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A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

Chachan, Yayaati and Knutson, Heather A. and Gao, Peter and Kataria, Tiffany and Wong, Ian and Henry, Gregory W. and Benneke, Björn and Zhang, Michael and Barstow, Joanna and Bean, Jacob L. and Mikal-Evans, Thomas and Lewis, Nikole K. and Mansfield, Megan and López-Morales, Mercedes and Nikolov, Nikolay and Sing, David K. and Wakeford, Hannah (2019) A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity. Astronomical Journal, 158 (6). Art. No. 244. ISSN 1538-3881.

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We present the first comprehensive look at the 0.35–5 μm transmission spectrum of the warm (~800 K) Neptune HAT-P-11b derived from 13 individual transits observed using the Hubble and Spitzer Space Telescopes. Along with the previously published molecular absorption feature in the 1.1–1.7 μm bandpass, we detect a distinct absorption feature at 1.15 μm and a weak feature at 0.95 μm, indicating the presence of water and/or methane with a combined significance of 4.4σ. We find that this planet's nearly 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 μm Hubble Space Telescope (HST) transmission spectrum yield strong constraints on atmospheric cloud-top pressure and metallicity, but we are unable to match the relatively shallow Spitzer transit depths without underpredicting the strength of the near-infrared molecular absorption bands. HAT-P-11b's HST transmission spectrum is well matched by predictions from our microphysical cloud models. Both forward models and retrievals indicate that HAT-P-11b most likely has a relatively low atmospheric metallicity (<4.6 Z⊙ and <86 Z⊙ at the 2σ and 3σ levels respectively), 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 HST STIS data is critical for making these inferences.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Chachan, Yayaati0000-0003-1728-8269
Knutson, Heather A.0000-0002-0822-3095
Gao, Peter0000-0002-8518-9601
Kataria, Tiffany0000-0003-3759-9080
Wong, Ian0000-0001-9665-8429
Henry, Gregory W.0000-0003-4155-8513
Benneke, Björn0000-0001-5578-1498
Zhang, Michael0000-0002-0659-1783
Barstow, Joanna0000-0003-3726-5419
Bean, Jacob L.0000-0003-4733-6532
Mikal-Evans, Thomas0000-0001-5442-1300
Lewis, Nikole K.0000-0002-8507-1304
Mansfield, Megan0000-0003-4241-7413
López-Morales, Mercedes0000-0003-3204-8183
Nikolov, Nikolay0000-0002-6500-3574
Sing, David K.0000-0001-6050-7645
Wakeford, Hannah0000-0003-4328-3867
Additional Information:© 2019 The American Astronomical Society. Received 2019 May 29; revised 2019 September 30; accepted 2019 October 15; published 2019 November 22. This work is based on observations from the Hubble Space Telescope, operated by AURA, Inc. on behalf of NASA/ESA. This work also includes observations made with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through Space Telescope Science Institute grants GO-14260 and GO-14767. P. Gao and I. Wong acknowledge the generous support of the Heising-Simons Foundation via the 51 Pegasi b fellowships in Planetary Astronomy. J. K. Barstow acknowledges funding support from the Royal Astronomical Society Research Fellowship. G. W. Henry acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence program. We would like to thank D. Deming and J. Fraine for exchanges regarding the WFC3 G141 data set. Y. C. also thanks C. V. Morley, S. Hörst, Y. Kawashima, Y. Wu, E. Lee, and Y. Yung for inspiring conversations.
Group:Astronomy Department
Funding AgencyGrant Number
Heising-Simons Foundation51 Pegasi b Fellowship
Royal Astronomical SocietyUNSPECIFIED
Tennessee State UniversityUNSPECIFIED
State of TennesseeUNSPECIFIED
Subject Keywords:Exoplanet atmospheres; Exoplanet atmospheric composition
Issue or Number:6
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet atmospheres (487); Exoplanet atmospheric composition (2021)
Record Number:CaltechAUTHORS:20191119-113728766
Persistent URL:
Official Citation:Yayaati Chachan et al 2019 AJ 158 244
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99933
Deposited By: Tony Diaz
Deposited On:19 Nov 2019 21:29
Last Modified:20 Apr 2020 08:47

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