Precision wavelength calibration for radial velocity measurements using uranium lines between 3800 and 6900 Å

We present the precise wavelength calibration of a high-resolution spectrum using uranium (U) lines in the wavelength range of 3809 to 6833 Å for precision radial velocity (RV) measurements for exoplanet detection or related astrophysical sciences. We identify 1540 well-resolved U lines from a high-resolution (R  =  67  ,  000) spectrum of the uranium-argon hollow cathode lamp (UAr HCL) using PARAS spectrograph in the aforesaid wavelength range. We calculate the neutral and first allowed transitions (Ritz wavelength) of U from its known energy levels and compare them with our observed central wavelengths. We measure an offset of −0.15  mÅ in our final U line list. The line list has an average measurement uncertainty of 15  ms  −  1 (0.013 pixels or 0.28 mÅ). We included these lines to the PARAS data analysis framework to perform the wavelength calibration and then calculate the multi-order RV of PARAS spectra. The typical dispersion of residuals around the wavelength solution of a UAr spectrum, using U lines, is found to be 0.8 mÅ (∼45  m s  −  1). With the use of this line list, we present our results for the precision RV of an on-sky source (an RV standard star) and an off-sky source (an HCL) observed with PARAS along with UAr HCL. We measure the dispersion in absolute drift difference between two fibers (inter-fiber drift) for a span of 6.5 h to be 88  cm s  −  1, and the RV dispersion (σRV) for an RV standard star, HD55575 over the course of ∼450 days to be 3.2  m s  −  1. These results are in good agreement with the previous ones measured using the ThAr HCL. It proves that the ThAr HCL with ∼99  %   pure-Th is replaceable with the UAr HCL for the wavelength calibration of the high-resolution spectrographs such as PARAS (R  ≤  67,000) to achieve an RV precision of 1 to 3  m s  −  1 in the visible region.