Retrieving the molecular composition of planet-forming material: an accurate non-LTE radiative transfer code for jwst based on the observed distributions of exoplanets and dynamical models of their…

This federal contract award, valued at $447,149.21, was granted by NASA to the Association of Universities for Research in Astronomy, Inc. (AURA), doing business as the Space Telescope Science Institute (STScI). The award is for developing a numerically accurate, non-LTE (non-local thermodynamic equilibrium) 3D line radiative transfer code to interpret mid-infrared molecular line observations of protoplanetary and debris disks in preparation for the James Webb Space Telescope (JWST). This work is critical for understanding the chemical composition of planet-forming regions and supporting NASA's broader astrophysics objectives. The award does not have a set-aside designation. AURA, a non-profit organization, has extensive experience managing and operating major astronomical observatories and telescopes on behalf of NASA and the National Science Foundation, and frequently collaborates with subcontractors on such projects.

Retrieving the molecular composition of planet-forming material: an accurate non-LTE radiative transfer code for jwst based on the observed distributions of exoplanets and dynamical models of their evolution, the primary planet-forming regions of protoplanetary disks are thought to span distances of 1-20 AU from typical STARS. A key observational challenge of the next decade will be to understand the links between the formation of planets in protoplanetary disks and the chemical composition of exoplanets. Potentially habitable planets in particular are likely formed by solids growing within radii of a few AU, augmented by unknown contributions from volatiles formed at larger radii of 10-50 AU. The basic chemical composition of these inner disk regions is characterized by near- to far-infrared (2-200 micron) emission lines from molecular gas at temperatures of 50-1500 k. A critical step toward measuring the chemical composition of planet-forming regions is therefore to convert observed infrared molecular line fluxes, profiles and images to gas temperatures, densities and molecular abundances. However, current techniques typically employ approximate radiative transfer methods and assumptions of local thermodynamic equilibrium (LTE) to retrieve abundances, leading to uncertainties of orders of magnitude and inconclusive comparisons to chemical models. Ultimately, the scientific impact of the high quality spectroscopic data expected from the james webb space telescope (jwst) will be limited by the availability of radiative transfer tools for infrared molecular lines. We propose to develop a numerically accurate, non-LTE 3D line radiative transfer code, needed to interpret mid-infrared molecular line observations of protoplanetary and debris disks in preparation for the james webb space telescope (jwst). This will be accomplished by adding critical functionality to the existing monte carlo code lime, which was originally developed to support (sub)millimeter interferometric observations. In contrast to existing infrared codes, lime calculates the exact statistical balance of arbitrary collections of molecular lines, and does not use large velocity gradient (LVG) or escape probability approximations. However, to use lime for infrared line radiative transfer, new functionality must be added and tested, such as dust scattering, UV fluorescence, and interfaces with public state-of-the art 3D dust radiative transfer codes (e.g., RADMC3D) and thermo-chemical codes (e.g, prodimo). Infrared transitions of molecules expected to be ubiquitous in jwst spectra currently do not have good databases applicable to astrophysical modeling and protoplanetary disks, including water, OH, CO2, NH3, CH4, HCN, etc. Obtaining accurate solutions of the non-LTE line transfer problem in 3D in the infrared is computationally intensive. We propose to benchmark the new code relative to existing, approximate methods to determine whether they are accurate, and under what conditions. We will also create conversion tables between mid-infrared line strengths of water, OH, CH4, NH3, CH3OH, CO2 and other species expected to be observed with jwst, and their relative abundances in planet-forming regions. We propose to apply the new IR-lime to retrieve molecular abundances from archival and new spectroscopic observations with spitzer/herschel/keck/VLT of CO, water, OH and organic molecules, and to publish comprehensive tables of retrieved molecular abundances in protoplanetary disks. The proposed research is relevant to the XRP call, since IT addresses a critical step in inferring the chemical abundances of planet-forming material, which in turn can be compared to the observed compositions of exoplanets, thereby improving our understanding of the origins of exoplanetary systems. The proposed research is particularly timely as the first jwst science data are expected to become available toward the end of the three-year duration of the project.