A quantitative assessment of the VO line list: Inaccuracies hamper high-resolution VO detections in exoplanet atmospheres

Abstract: Metal hydrides and oxides are important species in hot-Jupiters since they can affect their energy budgets and the thermal structure of their atmospheres. One such species is VO, which is prominent in stellar M-dwarf spectra. Evidence for VO has been found in the low-resolution transmission spectrum of WASP-121b, but this has not been confirmed at high resolution. It has been suggested that this is due to inaccuracies in its line list. In this paper, we quantitatively evaluate the VO line list and assess whether inaccuracies are responsible for the non-detections at high resolution in WASP-121b. Furthermore, we investigate whether the detectability can be improved by selecting only those lines associated with the most accurate quantum transitions. A cross-correlation analysis was applied to archival HARPS and CARMENES spectra of several M dwarfs. VO cross-correlation signals from the spectra were compared with those in which synthetic VO models were injected, providing an estimate of the ratio between the potential strength (in case of a perfect model) and the observed strength of the signal. This was repeated for the reduced model covering the most accurate quantum transitions. The findings were fed into injection and recovery tests of VO in a UVES transmission spectrum of WASP-121b. We find that inaccuracies cause VO cross-correlation signals in M-dwarf spectra to be suppressed by about a factor 2.1 and 1.1 for the complete and reduced line lists, respectively, corresponding to a reduced observing efficiency of a factor 4.3 and 1.2. The reduced line list outperforms the complete line list in recovering the actual VO signal in the M-dwarf spectra by about a factor of 1.8. Neither line list results in a VO detection in WASP-121b. Injection tests show that with the reduced efficiency of the line lists, the potential signal as seen at low resolution is not detectable in these data.