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An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b

Lothringer, Joshua D. and Benneke, Björn and Crossfield, Ian J. M. and Henry, Gregory W. and Morley, Caroline and Dragomir, Diana and Barman, Travis and Knutson, Heather and Kempton, Eliza and Fortney, Jonathan and McCullough, Peter and Howard, Andrew W. (2018) An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b. Astronomical Journal, 155 (2). Art. No. 66. ISSN 1538-3881.

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GJ 436b is a prime target for understanding warm Neptune exoplanet atmospheres and a target for multiple James Webb Space Telescope (JWST) Guaranteed Time Observation programs. Here, we report the first space-based optical transmission spectrum of the planet using two Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) transit observations from 0.53 to 1.03 μm. We find no evidence for alkali absorption features, nor evidence of a scattering slope longward of 0.53 μm. The spectrum is indicative of moderate to high metallicity (~100–1000× solar), while moderate-metallicity scenarios (~100× solar) require aerosol opacity. The optical spectrum also rules out some highly scattering haze models. We find an increase in transit depth around 0.8 μm in the transmission spectra of three different sub-Jovian exoplanets (GJ 436b, HAT-P-26b, and GJ 1214b). While most of the data come from STIS, data from three other instruments may indicate this is not an instrumental effect. Only the transit spectrum of GJ 1214b is well fit by a model with stellar plages on the photosphere of the host star. Our photometric monitoring of the host star reveals a stellar rotation rate of 44.1 days and an activity cycle of 7.4 years. Intriguingly, GJ 436 does not become redder as it gets dimmer, which is expected if star spots were dominating the variability. These insights into the nature of the GJ 436 system help refine our expectations for future observations in the era of JWST, whose higher precision and broader wavelength coverage will shed light on the composition and structure of GJ 436b's atmosphere.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Lothringer, Joshua D.0000-0003-3667-8633
Benneke, Björn0000-0001-5578-1498
Crossfield, Ian J. M.0000-0002-1835-1891
Henry, Gregory W.0000-0003-4155-8513
Morley, Caroline0000-0002-4404-0456
Dragomir, Diana0000-0003-2313-467X
Barman, Travis0000-0002-7129-3002
Knutson, Heather0000-0002-0822-3095
Kempton, Eliza0000-0002-1337-9051
Fortney, Jonathan0000-0002-9843-4354
McCullough, Peter0000-0001-9165-9799
Howard, Andrew W.0000-0001-8638-0320
Additional Information:© 2018. The American Astronomical Society. Received 2017 August 15; revised 2017 November 15; accepted 2017 December 4; published 2018 January 16. We thank the anonymous referee for useful comments and suggestions. We thank David Sing and Nikolay Nikolov for helpful discussion. We also thank Ron Gilliland for help with the observing proposal. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Support for this work was provided by NASA through grants HST-GO-13308 and HST-GO-13665 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. G.W.H. acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence Program. An allocation of computer time from the UA Research Computing High Performance Computing (HPC) and High Throughput Computing (HTC) at the University of Arizona is gratefully acknowledged. This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013), and PyAstronomy.13 This research has made use of NASA's Astrophysics Data System. IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.
Funding AgencyGrant Number
NASANAS 5-26555
Tennessee State UniversityUNSPECIFIED
State of Tennessee Centers of Excellence programUNSPECIFIED
Subject Keywords:planetary systems; planets and satellites: atmospheres; planets and satellites: individual (GJ 436b) ; stars: individual (GJ 436) ; techniques: spectroscopic
Issue or Number:2
Record Number:CaltechAUTHORS:20180201-112825576
Persistent URL:
Official Citation:Joshua D. Lothringer et al 2018 AJ 155 66
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84628
Deposited By: George Porter
Deposited On:01 Feb 2018 19:40
Last Modified:11 Oct 2019 18:22

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