Simulations and Theory

Today, the state-of-the-art in numerical simulations is defined by codes that have been developed and refined over decades. It is recognized that a scientific return of large instruments will only be achieved if the community has access to these codes to prepare missions and to interpret observations.

In this context, VO-Theory activities at VO-Paris cover a wide range of activities to provide theoretical services to the community. This includes publication of source codes and libraries, development of VO services to provide access to online codes, theoretical databases, and tools to compare simulated data and observations.

VO-Theory members of VO-Paris also participates to the development of VO standards at the IVOA and to their implementation.

VO Services

  - Galmer database
GalMer is a database in the frame of the Horizon project. It provides access to N-body simulation of galaxy mergers (Di Matteo et al. 2007). The N-body simulations are made using the realization of the Tree-SPH code, the star formation and metal enrichment histories in merging galaxies are followed by means of hybrid particles. The first release of the GalMer data comprises results of ~900 experiments representing mergers of giant galaxies of 4 different morphological types (E0 to Sd) represented by 120000 particles.
A number of value-added services is provided, such as (1) generation of maps of various physical parameters; (2) generation of synthetic spectra of stellar populations in the merging galaxies; (3) generation of synthetic broad-band images.
Preview images and their manipulation can be done thanks to VO software such as TOPCAT, CDS Aladin, ESA VOSpec.
Developments have been supported by the HORIZON ANR and EURO-VO DCA.
  - Meudon PDR services
The Meudon PDR code computes the atomic and molecular structure of a stationary plane-parallel slab of gas and dust illuminated by a radiation field (ISRF or star). The code computes the cloud structure (chemical abundances, excitations, temperatures, ...) and provide access to line intensities and column densities.
The Meudon PDR services can be used to study the physics and chemistry of diffuse clouds, photodissociation regions (PDRs), dark clouds, circumstellar regions, ...
VO services are developed in perspective to HERSCHEL and ALMA observations :
  • PDR database (based on SimDB)
  • Online code through the Virtual Observatory
Services above the database permits queries on column densities for inverse problems and provide access to the full physics in the simulations (abundance profiles, spectra, line intensities, chemistry analyser, ...).
Development of VO services are supported by EURO-VO DCA/Aida and the ASTRONET project STARFORMAT.


  - Dalia
DALIA is a prototype of the MAGIX concept (Modelization of Astronomical data with a Generic Interface to eXternal numerical simulation codes), a versatile model optimizer. It aims at providing a generic interface to interpret observations thanks to numerical simulations. This tool can compare simulation outputs to data (spectra, cubes, ...), and iterates on the model inputs to find the best fitting parameter set, as well as confidence intervals on those parameters. Several minimization algorithms will be implemented. Implementations of simulation codes in DALIA are made easy thanks to the generation of a generic interface created from an XML description of the input parameters of the code.
The development of MAGIX is part of the ASTRONET project CATS.

Public codes and webservices

Sources for several codes and libraries are published by scientists of VO-Paris.

  - APPLawD
APPlawD ("Accurate Potentials in Power Law Disks") determines the gravitational potential in the equatorial plane of a flat axially symmetric disk (inside and outside) with finite size and power law surface density profile. Potential values are computed on the basis of the "density splitting method" (Hure and Pierens, 2005, ApJ, 624, 289), were the residual Poisson integral is expanded over the modulus of the complete elliptic integral of the first kind. In contrast with classical multipole expansions of potential theory, the residual series has a cubic convergence rate inside sources, leading to very accurate potential valuesfor low order truncations of the series.
Object Language for Numerical Relativity to solve various problems arising in numerical relativity, and more generally in computational astrophysics. Using multi-domain spectral methods, LORENE can implement matrices, tensors, and astrophysical objects, such as stars and black holes.
  - MHD Shocks models
Code computing the atomic and molecular structure of a slab of gas and dust submitted to a shock. Physics include 1) stationary flow of gas throughout the shock (in the shock frame), 2) time evolution of a single gas cell as it crosses the shock, 3) possible magnetic field (the shock may be "MHD" or not) 4) 3 fluids model (neutral gas, ions, electrons).
The code provides abundance profiles, column densities and level excitations.
  - Monte-Carlo Radiative Transfer code
A Monte-Carlo radiative transfer code in sub-millimeter optimized for the hyperfine structure of N2H+/N2D+ with lines overlapping. The code is based on the original code by Bernes (1979). Recent collision rates with He have been implemented.
To get the code, contact : Laurent Pagani at Paris Observatory.
  - Stokes
The computer program STOKES is a publicly available radiative transfer code to model optical and ultraviolet polarization. It was written by Rene Goosmann and is designed to model astrophysical objects of various geometries. To obtain further information about STOKES just follow the web link above.

In development

  - Cifist Legacy
The CIFIST legacy project will give access to data products of the 3D hydrodynamical simulations of stellar atmospheres computed in the course of the EU funded CIFIST (Cosmological Impac of the FIrst STars) project. The data products of the 3D model runs will be provided into a format which allows easy application and easy access for interested researchers, particularly those of interest to spectral synthesis applications. The data base consists at present of about 70 simulations and associated auxiliary data.
  - Dark Energy Universes Virtual Observatory
The aim of this project is to develop a VO Theory database to share RAMSES large scale simulations with different cosmological models (LCDM, SUCDM, RPCDM). The first version of the database will provide access to recent simulations ran on Bluegene at IDRIS aiming at discovering the properties of dark energy. Services will provide access to properties of halos (position, mass, velocity, size) with statistical studies, cut-out for third-party analysis, ...
  - Starformat
STARFORMAT will be a database of MHD simulations of the dynamical evolution of interstellar gas. Several services above the database will provide access to images, cuts, catalogues of dense codes as well as properties of lines of sight (as densities and velocities). These services will be interoperable with VO-tools and other theoretical services such as the Meudon PDR code and radiative transfer codes. One of the aim is to determine observables from these detailled dynamical simulations for comparison to HERSCHEL and ALMA observations.
This project is funded by ASTRONET.
  - Titan
TITAN calculates the interactions of a dilute stationnary non LTE plane-parallel medium with electromagnetic radiation. It includes atomic processes: absorption, recombination, diffusion, excitation, deexcitation of atoms and ions, heating and cooling of the gas, and it solves the radiation transfer, in order to obtain the spectra reemitted by the medium. It is specifically designed for warm-hot (8000 to 10**8 K) and thick media (till an electron scattering optical depth of several tens) emitting and absorbing in the X-ray range (density from 10**5 to 10**14 cm-3).
It computes the physical parameters, ionisation degrees, temperature, density, and the spectrum of the radiated light in each point of the slab, by solving simultaneously the ionisation equations, the equations of statistical equilibrium, the thermal equations and the radiation transfer, using iteration processes. The most important features are :
  • the radiation transfer equation is solved for the continuum and for the lines (without resorting to escape probabilities), using a competitive method called "accelerated lambda iteration" (ALI), which assures a very accurate result for the transfer inside thick clouds, at the expense of a long computation time
  • the radiation transfer equation is solved for several directions, thus permitting various direction illuminations
  • the whole specific intensity is treated, without distinction between diffuse or reemitted radiation and incident absorbed radiation


People of VO-Paris involved in the VO-Theory aspects also work on VO standards:

  • Characterisation DM
  • Simulation Datamodel (SimDM and SimDB)
  • Simulation Data Access Protocol (SimDAP)
  • Semantics for theory

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