Open Access

Absorption cells filled with diatomic iodine are frequently employed as wavelength reference for high-precision stellar radial velocity determination due to their long-term stability and low cost. Despite their wide-spread usage in the community, there is little documentation on how to determine the ideal operating temperature of an individual cell. We have developed a new approach to measuring the effective molecular temperature inside a gas absorption cell and searching for effects detrimental to a high-precision wavelength reference, utilizing the Boltzmann distribution of relative line depths within absorption bands of single vibrational transitions. With a high-resolution Fourier transform spectrometer, we took a series of 632 spectra at temperatures between 23 and 66◦C. These spectra provide a sufficient basis to test the algorithm and demonstrate the stability and repeatability of the temperature determination via molecular lines on a single iodine absorption cell. The achievable radial velocity precision σ RV is found to be independent of the cell temperature and a detailed analysis shows a wavelength dependence, which originates in the resolving power of the spectrometer in use and the signal- to-noise ratio. Two effects were found to cause apparent absolute shifts in radial velocity, a temperature-induced shift of the order of ∼1 m s−1 K−1 and a more significant effect resulting in abrupt jumps of ≥50 m s−1 is determined to be caused by the temperature crossing the dew point of the molecular iodine