Ceres’ sunlight atlas⋆

Context. Astronomical research dealing with accurate radial velocity measurements need reliable astronomical standards to calibrate the spectrographs and to assess possible systematics. Stellar radial velocity standards offer a reference at the level of a few hundred m s−1 and are not adequate for most present needs. Aims. We aim to show that sunlight reflected by asteroids is a fairly accessible way to record a high-resolution solar spectrum from the whole disk, which can therefore be used as a radial velocity standard and can improve the uncertainties of solar line positions. Methods. We used solar light reflected by the asteroid Ceres observed with HARPS to measure solar lines’ wavelengths. Results. We provide a new solar atlas with 491 line wavelengths in the range 540−690 nm and 222 lines in the range 400−410 nm obtained from reflected solar spectrum of Ceres. These measurements are consistent with those of Allende Prieto & Garcia Lopez (1998b) based on FTS solar atlases but with a factor 3 higher precision. Conclusions. This atlas provides a benchmark for wavelength calibration to check radial velocity accuracy down to 44 m s−1 locally and a few m s−1 globally. The asteroid-based technique could provide a new way to track radial velocity shifts with solar activity cycle, as well as to derive convective shifts suitable for comparison with theoretical atmospheric models. It could also be used to study radial velocity deviations in spectrographs such as those recently detected in HIRES and UVES. Dedicated HARPS observations of other asteroids could improve present results substantially and these investigations have been solicited.