LRG-BEASTS: Transmission Spectroscopy and Retrieval Analysis of the Highly Inflated Saturn-mass Planet WASP-39b

Abstract

We present a ground-based transmission spectrum and comprehensive retrieval analysis of the highly inflated Saturn-mass planet WASP-39b. We obtained low-resolution spectra (R ≈ 400) of a transit of WASP-39b using the ACAM instrument on the 4.2 m William Herschel Telescope as part of the LRG-BEASTS survey. Our transmission spectrum is in good agreement with previous ground-and spacebased observations of WASP-39b, and covers a wavelength range of 4000-9000Å. Previous analyses of this exoplanet have retrieved water abundances that span more than four orders of magnitude, which in turn lead to conclusions of a subsolar or highly supersolar atmospheric metallicity. In order to determine the cause of the large discrepancies in the literature regarding WASP-39b's atmospheric metallicity, we performed retrieval analyses of all literature data sets. Our retrievals, which assume equilibrium chemistry, recovered highly supersolar metallicities for all data sets. When running our retrievals on a combined spectrum, spanning 0.3-5 µm, we recovered an atmospheric metallicity of 282 +65 −58 × solar. We find that stellar activity has a negligible effect on the derived abundances and instead conclude that different assumptions made during retrieval analyses lead to the reported water abundances that differ by orders of magnitude. This in turn has significant consequences for the conclusions we draw. This is the fourth planet to be observed as part of the LRG-BEASTS survey, which is demonstrating that 4 m class telescopes can obtain low-resolution transmission spectra with precisions of around one atmospheric scale height.

PLATON II: New Capabilities and a Comprehensive Retrieval on HD 189733b Transit and Eclipse Data

“…A similar lack of retrieval codes for transit spectra motivated us to write PLATON (Zhang et al 2019), a fast, open source, easy to use, and easy to understand forward modeling and retrieval code that traces its lineage back to Exo-Transmit (Kempton et al 2017). PLATON has since been used in several papers: a few exploring the atmospheric properties of observed planets (Chachan et al 2019;Kirk et al 2019;Guo et al 2020), and one demonstrating the possibility of using K-means clustering to speed up retrievals by 40% (Hayes et al 2020). In the latter study, the speed of PLATON was especially useful due to the necessity of running many retrievals.…”

Two Transiting Hot Jupiters from the WASP Survey: WASP-150b and WASP-176b

“…WASP-176b is a potential target for atmospheric characterization via transmission spectroscopy due to its low density and high equilibrium temperature. If we assume an atmosphere similar in composition to Jupiter (μ=2.2u, where u=1.66×10 −27 kg is the atomic mass units) then the scale height is found to be H=k B T/μg∼500 km which is smaller, but comparable, to values seen in recent successful atmospheric retrievals (Kirk et al 2019). However, with a magnitude of only V=12.01, the follow-up would be a significant challenge.…”

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

“…We fit the light curves following the procedure detailed in Kirk et al (2017Kirk et al ( , 2018Kirk et al ( , 2019, which we briefly describe here. 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.…”

“…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).…”

“…Close-in gas giant exoplanets that are cloud-free/haze-free in the observable atmosphere appear to be rare (<7% of cases), with the vast majority of hot Jupiters showing substantial opacity from condensation clouds and photochemical hazes (Wakeford et al 2019). The few benchmark cases of clear atmospheres, such as WASP-96b (Nikolov et al 2018) and WASP-39b (Fischer et al 2016;Nikolov et al 2016;Wakeford et al 2018;Kirk et al 2019), have produced some of the best constraints on atmospheric metallicities and abundances of H 2 O and Na to date (e.g., Welbanks et al 2019). The rare class of cloud-free planets at the pressures probed via low-resolution transmission spectroscopy (∼10 −3 −1 bar for optical and infrared observations) permit precision measurements of atomic and molecular abundances, unhindered by cloud-composition degeneracies (e.g., Fraine et al 2014;Kreidberg et al 2014;Sing et al 2016;Kilpatrick et al 2018).…”