A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter

Abstract

With JWST, we are directly imaging cold (~200-300K), solar-age giant exoplanets for the first time. At these temperatures many molecular features appear and water-ice clouds may condense and affect the emission spectrum; early photometric measurements of cold giant planets are already showing some tension with the predictions of cloud-free, solar-metallicity atmosphere models. Here we present new JWST/MIRI coronagraphic observations of the cold giant exoplanet Eps Ind Ab at 11.3um. Together with archival data, we use these new observations to study the atmosphere of this cold exoplanet, and we also re-fit its orbit, finding an updated mass of $7.6\pm0.7$ Mj and an eccentricity of $0.24^{+0.11}_{-0.08}$. The planet is significantly brighter (by $0.88\pm0.08$ mag) at 11.3um than at 10.6um, indicating the presence of ammonia. However, this ammonia feature is shallower than expected. This could indicate a low-metallicity or nitrogen-depleted atmosphere, but our preferred explanation is the presence of thick water-ice clouds that suppress the ammonia feature and the near-IR emission of Eps Ind Ab. Photometry of the small but growing sample of cold, giant exoplanets demonstrates that they are consistently fainter than expected between 3 to 5um, consistent with the water-ice cloud hypothesis. 10.6um and 11.3um photometry of this cold exoplanet sample would be valuable to determine whether the suppressed ammonia feature is universal, and to frame a new open question about the underlying physical cause.

Figures (10)

• Figure 1: Coronagraphic images of Eps Ind A, collected with the F1140C filter of JWST/MIRI. The planet is detected as a bright point source in upper left of this image.
• Figure 2: Opacity of ammonia as a function of wavelength (left axis), plotted against the transmission of the MIRI coronagraphic filters (right axis). The deep, double-peaked 10-11 ammonia absorption feature falls primariy in the F1065C filter, and the F1065C-F1140C color is a proxy for the depth of the feature.
• Figure 3: CMD positions of Eps Ind Ab, as well as a number of other cold brown dwarfs (with details provided in Section \ref{['sec:archivalbds']}), considering the three JWST filters as well as the NaCo L' filter (for which there is a non-detection of Eps Ind Ab; the base of the arrow is the 5$\sigma$ lower limit in L'-F1065C color for this planet). Models are from the Sonora Flame Skimmer grid (Mang et al. in prep.), and have solar metallicity and C/O and $K_{\rm zz}=10^7$ cm$^2$/s. These models broadly explain the population of warmer brown dwarfs, and in particular the trend whereby the F1065C-F1140C color becomes increasingly red, tracing an increasingly deep ammonia feature, in cooler brown dwarfs. However, both Eps Ind Ab and WISE 0855 show significantly bluer F1065C-F1140C colors than these simple models, indicating that the ammonia feature is smaller than expected. Eps Ind Ab and WISE 0855 have remarkably similar mid-IR magnitudes and colors, but Eps Ind Ab is significantly redder in L'-F1065C (i.e., it is significantly fainter at L'), and we discuss potential explanations in the text.
• Figure 4: Same as Figure \ref{['fig:ammonia_cmd']}, but for models with a range of metallicities. While a significantly sub-solar metallicity would explain the shallow ammonia feature of Eps Ind Ab (left), the models with the lowest metallicity have the brightest NaCo L' magnitude. Sub-solar metallicity is not (by itself) sufficient to explain the in-hand photometric constraints for Eps Ind Ab. All plotted metals have solar C/O and $K_{\rm zz}=10^7$ cm$^2$/s, and the model with [Fe/H]=0 (identical to Figure \ref{['fig:ammonia_cmd']}) is highlighted with a dashed line.
• Figure 5: Same as Figure \ref{['fig:ammonia_cmd']}, but for models depleted in nitrogen. The top and bottom rows show respectively the case of solar metallicity and C/O, and of enhanced metallicity and C/O as found for Eps Ind Ab in Matthews2024. Percentage labels indicate the nitrogen content of each atmosphere as a fraction of equilibrium models, and the model from Fig. \ref{['fig:ammonia_cmd']} is highlighted with a dashed line. Atmospheres strongly depleted (to 5%) in nitrogen are consistent with both the F1065C-F1140C color of Eps Ind Ab and WISE 0855, and strongly N-depleted atmospheres also have bluer L'-F1065C colors. However, only in models with both (1) depleted nitrogen and (2) enhanced metallicity and C/O is the L'-F1065C color sufficiently blue to match the observational constraints for Eps Ind Ab.