UV Exoplanet Transmission Spectral Features as Probes of Metals and Rainout

Abstract

The transmission spectra of ultra-hot Jupiters observed shortward of 0.5 μm indicate strong absorption. Previous explanations have included scattering, photochemistry, escaping metals, and disequilibrium chemistry. In this Letter, we show that slopes and features shortward of 0.5 μm can be caused by opacity not commonly considered in atmosphere models of exoplanets but guaranteed to be present if conditions are near chemical equilibrium including, but not limited to, atoms and ions of Fe, Ti, Ni, Ca, Cr, Mn, and SiO. Using the PHOENIX atmosphere model, we describe how the short-wavelength transit spectrum varies with equilibrium temperature between 1000 K and 4000 K, as well as the effect that the rainout of condensates has at these wavelengths. We define two spectral indices to quantify the strength of the NUV and blue absorption compared to that in the red-optical, finding that the NUV transit depth will significantly exceed the transit depth from Rayleigh scattering alone for all hot Jupiters down to around 1000 K. In the blue-optical, hot Jupiters warmer than 2000 K will have transit depths larger than that from Rayleigh scattering, but below 2000 K, Rayleigh scattering can dominate, if present. We further show that these spectral indices may be used to trace the effects of rainout. We then compare our simulated transit spectra to existing observations of WASP-12b, WASP-33b, WASP-76b, and WASP-121b. Further observation of exoplanets at these wavelengths should be prioritized in the coming years as the Hubble Space Telescope nears the end of its operational capability.

An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy

“…The existence of transition metals such as Fe, V and Cr in UHJ atmospheres was predicted by theoretical work using stellar models by Lothringer et al (2018), Lothringer & Barman (2019 and Lothringer et al (2020), who posited that the strong optical opacity of these species would be sufficient to drive the thermal inversion observed in emission (Evans et al 2017;Mikal-Evans et al 2019). It has also been posited (e.g., by Lothringer et al 2020) that a forest of transition metal lines could be responsible for the strong opacity seen at wavelengths < 3000 Åin transmission spectra of WASP-121b (Evans et al 2018), WASP-76b (Fu et al 2020), and WASP-12b (Fossati et al 2010;Haswell et al 2012;Sing et al 2013). Due to the dense region of overlapping lines they create, these species are extremely difficult to isolate at low-resolution, highlighting the importance of high-resolution observations for confirming the presence of such species.…”

“…To date, Mn has not been confirmed in an exoplanet atmosphere. However, it was theorised by Lothringer et al (2020) to be one of many species contributing to the increased transit depth at short wavelengths seen in low-resolution transmission spectra of UHJs, including WASP-121b (Evans et al 2018).…”

“…Additionally, when the terminator of the atmosphere is probed in transit observations, multiple temperature regimes are explored in a transition from the hot day-side to the cooler night-side (Parmentier et al 2018), where both ionised and neutral metals/atoms and recombined molecules may be observable at the same time. Simulations of UHJs using stellar models have predicted a plethora of neutral and ionised atomic species in their atmospheres (Lothringer et al 2018(Lothringer et al , 2020Lothringer & Barman 2019).…”

Aerosols in Exoplanet Atmospheres

“…In particular, as several groups of gasses are associated with clouds that condense at similar temperatures on hot Jupiters (e.g. TiO/VO, aluminum, and calcium at the highest temperatures, iron, magnesium, silicon, chromium, and manganese at moderate temperatures, and potassium and sodium at lower temperatures, see Figure 1), measuring the absolute abundances of these gases and their ratios as a function of planetary temperature and gravity could help constrain the condensation sequence in exoplanet atmospheres (Lothringer et al, 2020). However, while many species have been detected for ultra hot Jupiters (e.g.…”

“…An alternative strategy for probing the composition of clouds is to look for the existence or absence of gas species that have been hypothesized to condense. In particular, as several groups of gasses are associated with clouds that condense at similar temperatures on hot Jupiters (e.g., TiO/VO, aluminum, and calcium at the highest temperatures, iron, magnesium, silicon, chromium, and manganese at moderate temperatures, and potassium and sodium at lower temperatures, see Figure 1), measuring the absolute abundances of these gases and their ratios as a function of planetary temperature and gravity could help constrain the condensation sequence in exoplanet atmospheres (Lothringer et al, 2020). However, while many species have been detected for ultra-hot Jupiters (e.g., Ben-Yami et al, 2020;Cabot et al, 2020;Fossati et al, 2010;Haswell et al, 2012;Hoeijmakers et al, 2018;Nugroho et al, 2020;Sing et al, 2019;von Essen et al, 2019;Yan et al, 2019), suggesting largely cloud-free atmospheres, efforts at lower temperatures have yielded mixed results due to controversial detections that are difficult to replicate (e.g., Chen et al, 2018;Cubillos et al, 2020;Espinoza et al, 2019;Gibson et al, 2019Gibson et al, , 2017McGruder et al, 2020;Sedaghati et al, 2017;Seidel et al, 2020;Sing et al, 2015;Vidal-Madjar et al, 2013) and aerosol opacity at optical wavelengths that reduce the amplitudes of atomic and molecular absorption features (Charbonneau et al, 2002;Heng, 2016;Pont et al, 2008;Sing et al, 2016).…”