Thermal emission spectra of the ultra-hot Jupiter WASP-33 b

TL;DR

This work presents a dayside emission study of the ultra-hot Jupiter WASP-33 b by combining two CFHT/WIRCam secondary eclipses in narrow CO and CH4_on bands with archival HST/WFC3 and Spitzer data. Using a parallelized data pipeline, the team extracts high-precision eclipse depths and performs two atmospheric retrievals (FREE and EQ) to infer composition and temperature structure, with the EQ model indicating a thermal inversion and a high C/O ratio of $0.78^{+0.03}_{-0.04}$ alongside a metallicity of $1.51^{+0.44}_{-0.35}$ dex (≈$26 imes$ solar). The results show H$^{-}$-driven opacity as a key factor in shaping the dayside spectrum, with CO largely dissociated-free in the inversion region, and only weak evidence for TiO; CH$_{4}$ is not detected. The inferred high metallicity and C/O ratio support formation via accretion of metal-rich gas, potentially near the CO$_2$ snowline, followed by inward migration possibly driven by dynamical processes; the authors advocate JWST observations to better constrain oxygen-bearing species and refractory elements, refining formation and evolutionary narratives for WASP-33 b.

Abstract

Observations of exoplanetary atmospheres provide critical insights into their chemical composition, formation and evolution history. Ultra-hot Jupiters serve as excellent targets for atmospheric characterization; studies of these planets may yield key understanding of gas giant's formation and evolution history. We present a thermal emission study of WASP-33 b's dayside atmosphere, based on two secondary eclipse observations with CFHT/WIRCam in two specific narrow band filters, namely the CO and CH4$_{\rm on}$ filters, and archival data with HST/WFC3 and Spitzer. Stellar pulsations of the host star induce some quasi-periodic photometric variations, particularly in the CH4$_{\rm on}$ band, which are modelled and corrected in the high-precision differential light curves. An eclipse depth of $1565.2^{+228.6}_{-237.5}$ ppm and $914.3^{+56.1}_{-57.0}$ ppm is determined for the CO and CH4$_{\rm on}$ bands, respectively. Combined with HST/WFC3 and Spitzer data, our joint retrieval of WASP-33 b's dayside atmosphere reveals a high metallicity ([Fe/H] $= 1.52^{+0.35}_{-0.52}$), high C/O ratio (C/O $= 0.78^{+0.03}_{-0.04}$), and a thermal inversion layer, suggesting a formation history involving metal-rich gas accretion. We confirm the presence of the molecules H$_{2}$O, H$^{-}$ and CO, and report a tentative detection of TiO in the dayside atmosphere of WASP-33 b. Future higher precision observations with JWST may provide better understand constraints on the chemical abundances of oxygen and refractory element abundances to better WASP-33 b's formation and evolutionary pathway.

Figures (16)

• Figure 1: The orbit phases of our two observations. Specifically, Night 1 covered phase 0.410-0.589 (October 25 2015), Night 2 covered phase 0.403-0.573 (December 24 2015).
• Figure 2: The transmission curves of the CO and CH4$_{\rm on}$ filters used in this work.
• Figure 3: The CO filter reduced full-frame WIRCam image of WSAP-33 taken on 25 October 2015. The target star is marked as the white square and the final chosen reference stars are marked as the grey circles.
• Figure 4: Normalized light curves of target star and the candidate reference stars.
• Figure 5: Contour maps of the normalized rms$*\beta^2$ distribution are superimposed on the two-dimensional parameter space defined by $D$ and $N_{\rm RSG}$ in the CO filter data. The pixel scale is 0.306 arcsec $\rm pixel^{-1}$. The minimum value of rms$*\beta^2$ is reached with $D=29$ and $N_{\rm RSG}=3$.