Testing the limits of Doppler spectroscopy

Doppler spectroscopy is a method of exoplanet atmosphere characterisation which utilises the many thousands of molecular lines present in a transmission spectrum. Utilising high-resolution (>20,000) ground-based spectroscopy, this method combines the signal of many molecular lines by cross-correlating the spectra with template models of the exoplanetary atmosphere. This technique has been used very successfully to detect water and CO in the atmospheres of large hot Jupiters, as well as adding to the evidence of a temperature in version in WASP-121b via the detection of titanium oxide. Doppler spectroscopy has so far been used almost exclusively by larger ground-based telescopes and on very bright stars, though a recent paper by Brogi et al. (2018) has demonstrated that this technique can also be applied to smaller facilities.

I will present simulations using real high-resolution spectra to demonstrate the power of this technique in detecting molecular species in exoplanet atmospheres. Using spectra of WASP-121b obtained by UVES, randomised artificial signals are injected at the expected level (~10-4) and at randomised system and planetary radial velocities: these signals are retrieved with a high significance by the cross-correlation code. In addition, I hope to demonstrate the effects of inaccurate or inadequate atmospheric templates or unexpected spectral effects on retrieval of the exoplanetary signal. This work in testing and defining the limits of Doppler spectroscopy will prove especially valuable when larger ground-based facilities see first light, opening up more potential targets for this valuable technique.

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