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Theory: radiation transfer and atmosphere modeling

1-D models

The 1-D model grids will remain the backbone of spectral analysis for a long time, in particular for the host stars which can be approximated very well as spherical stars. An important reason for 1-D models is the high level of physical realism that such models have reached within the last 5­10 years. The 1-D structures will also serve as starting points for 3-D models (e.g., modeling the spectra and light curves of irradiated planets and predicting images of irradiated exoplanets). Both G¨ötingen and Hamburg have very o strong 1-D NLTE atmosphere groups. We will leverage this experience to develop a better treatment of non-isotropic and non-coherent scattering processes due to dust particles found in the atmospheres of cool stars, brown dwarfs and giant planets. This will include treatment of polarization, which may become an important diagnostic tool in the future. In addition, we will continue to improve the equation of state and opacity sets (dust, molecular lines etc.) in the 1-D models, compute improved model grids and test such models against observed data. All these developments will be immediately available to the 3-D simulations due to the modular construction of our code packages and through the availability of improved 1.5D starting models for the 3-D simulations.

3-D radiative transfer

Our theoretical work aims at extending the current 1.5D modeling capabilities towards full 3-D radiative transfer models of, e.g., irradiated exoplanets and cool accretion disks. This is fundamental to a wide range of astrophysical scenarios. Presently, we simulate a model spectrum of an irradiated planet by using a number of differently illuminated 1-D atmosphere patches, distributed over the observable face of the planet to compute an approximate spatially integrated spectrum. This allows for a very good treatment of the 1-D patches, with full line lists, complicated equation of state and NLTE. This approach allows treatment of radiation passing through the planet's atmosphere near the limb, an important diagnostic tool for transit observations. However, the important horizontal energy transfer cannot be treated properly by the simplified method, but requires full 3-D radiation transfer. Currently, our accretion disk models assume a separation of the disk into individual rings approximated as independent 1-D (irradiated) atmospheres. Similarly we will extend this 1.5D accretion disk modeling by solving the full 3-D radiation transport problem. A first major goal is to model the impinging radiation across the irradiated parts of the planet or the accretion disk. This will include the treatment of scattered radiation so that more accurate reflection spectra will be available. The computation of transmission spectra and light curves will be substantially improved by this approach. In addition, we can model horizontal radiation energy transport and provide the results as input to hydrodynamical models, allowing us to model the possible contribution of a disk to planetary transmission and reflection spectra. This work will combine the strong radiation transport expertise available in Göttingen and Hamburg. Hamburg has an ongoing collaboration with the University of Oklahoma (E. Baron) to develop a 3-D radiation transport framework for relativistic configurations. The static version of this 3-D radiation transport code has already been successfully tested and is currently available for the project as starting point. An important application will be to allow for the variation of the incident angle of the irradiation through the geometry of the system. At first, we will only be able to use a restricted set of wavelength regions for the 3-D modeling due to the increase in computer resources required by the 3-D radiation transport. However, both G¨ttino gen and Hamburg have access to regional supercomputer centers (GWDG, HLRN) so that we will be able to improve the input micro-physics over the course of the GC work.

Molecular Opacities and Databases

Our ability to analyze spectra depends critically on the knowledge of Einstein A-values and line frequencies for atomic and molecular transitions. While atomic line opacities are satisfactorily known, molecular opacities are still very uncertain. Crucial opacity sources at the relatively low exoplanet temperatures are water vapor, methane, and the abundant hydrides (FeH, CaH, MgH, LiH, TiH). Recent progress concerned water vapor (Barber et al. 2005), methane (Homeier et al. 2003), MgH and LiH (Weck et al. 2003, 2004), but was insufficient for CaH (judged by comparison to observed spectra). Data for important molecules such as FeH remain virtually unknown and semi-empirical data derived from, e.g., the Sun are only of limited value for our applications. Through the close collaboration of data providers and data users (e.g., our model atmosphere groups) already during the process we expect much improved timeliness and accuracy of previously inadequate molecular data. P. Hauschildt has very good experience with this type of collaboration via an ongoing project with P. Stancil at UGA and we intend to enhance these external collaborations in the future. The line data will be electronically available on the Göttingen and Hamburg ftp and web servers.