Solar-to-supersolar sodium and oxygen absolute abundances for a ‘hot Saturn’ orbiting a metal-rich star

Nikolov, Nikolay K; Sing, David K; Spake, Jessica J; Smalley, Barry; Goyal, Jayesh M; Mikal-Evans, Thomas; Wakeford, Hannah R; Rustamkulov, Zafar; Deming, Drake; Fortney, Jonathan J; Carter, Aarynn; Gibson, Neale P; Mayne, Nathan J

Abstract

We present new analysis of infrared transmission spectroscopy of the cloud-free hot-Saturn WASP-96b performed with the Hubble and Spitzer Space Telescopes (HST and Spitzer). The WASP-96b spectrum exhibits the absorption feature from water in excellent agreement with synthetic spectra computed assuming a cloud-free atmosphere. The HST-Spitzer spectrum is coupled with Very Large Telescope (VLT) optical transmission spectroscopy which reveals the full pressure-broadened profile of the sodium absorption feature and enables the derivation of absolute abundances. We confirm and correct for a spectral offset of $\Delta R_{{\rm p}}/R_{\ast }=(-4.29^{+0.31}_{-0.37})\, \times 10^{-3}$ of the VLT data relative to the HST-Spitzer spectrum. This offset can be explained by the assumed radius for the common-mode correction of the VLT spectra, which is a well-known feature of ground-based transmission spectroscopy. We find evidence for a lack of chromospheric and photometric activity of the host star which therefore make a negligible contribution to the offset. We measure abundances for Na and O that are consistent with solar to supersolar, with abundances relative to solar values of $21^{+27}_{-14}$ and $7^{+11}_{-4}$, respectively. We complement the transmission spectrum with new thermal emission constraints from Spitzer observations at 3.6 and 4.5 $\mu$m, which are best explained by the spectrum of an atmosphere with a temperature decreasing with altitude. A fit to the spectrum assuming an isothermal blackbody atmosphere constrains the dayside temperature to be Tp = 1545 ± 90 K.

• Journal Article Type: Article
• Acceptance Date: May 30, 2022
• Online Publication Date: Jun 7, 2022
• Publication Date: Aug 4, 2022
• Deposit Date: May 30, 2023
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 515
• Issue: 2
• Pages: 3037-3058
• DOI: https://doi.org/10.1093/mnras/stac1530
• Keywords: Space and Planetary Science, Astronomy and Astrophysics