Pushing through the m/s RV regime in the HARPS-N RPS targets using TWEAKS (kima+SCALPELS)

In the hunt for Earth twins, which demands a radial-velocity (RV) precision of 10 cm/s, minimizing the impact of stellar variability in RV measurements is a critical challenge. Since 2012, a dedicated programme has been deployed to use the high-resolution echelle spectrograph HARPS-N, mounted on the Telescopio Nazionale Galileo to conduct a blind RV Search for Rocky Planets (RPS) around bright stars in the Northern Hemisphere. Here we describe the results of a comprehensive search for planetary systems in two RPS targets HD 166620 and HD 144579, of which HD 166620 was recently identified as the first unambiguous Maunder minimum analogue. We used a set of wavelength-domain line profile decorrelation vectors to mitigate the stellar activity and performed a deep search for planetary reflex motions using a transdimensional nested sampler. This pipeline, TWEAKS, is designed for attaining a sub-m/s detection threshold at long orbital periods, by combining the wavelength-domain and time-domain stellar activity mitigation.

We found no significant detections in either of the stars. The results were validated by performing data-splitting exercises and testing them with injection and recovery tests. Additionally, we obtained the detection limits on the HARPS-N RVs and found that the likelihood of finding a low-mass planet increases noticeably across a wide period range when the inherent stellar variability is corrected for using SCALPELS U-vectors. The 54 cm/s detection threshold achieved based on the aforementioned decorrelations brings us closer to the theoretical precision value provided by the HARPS-N instrument (50 cm/s). Moreover, we are now able to detect planets with masses ≤1M⊕ in stars with comparable activity levels to HD 166620 and HD 144579.