Secondary eclipses of WASP-18b — Near Infrared observations with the Anglo Australian Telescope, the Magellan Clay Telescope and the LCOGT network. (arXiv:1812.10623v1 [astro-ph.EP])

Secondary eclipses of WASP-18b — Near Infrared observations with the Anglo Australian Telescope, the Magellan Clay Telescope and the LCOGT network. (arXiv:1812.10623v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kedziora_Chudczer_L/0/1/0/all/0/1">L. Kedziora-Chudczer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_G/0/1/0/all/0/1">G. Zhou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bailey_J/0/1/0/all/0/1">J. Bailey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bayliss_D/0/1/0/all/0/1">D.D.R. Bayliss</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tinney_C/0/1/0/all/0/1">C.G. Tinney</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Osip_D/0/1/0/all/0/1">D. Osip</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Colon_K/0/1/0/all/0/1">K.D. Colon</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shporer_A/0/1/0/all/0/1">A. Shporer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dragomir_D/0/1/0/all/0/1">D. Dragomir</a>

We present new eclipse observations for one of the hottest “hot Jupiters”
WASP-18b, for which previously published data from HST WFC3 and Spitzer have
led to radically conflicting conclusions about the composition of this planet’s
atmosphere. We measure eclipse depths of $0.15pm0.02%$ at $Ks$ and
$0.07pm0.01%$ at $z’$ bands. Using the VSTAR line-by-line radiative transfer
code and both these new observations with previously published data, we derive
a new model of the planetary atmosphere. We have varied both the metallicity
and C/O ratio in our modelling, and find no need for the extreme metallicity
suggested by Sheppard et al.(2017). Our best fitting models slightly
underestimate the emission at $z’$ band and overestimate the observed flux at
$Ks$-band. To explain these discrepancies, we examine the impact on the
planetary emission spectrum of the presence of several types of hazes which
could form on the night-side of the planet. Our $Ks$ band eclipse flux
measurement is lower than expected from clear atmosphere models and this could
be explained by a haze particles larger than 0.2 $mu$m with the optical
properties of Al$_{2}$O$_{3}$, CaTiO$_{3}$ or MgSiO$_{3}$. We find that $z’$
band measurements are important for understanding the contribution of
photochemical hazes with particles smaller than 0.1 $mu$m at the top of the
atmosphere.

We present new eclipse observations for one of the hottest “hot Jupiters”
WASP-18b, for which previously published data from HST WFC3 and Spitzer have
led to radically conflicting conclusions about the composition of this planet’s
atmosphere. We measure eclipse depths of $0.15pm0.02%$ at $Ks$ and
$0.07pm0.01%$ at $z’$ bands. Using the VSTAR line-by-line radiative transfer
code and both these new observations with previously published data, we derive
a new model of the planetary atmosphere. We have varied both the metallicity
and C/O ratio in our modelling, and find no need for the extreme metallicity
suggested by Sheppard et al.(2017). Our best fitting models slightly
underestimate the emission at $z’$ band and overestimate the observed flux at
$Ks$-band. To explain these discrepancies, we examine the impact on the
planetary emission spectrum of the presence of several types of hazes which
could form on the night-side of the planet. Our $Ks$ band eclipse flux
measurement is lower than expected from clear atmosphere models and this could
be explained by a haze particles larger than 0.2 $mu$m with the optical
properties of Al$_{2}$O$_{3}$, CaTiO$_{3}$ or MgSiO$_{3}$. We find that $z’$
band measurements are important for understanding the contribution of
photochemical hazes with particles smaller than 0.1 $mu$m at the top of the
atmosphere.

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