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Detection of a high-velocity sodium feature on the ultra-hot Jupiter WASP-121 b

J. V. Seidel (1)F. Borsa (2)L. Pino (3) ...+78 Italy)
Mar 2023
Ultra-hot Jupiters, with their high equilibrium temperatures and resolvedspectral lines, have emerged as a perfect testbed for new analysis techniquesin the study of exoplanet atmospheres. In particular, the resolved sodiumdoublet as a resonant line has proven a powerful indicator to probe theatmospheric structure over a wide pressure range. We explore an atmosphericorigin of the observed blueshifted feature next to the sodium doublet of theultra-hot Jupiter WASP-121~b, using a partial transit obtained with the 4-UTmode of ESPRESSO. We study its atmospheric dynamics visible across theterminator by splitting the data into mid-transit and egress. We determine thatthe blueshifted high-velocity absorption component is generated only during theegress part of the transit when a larger fraction of the day side of the planetis visible. For the egress data, MERC retrieves the blueshifted high-velocityabsorption component as an equatorial day-to-night side wind across the eveninglimb, with no zonal winds visible on the morning terminator with weak evidencecompared to a model with only vertical winds. For the mid-transit data, theobserved line broadening is attributed to a vertical, radial wind. We attributethe equatorial day-to-night side wind over the evening terminator to alocalised jet and restrain its existence between the substellar point and up to$10^\circ$ to the terminator in longitude, an opening angle of the jet of atmost $60^\circ$ in latitude, and a lower boundary in altitude between [1.08,1.15] $R_p$. Due to the partial nature of the transit, we cannot make anystatements on whether the jet is truly super-rotational and one-sided or partof a symmetric day-to-night side atmospheric wind from the hotspot.