HST PanCET Program: A Complete Near-UV to Infrared Transmission Spectrum for the Hot Jupiter WASP-79b

Abstract

We present a new optical transmission spectrum of the hot Jupiter WASP-79b. We observed three transits with the STIS instrument mounted on the Hubble Space Telescope (HST), spanning 0.3-1.0 μm. Combining these transits with previous observations, we construct a complete 0.3-5.0 μm transmission spectrum of WASP-79b. Both HST and ground-based observations show decreasing transit depths toward blue wavelengths, contrary to expectations from Rayleigh scattering or hazes. We infer atmospheric and stellar properties from the full near-UV to infrared transmission spectrum of WASP-79b using three independent retrieval codes, all of which yield consistent results. Our retrievals confirm previous detections of H 2 O (at 4.0σ confidence) while providing moderate evidence of H − bound-free opacity (3.3σ) and strong evidence of stellar contamination from unocculted faculae (4.7σ). The retrieved H 2 O abundance (∼1%) suggests a superstellar atmospheric metallicity, though stellar or substellar abundances remain consistent with present observations (O/H = 0.3-34× stellar). All three retrieval codes obtain a precise H − abundance constraint: log( -X H ) ≈ −8.0 ± 0.7. The potential presence of H − suggests that James Webb Space Telescope observations may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred faculae are ∼500 K hotter than the stellar photosphere, covering ∼15% of the stellar surface. Our analysis underscores the importance of observing UV-optical transmission spectra in order to disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra.

Hiding in plain sight: observing planet-starspot crossings with the James Webb Space Telescope

“…Our simulations show that JWST can be expected to be competitive for the determination of starspot temperatures from transit data with respect to HST. This latter was able to provide < ∼ 250 K uncertainties on starspots occulted by planets transiting very bright stars, such as the mag 𝐾 5.5 HD 189733, by using observations in the visible (Sing et al 2011); about 400 K precision was also shown to be achievable, by stitching together visible and near-infrared spectra affected by unocculted spots (Bruno et al 2020;Rathcke et al 2021). The broad wavelength coverage of JWST will be able to place significant constraints by relying on near-infrared observations alone.…”

A Comprehensive Analysis of WASP-17b's Transmission Spectrum from Space-Based Observations

“…Retrieval frameworks can operate using a free chemistry model (e.g., POSEIDON MacDonald & Madhusudhan (2017)), in which the volume mixing ratios of each chemical species included within a model atmosphere are treated as separate free parameters, or using a chemical equilibrium model (e.g., ATMO Amundsen et al (2014); Tremblin et al (2015); Wakeford et al (2017)), in which the abundances of the complete chemical inventory are solved for assuming that the chemical processes governing the atmosphere are in equilibrium. By utilising multiple retrieval methods, the impacts of different modelling choices can be quantified, while comparison between them can hint at processes that would be difficult to detect with a single method (e.g., Baudino et al 2017;Barstow et al 2018;Lewis et al 2020;Rathcke et al 2021;Barstow et al 2022). For example, comparing the results of free and equilibrium chemistry retrievals can help to ascertain whether disequilibrium processes are at play (e.g., Baudino et al 2017;Lewis et al 2020).…”

“…We consider both an isothermal, gradient and a parametric pressure-temperature (P-T) profile (Madhusudhan & Seager 2009). The impacts of stellar contamination (Rackham et al 2017) on WASP-17b's transmission spectrum are parameterised by the temperature and covering fraction of stellar heterogeneities and the photosphere temperature (see Rathcke et al 2021). All retrievals also fit for the planetary radius at the 10 bar reference pressure, 𝑅 𝑝, ref .…”

“…To better assess the physical nature of our free-chemistry retrievals, we also explore the implications of equilibrium chemistry on the atmosphere of WASP-17b. To do so, we utilised the ATMO retrieval code (Amundsen et al 2014;Tremblin et al 2015;Wakeford et al 2017) which has been previously bench-marked against POSEIDON's free chemistry (Lewis et al 2020;Rathcke et al 2021). ATMO computes the chemistry for each atmospheric layer and wavelength such Comparison between the retrieved transmission spectra of a POSEIDON free chemistry retrieval of the full 0.3-5 𝜇m transmission spectrum using STIS, WFC3/IR and Spitzer (blue), and the of same retrieval without the inclusion of STIS data (orange).…”

Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7 b

“…In addition, the Hubble Space Telescope has collected spectroscopic data in order to learn more about the atmospheric composition of exoplanets (e.g. Tinetti et al 2007, Kreidberg et al 2014, Knutson et al 2014, Sing et al 2016, Barstow et al 2017, Skaf et al 2020, Sing et al 2019, Rathcke et al 2021, Foote et al 2022; for an overview, see Kreidberg 2018).…”