Ultra-hot Jupiters are tidally locked gas giants with dayside temperatures high enough to dissociate hydrogen and other molecules. Their atmospheres are vastly non-uniform in terms of chemistry, temperature and dynamics, and this makes their high-resolution transmission spectra and cross-correlation signal difficult to interpret. In this work, we use the SPARC/MITgcm global circulation model to simulate the atmosphere of the ultra-hot Jupiter WASP-76b under different conditions, such as atmospheric drag and the absence of TiO and VO. We then employ a 3D Monte Carlo radiative transfer code, hires-mcrt, to self-consistently model high-resolution transmission spectra with iron (Fe i) lines at different phases during the transit. To untangle the structure of the resulting cross-correlation map, we decompose the limb of the planet into four sectors, and we analyse each of their contributions separately. Our experiments demonstrate that the cross-correlation signal of an ultra-hot Jupiter is primarily driven by its temperature structure, rotation and dynamics, while being less sensitive to the precise distribution of iron across the atmosphere. We also show that the previously published iron signal of WASP-76b can be reproduced by a model featuring iron condensation on the leading limb. Alternatively, the signal may be explained by a substantial temperature asymmetry between the trailing and leading limb, where iron condensation is not strictly required to match the data. Finally, we compute the Kp–Vsys maps of the simulated WASP-76b atmospheres, and we show that rotation and dynamics can lead to multiple peaks that are displaced from zero in the planetary rest frame.

The emergent spectra of close-in, giant exoplanets ("hot Jupiters") are believed to be distinct from those of young gas giants and brown dwarfs with similar effective temperatures because these objects are primarily heated from above by their host stars rather than internally from the release of energy from their formation 1 . Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions, and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H − ion 2-4 . Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters 5,6 , and population-level trends have been observed in photometric measurements [7][8][9][10][11] . However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters such as has been done for brown dwarfs 12 and transmission spectra of hot Jupiters 13 . Here we show that hot Jupiter secondary eclipse spectra centered around a water absorption band at 1.4 µm follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with the predictions of self-consistent one-dimensional models for how the thermal structures of solar composition planets vary with irradiation level. Nevertheless, the ensemble of planets exhibits significant scatter around the mean trend. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models 14-17 . 42 side temperatures in the HST/WFC3+G141 bandpass between 43 1450 − 3100 K and radii between 0.9 − 2.0 Jupiter radii. The 44 full set of 14 spectra are shown in Figure 1. 45 Baxter et al. (2020) 9 presented an analysis of changes in 46 the thermal emission spectra of a subset of planets observed 47 with HST. This study expands on that work by uniformly an-366 brown dwarf models 42 and analytic models 22 ) assuming cloud-367 free, radiative-convective-thermochemical equilibrium atmo-368 spheres. The models' assumption of chemical equilibrium 369 is likely a good approximation for the highly irradiated plan-370 ets that make up the majority of our observed population 43 . 371 A two stream source function technique 44 is employed to 372 solve for the planetary thermal fluxes at each atmospheric 373 level (under the hemispheric mean approximation). We mod-374 eled the stellar flux via a standard two stream approximation 375 (for both direct and diffuse fluxes, under the quadrature ap-376 proximation) assuming cosine incident angle of 0.5, utilizing 377 / the PHOENIX models for the stellar spectra 45 . A Newton-378 Raphson iteration 46 is used to determine the temperature at 379 each model layer which ensures zero net flux divergence. We 380 include absorption cross-sections from 0.1 -100 µm...

Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, by approximately 80 and 100‰, respectively, relative to Earth’s oceans, with inferred photospheric values of about −60‰ for both isotopes. Direct astronomical measurements of CO absorption lines in the solar photosphere have previously yielded a wide range of O isotope ratios. Here, we reanalyze the line strengths for high-temperature rovibrational transitions in photospheric CO from ATMOS FTS data, and obtain an 18O depletion of δ18O = −50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ13C = −48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in 13C relative to the bulk material from which the solar system formed, possibly as a result of CO self-shielding or inheritance from the parent cloud.

Planet formation models suggest broad compositional diversity in the sub-Neptune/super-Earth regime, with a high likelihood for large atmospheric metal content (≥ 100 × Solar). With this comes the prevalence of numerous plausible bulk atmospheric constituents including N 2 , CO 2 , H 2 O, CO, and CH 4 . Given this compositional diversity there is a critical need to investigate the influence of the background gas on the broadening of the molecular absorption cross-sections and the subsequent influence on observed spectra. This broadening can become significant and the common H 2 /He or "air" broadening assumptions are no longer appropriate. In this work we investigate the role of water self-broadening on the emission and transmission spectra as well as on the vertical energy balance in representative sub-Neptune/super-Earth atmospheres. We find that the choice of the broadener species can result in a 10 -100 parts-per-million difference in the observed transmission and emission spectra and can significantly alter the 1-dimensional vertical temperature structure of the atmosphere. Choosing the correct background broadener is critical to the proper modeling and interpretation of transit spectra observations in high metallicity regimes, especially in the era of higher precision telescopes such as JWST.

Stellar, substellar, and planetary atmosphere models are all highly sensitive to the input opacities. Generational differences between various state-of-the-art stellar/planetary models arise primarily because of incomplete and outdated atomic/molecular line lists. Here we present a database of precomputed absorption cross sections for all isotopologues of key atmospheric molecules relevant to late-type stellar, brown dwarf, and planetary atmospheres: MgH, AlH, CaH, TiH, CrH, FeH, SiO, TiO, VO, and H2O. The pressure and temperature ranges of the computed opacities are 10−6–3000 bar and 75–4000 K, and their spectral ranges are 0.25–330 μm for many cases where possible. For cases with no pressure-broadening data, we use collision theory to bridge the gap. We also probe the effect of absorption cross sections calculated from different line lists in the context of ultrahot Jupiter and M-dwarf atmospheres. Using 1D self-consistent radiative–convective thermochemical equilibrium models, we report significant variations in the theoretical spectra and thermal profiles of substellar atmospheres. With a 2000 K representative ultrahot Jupiter, we report variations of up to 320 and 80 ppm in transmission and thermal emission spectra, respectively. For a 3000 K M-dwarf, we find differences of up to 125% in the spectra. We find that the most significant differences arise as a result of the choice of TiO line lists, primarily below 1 μm. In summary, (1) we present a database of precomputed molecular absorption cross sections, and (2) we quantify biases that arise when characterizing substellar/exoplanet atmospheres as a result of differences in the line lists, therefore highlighting the importance of correct and complete opacities for eventual applications to high-precision spectroscopy and photometry.