The Metal-Rich Atmosphere of the Neptune HAT-P-26b. (arXiv:1903.09151v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+MacDonald_R/0/1/0/all/0/1">Ryan J. MacDonald</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Madhusudhan_N/0/1/0/all/0/1">Nikku Madhusudhan</a>
Transmission spectroscopy is enabling precise measurements of atmospheric H2O
abundances for numerous giant exoplanets. For hot Jupiters, relating H2O
abundances to metallicities provides a powerful probe of their formation
conditions. However, metallicity measurements for Neptune-mass exoplanets are
only now becoming viable. Exo-Neptunes are expected to possess super-solar
metallicities from accretion of H2O-rich and solid-rich planetesimals. However,
initial investigations into the exo-Neptune HAT-P-26b suggested a significantly
lower metallicity than predicted by the core-accretion theory of planetary
formation and solar system expectations from Uranus and Neptune. Here, we
report an extensive atmospheric retrieval analysis of HAT-P-26b, combining all
available observations, to reveal its composition, temperature structure, and
cloud properties. Our analysis reveals an atmosphere containing
1.5(+2.1)(-0.9)% H2O, an O/H of 18.1(+25.9)(-11.3)x solar, and C/O < 0.33 (to
2$sigma$). This updated metallicity, the most precise exo-Neptune metallicity
reported to date, suggests a formation history with significant planetesimal
accretion, albeit below that of Uranus and Neptune. We additionally report
evidence for metal hydrides at 4.1$sigma$ confidence. Potential candidates are
identified as TiH (3.6$sigma$), CrH (2.1$sigma$), or ScH (1.8$sigma$).
Maintaining gas-phase metal hydrides at the derived temperature (~560 K)
necessitates strong disequilibrium processes or external replenishment.
Finally, we simulate the JWST Guaranteed Time Observations for HAT-P-26b.
Assuming a composition consistent with current observations, we predict JWST
can detect H2O (at 29$sigma$), CH4 (6.2$sigma$), CO2 (13$sigma$), and CO
(3.7$sigma$), improving metallicity and C/O precision to 0.2 dex and 0.35 dex.
Furthermore, NIRISS observations could detect several metal hydrides at
>5$sigma$ confidence.
Transmission spectroscopy is enabling precise measurements of atmospheric H2O
abundances for numerous giant exoplanets. For hot Jupiters, relating H2O
abundances to metallicities provides a powerful probe of their formation
conditions. However, metallicity measurements for Neptune-mass exoplanets are
only now becoming viable. Exo-Neptunes are expected to possess super-solar
metallicities from accretion of H2O-rich and solid-rich planetesimals. However,
initial investigations into the exo-Neptune HAT-P-26b suggested a significantly
lower metallicity than predicted by the core-accretion theory of planetary
formation and solar system expectations from Uranus and Neptune. Here, we
report an extensive atmospheric retrieval analysis of HAT-P-26b, combining all
available observations, to reveal its composition, temperature structure, and
cloud properties. Our analysis reveals an atmosphere containing
1.5(+2.1)(-0.9)% H2O, an O/H of 18.1(+25.9)(-11.3)x solar, and C/O < 0.33 (to
2$sigma$). This updated metallicity, the most precise exo-Neptune metallicity
reported to date, suggests a formation history with significant planetesimal
accretion, albeit below that of Uranus and Neptune. We additionally report
evidence for metal hydrides at 4.1$sigma$ confidence. Potential candidates are
identified as TiH (3.6$sigma$), CrH (2.1$sigma$), or ScH (1.8$sigma$).
Maintaining gas-phase metal hydrides at the derived temperature (~560 K)
necessitates strong disequilibrium processes or external replenishment.
Finally, we simulate the JWST Guaranteed Time Observations for HAT-P-26b.
Assuming a composition consistent with current observations, we predict JWST
can detect H2O (at 29$sigma$), CH4 (6.2$sigma$), CO2 (13$sigma$), and CO
(3.7$sigma$), improving metallicity and C/O precision to 0.2 dex and 0.35 dex.
Furthermore, NIRISS observations could detect several metal hydrides at
>5$sigma$ confidence.
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