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

Abstract

High-resolution Doppler-resolved spectroscopy has opened up a new window into the atmospheres of both transiting and non-transiting exoplanets. Here, we present VLT/UVES observations of a transit of WASP-121b, an 'ultra-hot' Jupiter previously found to exhibit a temperature inversion and detections of multiple species at optical wavelengths. We present initial results using the blue arm of UVES (≈3700 -5000Å), recovering a clear signal of neutral Fe in the planet's atmosphere at >8 σ, which could contribute to (or even fully explain) the temperature inversion in the stratosphere. However, using standard cross-correlation methods, it is difficult to extract physical parameters such as temperature and abundances. Recent pioneering efforts have sought to develop likelihood 'mappings' that can be used to directly fit models to high-resolution datasets. We introduce a new framework that directly computes the likelihood of the model fit to the data, and can be used to explore the posterior distribution of parameterised model atmospheres via MCMC techniques. Our method also recovers the physical extent of the atmosphere, as well as account for time-and wavelength-dependent uncertainties. We measure a temperature of 3710 +490 −510 K, indicating a higher temperature in the upper atmosphere when compared to low-resolution observations. We also show that the Fe i signal is physically separated from the exospheric Fe ii. However, the temperature measurements are highly degenerate with aerosol properties; detection of additional species, using more sophisticated atmospheric models, or combining these methods with low-resolution spectra should help break these degeneracies.

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

“…Our analysis showed that the other detected species are at a higher altitude than Fe I, and the line contrast is underestimated by our chemical equilibrium model, which might indicate a higher temperature in the upper layer or, in other words, an inversion layer. Furthermore, this possibility has been supported observationally for WASP-121b, which shows clear evidence of thermal inversion layer (Evans et al 2017) and strong absorption from Fe I and Fe II (Gibson et al 2020;Sing et al 2019), but has no TiO/VO as shown by Merritt et al (2020) although again, subject to the accuracy of line lists. Further, the investigation should be done using the secondary eclipse technique or emission spectroscopy in the near-infrared to reveal the temperature structure on the dayside of the planet using either HST, JWST or ground-based facilities.…”

“…The result is the integrated CCF at various combination of covers too narrow a velocity range, then the S/N of the possible detected signal using this method might not represent the true S/N. We also generated a likelihood map for each of the detected species by using the new likelihood-based approach outlined by Gibson et al (2020, see also Brogi & Line 2019 for a related approach). The log-likelihood (ln L) is defined as…”

“…One advantage of using the new likelihood mapping developed by Gibson et al (2020) is that we do not need injection tests to constrain the α value. The deviation from our chemical equilibrium assumption is also reflected in the α value of each detected species.…”

“…This has become one of the most robust approaches in the attempt to characterize the atmospheres of exoplanets (e.g. Redfield et al 2008;Snellen et al 2008Snellen et al , 2010Jensen et al 2011;Brogi et al 2012Brogi et al , 2013Birkby et al 2013;Brogi et al 2014;Lockwood et al 2014;Snellen et al 2014;Hoeijmakers et al 2015;Schwarz et al 2015;Wyttenbach et al 2015;Birkby et al 2017;Brogi et al 2018;Hawker et al 2018;Pino et al 2018;Wang et al 2018;Alonso-Floriano et al 2019;Brogi & Line 2019;Hoeijmakers et al 2019;Sánchez-López et al 2019;Gibson et al 2020;Merritt et al 2020). Using this technique, Nugroho et al (2017) directly detected a high-resolution emission signature of TiO on the dayside of WASP-33b using the Subaru telescope, providing simultaneous evidence of the existence of both this molecule and a hot stratosphere in the atmosphere of the planet.…”

Neutral Iron Emission Lines from the Dayside of KELT-9b: The GAPS Program with HARPS-N at TNG XX

“…Detections of atomic metals at the day-night transition of their atmospheres (WASP-12b, Fossati et al 2010;Haswell et al 2012;KELT-9b, Hoeijmakers et al 2018Cauley et al 2019;MASCARA-2b, Casasayas-Barris et al 2019;WASP-121b, Sing et al 2019;Gibson et al 2020) show that heavy elements are not necessarily sequestered deep in the atmospheres of these planets. This may also indicate the presence of a shallow radiativeconvective boundary (Thorngren et al 2019).…”