Determining Benchmark Precision Abundances for Ultracool Companions
Abstract
With sensitive HST spectroscopy coupled with advanced atmospheric retrieval techniques, we propose to measure the 0.8-1.7um spectra of four planetary temperature T/Y companions and to determine abundances, metallicities, C/O ratios, effective temperatures, surface gravities, and temperature-pressure profiles. Models of star and planet formation suggest that the compositions of the atmospheres of companions relative to their primaries represent an important pathway to understanding the formation mechanism. This program will establish the first high precision atmospheric composition measurements for a set of three benchmark substellar systems with mass ratios comparable to binary systems and test the hypothesis that the C/O ratios are the same for each component, distinct from the prediction for star-planet pairs. The results will provide an important comparison to imaged or transiting exoplanet companions, and we will compare the the benchmark systems to the fourth target, GJ 504 b, the coolest directly imaged companion that may be planetary. The target mass ratios extend from high values comparable to binary stars down to low ratios analogous to exoplanet systems. Covering this broad continuum of mass ratios is essential to test the models in different regimes. Only space-based observations will provide the high sensitivity required to perform the retrieval analysis and robustly determine the atmospheric properties. Another core project goal is developing a comprehensive, high sensitivity spectral library of the coolest companions with legacy value for advancing theoretical atmosphere/evolution models and complementing future JWST observations.
- Publication:
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HST Proposal
- Pub Date:
- June 2019
- Bibcode:
- 2019hst..prop15819P