# Atmospheric Retrieval of L Dwarfs: Benchmarking Results and Characterizing the Young Planetary Mass Companion HD 106906 b in the Near-Infrared

**Authors:** Arthur D. Adams, Michael R. Meyer, Alex R. Howe, Ben Burningham, Sebastian Daemgen, Jonathan Fortney, Mike Line, Mark Marley, Sascha P. Quanz, Kamen Todorov  
**arXiv:** [2309.10188](https://arxiv.org/abs/2309.10188)  
**Date:** 2023-09-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2309.10188v1/

## Key points

- The study focuses on the atmospheric structure of HD 106906 b, a planetary-mass companion located at a projected separation of approximately 700 AU around a 15 million-year-old stellar binary.
- The authors use the APOLLO retrieval code to analyze spectral data spanning the wavelength range of 1.1-2.5 μm.
- They aim to investigate the carbon-to-oxygen (C/O) ratios to gain insights into the formation pathways of companions.
- The retrieval code is benchmarked against an existing retrieval of the field L dwarf 2M2224-0158 and shows consistent C/O ratio but disagreements in thermal structure, cloud properties, and atmospheric scale height.
- For HD 106906 b, a C/O ratio of $0.53^{+0.15}_{-0.25}$ is retrieved, suggesting a stellar-like formation for this companion.
- Abundances of H$_2$O and CO near chemical equilibrium values are found for solar metallicity, but there are deviations in surface gravity and temperature gradient within its thermal profile.
- More accurate constraints require data across a broader wavelength range.
- This work serves as preparation for future retrievals using JWST (James Webb Space Telescope) to enhance understanding of atmospheric properties and formation mechanisms of planetary-mass companions.
- Expanding and refining analysis in this era is essential to ensure robustness and reliability when characterizing these objects through simultaneous comparative retrievals on L dwarf companions using multiple retrieval codes.

## Summary

In this study, the authors present model constraints on the atmospheric structure of HD 106906 b, a planetary-mass companion located at a projected separation of approximately 700 AU around a 15 million-year-old stellar binary. They use the APOLLO retrieval code to analyze spectral data spanning the wavelength range of 1.1-2.5 μm with the aim of investigating the carbon-to-oxygen (C/O) ratios in order to gain insights into the formation pathways of companions. To validate their retrieval code, they benchmark it against an existing retrieval of the field L dwarf 2M2224-0158 and find a consistent C/O ratio with previous fits to the same JHKs data. However, there are disagreements in terms of thermal structure, cloud properties and atmospheric scale height. For HD 106906 b, they retrieve a C/O ratio of $0.53^{+0.15}_{-0.25}$, which is consistent with the expected C/O ratios for HD 106906's stellar association and suggests a stellar-like formation for this companion. The authors also find abundances of H$_2$O and CO near chemical equilibrium values for solar metallicity but observe a lower surface gravity than expected as well as sharp transitions in its temperature gradient within its thermal profile. Despite high signal-to-noise ratio and spectral resolution in their analysis, more accurate constraints require data across a broader wavelength range. This work serves as preparation for future retrievals using JWST (James Webb Space Telescope), which offers promising opportunities to resolve difficulties in fitting low-gravity L dwarfs and enhance our understanding of the atmospheric properties and formation mechanisms of planetary-mass companions. The authors emphasize that expanding and refining their analysis in this era is essential to ensure robustness and reliability when characterizing these objects through simultaneous comparative retrievals on L dwarf companions using multiple retrieval codes.