Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere

Abstract

Phase curve measurements provide a global view of the composition, thermal structure, and dynamics of exoplanet atmospheres. Although most of the dozens of phase curve measurements made to date are of large, massive hot Jupiters, there is considerable interest in probing the atmospheres of the smaller planets that are the more typical end product of planet formation. One such planet is the ultra-hot Neptune LTT 9779b, a rare denizen of the Neptune desert. A companion paper presents the planet's secondary eclipses and day-side thermal emission spectrum; in this work we describe the planet's optical and infrared phase curves, characterized using Spitzer and TESS photometry. We detect LTT 9779b's thermal phase variations at 4.5um, finding a phase amplitude of 358+/-106 ppm and a longitude of peak emission -10 deg +/- 21 deg east of the substellar point. Combined with our secondary eclipse observations, these phase curve measurements imply a 4.5um day-side brightness temperature of 1800+/-120 K, a night-side brightness temperature of 700+/-430 K (<1350 K at 2 sigma confidence), and a day-night brightness temperature contrast of 1110+/-460 K. We compare our data to the predictions of 3D GCMs and to similar observations of hot Jupiters experiencing similar levels of stellar irradiation. Though not conclusive, our measurement of its small 4.5um phase offset, the relatively large amplitude of the phase variation, and the qualitative differences between our target's day-side emission spectrum and those of hot Jupiters of similar temperatures all suggest a super-Solar atmospheric metallicity for LTT 9779b, as might be expected given its size and mass. Finally, we provide a refined ephemeris (P=0.79207022+/-0.00000069 d, T0=2458783.51636+/-0.00027, BJD_TDB) to enable efficient scheduling of future observations to further characterize the atmosphere of this intriguing planet. (abstract abridged)

27 pages, 14 pages, 1 data table. ApJL in press. Companion paper to Dragomir et al. 2020

References in corpus (19)

• Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy
• HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance
• Atmospheric Dynamics of Hot Exoplanets
• Spitzer Phase Curve Constraints for WASP-43b at 3.6 and 4.5 microns
• The Atmospheric Circulation of the Hot Jupiter WASP-43b: Comparing Three-Dimensional Models to Spectrophotometric Data
• The 4.5 $\mu$m full-orbit phase curve of the hot Jupiter HD 209458b
• Atmospheric Circulation of Eccentric Hot Neptune GJ436b
• Hot Nights on Extrasolar Planets: Mid-IR Phase Variations of Hot Jupiters
• Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star
• Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3
• An Ultra-Hot Neptune in the Neptune desert
• Detection of a Westward Hotspot Offset in the Atmosphere of a Hot Gas Giant CoRoT-2b
• Evidence for H${}_2$ Dissociation and Recombination Heat Transport in the Atmosphere of KELT-9b
• A transition between the hot and the ultra-hot Jupiter atmospheres
• The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with $\text{Hubble}$
• Phase Offsets and the Energy Budgets of Hot Jupiters
• Introducing a New Spitzer Master BLISS Map to Remove the Instrument-Systematic -- Phase-Curve-Parameter Degeneracy, as Demonstrated by a Reanalysis of the 4.5$\mu m$ WASP-43b Phase Curve
• Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9979b
• A dayside thermal inversion in the atmosphere of WASP-19b

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• HST Transmission Spectra of the Hot-Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane
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• Spitzer phase curve observations and circulation models of the inflated ultra-hot Jupiter WASP-76b
• HST/WFC3 Constraints on the Abundances of OH and FeH in the Atmosphere of the Ultra-Hot Neptune LTT-9779 b
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