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Dix Planetary Science Seminar

Tuesday, June 11, 2019
4:00pm to 5:00pm
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South Mudd 365
"A Panchromatic Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity" & "Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses"
Yayaati Chachan, Graduate Student, Department of Planetary Science, Caltech,
Nicole Wallack, Graduate Student, Department of Planetary Science, Caltech,

Abstract 1: We present the first comprehensive look at the $0.35-5$ $\mu$m transmission spectrum of the warm ($\sim 800$ K) Neptune HAT-P-11b derived from thirteen individual transits observed using the \emph{Hubble} and \emph{Spitzer Space Telescopes}. Along with the previously published molecular absorption feature in the $1.1-1.7$~$\mu$m bandpass, we detect a distinct absorption feature at 1.15~$\mu$m and a weak feature at 0.95~$\mu$m, indicating the presence of water and/or methane with a combined significance of 4.4 $\sigma$. We find that this planet's flat optical transmission spectrum and attenuated near-infrared molecular absorption features are best-matched by models incorporating a high altitude cloud layer. Atmospheric retrievals using the combined $0.35-1.7$~$\mu$m \emph{HST} transmission spectrum yield strong constraints on atmospheric cloud-top pressure and metallicity, but we are unable to match the relatively shallow \emph{Spitzer} transit depths without under-predicting the strength of the near-infrared molecular absorption bands. HAT-P-11b's \emph{HST} transmission spectrum is well-matched by predictions from our microphysical cloud models when sulphide condensates are removed, and that the physical properties of the condensates in these models are in good agreement with constraints obtained from retrievals using the Mie scattering formalism. Both forward models and retrievals indicate that HAT-P-11b most likely has a relatively low atmospheric metallicity ($<55 \; Z_{\odot}$ or $<28 \; Z_*$ with 95\% confidence), in contrast to the expected trend based on the solar system planets. Our work also demonstrates that the wide wavelength coverage provided by the addition of the \emph{HST} STIS data is critical for making these inferences.

Abstract 2: We present new 3.6 and 4.5 micron secondary eclipse measurements for five cool (T < ~1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to predictions from 1D atmosphere models for each planet. For planets with hydrogen-dominated atmospheres and equilibrium temperatures cooler than ~1000 K, this ratio should vary as a function of both atmospheric metallicity and carbon-to-oxygen ratio. We see no evidence for a solar system-like correlation between planet mass and atmospheric metallicity, but instead identify a potential (1.9 sigma) correlation between the inferred CH4/(CO+CO2) ratio and stellar metallicity. Our ability to characterize this potential trend is limited by the relatively large uncertainties in the stellar metallicity values. Our observations provide a first look at the brightness of these planets at wavelengths accessible to the James Webb Space Telescope, which will be able to resolve individual CH4, CO, and CO2 bands and provide much stronger constraints on their atmospheric compositions.

For more information, please contact Nicole Wallack by email at nwallack@caltech.edu.