Testing the Detectability of Extraterrestrial O2 with the Extremely Large Telescopes Using Real Data with Real Noise

The future extremely large telescopes (ELTs) are expected to be powerful tools to probe the atmospheres of extrasolar planets using high-dispersion spectroscopy, with the potential to detect molecular oxygen in Earth-like planets transiting nearby, late-type stars. So far, simulations have concentrated on the optical 7600 \AA{} A-band of oxygen using synthetic noise distributions. In this paper, we build upon previous work to predict the detectability of molecular oxygen in nearby, temperate planets by using archival, time-series data of Proxima Centauri from the high-dispersion UVES spectrograph on ESO's Very Large Telescope (VLT). The brightest transiting M-dwarfs are expected to be about 25 times fainter than Proxima, a factor that is similar to the difference in light-gathering power between the VLT and the future ELTs. By injecting synthetic oxygen transmission signals into the UVES data, the $\mathrm{O}_2$ detectability can be studied in the presence of real data with real noise properties. Correcting for the relatively low throughput ($\sim$4%) of the Proxima spectra to an assumed 20% throughput for a high-dispersion spectrograph on the European ELT, we find that the molecular oxygen signature of an Earth-twin transiting a nearby ($d \approx 7 \,\mathrm{pc}$) M5V star can be detected in 20-50 transits (a total of 70-175 hours of observing time). This estimate using more realistic simulations is close to previous predictions. Novel concepts that increase the instrumental throughput can further reduce the time span over which such observations need to be taken.