The GTC exoplanet transit spectroscopy survey

Abstract

Aims. We set out to study the atmosphere of WASP-80b, a warm inflated gas giant with an equilibrium temperature of ∼800 K, using ground-based transmission spectroscopy covering the spectral range from 520 to 910 nm. The observations allow us to probe the existence and abundance of K and Na in WASP-80b's atmosphere, existence of high-altitude clouds, and Rayleigh-scattering in the blue end of the spectrum. Methods. We observed two spectroscopic time series of WASP-80b transits with the OSIRIS spectrograph installed in the Gran Telescopio CANARIAS, and use the observations to estimate the planet's transmission spectrum between 520 nm and 910 nm in 20 nm-wide passbands, and around the K I and Na I resonance doublets in 6 nm-wide passbands. We model three previously published broadband datasets consisting of 27 light curves jointly prior to the transmission spectroscopy analysis in order to obtain improved prior estimates for the planet's orbital parameters, average radius ratio, and stellar density. The parameter posteriors from the broadband analysis are used to set informative priors on the transmission spectroscopy analysis. The final transmission spectroscopy analyses are carried out jointly for the two nights using a divide-by-white approach to remove the common-mode systematics and Gaussian processes to model the residual wavelength-dependent systematics. Results. We recover a flat transmission spectrum with no evidence of Rayleigh scattering or K I or Na I absorption, and obtain an improved system characterisation as a by-product of the broadband-and GTC-dataset modelling. The transmission spectra estimated separately from the two observing runs are consistent with each other, as are the transmission spectra estimated using either a parametric or nonparametric systematics models. The flat transmission spectrum favours an atmosphere model with high-altitude clouds over cloud-free models with stellar or sub-stellar metallicities. Conclusions. Our results disagree with the recently published discovery of strong K I absorption in WASP-80b's atmosphere based on ground-based transmission spectroscopy with FORS2 at VLT.

The Transit Light Source Effect. II. The Impact of Stellar Heterogeneity on Transmission Spectra of Planets Orbiting Broadly Sun-like Stars

“…This last point is interesting to consider in the context of the flat visual transmission spectra that are commonly observed for hot Jupiters (e.g., Gibson et al 2013;Huitson et al 2017;Parviainen et al 2018).…”

Evidence of a Clear Atmosphere for WASP-62b: The Only Known Transiting Gas Giant in the JWST Continuous Viewing Zone

“…We modeled the analytic transit light curves of Mandel & Agol (2002) using the Batman package (Kreidberg 2015), combined with a Gaussian process (GP) implemented with the george (Ambikasaran et al 2015) code to model noise in the data. GPs are increasingly used in transmission spectroscopy (e.g., Gibson et al 2012aGibson et al , 2012bEvans et al 2015Evans et al , 2017Kirk et al 2017Kirk et al , 2018Kirk et al , 2019Louden et al 2017;Parviainen et al 2018).…”

“…This choice assures uninformative priors since a uniform prior in log-space is akin to fitting for a 1/x prior in nonlogarithmic space. Since we are fitting for the natural log of the inverse length scale, this choice encourages the length scale to longer length scales so as to ensure that the GP does not over-fit the data (e.g., Evans et al 2018;Parviainen et al 2018;Gibson et al 2019). The GP amplitude was bounded between 0.01 and 100× the variance of the outof-transit data, and the length scales were bounded by the typical spacing between data points and 5× the maximum length scale.…”

Evidence of a Clear Atmosphere for WASP-62b: the Only Known Transiting Gas Giant in the JWST Continuous Viewing Zone

“…We modeled the analytic transit light curves of Mandel & Agol (2002) using the Batman package (Kreidberg 2015), combined with a Gaussian process (GP) implemented with the george (Ambikasaran et al 2014) code to model noise in the data. GPs are increasingly used in transmission spectroscopy (e.g., Gibson et al 2012a,b;Evans et al 2015Evans et al , 2017Kirk et al 2017Kirk et al , 2018Kirk et al , 2019Louden et al 2017;Parviainen et al 2018).…”

“…This choice assures uninformative priors since a uniform prior in log-space is akin to fitting for a 1/x prior in non-logarithmic space. Since we are fitting for the natural log of the inverse length scale, this choice encourages the length scale to longer length scales so as to ensure that the GP does not over-fit the data (e.g., Parviainen et al 2018;Evans et al 2018;Gibson et al 2019). The GP amplitude was bounded between 0.01 and 100× the variance of the out-of-transit data, and the length scales were bounded by the typical spacing between data points and 5× the maximum length scale.…”