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Illustration of the ultra-hot Jupiter MASCARA-2b. Image by S. Cabot.

Hot Jupiters are close-in gas giants which make excellent laboratories for studying chemistry and physical processes inside exoplanet atmospheres. They are amenable to both emission spectroscopy, since their equilibrium temperatures correspond to blackbodies that peak in the near infrared, and transmission spectroscopy, thanks to their large scale heights and extended atmospheres.

My specialization lies in high-resolution spectroscopy, a method pioneered by Snellen et al. (2010), Brogi et al. (2012), and Birkby et al. (2013). Atmospheric spectral features are resolved in high-resolution spectra (R ~ 10,000 to 100,000), and distinguished from telluric and stellar absorption lines by detrending algorithms (e.g. PCA, SYSREM) or physical models. Cross-correlating an atmospheric spectral model yields an aggregate signal from all atmospheric features, and simultaneously reveals the semi-amplitude of the planet's orbit, which can reach ~100 km/s. Recently, I contributed to a study of telluric correction methods in the optical regime (Langeveld et al. 2021). 

I have made novel detections of HCN in the atmospheres of HD 189733b and HD 209458b (Cabot et al. 2019, Hawker et al. 2018) using the CRIRES infrared spectrograph. HCN forms in high temperature gas with C/O > 1, making it a tracer of hot Jupiter formation conditions (Madhusudhan 2012). Specifically, the presence of HCN favors planetary migration from past the water and carbon dioxide snowlines, compared to in situ formation. However, the C/O of WASP-77Ab was recently found to be less than unity (Line et al. 2022), which may indicate multiple formation pathways for hot Jupiters.


Novel detection of HCN in an exoplanet atmosphere (Cabot et al. 2019) and confirmations of water and CO in the atmosphere of HD189733b

In the optical regime, atoms, ions, hydrides, and oxides, have strong absorption and emission features. Iron, titanium, and vanadium, as well the molecules containing them, may be responsible for thermal inversions. I detected atmospheric absorption from iron in archival HARPS observations of WASP-121b (Cabot et al. 2020). After generating additional model templates, cross-correlation revealed additional species Fe II, V I and Cr I --- the latter two representing the first detections in an exoplanet (Ben-Yami et al. 2020).


Cross-correlation functions (CCFs) showing detections of four atomic/ionic species in WASP-121b. Figure from Ben-Yami et al. (2021).

I was also involved in the first atmospheric detections made with EXPRES, a recently commissioned spectrograph developed at Yale (P.I. Debra Fischer). I performed the cross-correlation analysis that made novel detections in the atmosphere of MASCARA-2b using EXPRES (Hoeijmakers et al. 2019). Also, I used EXPRES to confirm the new hot Jupiter TOI-1518b by resolving its Doppler shadow through the Rossiter-McLaughlin (RM) effect (Cabot et al. 2021). I also performed an atmospheric analysis which resulted in the detection of neutral iron in its atmosphere, plus evidence of Fe+. TOI-1518b has an equilibrium temperature of ~2500 K, which places it in a regime that often exhibits thermal inversions.


Cross-correlation analysis for transmission spectra of TOI-1518b, obtained with EXPRES. Left: the cross-correlation function revealing the planet's Doppler shadow (dark trail) and atmospheric signature (light trail). Right: stacked cross-correlation function for different combinations of orbital parameters. An enhancement is visible at the planet's semi-amplitude. A small blueshift is apparent, likely from high-altitude winds. Figures from Cabot et al. (2021).

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