Gemini/GMOS Optical Transmission Spectroscopy of WASP-121b: signs of variability in an ultra-hot Jupiter?

Abstract

We present ground-based, spectroscopic observations of two transits of the ultra-hot Jupiter WASP-121b covering the wavelength range $\approx$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 generalised 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 shortward of $\approx$650 nm. In contrast to the STIS results, we find evidence for an increasing blueward 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), similarly 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.

17 pages, 10 Figures. Accepted for publication in MNRAS

References in corpus (36)

• Array Programming with NumPy
• A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
• Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
• An ultrahot gas-giant exoplanet with a stratosphere
• SOAP 2.0: A tool to estimate the photometric and radial velocity variations induced by stellar spots and plages
• Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
• HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering
• A spectral survey of an ultra-hot Jupiter: Detection of metals in the transmission spectrum of KELT-9 b
• Detection of Fe I in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy
• Detection of titanium oxide in the atmosphere of a hot Jupiter
• The Complete transmission spectrum of WASP-39b with a precise water constraint
• Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b
• High-Resolution Spectroscopic Detection of TiO and Stratosphere in the Day-side of WASP-33b
• The Transit Light Source Effect II: The Impact of Stellar Heterogeneity on Transmission Spectra of Planets Orbiting Broadly Sun-like Stars
• Atmospheric Rossiter-McLaughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO
• Neutral Iron Emission Lines From The Day-side Of KELT-9b -- The GAPS Programme With HARPS-N At TNG XX
• Variability in the Atmosphere of the Hot Giant Planet HAT-P-7 b
• VLT FORS2 comparative transmission spectroscopy: Detection of Na in the atmosphere of WASP-39b from the ground
• New Avenues for Thermal Inversions in hot Jupiters
• An emission spectrum for WASP-121b measured across the 0.8-1.1 micron wavelength range using the Hubble Space Telescope
• Model Atmospheres From Very Low Mass Stars to Brown Dwarfs
• VLT/FORS2 comparative transmission spectroscopy II: confirmation of a cloud-deck and Rayleigh scattering in WASP-31b, but no potassium?
• The Influence of Host Star Spectral Type on Ultra-Hot Jupiter Atmospheres
• Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b
• Temporal Variability in Hot Jupiter Atmospheres
• Neutral Cr and V in the atmosphere of ultra hot jupiter WASP-121 b
• Swift UVOT near-UV transit observations of WASP-121 b
• UV Exoplanet Transmission Spectral Features as Probes of Metals and Rainout
• Variability in the Atmosphere of the Hot Jupiter Kepler-76b
• A high resolution spectral atlas of brown dwarfs
• The HST PanCET Program: An Optical to Infrared Transmission Spectrum of HAT-P-32Ab
• Search for TiO and Optical Night-side Emission from the Exoplanet WASP-33b
• Ground-Based Transmission Spectroscopy with FORS2: A featureless optical transmission spectrum and detection of H${}_2$O for the ultra-hot Jupiter WASP-103b
• The PHOENIX Exoplanet Retrieval Algorithm and Using H${}^{-}$ Opacity as a Probe in Ultra-hot Jupiters
• Upgrading from Gaussian Processes to Student's-T Processes
• Storms or Systematics? The changing secondary eclipse depth of WASP-12b

Cited by in corpus (7)

• Empirical limb-darkening coefficients & transit parameters of known exoplanets from TESS
• An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy
• Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7 b
• Detection of Aerosols at Microbar Pressures in an Exoplanet Atmosphere
• WASP-35 and HAT-P-30/WASP-51: re-analysis using TESS and ground-based transit photometry
• Generic Models for Disk-Resolved and Disk-Integrated Phase Dependent Linear Polarization of Light Reflected from Exoplanets
• UV Absorption by Silicate Cloud Precursors in Ultra-hot Jupiter WASP-178b