Correction of telluric absorption in the spectra of stars to detect exoplanets from variations of star radial velocities
Résumé
One classical method for detecting exoplanets is the radial velocity (RV) method. The RV changes are measured from Doppler effect affecting the stellar lines of the star spectrum. Ground-based spectra contain also telluric absorption lines, and, due to the rotation of the Earth and its annual motion, the two systems of lines are moving differently with time. The classical approach to overcome this problem consists in excluding from consideration the parts of the spectrum affected by non-negligible telluric lines, therefore losing a substantial part of the stellar spectrum, with an associated decrease of accuracy. We have developed a procedure to correct the stellar spectra form telluric absorption (mainly H _{2}O and O _{2}), using the TAPAS on-line web service (http://cds-espri.ipsl.fr/tapas/) which accounts from the meteorological field of pressure, temperature and humidity. We are testing the method on a series of spectra collected by Espresso/VLT, which RV intrinsic performance is better than 1 m/s. The use of a larger range of wavelengths should increase the accuracy of measurement in the radial velocity of the star. We will report on the gain of accuracy which is brought by our correction method, which depends also on the type and details of RV retrieval algorithm which is used. Two types of RV retrieval algorithm are considered: the classical use of a binary mask adapted to the spectral type of the star, and a best-fit algorithm to a high SNR template of the star's spectrum.