Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum. Issue 1 (4th May 2019)

• Record Type: Journal Article
• Authors: Collier Cameron, A; Mortier, A; Phillips, D; Dumusque, X; Haywood, R D; Langellier, N; Watson, C A; Cegla, H M; Costes, J; Charbonneau, D; Coffinet, A; Latham, D W; Lopez-Morales, M; Malavolta, L; Maldonado, J; Micela, G; Milbourne, T; Molinari, E; Saar, S H; Thompson, S; Buchschacher, N; Cecconi, M; Cosentino, R; Ghedina, A; Glenday, A; Gonzalez, M; Li, C-H; Lodi, M; Lovis, C; Pepe, F; Poretti, E; Rice, K; Sasselov, D; Sozzetti, A; Szentgyorgyi, A; Udry, S; Walsworth, R;
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 487:Issue 1(2019)
• Page Start: 1082
• Page End: 1100
• Publication Date: 2019-05-04
• Subjects: techniques: radial velocities -- Sun: activity -- Sun: faculae, plages -- Sun:granulation -- sunspots -- planets and satellites: detection; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1215
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: The time-variable velocity fields of solar-type stars limit the precision of radial-velocity determinations of their planets' masses, obstructing detection of Earth twins. Since 2015 July, we have been monitoring disc-integrated sunlight in daytime using a purpose-built solar telescope and fibre feed to the HARPS-N stellar radial-velocity spectrometer. We present and analyse the solar radial-velocity measurements and cross-correlation function (CCF) parameters obtained in the first 3 yr of observation, interpreting them in the context of spatially resolved solar observations. We describe a Bayesian mixture-model approach to automated data-quality monitoring. We provide dynamical and daily differential-extinction corrections to place the radial velocities in the heliocentric reference frame, and the CCF shape parameters in the sidereal frame. We achieve a photon-noise-limited radial-velocity precision better than 0.43 m s −1 per 5-min observation. The day-to-day precision is limited by zero-point calibration uncertainty with an RMS scatter of about 0.4 m s −1 . We find significant signals from granulation and solar activity. Within a day, granulation noise dominates, with an amplitude of about 0.4 m s −1 and an autocorrelation half-life of 15 min. On longer time-scales, activity dominates. Sunspot groups broaden the CCF as they cross the solar disc. Facular regions temporarily reduce the intrinsic asymmetry of the CCF. The radial-velocity increase that accompanies an active-region passage has a typical amplitude of 5 m s −1 and is correlated with the line asymmetry, but leads it by 3 d. Spectral line-shape variability thus shows promise as a proxy for recovering the true radial velocity.