Abstract
ABSTRACT We present ground-based, spectroscopic observations of two transits of the ultra-hot Jupiter WASP-121b covering the wavelength range ≈500–950 nm using Gemini/GMOS. We use a Gaussian process framework to model instrumental systematics in the light curves, and also demonstrate the use of the more generalized Student’s-T process to verify our results. We find that our measured transmission spectrum, whilst showing overall agreement, is slightly discrepant with results obtained using HST/STIS, particularly for wavelengths shortwards of ≈650 nm. In contrast to the STIS results, we find evidence for an increasing bluewards slope and little evidence for absorption from either TiO or VO in our retrieval, in agreement with a number of recent studies performed at high-resolution. We suggest that this might point to some other absorbers, particularly some combination of recently detected atomic metals, in addition to scattering by hazes, being responsible for the excess optical absorption and observed vertical thermal inversion. Our results are also broadly consistent with previous ground-based photometry and 3D GCM predictions, however, these assumed different chemistry to our retrievals. In addition, we show that the GMOS observations are repeatable over short periods (days), similar to the HST/STIS observations. Their difference over longer periods (months) could well be the result of temporal variability in the atmospheric properties (i.e. weather) as predicted by theoretical models of ultra-hot Jupiters; however, more mundane explanations such as instrumental systematics and stellar activity cannot be fully ruled out, and we encourage future observations to explore this possibility.
Highlights
Close-in giant exoplanets are amongst the most promising targets for detailed atmospheric studies of planets outside our own Solar System, owing to their favourable combinations of short periods, extended atmospheres and large planet-to-star radius ratios – all of which helps to maximise the achievable signal-to-noise of what are very challenging measurements
We have presented ground-based Gemini/Gemini Multi-Object Spectrograph (GMOS) observations of the ultra-hot Jupiter WASP-121b covering two full transits and extracting a transmission spectrum over the wavelength range ≈ 500 – 950 nm using the technique of differential spectrophotometry
We introduced a new analysis technique for dealing with instrumental systematics in exoplanet transit light curves using Student’s-T processes, which can be thought of as a generalisation of the more commonly employed Gaussian processes
Summary
Close-in giant exoplanets are amongst the most promising targets for detailed atmospheric studies of planets outside our own Solar System, owing to their favourable combinations of short periods, extended atmospheres and large planet-to-star radius ratios – all of which helps to maximise the achievable signal-to-noise of what are very challenging measurements. Beatty et al (2017) and Parmentier et al (2013) both show how cold-trap processes can interfere with the circulation of TiO/VO and suppress the formation of inversions, and it has been demonstrated (Parmentier et al 2018; Lothringer et al 2018) that for the very hottest planets much of the TiO and VO will be thermally dissociated on the dayside and in these cases thermal inversions could instead be driven by NUV and optical absorption by gas phase metals such as Fe and Mg. at high-resolution, the atmosphere of WASP-121b has been observed using both the HARPS (Bourrier et al 2020a; Cabot et al 2020) and UVES (Gibson et al 2020; Merritt et al 2020) spectrographs, which has resulted in significant detections of the atomic metals FeI and NaI and absorption from H-alpha but non-detections of both.
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