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

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 Fe I (11.9 σ ) and Fe II (23.7 σ ) and tentatively detect Na I (3.4 σ ) and Cr I (3.3 σ ). The full-transit velocity offsets of the strongest absorbers are Δ V Fe I = −1.0 ± 0.7 km s −1 and Δ V Fe II = 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 −1 between studies, our Fe II offset becomes consistent with previous measurements (≤1.7 σ ), while Fe I remains significantly less blueshifted than in earlier studies (≥2.2−4.5 σ ). We propose a set of detection criteria to improve future reproducibility in HRCCTS work. Phase-resolving the Fe I and Fe II absorption signatures into eight orbital phase bins reveals distinct dynamical regimes: Fe II exhibits a strong phase-dependent blueshift from ingress to egress along with significant limb asymmetry, while Fe I shows 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.