Thorium line list

The most significant improvement brought by the new data reduction is a strong decrease in the day-to-day radial-velocity scatter, from 1.28 to 1.09m/s; this is thanks to a more stable method to derive wavelength solutions, but also to the use of calibrations closer in time. We also demonstrate that the current HARPS-N data reduction software induces a long-term drift of 1.2m/s, due to the use of non-stable thorium lines.

As a result, the old solar RVs are weakly correlated to the solar magnetic cycle, which is not expected. On the contrary, the newly derived radial velocities are much more correlated, with a Pearson correlation coefficient of 0.93. We also discuss a special correction for ghost contamination, to extract a calcium activity index free from instrumental systematics.

Our work leads toward a better understanding of the instrumental and data reduction systematics affecting the HARPSN spectrograph. The new solar data released, representing an unprecedented time-series of 34550 high-resolution spectra and precise radial velocities will be crucial to understanding stellar activity signals of solar-type stars, with the goal of enabling the detection of other Earths.