Line-by-line velocity measurements, an outlier-resistant method for precision velocimetry

TL;DR

The paper introduces a line-by-line algorithm for precision radial velocity measurements that effectively handles outliers, demonstrating its accuracy on stellar datasets and its ability to detect exoplanets while providing activity indicators.

Contribution

A novel line-by-line approach for radial velocity measurements that improves outlier resistance and activity diagnostics in stellar spectra.

Findings

Successfully applied to datasets from SPIRou and HARPS.

Confirmed the Proxima b exoplanet signal and detected Proxima d.

Provided evidence against the Proxima c candidate due to systematic effects.

Abstract

We present a new algorithm for precision radial velocity (pRV) measurements, a line-by-line (LBL) approach designed to handle outlying spectral information in a simple but efficient manner. The effectiveness of the LBL method is demonstrated on two datasets, one obtained with SPIRou on Barnard's star, and the other with HARPS on Proxima Centauri. In the near-infrared, the LBL provides a framework for m/s-level accuracy in pRV measurements despite the challenges associated with telluric absorption and sky emission lines. We confirm with SPIRou measurements spanning 2.7 years that the candidate super-Earth on a 233-day orbit around Barnard's star is an artifact due to a combination of time-sampling and activity. The LBL analysis of the Proxima Centauri HARPS post-upgrade data alone easily recovers the Proxima b signal and also provides a 2-sigma detection of the recently confirmed 5-day Proxima d planet, but argues against the presence of the candidate Proxima c with a period of 1900 days. We provide evidence that the Proxima c signal is associated with small, unaccounted systematic effects affecting the HARPS-TERRA template matching RV extraction method for long-period signals. Finally, the LBL framework provides a very effective activity indicator, akin to the full width at half maximum derived from the cross-correlation function, from which we infer a rotation period of $92.1^{+4.2}_{-3.5}$ days for Proxima.