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 ($T_{\text{eq}}=2262$ K) ultra hot 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 PEPSI on the LBT. Upon combining nineteen in-transit exposures, we detect Fe I $(11.9\ddot{I}ƒ)$ and Fe II $(23.7\ddot{I}ƒ)$ and tentatively detect Na I $(3.4\ddot{I}ƒ)$ and Cr I $(3.3\ddot{I}ƒ)$. The full-transit velocity offsets of the strongest absorbers are $\hat{I}”V_{\text{Fe I}}=-1.0\pm 0.7$ km s${}^{-1}$ and $\hat{I}”V_{\text{Fe II}}=0.0\pm 0.5$ 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 ($\le 1.7\ddot{I}ƒ$), while Fe I remains significantly less blueshifted than in earlier studies ($\ge 2.2-4.5\ddot{I}ƒ$). 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.

Accepted for publication in AJ 18th November 2025. 24 pages, 15 figures