Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum
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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.
Cameron , A C , 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 2019 , ' Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum ' Monthly Notices of the Royal Astronomical Society , vol. 487 , no. 1 , pp. 1082–1100 . https://doi.org/10.1093/mnras/stz1215
Monthly Notices of the Royal Astronomical Society
© 2019, the Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This work has been made available online in accordance with the publisher's policies. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnras/stz1215
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