The Hubble/STIS Near-ultraviolet Transmission Spectrum of HD 189733b

Abstract

The benchmark hot Jupiter HD 189733 b has been a key target to lay out the foundations of comparative planetology for giant exoplanets. As such, HD 189733 b has been extensively studied across the electromagnetic spectrum. Here, we report the observation and analysis of three transit light curves of HD 189733 b obtained with Hubble/STIS in the near ultraviolet, the last remaining unexplored spectral window to be probed with present-day instrumentation for this planet. The NUV is a unique window for atmospheric massloss studies owing to the strong resonance lines and large photospheric flux. Overall, from a low-resolution analysis (R = 50) we found that the planet's near-ultraviolet spectrum is well characterized by a relatively flat baseline, consistent with the optical-infrared transmission, plus two regions at ∼2350 and ∼2600 Å that exhibit a broad and significant excess absorption above the continuum. From an analysis at a higher resolution (R = 4700), we found that the transit depths at the core of the magnesium resonance lines are consistent with the surrounding continuum. We discarded the presence of Mg ii absorption in the upper atmosphere at a ∼2-4σ confidence level, whereas we could place no significant constraint for Mg i absorption. These broad absorption features coincide with the expected location of Fe ii bands; however, solar-abundance hydrodynamic models of the upper atmosphere are not able to reproduce the amplitude of these features with iron absorption. Such scenario would require a combination of little to no iron condensation in the lower-atmosphere, super-solar metallicities, and a mechanism to enhance the absorption features (such as zonal wind broadening). The true nature of this feature remains to be confirmed.