Assessing the accuracy of telluric corrections

Abstract

Observing transiting exoplanets with ground-based telescopes and high-resolution spectrographs enables the resolution of individual absorption lines in the exoplanet transmission spectra. However, observing from the ground inherently introduces telluric contamination: spectral contamination from absorption due to molecules in the Earth’s atmosphere. We take high-resolution observations of a transiting exoplanet around a bright A-type star as a case study and use synthetic telluric molecfit models to remove contamination from water and oxygen molecules in Earth’s atmosphere. The quality of the telluric corrections was statistically assessed for several different telluric regions based on absorption depth and molecular absorption species. We find that corrections for shallow telluric lines are more robust than deeper telluric lines, though both depend similarly on airmass. Corrections for different molecular bands varied by region. Some regions demonstrate a higher dependency on airmass, potentially due to the wavelength, depth, or quantity of telluric lines. Finally, we determine that the most accurate corrections are performed at observations with airmass under 1.07 corresponding to a zenith angle of approximately 20.84 degrees. Whilst this is a somewhat limited airmass range, these results highlight the need for improving telluric models for future searches of water and oxygen features in Earth-like exoplanet transmission spectra with 30m-class telescopes. These results may assist in optimizing observations for retrieving and preserving more data in an exoplanet transmission spectrum, especially when absorption features from the same molecules–and potential biosignatures–in Earth-like atmospheres fall in these highly contaminated regions. Key Words: astronomy; astrobiology; exoplanets; atmosphere; spectroscopy; telluric contamination; telluric corrections