The Hubble/STIS near-ultraviolet transmission spectrum of HD 189733 b

Abstract

The benchmark hot Jupiter HD 189733 b has been a key target, used to lay out the foundations of comparative planetology for giant exo-planets. 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 (NUV), the last remaining unexplored spectral window to be probed with present-day instrumentation for this planet. The NUV is a unique window for atmospheric mass-loss 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 NUV 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. This 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.

CUTE Reveals Escaping Metals in the Upper Atmosphere of the Ultrahot Jupiter WASP-189b

“…Our data analysis scheme follows a similar methodology as described by Cubillos et al (2020Cubillos et al ( , 2023. For each visit, we first construct "white" light curves by integrating the flux in each exposure over the wavelength range to increase our signal-to-noise ratio (Kreidberg et al 2014).…”

“…This assumes that the instrumental systematics behave similarly over the integrated range (Kreidberg et al 2014). We fit the raw light curves (F λ (t)) with a parametric transit and systematics models similar to Cubillos et al (2023) as a function of time (t), CUTE orbital phase (f), and jitter vector ( j i ): (2008). This enables us to arrive at statistically robust parameter estimations.…”

“…10 UV transmission spectroscopy observations are typically conducted at far-UV (FUV) wavelengths (e.g., Lyα), but in the near-UV (NUV) band stars have a significantly stronger and more uniform emission than in the FUV (Haswell et al 2012;Llama & Shkolnik 2015). Furthermore, the NUV range contains strong lines of several abundant metals, such as Mg and Fe, that effectively probe upper atmospheres and escape (Fossati et al 2010;Haswell et al 2012;Sing et al 2019;Cubillos et al 2020Cubillos et al , 2023, as well as molecular bands probing atmospheric haze and cloud condensations (Lothringer et al 2022).…”

Hydrodynamic Atmospheric Escape in HD 189733 b: Signatures of Carbon and Hydrogen Measured with the Hubble Space Telescope

“…In this context, Ben-Jaffel & Ballester (2013) reported on the detection of neutral oxygen (O I) in the exosphere of HD 189733 b, which the authors attribute to atmospheric escape, but require super-solar abundances and super-thermal line broadening to be explained; they also report the nondetection of singly ionized carbon (C II). More recently, Cubillos et al (2023) ruled out the presence of singly ionized magnesium (Mg II) in the outflow of this planet and, although they reported the nondetection of Mg I, they did not rule out the presence of this species.…”

Expanding the inventory of spectral lines used to trace atmospheric escape in exoplanets

“…Similarly for some planets, ongoing atmospheric mass loss has been observed in one or more spectral tracers, but has eluded detection in others. For example, in HD 189733 b, Lyα (Lecavelier des Etangs et al 2010), metastable helium (Salz et al 2018) and O I (Ben-Jaffel & Ballester 2013) have been detected, but Mg II absorption has not (Cubillos et al 2023). The interpretation of (non-)detected signatures has also not been straightforward.…”