Improved precision of radial velocity measurements after correction for telluric absorption

Abstract

Context. The detection of planets around other stars by the measurement of the stellar Radial Velocity (RV) variations benefits from improvements of dedicated spectrographs, allowing to achieve a precision of 1 ms −1 or better. Spectral intervals within which stellar lines are contaminated by telluric lines are classically excluded from the RV processing. Aims. We aim at estimating the potential improvement of telluric absorption removal and subsequent extension of the useful spectral domain on the precision of radial velocity measurements. Methods. We developed a correction method based on the on-line web service TAPAS, allowing to determine a synthetic atmospheric transmission spectrum for the time and location of observations. This method was applied to the telluric H 2 O and O 2 absorption removal from a series of 200 ESPRESSO consecutive exposures of the K2.5V star HD40307, available in ESO archives. We calculated the radial velocity using the standard Cross-Correlation Function (CCF) method and Gaussian fit of the CCF, with uncorrected spectra and the ESPRESSO standard stellar binary mask on one hand, and telluric-corrected spectra and an augmented binary mask with 696 additional lines on the other hand. Results. We find that the precision of radial velocity measurements is improved in the second case, with a reduction of the average formal error from 1.04 ms −1 down to 0.78 ms −1 in the case of these ESPRESSO data and this stellar type for the red arm. Using an estimator of the minimal error based on photon noise limit applied to the full CCF, the error is reduced from 0.89 ms −1 down to 0.78 ms −1 . This corresponds to a significant decrease of about 35% of observing time to reach the same precision in the red part.

There’s More to Life than O2: Simulating the Detectability of a Range of Molecules for Ground-based, High-resolution Spectroscopy of Transiting Terrestrial Exoplanets

“…Molecfit as a telluric line removal tool is demonstrated in Allart et al (2017), where it is used to fit and subtract the telluric lines down to the noise level in high-resolution observations of water lines in the atmosphere of HD 189733 b. More recently, TAPAS has been used on ESPRESSO data of HD 40307 to fit and remove telluric lines in wavelength regions with significant telluric absorption traditionally excluded by precision radial-velocity surveys, improving the precision of the resulting stellar spectrum by up to 25% (Ivanova et al 2023). While using a radiative-transfer tool to fit the telluric lines of our synthetic data would be more realistic, here we have chosen to present the ideal case with perfect subtraction; however, we look forward to including a more realistic telluric removal process in future iterations of this work.…”

Precise Radial Velocities Using Line Bisectors

“…Contamination by telluric water vapor absorption (Cunha et al 2014;Bedell et al 2019;Wang et al 2022;Ivanova et al 2023) can, in principle, affect many of the solar absorption lines in the SOLIS/ISS bands. To evaluate what lines are affected, we measure the strength of the time-variable telluric absorption and then seek correlations of the varying solar line properties with the varying telluric strength.…”

Radial velocities: Direct application of Pierre Connes’ shift-finding algorithm to cross-correlation functions

“…This 16 is the case of new near-IR (NIR) spectrographs like SPIROU (Donati et al 2020) and NIRPS (Bouchy et al 2017). Instead of discarding spectral regions contaminated by telluric absorptions, their correction and use is now also increasing the RV accuracy, moderately in the visible (see, e.g., Ivanova et al (2023) for ESPRESSO), and strongly in the NIR (see Cook et al (2022) for SPIROU).…”