Elemental abundance pattern and temperature inversion on the dayside of HAT-P-70b observed with CARMENES and PEPSI

TL;DR

The paper presents the first dayside high-resolution emission spectrum of the ultra-hot Jupiter HAT-P-70b using CARMENES and PEPSI, revealing a rich array of refractory species through cross-correlation that includes the first exoplanet detections of Ali and AlH. Through atmospheric retrieval under both free-VMR and chemical-equilibrium frameworks, the authors infer a strong dayside temperature inversion and a near-solar metallicity, with Ni enrichment and disproportionately strong Ca II and Fe II signals pointing to ionization-dominated, high-altitude layers and possible hydrodynamic escape. The results indicate partial depletion of high-condensation-temperature elements (Ti, Ca, V) likely due to nightside cold trapping, while Al remains solar-like, suggesting formation-related enrichment or disequilibrium storage in Al-containing species. The work also discusses limitations due to LTE assumptions and potential NLTE effects, emphasizing the need for future infrared and space-based observations to tighten constraints on C/O, complete elemental abundances, and the global atmospheric dynamics of HAT-P-70b.

Abstract

Ground-based high-resolution spectroscopy has identified various chemical species in the atmospheres of ultra-hot Jupiters, including neutral and ionized metals, providing key insights into planet formation through refractory element abundances. We observed the dayside thermal emission spectrum of the UHJ HAT-P-70b using the high-resolution spectrographs CARMENES and PEPSI. Through cross-correlation analysis, we detect emission signals of Al i, AlH, Ca ii, Cr i, Fe i, Fe ii, Mg i, Mn i, and Ti i, marking the first detection of Al i and AlH in an exoplanetary atmosphere. Tentative signals of C i, Ca i, Na i, NaH, and Ni i are also identified. These detections enable atmospheric retrievals to constrain the thermal profile and elemental abundances of the planet's dayside hemisphere. The retrieved temperature-pressure profile reveals a strong thermal inversion. The chemical free retrieval yields a metallicity of [Fe/H] = 0.38(+0.74/-1.11), while the chemical equilibrium retrieval gives [Fe/H] = 0.23(+1.08/-0.98), both consistent with solar metallicity. We also tentatively find an enhanced abundance of Ni, possibly due to the accretion of Ni-rich planetesimals during formation. On the other hand, elements with condensation temperatures above 1400 K (e.g., Ca, Ti, and V) appear slightly depleted, which may be caused by nightside cold trapping. However, Al, with the highest condensation temperature at 1653K, displays a solar like abundance, which might reflect the formation-related enrichment of Al. Our retrieval indicates extremely high volume mixing ratios of metal ions (Fe ii and Ca ii), which are significantly inconsistent with predictions from chemical equilibrium models. This disequilibrium suggests that the atmosphere is likely undergoing significant hydrodynamic escaping, which enhances the atmospheric density at high altitudes where the ionic lines are formed.

Figures (15)

• Figure 1: CCF for Fei in the observer's rest frame, obtained from PEPSI observations. Horizontal dashed lines indicate the temporal boundaries of the secondary eclipse. The hatched region highlights the moonlight-contaminated zone, which was systematically masked during the construction of the $K_\mathrm{p}$--$\Delta v$ map.
• Figure 2: Model spectra and S/N maps for Ali, AlH, Caii, and Cri. Left panels: Model spectra for each chemical species. Middle panels: Corresponding S/N maps, combining data from both CARMENES and PEPSI instruments. The white dotted lines indicate the position of $K_\mathrm{p}$--$\Delta v$ where the S/N reaches its maximum. Right panels: CCFs at the $K_\mathrm{p}$ values corresponding to the maximum S/N detections.
• Figure 3: Same as Fig. \ref{['Kpmap_signal1']}, but for Fei, Feii, Mgi, Mni, and Tii.
• Figure 4: Caii triplet emission lines observed with PEPSI. They were combined over two nights and shifted to the planetary rest frame using the best-fit velocity from the Caii CCF signal. The dashed blue lines indicate the expected positions of the Caii triplet lines. Among them, only the lines near 8500.36 $\mathrm{\AA}$ and 8664.52 $\mathrm{\AA}$ (vacuum wavelengths) were detected, with their Gaussian fits in red.
• Figure 5: Volume mixing ratios of Ali, Alii, AlO, and AlH as a function of pressure, calculated under the assumption of chemical equilibrium. An isothermal temperature profile of 2500 K was adopted.