Autodifferentiable Spectrum Model for High-dispersion Characterization of Exoplanets and Brown Dwarfs

We present an autodifferentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high-dispersion data to fit the ab initio line-by-line spectral computation to the observed spectrum by combining it with the Hamiltonian Monte Carlo in recent prob...

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Bibliographic Details
Published in:The Astrophysical journal. Supplement series 2022-02, Vol.258 (2), p.31
Main Authors: Kawahara, Hajime, Kawashima, Yui, Masuda, Kento, Crossfield, Ian J. M., Pannier, Erwan, van den Bekerom, Dirk
Format: Article
Language:English
Subjects:
ASTRONOMY AND ASTROPHYSICS
Bayesian analysis
Brown dwarf stars
Brown dwarfs
Calculators
Carbon monoxide
Dispersion
Exoplanet atmospheres
Extrasolar planets
High resolution spectroscopy
Line spectra
Linear algebra
Markov chain Monte Carlo
Modelling
Opacity
Programming languages
Radiative transfer
Source code
Spectrographs
Statistical inference
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Summary:We present an autodifferentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high-dispersion data to fit the ab initio line-by-line spectral computation to the observed spectrum by combining it with the Hamiltonian Monte Carlo in recent probabilistic programming languages. An open-source code, ExoJAX ( https://github.com/HajimeKawahara/exojax ), developed in this study, was written in Python using the GPU/TPU compatible package for automatic differentiation and accelerated linear algebra, JAX . We validated the model by comparing it with existing opacity calculators and a radiative transfer code and found reasonable agreements for the output. As a demonstration, we analyzed the high-dispersion spectrum of a nearby brown dwarf, Luhman 16 A, and found that a model including water, carbon monoxide, and H 2 /He collision-induced absorption was well fitted to the observed spectrum ( R = 10 5 and 2.28–2.30 μ m). As a result, we found that T 0 = 1295 − 32 + 35 K at 1 bar and C/O = 0.62 ± 0.03, which is slightly higher than the solar value. This work demonstrates the potential of a full Bayesian analysis of brown dwarfs and exoplanets as observed by high-dispersion spectrographs and also directly imaged exoplanets as observed by high-dispersion coronagraphy.
ISSN:0067-0049
1538-4365
DOI:10.3847/1538-4365/ac3b4d