First Light LBT AO Images of HR 8799 bcde at 1.65 and 3.3 Microns: New Discrepancies between Young Planets and Old Brown Dwarfs

TL;DR

This study uses advanced imaging to analyze HR 8799 planets, revealing unexpected brightness at 3.3 microns and proposing mixed cloud atmosphere models to explain their spectral properties, challenging existing brown dwarf models.

Contribution

First direct imaging of all four HR 8799 planets at H-band and 3.3 microns with unprecedented detail, and development of mixed cloud atmosphere models to explain observed discrepancies.

Findings

All four planets are unexpectedly bright at 3.3 microns.

Standard equilibrium models cannot explain the brightness; non-equilibrium chemistry models are inconsistent.

Mixed cloud atmosphere models can reproduce the observed spectral energy distributions.

Abstract

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 3.3 microns with the new LBT adaptive optics system, PISCES, and LBTI/LMIRCam. Our images offer an unprecedented view of the system, allowing us to obtain H and 3.3$ micron 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 3.3 microns compared to the equilibrium chemistry models used for field brown dwarfs, which predict that planets should be faint at 3.3 microns 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 3.3 micron photometry increases the predicted L' (3.8 microns) flux enough that it is inconsistent with observations. In an effort to fit the SED 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.