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Abstract Recent observations have shown that the atmospheres of ultrahot Jupiters (UHJs) commonly possess temperature inversions, where the temperature increases with increasing altitude. Nonetheless, which opacity sources are responsible for the presence of these inversions remains largely observationally unconstrained. We used LBT/PEPSI to observe the atmosphere of the UHJ KELT-20 b in both transmission and emission in order to search for molecular agents which could be responsible for the temperature inversion. We validate our methodology by confirming a previous detection of Feiin emission at 16.9σ. Our search for the inversion agents TiO, VO, FeH, and CaH results in non-detections. Using injection-recovery testing we set 4σupper limits upon the volume mixing ratios for these constituents as low as ∼1 × 10⁻⁹for TiO. For TiO, VO, and CaH, our limits are much lower than expectations from an equilibrium chemical model, while we cannot set constraining limits on FeH with our data. We thus rule out TiO and CaH as the source of the temperature inversion in KELT-20 b, and VO only if the line lists are sufficiently accurate. 

Abstract Decline and recovery timescales surrounding eclipse are indicative of the controlling physical processes in Io’s atmosphere. Recent studies have established that the majority of Io’s molecular atmosphere, SO₂and SO, condenses during its passage through Jupiter’s shadow. The eclipse response of Io’s atomic atmosphere is less certain, having been characterized solely by ultraviolet aurorae. Here we explore the response of optical aurorae for the first time. We find oxygen to be indifferent to the changing illumination, with [Oi] brightness merely tracking the plasma density at Io’s position in the torus. In shadow, line ratios confirm sparse SO₂coverage relative to O, since their collisions would otherwise quench the emission. Io’s sodium aurora mostly disappears in eclipse and e-folding timescales, for decline and recovery differ sharply: ∼10 minutes at ingress and nearly 2 hr at egress. Only ion chemistry can produce such a disparity; Io’s molecular ionosphere is weaker at egress due to rapid recombination. Interruption of a NaCl⁺photochemical pathway best explains Na behavior surrounding eclipse, implying that the role of electron impact ionization is minor relative to photons. Auroral emission is also evident from potassium, confirming K as the major source of far red emissions seen with spacecraft imaging at Jupiter. In all cases, direct electron impact on atomic gas is sufficient to explain the brightness without invoking significant dissociative excitation of molecules. Surprisingly, the nonresponse of O and rapid depletion of Na is opposite the temporal behavior of their SO₂and NaCl parent molecules during Io’s eclipse phase.