PHOENIX/3D NLTE Calculations
Hamburger Sternwarte, Universität Hamburg (Germany)
Local Project ID:
HPC Platform used:
JUQUEEN of JSC
Researchers finished the implementation and verification of a 3D non-local thermodynamic equilibrium (NLTE/3D) module for the PHOENIX/3D model atmosphere simulation code. The methods were extended to also allow NLTE modelling of molecular lines (here: CO) and then used to model the radiation from parameterized star-spots to investigate the effects of detailed 3D radiation transport on observables.
The PHOENIX code is a general-purpose model atmosphere simulation package. It is designed to model the structures and spectra of a wide range of astrophysical objects, from extrasolar planets (both terrestrial and gas giant planets) to brown dwarfs and all classes of stars, extending to novae and supernovae. The main results from the calculations are synthetic spectra (and derived quantities, such as colours), these can be directly compared to observed spectra and, in 3D simulations, images. By adjusting the simulation parameters and comparing the results to the observed data, the physical parameters of the stars or planets can be determined. The PHOENIX package has been designed for parallel computers since about 1993, the current version uses a hierarchical MPI+OpenMP based parallelization scheme
Over the last decade, the scientists of the Hamburger Sternwarte at Universität Hamburg have developed a 3D radiative transfer (3DRT) framework and integrated it into their PHOENIX model atmosphere package, creating a PHOENIX/3D mode in addition to the 1D models with the PHOENIX/1D mode. The main scientific drivers of PHOENIX/3D are models of: (a) irradiated extrasolar planets (e.g., reflection & transmission spectra, light curves); (b) structures in the ejecta of supernovae, classical novae, and stellar winds; and (c) "surface'' structure of stars (e.g., spots) and planets. Furthermore, there are many additional applications of PHOENIX/3D, e.g., true image calculation for hydrodynamical models and cloud modelling. In technical terms, the science drivers require not only the basic 3DRT framework, but also modules for 3D non-local thermodynamic equilibrium (NLTE) calculations, spectral line opacities for atoms and molecules, the equation of state and so on.
With the allocation of computing time on HPC system JUQUEEN of JSC, the scientists were able to finish the implementation and verification of the NLTE/3D module (A&A 566, A89), improve the overall performance by adding a GNU gmp based solver for the rate equations (up to a factor of 10 faster than the previous solver, leading to an overall speedup by about a factor of 2 on JUQUEEN) and finish the development of a super-level NLTE/3D method for molecules and its first application to the simulation of the CO molecule radiation from (parameterized) star-spots (PhD thesis A. Berkner 2014).
The PHOENIX/3D calculations on JUQUEEN show excellent scaling properties. The researchers were able to demonstrate both weak (from 512 processes) and strong (from 8192 processes) scaling up to 131072 MPI processes. This is a crucial milestone for (a) much larger simulations and (b) scaling to several million MPI processes.