A new list of thorium and argon spectral lines in the visible

Aims. We present a new list of thorium and argon emission lines in the visible obtained by analyzing high-resolution (R = 110 000) spectra of a ThAr hollow cathode lamp. The aim of this new line list is to allow significant improvements in the quality of wavelength calibration for medium- to high-resolution astronomical spectrographs. Methods. We use a series of ThAr lamp exposures obtained with the HARPS instrument (High Accuracy Radial-velocity Planet Searcher) to detect previously unknown lines, perform a systematic search for blended lines and correct individual wavelengths by determining the systematic offset of each line relative to the average wavelength solution. Results. We give updated wavelengths for more than 8400 lines over the spectral range 3785-6915 angstrom. The typical internal uncertainty on the line positions is estimated to be similar to 10 m s(-1) (3.3 parts in 10(8) or 0.18 m angstrom), which is a factor of 2-10 better than the widely used Los Alamos Atlas of the Thorium Spectrum (Palmer & Engleman 1983). The absolute accuracy of the global wavelength scale is the same as in the Los Alamos Atlas. Using this new line list on HARPS ThAr spectra, we are able to obtain a global wavelength calibration which is precise at the 20 cm s-1 level (6.7 parts in 10(10) or 0.0037 m angstrom). Conclusions. Several research fields in astronomy requiring high-precision wavelength calibration in the visible (e.g. radial velocity planet searches, variability of fundamental constants) should benefit from using the new line list.