First light LBT AO images of HR 8799 bcde at 1.6 and 3.3 μm: New discrepancies between young planets and old brown dwarfs

As the only directly imaged multiple planet system, HR 8799 provides a unique opportunity to study the physical properties of several planets in parallel. In this paper, we image all four of the HR 8799 planets at H band and 3.3μm with the new Large Binocular Telescope adaptive optics system, PISCES, and LBTI/LMIRCam. Our images offer an unprecedented view of the system, allowing us to obtain H and 3.3μm photometry of the innermost planet (for the first time) and put strong upper limits on the presence of a hypothetical fifth companion. We find that all four planets are unexpectedly bright at 3.3μm compared to the equilibrium chemistry models used for field brown dwarfs, which predict that planets should be faint at 3.3μm due to CH₄ opacity. We attempt to model the planets with thick-cloudy, non-equilibrium chemistry atmospheres but find that removing CH₄ to fit the 3.3μm photometry increases the predicted L′ (3.8μm) flux enough that it is inconsistent with observations. In an effort to fit the spectral energy distribution of the HR 8799 planets, we construct mixtures of cloudy atmospheres, which are intended to represent planets covered by clouds of varying opacity. In this scenario, regions with low opacity look hot and bright, while regions with high opacity look faint, similar to the patchy cloud structures on Jupiter and L/T transition brown dwarfs. Our mixed-cloud models reproduce all of the available data, but self-consistent models are still necessary to demonstrate their viability.