High-resolution emission spectroscopy retrievals of MASCARA-1b with CRIRES+: Strong detections of CO, H${}_2$O and Fe emission lines and a C$\mathrm{/}$O consistent with solar

The characterization of exoplanet atmospheres has proven to be successful using high-resolution spectroscopy. Phase curve observations of hot/ultra-hot Jupiters can reveal their compositions and thermal structures, thereby allowing the detection of molecules and atoms in the planetary atmosphere using the cross-correlation technique. We present pre-eclipse observations of the ultra-hot Jupiter, MASCARA-1b, observed with the recently upgraded CRIRES+ high-resolution infrared spectrograph at the VLT. We report a detection of $\mathrm{F}\mathrm{e}$ ($\approx$8.3$\sigma$) in the K-band and confirm previous detections of $\mathrm{C}\mathrm{O}$ (>15$\sigma$) and ${\mathrm{H}}_2\mathrm{O}$ (>10$\sigma$) in the day-side atmosphere of MASCARA-1b. Using a Bayesian inference framework, we retrieve the abundances of the detected species and constrain planetary orbital velocities, $T$-$P$ profiles, and the carbon-to-oxygen ratio ($\mathrm{C}\mathrm{/}\mathrm{O}$). A free retrieval results in an elevated $\mathrm{C}\mathrm{O}$ abundance (${\log }_{10}$(${\chi }_{{}^{12}\mathrm{C}\mathrm{O}}$) = $-2.85_{-0.69}^{+0.57}$), leading to a super-solar $\mathrm{C}\mathrm{/}\mathrm{O}$ ratio. More realistically, allowing for vertically-varying chemistry in the atmosphere by incorporating a chemical-equilibrium model results in a $\mathrm{C}\mathrm{/}\mathrm{O}$ of $0.68_{-0.22}^{+0.12}$ and a metallicity of $[\mathrm{M}\mathrm{/}\mathrm{H}]=0.62_{-0.55}^{+0.28}$, both consistent with solar values. Finally, we also report a slight offset of the $\mathrm{F}\mathrm{e}$ feature in both K${}_{\mathrm{p}}$ and v${}_{\mathrm{s}\mathrm{y}\mathrm{s}}$ that could be a signature of atmospheric dynamics. Due to the 3D structure of exoplanet atmospheres and the exclusion of time/phase dependence in our 1D forward models, further follow-up observations and analysis are required to confirm or refute this result.