© 2022 The American Astronomical SocietyRathcke, A. D.MacDonald, R. J.Barstow, J. K.Goyal, J. M.López Morales, M.Mendoça, J. M.Sanz Forcada, J.Henry, G. W.Sing, D. K.Alam, M. K.2022-05-062022-05-062021-09-10The Astronomical Journal 162(4): 138(2021)0004-637Xhttps://iopscience.iop.org/article/10.3847/1538-3881/ac0e99http://hdl.handle.net/20.500.12666/752We 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.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttps://creativecommons.org/licenses/by-nc-nd/4.0/Exoplanet atmospheresExoplanets atmospheric compositionObservational astronomyHot JupitersTransmission spectroscopyinfo:eu-repo/semantics/article10.3847/1538-3881/ac0e991538-4357http://dx.doi.org/10.13039/501100011033info:eu-repo/semantics/openAccess