2025ApJ...986...63W

High-resolution spectroscopy has provided a wealth of information about the climate and composition of ultrahot Jupiters (UHJs). However, the 3D structure of their atmospheres makes observations more challenging to interpret, necessitating 3D forward-modeling studies. In this work, we model phase-dependent thermal emission spectra of the archetype UHJ WASP-76b to understand how the line strengths and Doppler shifts of Fe, CO, H₂O, and OH evolve throughout the orbit. We postprocess outputs of the SPARC/MITgcm global circulation model with the 3D Monte Carlo radiative transfer code gCMCRT to simulate emission spectra at 36 orbital phases. We then cross correlate the spectra with different templates to obtain cross-correlation function and K_p-V_sys maps. For each species, our models produce consistently negative K_p offsets in pre- and posteclipse, which are driven by planet rotation. The size of these offsets is similar to the equatorial rotation velocity of the planet. Furthermore, we demonstrate how the weak vertical temperature gradient on the nightside of UHJs mutes the absorption features of CO and H₂O, which significantly hampers their detectability in pre- and posttransit. We also show that the K_p and V_sys offsets in pre- and posttransit are not always a measure of the line-of-sight velocities in the atmosphere. This is because the cross-correlation signal is a blend of dayside emission and nightside absorption features. Finally, we highlight that the observational uncertainty in the known orbital velocity of UHJs can be multiple kilometers per second, which makes it hard for certain targets to meaningfully report absolute K_p offsets.