YARARA: Significant improvement in RV precision through post-processing of spectral time series

Abstract

Aims. Even the most precise radial-velocity instruments gather high-resolution spectra that present systematic errors that a data reduction pipeline cannot identify and correct for efficiently by simply analysing a set of calibrations and a single science frame. In this paper we aim at improving the radial-velocity precision of HARPS measurements by 'cleaning' individual extracted spectra using the wealth of information contained in spectral time series. Methods. We developed YARARA, a post-processing pipeline designed to clean high-resolution spectra of instrumental systematics and atmospheric contamination. Spectra are corrected for: tellurics, interference patterns, detector stitching, ghosts, and fibre B contaminations, as well as more advanced spectral line-by-line corrections. YARARA uses principal component analysis on spectral time series with prior information to disentangle contaminations from real Doppler shifts. We applied YARARA to three systems, HD10700, HD215152, and HD10180, and compared our results to the standard HARPS data reduction software and the SERVAL post-processing pipeline.Results. We ran YARARA on the radial-velocity dataset of three stars intensively observed with HARPS: HD10700, HD215152, and HD10180. For HD10700, we show that YARARA enables us to obtain radial-velocity measurements that present an rms smaller than 1 m s −1 over the 13 years of the HARPS observations, which is 20% and 10% better than the HARPS data reduction software and the SERVAL post-processing pipeline, respectively. We also injected simulated planets into the data of HD10700 and demonstrated that YARARA does not alter pure Doppler-shifted signals. For HD215152, we demonstrated that the 1-year signal visible in the periodogram becomes marginal after processing with YARARA and that the signals of the known planets become more significant. Finally, for HD10180, the six known exoplanets are well recovered, although different orbital parameters and planetary masses are provided by the new reduced spectra. Conclusions. The post-processing correction of spectra using spectral time series allows the radial-velocity precision of HARPS data to be significantly improved and demonstrates that for the extremely quiet star HD10700 a radial-velocity rms better than 1 m/s can be reached over the 13 years of HARPS observations. Since the processing proposed in this paper does not absorb planetary signals, its application to intensively followed systems is promising and will certainly result in advances in the detections of the lightest exoplanets.

Stellar activity correction using PCA decomposition of shells

“…We worked with HARPS 1D-merged spectra produced by the official data reduction software (DRS). The spectra were postprocessed using the YARARA pipeline (Cretignier et al 2021) to remove the known systematics present on HARPS spectra (instrumental and telluric contamination). In YARARA, the 1Dmerged spectra are first nightly stacked, before being continuum normalised by RASSINE (Cretignier et al 2020b).…”

“…The shell space has been divided in a 9 × 9 grid (black lines) to increase the S/N of the raw shell after binning all the points within each cell. strongly affect the overall stellar RV even if the contamination appears in a small portion of the spectrum (Cretignier et al 2021).…”

“…By averaging as in Dumusque (2018), the LBL RVs obtained in Cretignier et al (2021) produce the average RV m (t) time series that will be used in this work. Some concern may be raised about the number of PCA shell basis components that have to be fit.…”

“…Despite the technical challenges to reach extreme precision, the RV method remains the most promising technique for the detection of other Earths due to the low transit probability of those objects and the extremely dim light emitted or reflected by their surface (Zhu & Dong 2021). This has motivated the design of new ultra-stable spectrographs such as ESPRESSO (Pepe et al 2021) and EXPRES (Jurgenson et al 2016) that already demonstrated a 30 (Suárez Mascareño et al 2020) and 60 cm s −1 (Brewer et al 2020) RV precision, respectively.…”

The Rossiter–McLaughlin effect revolutions: an ultra-short period planet and a warm mini-Neptune on perpendicular orbits

“…Because upper envelop algorithms such as the "Jarvis March" used in RASSINE can yield inaccurate continuum levels, the master continuum was corrected for using a stellar template from the POLLUX database (Palacios et al 2020), selected for a star with similar effective temperature and gravity as HD 3167. Such an absolute correction of the continuum level is described in the appendix of Cretignier et al 2021 and can be understood as a matching of the master spectra and stellar template upper envelopes. More accurate line depths can be calculated with this corrected continuum, and we rejected shallow lines with contrast smaller than 10%.…”