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1. PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). IV. High-resolution Phased-resolved Spectroscopy of the Ultra-hot-Jupiter KELT-20 b

   https://doi.org/10.3847/1538-3881/ae21be  

   Bonidie, Victoria; Johnson, Marshall C; Wang_王, Ji 吉; Petz, Sydney; Kamen, Jake; Lenhart, Calder; Duck, Alison; Badenes, Carles; Strassmeier, Klaus; Ilyin, Ilya (December 2025, The Astronomical Journal)

   Abstract We present five datasets of high-resolution optical emission spectra of the ultra-hot-Jupiter KELT-20 b with the PEPSI spectrograph. Using a Bayesian retrieval framework, we constrain its dayside pressure–temperature profile and abundances of Fe, Ni, and Ca, providing the first measurements for Ni and Ca for KELT-20 b in emission. We retrieve the preeclipse and posteclipse datasets separately (corresponding to the evening and morning sides, respectively), and compare the constraints on their thermal structures and chemical abundances. We constrain lower abundances in the pre-eclipse datasets compared to the posteclipse datasets. We interpret these results with an equilibrium chemistry model which suggests ∼10–30× supersolar refractory abundances. Due to the well-known degeneracy between absolute abundances and continuum opacities, the abundance ratios are more precise probes of the planetary abundances. Therefore we measure the abundance ratios [Ni/Fe] and [Ca/Fe] across these datasets and find they agree within 1σ. We constrain [Ni/Fe] to be consistent with solar within 2σ, and [Ca/Fe] to be 0.001–0.01× solar, not accounting for ionization. We compare these abundance ratios with literature results for KELT-20 b in transmission, and find they agree within 2σ, suggesting that even though the abundances vary significantly as a function of phase, the abundance ratios of these species remain relatively constant. We find a ∼100 K difference in temperature at the top of the thermal inversion, suggesting a hotter evening side than morning side and underscoring the importance of considering 3D effects when studying ultrahot Jupiters. 

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2. The PEPSI Exoplanet Transit Survey (PETS) – IV. Assessing the atmospheric chemistry of KELT-20b

   https://doi.org/10.1093/mnras/stad3481  

   Petz, Sydney; Johnson, Marshall C; Asnodkar, Anusha Pai; Wang, Ji; Gaudi, B Scott; Henning, Thomas; Keles, Engin; Molaverdikhani, Karan; Poppenhaeger, Katja; Scandariato, Gaetano; et al (November 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Most ultra-hot Jupiters (UHJs) show evidence of temperature inversions, in which temperature increases with altitude over a range of pressures. Temperature inversions can occur when there is a species that absorbs the stellar irradiation at a relatively high level of the atmospheres. However, the species responsible for this absorption remains unidentified. In particular, the UHJ KELT-20b is known to have a temperature inversion. Using high resolution emission spectroscopy from LBT/PEPSI we investigate the atomic and molecular opacity sources that may cause the inversion in KELT-20b, as well as explore its atmospheric chemistry. We confirm the presence of Fe i with a significance of 17σ. We also report a tentative 4.3σ detection of Ni i. A nominally 4.5σ detection of Mg i emission in the PEPSI blue arm is likely in fact due to aliasing between the Mg i cross-correlation template and the Fe i lines present in the spectrum. We cannot reproduce a recent detection of Cr i, while we do not have the wavelength coverage to robustly test past detections of Fe ii and Si i. Together with non-detections of molecular species like TiO, this suggests that Fe i is likely to be the dominant optical opacity source in the dayside atmosphere of KELT-20b and may be responsible for the temperature inversion. We explore ways to reconcile the differences between our results and those in literature and point to future paths to understand atmospheric variability. 

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3. PEPSI Investigation, Retrieval, and Atlas of Numerous Giant Atmospheres (PIRANGA). II. Phase-resolved Cross-correlation Transmission Spectroscopy of KELT-20b

   https://doi.org/10.3847/1538-3881/ae2252  

   Lenhart, Calder; Johnson, Marshall C; Wang_王, Ji 吉; Asnodkar, Anusha Pai; Petz, Sydney; Duck, Alison; Strassmeier, Klaus G; Ilyin, Ilya (January 2026, The Astronomical Journal)

   Abstract KELT-20b is a well-studied (T_eq = 2262 K) ultrahot Jupiter, but its multidimensional atmospheric structure remains unconstrained. We performed high-resolution cross-correlation transmission spectroscopy (HRCCTS) on a single-transit time series of KELT-20b, observed with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope. Upon combining nineteen in-transit exposures, we detect FeI(11.9σ) and FeII(23.7σ) and tentatively detect NaI(3.4σ) and CrI(3.3σ). The full-transit velocity offsets of the strongest absorbers are ΔV_FeI = −1.0 ± 0.7 km s⁻¹and ΔV_FeII = 0.0 ± 0.5 km s⁻¹, which are mostly inconsistent with previously published values for KELT-20b, although the previous measurements are mostly inconsistent with each other. By correcting for discrepant systemic velocity solutions of up to 1.7 km s⁻¹between studies, our FeIIoffset becomes consistent with previous measurements (≤1.7σ), while FeIremains significantly less blueshifted than in earlier studies (≥2.2−4.5σ). We propose a set of detection criteria to improve future reproducibility in HRCCTS work. Phase-resolving the FeIand FeIIabsorption signatures into eight orbital phase bins reveals distinct dynamical regimes: FeIIexhibits a strong phase-dependent blueshift from ingress to egress along with significant limb asymmetry, while FeIshows weaker signals and a more modest blueshift with phase. These patterns indicate day-to-night winds and suggest scale height differences are a significant driver of limb asymmetry in KELT-20b. 

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4. A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

   https://doi.org/10.1038/s41586-023-06230-1  

   Coulombe, Louis-Philippe; Benneke, Björn; Challener, Ryan; Piette, Anjali_A A; Wiser, Lindsey S; Mansfield, Megan; MacDonald, Ryan J; Beltz, Hayley; Feinstein, Adina D; Radica, Michael; et al (August 2023, Nature)

   Abstract Close-in giant exoplanets with temperatures greater than 2,000 K (‘ultra-hot Jupiters’) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble Space Telescope (HST) and Spitzer Space Telescope^1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis^3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS¹³instrument on the JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at >6σconfidence) and evidence for optical opacity, possibly attributable to H⁻, TiO and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (‘metallicity’,$${\rm{M/H}}=1.0{3}_{-0.51}^{+1.11}$$ $\mathrm{M/H}=1.03_{-0.51}^{+1.11}$times solar) and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the substellar point that decreases steeply and symmetrically with longitude towards the terminators. 

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5. Cold molecular gas in the hot nuclear wind of the Milky Way

   https://doi.org/10.1051/0004-6361/202452875  

   Heyer, M; Di_Teodoro, E; Loinard, L; Lockman, F J; McClure-Griffiths, N M; Wang, Q D (March 2025, Astronomy & Astrophysics)

   Using the Large Millimeter Telescope and the SEQUOIA 3 mm focal plane array, we have searched for molecular line emission from two atomic clouds associated with the Fermi Bubble of the Milky Way. Neither¹²CO nor¹³CO J=1–0 emission is detected from the HIcloud, MW-C20.¹²CO J=1–0 emission is detected from MW-C21 that is distributed within 11 clumps with most of the CO luminosity coming from a single clump. However, we find no¹³CO emission to a 3σbrightness temperature limit of 0.3 K. Using this limit andRADEXnon-local thermodynamic equilibrium (non-LTE) excitation models, we derive H₂column density upper limits of (0.4–3)×10²¹cm⁻²for a set of physical conditions and a H₂to¹²CO abundance ratio of 10⁴. Model CO-to-H₂conversion factors are derived for each set of physical conditions. We find the maximum value is 1.6×10²⁰cm⁻²/(K km s⁻¹). Increasing [H₂/¹²CO] to 10⁵to account for photodissociation and cosmic ray ionization increases the column density and X(CO) upper limits by a factor of 10. Applying these X(CO) limits to the CO luminosities, the upper limit on the total molecular mass in MW-C21 is 132±2 M_⊙, corresponding to <27% of the neutral gas mass. For the three clumps that are fully resolved, lower limits to the virial ratios are 288±32, 68±28, and 157±39, which suggest that these structures are bound by external pressure to remain dynamically stable over the entrainment time of 2×10⁶years or are being disrupted by shear and expansion over the clump crossing times of 3–8×10⁵years. The observations presented in this study add to the growing census of cold gas entrained within the Galactic Center wind. 

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