An emission spectrum for WASP-121b measured across the 0.8–1.1 μm wavelength range using the Hubble Space Telescope. Issue 2 (27th June 2019)

• Record Type: Journal Article
• Authors: Mikal-Evans, Thomas; Sing, David K; Goyal, Jayesh M; Drummond, Benjamin; Carter, Aarynn L; Henry, Gregory W; Wakeford, Hannah R; Lewis, Nikole K; Marley, Mark S; Tremblin, Pascal; Nikolov, Nikolay; Kataria, Tiffany; Deming, Drake; Ballester, Gilda E
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 488:Issue 2(2019)
• Page Start: 2222
• Page End: 2234
• Publication Date: 2019-06-27
• Subjects: techniques: spectroscopic -- planets and satellites: atmospheres; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1753
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche limit, with an inflated radius nearly double that of Jupiter and a dayside temperature comparable to a late M dwarf photosphere. Secondary eclipse observations covering the 1.1–$1.6\, \mu{\rm m}$ wavelength range have revealed an atmospheric thermal inversion on the dayside hemisphere, likely caused by high-altitude absorption at optical wavelengths. Here we present secondary eclipse observations made with the Hubble Space Telescope Wide Field Camera 3 spectrograph that extend the wavelength coverage from $1.1\, \mu{\rm m}$ down to $0.8\, \mu{\rm m}$ . To determine the atmospheric properties from the measured eclipse spectrum, we performed a retrieval analysis assuming chemical equilibrium, with the effects of thermal dissociation and ionization included. Our best-fitting model provides a good fit to the data with reduced $\chi ^2_\nu =1.04$ . The data diverge from a blackbody spectrum and instead exhibit emission due to H − shortward of $1.1\, \mu{\rm m}$ . The best-fitting model does not reproduce a previously reported bump in the spectrum at $1.25\, \mu{\rm m}$, possibly indicating this feature is a statistical fluctuation in the data rather than a VO emission band as had been tentatively suggested. We estimate an atmospheric metallicity of $[{\rm M}/{\rm H}]= {1.09}_{-0.69}^{+0.57}$, and fit for the carbon and oxygen abundances separately, obtaining $[{\rm C}/{\rm H}]= {-0.29}_{-0.48}^{+0.61}$ and $[{\rm O}/{\rm H}]= {0.18}_{-0.60}^{+0.64}$ . The corresponding carbon-to-oxygen ratio is ${\rm C/O} = 0.49_{-0.37}^{+0.65}$, which encompasses the solar value of 0.54, but has a large uncertainty.