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Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum

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Author(s)

A. Collier Cameron, A. Mortier, D. Phillips, X. Dumusque, R. D. Haywood, N. Langellier, C. A. Watson, H. M. Cegla, J. Costes, D. Charbonneau, A. Coffinet, D. W. Latham, M. Lopez-Morales, L. Malavolta, J. Maldonado, G. Micela, T. Milbourne, E. Molinari, S. H. Saar, S. Thompson & 17 others N. Buchschacher, M. Cecconi, R. Cosentino, A. Ghedina, A. Glenday, M. Gonzalez, C. -H. Li, M. Lodi, C. Lovis, F. Pepe, E. Poretti, K. Rice, D. Sasselov, A. Sozzetti, A. Szentgyorgyi, S. Udry, R. Walsworth

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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.
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Original languageEnglish
Pages (from-to)1082–1100
JournalMonthly Notices of the Royal Astronomical Society
Volume487
Issue number1
Early online date4 May 2019
DOIs
Publication statusPublished - Jul 2019

    Research areas

  • Techniques: radial velocities, Sun: activity, Sun: faculae, plages, Sun: granulation, Sunspots, Planets and satellites: detection

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