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HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b

Sing, D. K. and Lecavelier des Etangs, A. and Fortney, J. J. and Burrows, A. S. and Pont, F. and Wakeford, H. R. and Ballester, G. E. and Nikolov, N. and Henry, G. W. and Aigrain, S. and Deming, D. and Evans, T. M. and Gibson, N. P. and Huitson, C. M. and Knutson, H. and Showman, A. P. and Vidal-Madjar, A. and Wilson, P. A. and Williamson, M. H. and Zahnle, K. (2013) HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b. Monthly Notices of the Royal Astronomical Society, 436 (4). pp. 2956-2973. ISSN 0035-8711. doi:10.1093/mnras/stt1782.

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We present Hubble Space Telescope (HST) optical transmission spectra of the transiting hot-Jupiter WASP-12b, taken with the Space Telescope Imaging Spectrograph instrument. The resulting spectra cover the range 2900–10 300 Å which we combined with archival Wide Field Camera 3 spectra and Spitzer photometry to cover the full optical to infrared wavelength regions. With high spatial resolution, we are able to resolve WASP-12A's stellar companion in both our images and spectra, revealing that the companion is in fact a close binary M0V pair, with the three stars forming a triple-star configuration. We derive refined physical parameters of the WASP-12 system, including the orbital ephemeris, finding the exoplanet's density is ∼20 per cent lower than previously estimated. From the transmission spectra, we are able to decisively rule out prominent absorption by TiO in the exoplanet's atmosphere, as there are no signs of the molecule's characteristic broad features nor individual bandheads. Strong pressure-broadened Na and K absorption signatures are also excluded, as are significant metal-hydride features. We compare our combined broad-band spectrum to a wide variety of existing aerosol-free atmospheric models, though none are satisfactory fits. However, we do find that the full transmission spectrum can be described by models which include significant opacity from aerosols: including Rayleigh scattering, Mie scattering, tholin haze and settling dust profiles. The transmission spectrum follows an effective extinction cross-section with a power law of index α, with the slope of the transmission spectrum constraining the quantity αT = −3528 ± 660 K, where T is the atmospheric temperature. Rayleigh scattering (α = −4) is among the best-fitting models, though requires low terminator temperatures near 900 K. Sub-micron size aerosol particles can provide equally good fits to the entire transmission spectrum for a wide range of temperatures, and we explore corundum as a plausible dust aerosol. The presence of atmospheric aerosols also helps to explain the modestly bright albedo implied by Spitzer observations, as well as the near blackbody nature of the emission spectrum. Ti-bearing condensates on the cooler night-side is the most natural explanation for the overall lack of TiO signatures in WASP-12b, indicating the day/night cold trap is an important effect for very hot Jupiters. These findings indicate that aerosols can play a significant atmospheric role for the entire wide range of hot-Jupiter atmospheres, potentially affecting their overall spectrum and energy balance.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Sing, D. K.0000-0001-6050-7645
Lecavelier des Etangs, A.0000-0002-5637-5253
Fortney, J. J.0000-0002-9843-4354
Burrows, A. S.0000-0002-3099-5024
Pont, F.0000-0003-0076-5444
Wakeford, H. R.0000-0003-4328-3867
Nikolov, N.0000-0002-6500-3574
Henry, G. W.0000-0003-4155-8513
Aigrain, S.0000-0003-1453-0574
Deming, D.0000-0001-5727-4094
Evans, T. M.0000-0001-5442-1300
Gibson, N. P.0000-0002-9308-2353
Knutson, H.0000-0002-0822-3095
Additional Information:© 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2013 September 19; received 2013 September 4; in original form 2013 July 29. First published online: October 17, 2013. This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. Support for this work was provided by NASA through grants under the HST-GO-12473 programme from the STScI. We thank I. Baraffe for useful discussions, and the anonymous referee for their comments. We also warmly thank Jason W. Ferguson for providing the optical constants for CaTiO3. CMH, PAW and HRW acknowledge support from the UK Science and Technology Facilities Council (STFC). DKS, FP and, NN acknowledge support from STFC consolidated grant ST/J0016/1. ALE and AVM acknowledge support from the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 ‘Exo-Atmos’.
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/J0016/1
Agence Nationale pour la Recherche (ANR)ANR-12-BS05-0012
Subject Keywords:techniques: spectroscopic; planets and satellites: atmospheres; planets and satellites: individual: WASP-12b; stars: individual: WASP-12; planetary systems
Issue or Number:4
Record Number:CaltechAUTHORS:20140113-101029319
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Official Citation:D. K. Sing, A. Lecavelier des Etangs, J. J. Fortney, A. S. Burrows, F. Pont, H. R. Wakeford, G. E. Ballester, N. Nikolov, G. W. Henry, S. Aigrain, D. Deming, T. M. Evans, N. P. Gibson, C. M. Huitson, H. Knutson, A. P. Showman, A. Vidal-Madjar, P. A. Wilson, M. H. Williamson, and K. Zahnle HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b MNRAS (December 21, 2013) Vol. 436 2956-2973 first published online October 17, 2013 doi:10.1093/mnras/stt1782
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43335
Deposited On:14 Jan 2014 21:49
Last Modified:10 Nov 2021 16:36

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