Extremely Irradiated Hot Jupiters: Non-oxide Inversions, H⁻ Opacity, and Thermal Dissociation of Molecules

Abstract

Extremely irradiated hot Jupiters, exoplanets reaching dayside temperatures >2000 K, stretch our understanding of planetary atmospheres and the models we use to interpret observations. While these objects are planets in every other sense, their atmospheres reach temperatures at low pressures comparable only to stellar atmospheres. In order to understand our a priori theoretical expectations for the nature of these objects, we self-consistently model a number of extreme hot Jupiter scenarios with the PHOENIX model atmosphere code. PHOENIX is well-tested on objects from cool brown dwarfs to expanding supernovae shells and its expansive opacity database from the UV to far-IR make PHOENIX well-suited for understanding extremely irradiated hot Jupiters. We find several fundamental differences between hot Jupiters at temperatures >2500 K and their cooler counterparts. First, absorption by atomic metals like Fe and Mg, molecules including SiO and metal hydrides, and continuous opacity sources like H − all combined with the short-wavelength output of early-type host stars result in strong thermal inversions, without the need for TiO or VO. Second, many molecular species, including H 2 O, TiO, and VO are thermally dissociated at pressures probed by eclipse observations, biasing retrieval algorithms that assume uniform vertical abundances. We discuss other interesting properties of these objects, as well as future prospects and predictions for observing and characterizing this unique class of astrophysical object, including the first self-consistent model of the hottest known jovian planet, KELT-9b.

The Intrinsic Temperature and Radiative–Convective Boundary Depth in the Atmospheres of Hot Jupiters

“…It has long been a convention in this field to use intrinsic temperatures similar to Jupiter's (e.g. Showman et al 2015;Amundsen et al 2016;Lothringer et al 2018;Flowers et al 2019, and many others), around 100 K (Li et al 2012). Our work shows that more realistic values for T int should depend strongly on the incident flux and will typically be several hundreds of Kelvin, as shown in Figure 1.…”

Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b

“…These calculations assume solar elemental abundances (Asplund et al 2009) and include the same species as Harrison et al (2018) plus gaseous, solid, neutral and ionic molecules and atomic forms of Fe, Ti, V, Cr and Mg. Figure 4 shows the equilibrium abundances of several neutral gaseous Fe species between 1000-3000 K at a pressure of 1 mbar nominally corresponding to the optical photosphere. At the equilibrium temperature of WASP-121 b (∼2400 K), neutral atomic Fe is the dominant form of Fe and should therefore be the most easily detected (also see Kitzmann et al 2018;Lothringer et al 2018). We note that the terminator temperature probed in the optical at high resolution, i.e., going as high up in the atmosphere as 1 mbar -1 µbar, can be significantly lower than the equilibrium temperature.…”

Detection of Fe i in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy

“…Haswell et al 2012;Hoeijmakers et al 2018Hoeijmakers et al , 2019Casasayas-Barris et al 2019), with Fe i in particular recently predicted to be an important atmospheric constituent in hot atmospheres (e.g. Kitzmann et al 2018;Lothringer et al 2018;Lothringer & Barman 2019).…”