2025MNRAS.537..134G

A growing number of directly-imaged companions have been recently characterized, with robust constraints on carbon-to-oxygen ratios and even isotopic ratios. Many companions and isolated targets have also shown spectral variability. In this work, we observed the super-Jupiter AB Pictoris b across four consecutive nights using VLT/CRIRES+ as part of the ESO SupJup survey, exploring how the constraints on chemical composition and temperature profile change over time using spectral line shape variations between nights. We performed atmospheric retrievals of the high-resolution observations and found broadly consistent results across all four nights, but there were differences for some parameters. We clearly detect H$_2$O, $^{12}$CO, and $^{13}$CO in each night, but abundances varied by ${\sim} 2\sigma$, which was correlated to the deep atmosphere temperature profiles. We also found differences in the $^{12}$C$/^{13}$C ratios in each night by up to ${\sim} 3\sigma$, which seemed to be correlated with the cloud deck pressure. Our combined retrieval simultaneously analysing all nights together constrained broadly the average of each night individually, with the C/O$=0.59\pm 0.01$, consistent with solar composition, and $^{12}$C$/^{13}$C $= 102\pm 8$, slightly higher than the Interstellar Medium (ISM) and Solar System values. We also find a low projected rotational velocity, suggesting that AB Pictoris b is either intrinsically a slow rotator due to its young age or that the spin axis is observed pole-on with a ${\sim} 90^\circ$ misalignment with its orbit inclination. Future observations will be able to further explore the variability and orbit of AB Pictoris b as well as for other companions.