A comprehensive study on radial velocity signals using ESPRESSO: Pushing precision to the 10 cm/s level^★

¹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁴ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Department of Physics and Astronomy G. Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122, Padova, Italy
⁶ Departamento de Física de la Tierra y Astrofísica & IPARCOSUCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁷ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec H3T 1J4, Canada
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134, Palermo, Italy
⁹ Instituto de Astrofí+ísica e Ciências do Espaço, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁰ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹¹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹² Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹⁴ Aix Marseille Univ, CNRS, CNES, LAM, 13007 Marseille, France
¹⁵ ESO – European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁶ Institute of Fundamental Physics of the Universe, IFPU, Via Beirut, 2, Trieste, 34151, Italy
¹⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy

^★★ Corresponding author: pedro.figueira@unige.ch

Received: 23 January 2025
Accepted: 20 June 2025

Abstract

Aims. We analyse ESPRESSO data for the stars HD 10700 (τ Ceti), HD 20794 (e Eridani), HD 102365, and HD 304636 acquired via its Guaranteed Time Observations (GTO) programme. We characterise the stars’ radial velocity (RV) signals down to a precision of 10 cm/s on timescales ranging from minutes to planetary periods falling within the host’s habitable zone (HZ). We study the RV signature of pulsation, granulation, and stellar activity, inferring the potential presence of planets around these stars. Thus, we outline the population of planets that while undetectable remain compatible with the available data.

Methods. We derived the stellar parameters through different methods for a complete characterisation of the star. We used these parameters to model the effects of stellar pulsations on intra-night RV variations and of stellar activity on nightly averaged values. The RVs were derived both with the cross-correlation method and template matching, as well as over the blue and red ESPRESSO detectors independently to identify colour-dependent parasitic effects of an instrumental or stellar nature. The study of RVs was complemented by an investigation of stellar activity indicators using photospheric information and chromospheric indexes.

Results. A simple model of stellar pulsations successfully reproduced the intra-night RV scatter of HD 10700 down to a few cm/s. For HD 102365 and HD 20794, an additional source of scatter at the level of several 10 cm/s remains necessary to explain the data. A kima analysis was used to evaluate the number of planets supported by the nightly averaged time series of each of these three stars, under the assumption that a quasi-periodic Gaussian process (GP) regression is able to model the activity signal. While a frequency analysis of HD 10700 RVs is able to identify a periodic signal at 20 d, when it is modelled along with the activity signal the signal is formally non-significant. Moreover, its physical origin remains uncertain due to the similarity with the first harmonic of the stellar rotation. ESPRESSO data on their own do not provide conclusive evidence to support the existence of planets around HD 10700, HD 102365, or HD 304636. In addition, the comparison of RVs with the contemporaneous indicators displays a strong correlation for HD 102365. The direct interpretation is that half of the RV variance on this star is directly attributed to activity.

Conclusions. ESPRESSO is shown to reach an on-sky RV precision of better than 10 cm/s on short timescales (<1h) and of 40 cm/s over 3.5 yr. A subdivision of the datasets showcases a precision reaching 20–30 cm/s over one year. These results impose stringent constraints on the impact of granulation mechanisms on RV. In spite of no detections, our analysis of HD 10700 RVs demonstrates a sensitivity to planets with a mass of 1.7 M_⊕ for periods of up to 100 d, and a mass of 2–5 M_⊕ for the star’s HZ.