Searching for thermal inversion agents in the transmission spectrum of KELT-20b/MASCARA-2b: detection of neutral iron and ionised calcium H&K lines

Abstract

We analyse the transmission spectra of KELT-20b/MASCARA-2b to search for possible thermal inversion agents. The data consist of three transits obtained using HARPSN and one using CARMENES. We removed stellar and telluric lines before cross-correlating the residuals with spectroscopic templates produced using a 1D plane-parallel model, assuming an isothermal atmosphere and chemical equilibrium at solar metallicity. Using a likelihood-mapping method, we detect Fe i at > 13σ, Ca ii H$\&$K at > 6σ and confirm the previous detections of Fe ii, Ca ii IR Triplet, and Na i D. The detected signal of Fe i is shifted by −3.4 ± 0.4 km s−1 from the planetary rest frame, which indicates a strong day–night wind. Our likelihood-mapping technique also reveals that the absorption features of the detected species extend to different altitudes in the planet’s atmosphere. Assuming that the line lists are accurate, we do not detect other potential thermal inversion agents (NaH, MgH, AlO, SH, CaO, VO, FeH, and TiO) suggesting that non-chemical equilibrium mechanisms (e.g. a cold-trap) might have removed Ti- and V-bearing species from the upper atmosphere. Our results, therefore, show that KELT-20b/MASCARA-2b cannot possess an inversion layer caused by a TiO/VO-related mechanism. The presence of an inversion layer would therefore likely be caused by metal atoms such as Fe i and Fe ii. Finally, we report a double-peak structure in the Fe i signal in all of our data sets that could be a signature of atmospheric dynamics. However, further investigation is needed to robustly determine the origin of the signal.

A temperature inversion with atomic iron in the ultra-hot dayside atmosphere of WASP-189b

“…We resampled the spectra into a wavelength grid with steps corresponding to ∼ 0.6 km s −1 . The continuum level of HARPS-N spectra exhibits a wavelength-dependent variation, which is mainly attributed to a problem of the atmospheric dispersion corrector (Berdiñas et al 2016;Casasayas-Barris et al 2019;Nugroho et al 2020). Other effects, including blaze function stability, atmospheric extinction, and stellar pulsations, can also introduce continuum variations.…”

Aerosols in Exoplanet Atmospheres

“…However, while many species have been detected for ultra hot Jupiters (e.g. Fossati et al, 2010;Haswell et al, 2012;Hoeijmakers et al, 2018;Yan et al, 2019;von Essen et al, 2019;Sing et al, 2019;Ben-Yami et al, 2020;Cabot et al, 2020;Nugroho et al, 2020), suggesting largely cloud-free atmospheres, efforts at lower temperatures have yielded mixed results due to controversial detections that are difficult to replicate (e.g. Vidal-Madjar et al, 2013;Cubillos et al, 2020;Sedaghati et al, 2017;Espinoza et al, 2019;Chen et al, 2018;Seidel et al, 2020;Sing et al, 2015;Gibson et al, 2017Gibson et al, , 2019McGruder et al, 2020) and aerosol opacity at optical wavelengths that reduce the amplitudes of atomic and molecular absorption features (Charbonneau et al, 2002;Pont et al, 2008;Heng, 2016;Sing et al, 2016).…”

“…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).…”

Silicon in the dayside atmospheres of two ultra-hot Jupiters

“…KELT-20b/MASCARA-2b (Lund et al 2017;Talens et al 2018) is a UHJ with T eq ∼ 2300 K that orbits an A-type star without pulsations. A number of metals, such as Ca, Cr, Fe, Na, and Mg, were found in the planetary transmission spectrum (Casasayas-Barris et al 2018Hoeijmakers et al 2020a;Stangret et al 2020;Nugroho et al 2020b;Rainer et al 2021). Recently, Yan et al (2021a) used the spectral emission lines of Fe i to retrieve the atmospheric temperature profile, claiming the presence of a temperature inversion on the planetary dayside.…”