Three years of HARPS-N high-resolution spectroscopy and precise radial velocity data for the Sun^⋆

¹ Astronomy Department of the University of Geneva, 51 ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: xavier.dumusque@unige.ch
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ SUPA School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁴ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
⁷ Fundación Galileo Galilei-INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁸ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁹ SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ Scotland, UK
¹⁰ INAF-Osservatorio Astrofisico di Torino, Strada Osservatorio 20 Pino Torinese, TO 10025, Italy
¹¹ Astrophysics Group, University of Exeter, Exeter EX4 2QL, UK
¹² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁴ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
¹⁵ Department of Physics and Astronomy, Universitá degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy

Received: 6 September 2020
Accepted: 21 January 2021

Abstract

Context. The solar telescope connected to HARPS-N has been observing the Sun since the summer of 2015. Such a high-cadence, long-baseline data set is crucial for understanding spurious radial-velocity signals induced by our Sun and by the instrument. On the instrumental side, this data set allowed us to detect sub- m s⁻¹ systematics that needed to be corrected for.

Aims. The goals of this manuscript are to (i) present a new data reduction software for HARPS-N, (ii) demonstrate the improvement brought by this new software during the first three years of the HARPS-N solar data set, and (iii) release all the obtained solar products, from extracted spectra to precise radial velocities.

Methods. To correct for the instrumental systematics observed in the data reduced with the current version of the HARPS-N data reduction software (DRS version 3.7), we adapted the newly available ESPRESSO DRS (version 2.2.3) to HARPS-N and developed new optimised recipes for the spectrograph. We then compared the first three years of HARPS-N solar data reduced with the current and new DRS.

Results. The most significant improvement brought by the new DRS is a strong decrease in the day-to-day radial-velocity scatter, from 1.27 to 1.07 m s⁻¹; this is thanks to a more robust method to derive wavelength solutions, but also to the use of calibrations closer in time. The newly derived solar radial-velocities are also better correlated with the chromospheric activity level of the Sun in the long term, with a Pearson correlation coefficient of 0.93 compared to 0.77 before, which is expected from our understanding of stellar signals. Finally, we also discuss how HARPS-N spectral ghosts contaminate the measurement of the calcium activity index, and we present an efficient technique to derive an index free of instrumental systematics.

Conclusions. This paper presents a new data reduction software for HARPS-N and demonstrates its improvements, mainly in terms of radial-velocity precision, when applied to the first three years of the HARPS-N solar data set. Those newly reduced solar data, representing an unprecedented time series of 34 550 high-resolution spectra and precise radial velocities, are released alongside this paper. Those data are crucial to understand stellar activity signals in solar-type stars further and develop the mitigating techniques that will allow us to detect other Earths.