Atmospheric characterization has become a crucial area of study for exoplanets. The exoplanets known as ultra-hot Jupiters (UHJs) offer a natural laboratory for studying extreme atmospheric physics that cannot be observed in the solar system. One way to analyze their atmospheres is by transmission spectroscopy. However, it can be challenging to obtain such information because a planet’s signal is too weak compared to that of its host star, resulting in the planetary contribution to the observed spectrum being negligible. Therefore, the minimum observational requirements must be assessed first to distinguish the planetary signal from the stellar one to study these planets. In this context, we obtained the transmission spectra of UHJs TOI-1431 b and WASP-189 b by observing each exoplanet for one night with BOAO Echelle Spectrograph (BOES) on the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO). We searched for various chemical species by cross-correlating the exoplanetary spectra with model synthetic spectra. Our search for atmospheric signal returned a detection confidence level less than 3 σ for both targets. Therefore, we applied model injection to recover the atmospheric signals of the planets and assessed the minimum signal-to-noise ratio (S/N) to achieve 5 σ detection. During our search, we successfully recovered the planet signals with detection significances of 5.11 σ after a 750% injection of the model signal for TOI-1431 b and 5.02 σ for a 90% injection forWASP-189 b. These signal injection exercises suggest that a higher S/N of the transmission spectra is required to detect the planetary absorption features, and this can be done by stacking data from the observations of more than three cycles of the transit of a planet with a small-scale height such as WASP-189 b at BOAO facilities.

Introduction
Target Observations
Data Preparation
    Removal of Stellar and Telluric Absorption Features
    Generation of Planet's Model Spectrum
Cross-Correlation Analysis of Transmission Spectra
    Methodology
    Results of Cross-Correlation Analysis of Our Transmission Spectra
Assessing Minimum S/N for Detecting Planet's Signal
    Injection of Model Signal
    Derivation of Minimum S/N Values for Our Targets
Discussion and Conclusions
Acknowledgments
References
Model Spectra

• Berfin Toğay(Department of Astronomy, Space Science, and Geology, Chungnam National University, Daejeon 34134, Republic of Korea)
• Young Sun Lee(Department of Astronomy and Space Science, Chungnam National University, Daejeon 34134, Republic of Korea) Corresponding author
• Jae-Joon Lee(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)
• Yeon-Ho Choi(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea, Department of Astronomy and Space Science, University of Science and Technology, Daejeon 34113, Republic of Korea)
• Changwoo Kye(Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea)
• Sol Yun(Department of Earth, Environmental & Space Sciences, Chungnam National University, Daejeon 34134, Republic of Korea)