France Allard
Tel: 04 72 72 87 07
Fax: 04 72 72 87 87
Curriculum vitae
Phoenix Web Simulator
Projet CNRS 98
46, Allée d'Italie
69364 Lyon Cedex 07

Model Atmospheres and Synthetic Spectra of Very Low Mass Stars,
Brown Dwarfs,
and Extrasolar Planets


The only type of information comes from most stars and brown dwarfs in the form of an electromagnetic spectrum. The modeling of their atmosphere remains therefore the preferred tool for understanding these objects among the most numerous of our Galaxy. This modeling is based upon physical principles and well as upon thermochemical and spectroscopic databases which must be as complete as possible. An organigram illustrates well the method by which we model atmospheres and obtain informations about these objects.

For this we use the model atmosphere code Phoenix, a static and radial (1D) code which is however general, modeling as well novae and supernovea envelopes in relativistic expansion phases and extrasolar planets irradiated by a star within the hydrostatic equilibrium and spherical symmetry approximations.

While these 1D, static, and spherical symmetry approximations (classical to stellar atmosphere models) of Phoenix may appear to be serious limitations, this allows a complete description of the atmospheric emission spectrum. In order to account for the formation of molecular bands such as those of the water vapor, methane, or titanium oxide, we do not hesitate to solve the transfer equation over more than 20000 wavelength points in average, producing synthetic spectra with 2 Å resolution. The line selection is repeated at each iteration of the model. When the model is converged and the thermal structure obtained, Phoenix can then be used to compute higher resolution spectra upon request. But mainly, the relatively high computing speed (2h/model) can therefore allow for the generation of large grids of model atmospheres and synthetic spectra which then serve respectively as surface boundary condition and convertion into observational planes for interior and evolution models.

Among the most useful model atmosphere grids that we have constructed, you can access these listed below. Note that the names and format of the files are explained. Other informations about the grids can be found in README. Choose and click the repertory corresponding to the model grid you need. See the README file for some help. The suggested models are the NextGen for Teff > 2700K, AMES-Dusty for 2700K > Teff > 1700K, and AMES-Cond for Teff < 1400K. The new BT-Settl models are valid across the regime from stars to brown dwarfs as cool as 400K. The range of T type brown dwarfs is behing studied.

      1. NextGen (Very Low Mass Stars, no dust formation)
      2. AMES-Cond (brown dwarfs/extrasolar planets without irradiation, no dust opacity)
      3. AMES-Dusty (brown dwarfs/extrasolar planets without irradiation, with dust opacity)
      4. BT-Settl (stars/brown dwarfs/planets (without irradiation), with a cloud model)
      5. EGPs (extrasolar giant planets)
      6. Rosseland/Planck opacity tables

You can alternatively use the Phoenix web simulator which allows to compute colors, isochrones and ssynthetic spectra. It also allows to find possible parameters for an object from its colors by chi-square minimisation into the isochrones.

Some publications to download

Derek Homeier
Bernd Freytag
Isabelle Baraffe
Nicole Allard
Travis Barman
Peter H. Hauschildt
Hans-Günter Ludwig

Other useful links:
Centre Informatique National de l'Enseignement Superieur
ADS Abstract Service
Google Search engine

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