Ultra-hot Jupiters offer interesting prospects for expanding our theories on dynamical evolution and the properties of extremely irradiated atmospheres. In this context, we present the analysis of new optical spectroscopy for the transiting ultra-hot Jupiter WASP-121b. We first refine the orbital properties of WASP-121b, which is on a nearly polar (obliquity ψNorth = 88.1 ± 0.25° or ψSouth = 91.11 ± 0.20°) orbit, and exclude a high differential rotation for its fast-rotating (P < 1.13 days), highly inclined (i⋆North = 8.1-2.6+3.0° or i⋆South = 171.9-3.4+2.5°) star. We then present a new method that exploits the reloaded Rossiter-McLaughlin technique to separate the contribution of the planetary atmosphere and of the spectrum of the stellar surface along the transit chord. Its application to HARPS transit spectroscopy of WASP-121b reveals the absorption signature from metals, likely atomic iron, in the planet atmospheric limb. The width of the signal (14.3 ± 1.2 km s-1) can be explained by the rotation of the tidally locked planet. Its blueshift (-5.2 ± 0.5 km s-1) could trace strong winds from the dayside to the nightside, or the anisotropic expansion of the planetary thermosphere.

The custom CCF mask built for WASP-121, and the EulerCam lightcurves are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/635/A205

Based on observations made at ESO 3.6 m telescope (La Silla, Chile) under ESO programme 100.C-0750.