Evaluating the stability of atmospheric lines with HARPS

TL;DR

This study demonstrates that atmospheric lines observed with HARPS are stable enough over six years to serve as reliable wavelength references, achieving precisions of 1-2 m/s after atmospheric correction, beneficial for high-precision radial velocity measurements.

Contribution

The paper provides the first long-term analysis of atmospheric line stability using HARPS data, showing their potential as stable wavelength references for exoplanet searches.

Findings

Atmospheric lines are stable to 10 m/s over six years.

Radial velocity variations can be modeled and corrected for atmospheric effects.

Precision of 1-2 m/s is achievable with simple atmospheric modeling.

Abstract

Context: In the search for extrasolar systems by radial velocity technique, a precise wavelength calibration is necessary for high-precision measurements. The choice of the calibrator is a particularly important question in the infra-red domain, where the precision and exploits still fall behind the achievements of the optical. Aims: We investigate the long-term stability of atmospheric lines as a precise wavelength reference and analyze their sensitivity to different atmospheric and observing conditions. Methods: We use HARPS archive data on three bright stars, Tau Ceti, Mu Arae and Epsilon Eri, spanning 6 years and containing high-cadence measurements over several nights. We cross-correlate this data with an O2 mask and evaluate both radial velocity and bisector variations down to a photon noise of 1 m/s. Results: We find that the telluric lines in the three data-sets are stable down to 10 m/s (r.m.s.) over the 6 years. We also show that the radial velocity variations can be accounted for by simple atmospheric models, yielding a final precision of 1-2 m/s. Conclusions: The long-term stability of atmospheric lines was measured as being of 10 m/s over six years, in spite of atmospheric phenomena. Atmospheric lines can be used as a wavelength reference for short-time-scales programs, yielding a precision of 5 m/s "out-of-the box". A higher precision, down to 2 m/s can be reached if the atmospheric phenomena are corrected for by the simple atmospheric model described, making it a very competitive method even on long time-scales.