H- and Dissociation in Ultra-hot Jupiters: A Retrieval Case Study of WASP-18b. (arXiv:2004.07252v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gandhi_S/0/1/0/all/0/1">Siddharth Gandhi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Madhusudhan_N/0/1/0/all/0/1">Nikku Madhusudhan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mandell_A/0/1/0/all/0/1">Avi Mandell</a>
Atmospheres of a number of ultra-hot Jupiters (UHJs) with temperatures
$gtrsim$2000~K have been observed recently. Many of these planets show largely
featureless thermal spectra in the near-infrared observed with the HST WFC3
spectrograph (1.1-1.7~$mu$m) even though this spectral range contains strong
H$_2$O opacity. Recent works have proposed the possibility of H- opacity
masking the H$_2$O feature and/or thermal dissociation of H$_2$O causing its
apparent depletion at the high temperatures of UHJs. In this work we test these
hypotheses using observations of the exoplanet WASP-18b as a case study. We
report detailed atmospheric retrievals of the planet using the HyDRA retrieval
code, extended to include the effects of H- opacity and thermal dissociation.
We report constraints on the H$_2$O, CO and H- abundances as well as the
pressure-temperature profile of the dayside atmosphere for retrievals with and
without H-/dissociation for each dataset. We find that the H$_2$O and H-
abundances are relatively unconstrained given the featureless WFC3 spectra. We
do not conclusively detect H- in the planet contrary to previous studies which
used equilibrium models to infer its presence. The constraint on the CO
abundance depends on the combination of WFC3 and Spitzer data, ranging from
solar to super-solar CO values. We additionally see signs of a thermal
inversion from two of the datasets. Our study demonstrates the potential of
atmospheric retrievals of UHJs including the effects of H- and thermal
dissociation of molecules.
Atmospheres of a number of ultra-hot Jupiters (UHJs) with temperatures
$gtrsim$2000~K have been observed recently. Many of these planets show largely
featureless thermal spectra in the near-infrared observed with the HST WFC3
spectrograph (1.1-1.7~$mu$m) even though this spectral range contains strong
H$_2$O opacity. Recent works have proposed the possibility of H- opacity
masking the H$_2$O feature and/or thermal dissociation of H$_2$O causing its
apparent depletion at the high temperatures of UHJs. In this work we test these
hypotheses using observations of the exoplanet WASP-18b as a case study. We
report detailed atmospheric retrievals of the planet using the HyDRA retrieval
code, extended to include the effects of H- opacity and thermal dissociation.
We report constraints on the H$_2$O, CO and H- abundances as well as the
pressure-temperature profile of the dayside atmosphere for retrievals with and
without H-/dissociation for each dataset. We find that the H$_2$O and H-
abundances are relatively unconstrained given the featureless WFC3 spectra. We
do not conclusively detect H- in the planet contrary to previous studies which
used equilibrium models to infer its presence. The constraint on the CO
abundance depends on the combination of WFC3 and Spitzer data, ranging from
solar to super-solar CO values. We additionally see signs of a thermal
inversion from two of the datasets. Our study demonstrates the potential of
atmospheric retrievals of UHJs including the effects of H- and thermal
dissociation of molecules.
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