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

**Authors:** S. de Regt, A. Y. Kesseli, I. A. G. Snellen, S. R. Merritt, K. L. Chubb  
**arXiv:** [2203.03585](https://arxiv.org/abs/2203.03585)  
**Date:** 2022-03-07  
**Summary page:** https://www.summarizepaper.com/en/paper/2203.03585v1/

## Key points

- Metal hydrides and oxides are important in hot-Jupiter exoplanet atmospheres
- Vanadium oxide (VO) is commonly observed in M-dwarf star spectra
- Previous studies detected VO in low-resolution spectrum of WASP-121b
- High-resolution detections of VO in WASP-121b have not been confirmed, possibly due to inaccuracies in the VO line list
- Authors quantitatively evaluate the VO line list to determine if inaccuracies hinder high-resolution detections
- Cross-correlation analysis applied to archival spectra from HARPS and CARMENES instruments obtained from M-dwarf stars
- Synthetic VO models injected into spectra for comparison with cross-correlation signals
- Inaccuracies in line list cause suppression of VO cross-correlation signals by approximately 2.1 times for complete line list and 1.1 times for reduced line list
- Reduced line list outperforms complete line list by approximately 1.8 times in recovering actual VO signal present in M-dwarf spectra
- Despite efforts to improve detectability, neither line list results in a VO detection in transmission spectrum of WASP-121b

## Summary

Metal hydrides and oxides play a crucial role in the energy budgets and thermal structures of hot-Jupiter exoplanet atmospheres. One such oxide, vanadium oxide (VO), is commonly observed in the spectra of M-dwarf stars. Previous studies have detected evidence of VO in the low-resolution transmission spectrum of WASP-121b, a hot-Jupiter exoplanet. However, these detections have not been confirmed at high resolution, leading to speculation that inaccuracies in the VO line list may be responsible. In this study, the authors quantitatively evaluate the VO line list to determine if inaccuracies are indeed hindering high-resolution detections of VO in WASP-121b. They also investigate whether selecting only lines associated with the most accurate quantum transitions can improve detectability. To accomplish this, a cross-correlation analysis is applied to archival spectra from HARPS and CARMENES instruments, which were obtained from several M-dwarf stars. The cross-correlation signals from these spectra are compared with synthetic VO models that were injected into them. This allows for an estimation of the ratio between the potential strength of the signal (assuming a perfect model) and its observed strength. The analysis is performed for both the complete line list and a reduced model that includes only lines associated with accurate quantum transitions. The findings reveal that inaccuracies in the line list cause VO cross-correlation signals in M-dwarf spectra to be suppressed by approximately 2.1 times for the complete line list and 1.1 times for the reduced line list. This corresponds to a reduced observing efficiency by factors of 4.3 and 1.2, respectively. Furthermore, it is found that the reduced line list outperforms the complete line list by approximately 1.8 times in recovering the actual VO signal present in M-dwarf spectra. However, despite these efforts to improve detectability, neither line list results in a VO detection in the transmission spectrum of WASP-121b.