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arXiv · 2611.08489

BeAR: The Bern Atmospheric Retrieval code - A GPU-accelerated framework for low-resolution, high-resolution, and joint retrievals of exoplanet atmospheres

Esther Wang·Daniel Kitzmann·H. Jens Hoeijmakers·Joost P. Wardenier·Helena Lecoq-Molinos
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Abstract

We present BeAR, the Bern Atmospheric Retrieval code and successor of Helios-r, and its new high-resolution cross-correlation framework for emission and transmission spectra. BeAR performs joint retrievals by adding the log-likelihoods of high- and low-resolution data computed from the same model atmosphere. The framework implements established cross-correlation log-likelihoods, with the line-depth scaling either fitted analytically or sampled as a free parameter, their per-pixel generalization, fast model filtering, and model re-injection adapted to this filtering. The forward model and likelihood run entirely on a graphics processing unit (GPU). One forward model takes about 1 ms on an NVIDIA GH200, allowing millions of models per minute on a GPU cluster. In separate retrievals of Gemini South/IGRINS and JWST/NIRSpec dayside spectra of the hot Jupiter WASP-77Ab, BeAR reproduces published abundances, C/O ratios, and metallicities to within 1.5 sigma, the preference for 13CO, and the thermal structure and line contrast to within 1 sigma. The joint retrieval confirms a sub-solar metallicity and a near-solar C/O ratio. A full IGRINS retrieval with 17 free parameters takes 27 to 42 minutes on a single NVIDIA GeForce RTX 5080 or about 14 minutes on eight RTX 4090 GPUs. Injection--recovery tests show that the posteriors underestimate the uncertainties of abundance ratios by about a factor of two.

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Esther Wang, Daniel Kitzmann, H. Jens Hoeijmakers, Joost P. Wardenier, Helena Lecoq-Molinos. 2026-11-05. BeAR: The Bern Atmospheric Retrieval code - A GPU-accelerated framework for low-resolution, high-resolution, and joint retrievals of exoplanet atmospheres. https://arxiv.org/abs/2611.08489

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