3D Radiative Transfer in Protoplanetary Disks

Protoplanetary disks are the sites of planet formation during the final stages of the star formation process. Disks consist of 99% gas and 1% dust, both components playing a pivotal role for disk opacity, radiative transfer, heating and cooling, as well as chemical processing. The evolution of the disk itself is strongly influenced by stellar radiation, and in particular by short-wavelength (ultraviolet, extreme ultraviolet, and X-ray) photons because of their ionization and heating power, leading to enhanced chemical processing, disk evaporation, and magnetic disk instabilities. A proper understanding of the internal structure of a protoplanetary disk and its chemical and physical processing requires advanced computational methods solving for radiative transfer, heating and cooling mechanisms, and chemical networks under the influence of external irradiation in a self-consistent way. In the past decade, a number of approximative schemes have been developed to compute observable characteristics of protoplanetary disks, especially through radiation from their surface layers. We have started to develop an entirely new approach to the fundamental problem of disk modeling by adapting the multi-purpose 3D radiative transfer code PHOENIX/3D to protoplanetary disk conditions and disk geometry, including non-LTE conditions, anisotropic scattering, full continuum and line transfer, and adaptation of comprehensive chemical networks. PHOENIX/3D solves the radiative transfer equation rigorously throughout all optical depth layers of the disk, thus permitting to compute the entire disk profile from the cool, dense interior where most of the mass to form planets resides, to the tenuous and warm disk surface layers irradiated by starlight. PHOENIX/3D thus treats both regions which are akin to stellar atmospheres as well as "nebular-like" disk surface layers. The present project will specifically address three crucial, related issues toward modeling of externally irradiated disks with PHOENIX/3D, namely i) the full 3D structure of a static disk to be iterated toward a self-consistent solution with PHOENIX/3D, ii) to treat, within this structure, gas and dust as two thermally decoupled components as required under conditions prevailing in some of the disk volume, and iii) to implement anisotropic scattering to correctly treat radiative transport through the disks also for short-wavelength external radiation. Other packages (e.g., chemical modules) will be provided to this project. Our new code will break new ground by computing the entire 3D disk structure rigorously and at the same time providing comprehensive observational diagnostics. We aim at applications for disk modeling and interpretation based on new observational data from Herschel, ALMA, and in the future the James Webb Space Telescope.

Protoplanetary disks are the sites of planet formation during the final stages of the star formation process. Disks consist of 99% gas and 1% dust, both components playing a pivotal role for disk opacity, radiative transfer, heating and cooling, as well as chemical processing. The evolution of the disk itself is strongly influenced by stellar radiation (X-rays, ultraviolet, optical) but also energetic particles (mainly protons). A proper understanding of the internal structure of a protoplanetary disk and its chemical and physical processing requires advanced computational methods solving for radiative transfer, heating and cooling mechanisms, and chemical networks.In this project we made use of two disk modelling codes called PHOENIX/3D and PRODIMO. The first one is mainly used to model the gaseous atmosphere of stars which we further developed towards a full 3D disk simulation code. The second one is based on physics of the interstellar medium and is already an advanced two dimensional disk simulation code. With both codes we aim towards a direct comparison of our results to observations and to make predictions to inform future observational campaigns.We extended PHOENIX/3D towards a 3D disk radiative transfer code by implementing a proper description for the full 3D disk density and temperature structure. Further we developed an interface to a hydrodynamics code to produce e.g. images which can be directly compared to observations of real telescopes. PHOENIX/3D/DISK still lacks a proper treatment of the disk chemistry. However, the already available simpler chemical model can be applied to the hot and dense disk regions close to the star. Such models may be used in a new project of our group dealing with the episodic accretion of material by the star.We used and further developed PRODIMO to study the impact of X-ray emission and stellar energetic particles on disk chemistry. The young Sun was likely more active than currently (e.g. strong X-ray emission, higher production of energetic particles). We know from meteoritic material that the disk, from which our solar system was formed, was likely irradiated by a large number of energetic particles. From our simulations we find that it is possible to infer the energetic particle flux from young stars by observing molecular ions in the surrounding disk. With such observations at hand, it is possible to investigate if the anomalies in meteoritic material are indeed a consequence of enhanced particle production of the young Sun.Until a star fully develops it needs to collect material from its environment. This process is called accretion and likely happens episodically, with quiet phases (low accretion) interrupted by short periods (100s of years) with strongly enhanced accretion. Although we can observe such accretion bursts, it is yet unclear if all young stars experience such bursts. During a burst we observe an enhanced energy output close to the star which heats the stellar environment.Due to the heating molecules frozen-out on dust grains are released back to the gas phase and start to freeze-out again as soon as the burst has stopped. This freeze-out happens slowly and causes distinct patterns in the abundances of molecules which can be observed up to 10 000 years after the burst. From our simulations we derived a new method to identify those patterns in real observations. With such observations more potential burst sources can be identified which is crucial to answer the question if all young stars go through the violent times of episodic accretion.