A New Window into Planet Formation and Migration: Refractory-to-Volatile Elemental Ratios in Ultra-hot Jupiters. (arXiv:2011.10626v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Lothringer_J/0/1/0/all/0/1">Joshua D. Lothringer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rustamkulov_Z/0/1/0/all/0/1">Zafar Rustamkulov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sing_D/0/1/0/all/0/1">David K. Sing</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gibson_N/0/1/0/all/0/1">Neale P. Gibson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilson_J/0/1/0/all/0/1">Jamie Wilson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schlaufman_K/0/1/0/all/0/1">Kevin C. Schlaufman</a>

A primary goal of exoplanet characterization is to use a planet’s current
composition to understand how that planet formed. For example, the C/O ratio
has long been recognized as carrying important information on the chemistry of
volatile species. Refractory elements, like Fe, Mg, and Si, are usually not
considered in this conversation because they condense into solids like Fe(s) or
MgSiO$_3$ and would be removed from the observable, gaseous atmosphere in
exoplanets cooler than about 2000~K. However, planets hotter than about 2000~K,
called ultra-hot Jupiters (UHJs), are warm enough to largely avoid the
condensation of refractory species. In this paper, we explore the insight that
the measurement of refractory abundances can provide into a planet’s origins.
Through refractory-to-volatile elemental abundance ratios, we can estimate a
planet’s atmospheric rock-to-ice fraction and constrain planet formation and
migration scenarios. We first relate a planet’s present-day
refractory-to-volatile ratio to its rock-to-ice ratio from formation using
various compositional models for the rocky and icy components of the
protoplanetary disk. We discuss potential confounding factors like the
sequestration of heavy metals in the core and condensation. We then show such a
measurement using atmospheric retrievals of the low-resolution UV-IR
transmission spectrum of WASP-121b with PETRA, from which we estimate a
refractory-to-volatile ratio of 5.0$^{+6.0}_{-2.7}times$ solar and a
rock-to-ice ratio greater than 2/3. This result is consistent with significant
atmospheric enrichment by rocky planetismals. Lastly, we discuss the rich
future potential for measuring refractory-to-volatile ratios in ultra-hot
Jupiters with the arrival of JWST and by combining observations at low- and
high-resolution.

A primary goal of exoplanet characterization is to use a planet’s current
composition to understand how that planet formed. For example, the C/O ratio
has long been recognized as carrying important information on the chemistry of
volatile species. Refractory elements, like Fe, Mg, and Si, are usually not
considered in this conversation because they condense into solids like Fe(s) or
MgSiO$_3$ and would be removed from the observable, gaseous atmosphere in
exoplanets cooler than about 2000~K. However, planets hotter than about 2000~K,
called ultra-hot Jupiters (UHJs), are warm enough to largely avoid the
condensation of refractory species. In this paper, we explore the insight that
the measurement of refractory abundances can provide into a planet’s origins.
Through refractory-to-volatile elemental abundance ratios, we can estimate a
planet’s atmospheric rock-to-ice fraction and constrain planet formation and
migration scenarios. We first relate a planet’s present-day
refractory-to-volatile ratio to its rock-to-ice ratio from formation using
various compositional models for the rocky and icy components of the
protoplanetary disk. We discuss potential confounding factors like the
sequestration of heavy metals in the core and condensation. We then show such a
measurement using atmospheric retrievals of the low-resolution UV-IR
transmission spectrum of WASP-121b with PETRA, from which we estimate a
refractory-to-volatile ratio of 5.0$^{+6.0}_{-2.7}times$ solar and a
rock-to-ice ratio greater than 2/3. This result is consistent with significant
atmospheric enrichment by rocky planetismals. Lastly, we discuss the rich
future potential for measuring refractory-to-volatile ratios in ultra-hot
Jupiters with the arrival of JWST and by combining observations at low- and
high-resolution.

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