H⁻ Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18b

Abstract

We present one of the most precise emission spectra of an exoplanet observed so far. We combine five secondary eclipses of the hot Jupiter WASP-18 b (T day ∼ 2900 K) that we secured between 1.1 and 1.7 µm with the WFC3 instrument aboard the Hubble Space Telescope. Our extracted spectrum (S/N=50, R∼40) does not exhibit clearly identifiable molecular features but is poorly matched by a blackbody spectrum. We complement this data with previously published Spitzer/IRAC observations of this target and interpret the combined spectrum by computing a grid of self-consistent, 1D forward models, varying the composition and energy budget. At these high temperatures, we find there are important contributions to the overall opacity from H − ions, as well as the removal of major molecules by thermal dissociation (including water), and thermal ionization of metals. These effects were omitted in previous spectral retrievals for very hot gas giants, and we argue that they must be included to properly interpret the spectra of these objects. We infer a new metallicity and C/O ratio for WASP-18 b, and find them well constrained to be solar ([M/H]= −0.01 ± 0.35, C/O < 0.85 at 3σ confidence level), unlike previous work but in line with expectations for giant planets. The best fitting selfconsistent temperature-pressure profiles are inverted, resulting in an emission feature at 4.5 µm seen in the Spitzer photometry. These results further strengthen the evidence that the family of very hot gas giant exoplanets commonly exhibit thermal inversions.

Detection of the phase curve and occultation of WASP-100b with TESS

“…The results presented in this paper suggest that the atmosphere of WASP-100b is likely to have a strong thermal redistribution efficiency indicative of atmospheric winds, with significant reflectivity in the TESS waveband. From the measured occultation depth and regression analysis of the phase curve, we measure a maximum dayside temperature of 2720 ± 150 K, placing WASP-100b in the "ultra hot" class of Jupiter-sized exoplanets Arcangeli et al 2018;Bell & Cowan 2018). Our study provides more insight into this relatively new class of exoplanets.…”

“…If the atmosphere of WASP-100b is in actuality composed of many particles that scatter photons in a preferential direction, the relative contributions of the modeled reflection and thermal components to the observed phase curve would have to be altered accordingly (Dyudina et al 2005). Approximating WASP-100b's spectrum as that of a blackbody could be an adequate model for the dayside spectrum, where temperatures are high enough to dissociate absorbent molecules such as H 2 O, TiO, and VO, and to support a H − continuum opacity (Arcangeli et al 2018;Lothringer et al 2018). Because we measure a high redistribution efficiency of heat from the day side to the night side on WASP-100b, the 2400 ± 200 K nightside temperature we derive from the model fit may also be high enough to justify the blackbody approximation, although the posterior distribution of this measurement indicates the nightside temperature is likely low enough to maintain the molecular bond of prominent visible-infrared absorbers such as titanium oxide and vanadium oxide (Lothringer et al 2018), in which case the blackbody assumption would not hold.…”

Non-detection of TiO and VO in the atmosphere of WASP-121b using high-resolution spectroscopy

“…However, later work by Hansen et al (2014) showed that several previous single-transit measurements made using Spitzer had much higher uncertainties than previously thought, and later work by Zellem et al (2014), Diamond-Lowe et al (2014), Schwarz et al (2015), Evans et al (2015) and Line et al (2016) showed results consistent with no temperature inversion layer. Further work found a surprising number of flat, blackbodylike emission spectra for a number of ultra-hot Jupiters (Swain et al 2013;Cartier et al 2017;Kreidberg et al 2018;Arcangeli et al 2018;Mansfield et al 2018) showing none of the emission or absorption features that would indicate the presence of the expected temperature inversion. Sedaghati et al (2017) reported a detection of TiO in the ground-based transmission spectrum of WASP-19b using the VLT, contradicting an earlier result from HST/STIS that found no evidence of TiO (Huitson et al 2013).…”

Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b

“…To date, several UHJs have been studied through transmission and emission spectroscopy. Some examples include KELT-9 b (Hoeijmakers et al 2018), MASCARA-2 b (Casasayas-Barris et al 2019), WASP-121 b (Evans et al 2017), WASP-33 b (Nugroho et al 2017), WASP-103 b (Cartier et al 2017), and WASP-18 b (Sheppard et al 2017;Arcangeli et al 2018;Espinoza et al 2019). A number of neutral and ionized atomic species have been predicted in the atmospheres of such UHJs (Kitzmann et al 2018;Lothringer et al 2018).…”