Unraveling the Mystery of the Peculiar and Young Hot Jupiter CoRoT-2 b. II. Phase-resolved Emission Spectroscopy with Very Large Telescope/CRIRES+ and Gemini South/IGRINS
September 2026 • 2026AJ....172..145K
Abstract
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Hot Jupiters are expected to be tidally locked and synchronously rotating due to their short orbital periods. These conditions create large day─night temperature contrasts and are thought to drive eastward superrotating jets. Indeed, the majority of hot Jupiters are observed to have the hottest region of the planet either at the substellar point or offset in the eastern direction. However, the full phase curve of CoRoT-2 b, observed with the Spitzer Space Telescope, exhibits robust evidence of a western hotspot offset. To determine the origin of this peculiar hotspot offset, we present phase-resolved high-resolution observations of CoRoT-2 b from the CRIRES+ spectrograph on the Very Large Telescope and the IGRINS spectrograph on Gemini South, covering both pre- and posteclipse phases (0.34─0.63). We detect the signal from the planet (signal-to-noise ratio > 4) in both pre- and posteclipse phases separately, and therefore perform separate cross-correlation and retrieval analyses at the two epochs. The phase-resolved retrievals show highly consistent abundances and C/Os, but prefer a hotter and more isothermal temperature─pressure profile at posteclipse phases, consistent with the phase-curve observations that indicated a western hotspot offset. By testing multiple hypotheses invoked to drive a western hotspot offset, we find the most likely explanation to be subsynchronous planetary rotation. We measure the planet's rotational broadening to be
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