The GAPS Programme with HARPS-N at TNG

XVII. Line profile indicators and kernel regression as diagnostics of radial-velocity variations due to stellar activity in solar-like stars^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78„ 95123 Catania, Italy
e-mail antonino.lanza@inaf.it
² Dipartimento di Fisica e Astronomia Galileo Galilei – Università di Padova, Via Francesco Marzolo 8, 35122, Padova, Italy
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁷ European Southern Observatory, Alonso de Córdova 3107, Santiago, Vitacura, Chile
⁸ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁰ Fundación Galileo Galilei - INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹¹ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, 38205 La Laguna, TF, Spain
¹⁴ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna TF, Spain
¹⁵ Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 20 April 2017
Accepted: 10 April 2018

Abstract

Aims. Stellar activity is the ultimate source of radial-velocity (hereinafter RV) noise in the search for Earth-mass planets orbiting late-type main-sequence stars. We analyse the performance of four different indicators and the chromospheric index log R^′_HK in detecting RV variations induced by stellar activity in 15 slowly rotating (υ sin i ≤ 5 km s⁻¹), weakly active (log R^′_HK ≤ −4.95) solar-like stars observed with the high-resolution spectrograph High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N).

Methods. We consider indicators of the asymmetry of the cross-correlation function (CCF) between the stellar spectrum and the binary weighted line mask used to compute the RV, that is the bisector inverse span (BIS), ΔV, and a new indicator V_asy(mod) together with the full width at half maximum (FWHM) of the CCF. We present methods to evaluate the uncertainties of the CCF indicators and apply a kernel regression (KR) between the RV, the time, and each of the indicators to study their capability of reproducing the RV variations induced by stellar activity.

Results. The considered indicators together with the KR prove to be useful to detect activity-induced RV variations in ~47 ± 18 percent of the stars over a two-year time span when a significance (two-sided p-value) threshold of one percent is adopted. In those cases, KR reduces the standard deviation of the RV time series by a factor of approximately two. The BIS, the FWHM, and the newly introduced V_asy(mod) are the best indicators, being useful in 27 ± 13, 13 ± 9, and 13 ± 9 percent of the cases, respectively. The relatively limited performances of the activity indicators are related to the very low activity level and υ sin i of the considered stars. For the application of our approach to sun-like stars, a spectral resolution allowing λ/Δλ ≥ 10⁵ and highly stabilized spectrographs are recommended.