectra have been generated for our models, to investigate distinguishing features of these spectra in the presence of the two different types of molecular outflow. The shallow-angle spectra matching the aspect angle of HH30 itself are examined and the link between outflow scenario and time variability discussed. Spectra from the same dual outflow systems observed at different aspect angles to the sky plane are given, to provide a means to confirm these senarios in other HH30-like T-Tauri stars. Using code written in-house to calculate emission using rate coefficients for photon production, we generated synthetic observations; spatially resolved images, velocity channel maps and position-velocity diagrams. The morphology of the synthetic images from the two scenarios when compared to HST R-band imaging of HH30 suggests that the Orbital case is unlikely, whilst the Precessional case is supported

Gravitational instability and fragmentation of self-gravitating accretion disks

We know from observations that supermassive black holes (SMBH) of masses up to 10^{10} \msol existed in quasars when the universe was only about $10^9$ years old. The rapid formation of SMBHs can be understood as the outcome of the collision of two large gas-rich galaxies followed by disk accretion. This model relies on a large enough turbulent viscosity in the disk. We show in a linear stability analysis of thin self-gravitating viscous disks that the gravitational instability can drive a turbulence generating the $\beta$-viscosity. For simulating a self-gravitating accretion disk in polar coordinates the hydrodynamics code NIRVANA2.0 is adapted for our needs which includes cooling. The results are disk fragmentation, strong accretion at the inner radial boundary of the calculation domain and strong outflow at the outer boundary which both come about by interactions between clumps. The accretion time scale for a disk mass of 6\ex{8} \msol in a radial extent of 29 \pc to 126 \pc is about 1.2\ex{7} \yr, corresponding to a viscosity parameter $\beta \approx 0.04$. We can confirm the $\beta$-viscosity interpretation by the turbulent velocity and length scale and by the scaling of the accretion time scale. All this supports the SMBH-formation model