Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode

Transit spectroscopy usually relies on the integration of one or several transits to achieve the signal-to-noise ratio (S/N) necessary to resolve spectral features. Consequently, high-S/N observations of exoplanet atmospheres, where we can forgo integration, are essential for disentangling the complex chemistry and dynamics beyond global trends. In this study, we combined two partial 4-UT transits of the ultrahot Jupiter WASP-121 b, observed with the ESPRESSO at the European Southern Observatory’s Very Large Telescope in order to revisit its titanium chemistry. Through cross-correlation analysis, we achieved detections of H <ns0:sc>I</ns0:sc>, Li <ns0:sc>I</ns0:sc>, Na <ns0:sc>I</ns0:sc>, K <ns0:sc>I</ns0:sc>, Mg <ns0:sc>I</ns0:sc>, Ca <ns0:sc>I</ns0:sc>, Ti <ns0:sc>I</ns0:sc>, V <ns0:sc>I</ns0:sc>, Cr <ns0:sc>I</ns0:sc>, Mn <ns0:sc>I</ns0:sc>, Fe <ns0:sc>I</ns0:sc>, Fe <ns0:sc>II</ns0:sc>, Co <ns0:sc>I</ns0:sc>, Ni <ns0:sc>I</ns0:sc>, Ba <ns0:sc>II</ns0:sc>, Sr <ns0:sc>I</ns0:sc>, and Sr <ns0:sc>II</ns0:sc>. Additionally, narrow-band spectroscopy allowed us to resolve strong single lines, resulting in significant detections of Hα, Hβ, Hγ, Li <ns0:sc>I</ns0:sc>, Na <ns0:sc>I</ns0:sc>, K <ns0:sc>I</ns0:sc>, Mg <ns0:sc>I</ns0:sc>, Ca <ns0:sc>II</ns0:sc>, Sr <ns0:sc>I</ns0:sc>, Sr <ns0:sc>II</ns0:sc>, and Mn <ns0:sc>I</ns0:sc>. Our most notable finding is the high-significance detection of Ti <ns0:sc>I</ns0:sc> (∼5σ per spectrum, and ∼19σ stacked in the planetary rest frame). Comparison with atmospheric models reveals that Ti <ns0:sc>I</ns0:sc> is indeed depleted compared to V <ns0:sc>I</ns0:sc>. We also resolve the planetary velocity traces of both Ti <ns0:sc>I</ns0:sc> and V <ns0:sc>I</ns0:sc>, with Ti <ns0:sc>I</ns0:sc> exhibiting a significant blueshift toward the end of the transit. This suggests that Ti <ns0:sc>I</ns0:sc> primarily originates from low-latitude regions within the super-rotating jet observed in WASP-121 b. Our observations suggest limited mixing between the equatorial jet and the mid-latitudes, in contrast with model predictions from General Circulation Models. We also report the non-detection of TiO, which we attribute to inaccuracies in the line list that could hinder its detection, even if present. Thus, the final determination of the presence of TiO must await space-based observations. We conclude that the 4-UT mode of ESPRESSO is an excellent testbed for achieving high S/N on relatively faint targets, paving the way for future observations with the Extremely Large Telescope.