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

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 H₂O (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 H₂O 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( ${X}_{{{\rm{H}}}^{-}}$ ) ≈ -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.