2025A&A...695A.110R

Aims. We present high-resolution emission spectroscopy observations of the ultra-hot Jupiter MASCARA-1b with CRIRES+ in the K-band, covering the post-eclipse phases of the planet's orbit. These observations complement previously published pre-eclipse data.
Methods. The stellar and telluric features were removed from the data using the SYSREM algorithm, and the planetary signal was analysed with the cross-correlation technique. After confirming the presence of chemical species in our atmospheric model, we combined the pre- and post-eclipse datasets for a joint analysis. By employing a Bayesian retrieval framework with various parametrisations, this joint retrieval enabled us to constrain the spatially varying temperature-pressure (T-P) profile and the atmospheric carbon-to-oxygen (C/O) ratio.
Results. We detected strong emission signatures of CO and H₂O in the post-eclipse and the combined datasets. A well-mixed retrieval model results in a super-solar C/O, whereas allowing for vertically varying chemistry by incorporating a chemical model results in C/O values consistent with solar within ≈1.1σ. The retrieved parameters are not only consistent across the datasets but also across different chemical regimes. We did not identify any significant velocity shifts between the detected species or across the datasets that could otherwise serve as proxies for possible atmospheric dynamics. We also explored potential phase dependence through the model scaling factor and found no substantial changes in the atmospheric properties throughout the observed phases.
Conclusions. Due to strong degeneracies between the temperature gradient and chemical abundances, our retrieved temperatures are broadly consistent with either a full redistribution of heat or strong day-night contrasts. While this complicates direct comparisons with recent Spitzer phase curve analyses suggesting inefficient recirculation, we find no clear evidence of spatial variation in the chemical or temperature structure of MASCARA-1b from pre- to post-eclipse, nor temporal variation over a period of ≈2 years.