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

By D. Sing, A. D. Étangs, Jon Fortney, Adam Burrows, F. Pont, H. Wakeford, Gilda E. Ballester, N. Nikolov, G. Henry, Suzanne Aigrain and 10 more. 2013

154 citations recorded by the source corpus.

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 A 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/5dd6ad995ee0d81cfcb55b42a73442fc4bd502e6

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. D. Sing
5. M. Line
6. D. Deming
7. J. D'esert
8. V. Parmentier

Citations in the corpus

• HST PanCET Program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b (2017, result): “This is an effect observed in many previous studies of WFC3 exoplanet transits data (e.g., Deming et al. 2013; Mandell et al. 2013; Wakeford et al. 2013; Sing et al. 2013; Ehrenreich et al. 2014; Sing et al. 2016).”
• The Hubble Space Telescope PanCET Program: Exospheric Mg ii and Fe ii in the Near-ultraviolet Transmission Spectrum of WASP-121b Using Jitter Decorrelation (2019, background): “Previous analyses of other HST transit observations (Berta et al. 2012; Line et al. 2013a; Sing et al. 2013; Nikolov et al. 2014) have found this to be a good approximation when fitting HST STIS data.”
• SPECTROSCOPIC EVIDENCE FOR A TEMPERATURE INVERSION IN THE DAYSIDE ATMOSPHERE OF HOT JUPITER WASP-33b (2015, background): “However, previous searches for TiO in other hot Jupiters using transmission spectroscopy have yielded either secure non-detections (Huitson et al. 2013; Sing et al. 2013) or inconclusive results (Désert et al. 2008).”
• A CONSISTENT RETRIEVAL ANALYSIS OF 10 HOT JUPITERS OBSERVED IN TRANSMISSION (2016, background): “WASP-12b As discussed by Sing et al. (2013), the WASP-12b transmission spectrum is extremely challenging to fit with Rayleigh scattering cloud models.”
• Transmission spectral properties of clouds for hot Jupiter exoplanets (2014, background): “A majority of current exoplanet spectra are constructed from ground-based measurements with additional low resolution spectra from HST at wavelengths short of 1.7 µm, and in the infrared from Spitzer (e.g. Gibson et al. 2013; Pont et al. 2013; Sing et al. 2013; Nascimbeni et al. 2013).”
• TRANSITIONS IN THE CLOUD COMPOSITION OF HOT JUPITERS (2016, background): “Despite several attempts to search for TiO and VO in hot Jupiters atmospheres (Désert et al. 2009; Sing et al. 2013) weak evidence of its presence has been found only in the hottest known planet (Haynes et al. 2015).”
• HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering (2014, methodology): “Similar to our previous STIS analysis (Sing et al. 2013; Huitson et al. 2013; Nikolov et al. 2014b), we intended to discard the entire first orbit to minimise the effect of the breathing trend, but found for two of the three HST visits, a few of the exposures taken toward the end of the first orbit…”
• A DETECTION OF WATER IN THE TRANSMISSION SPECTRUM OF THE HOT JUPITER WASP-12b AND IMPLICATIONS FOR ITS ATMOSPHERIC COMPOSITION (2015, methodology): “…molecular features have been confidently identified in the emission spectrum, and measurements of the planet’s transmission spectrum have yielded even less conclusive constraints on the C/O (Sing et al. 2013; Mandell et al. 2013; Swain et al. 2013; Stevenson et al. 2014c; Madhusudhan et al. 2014b).”
• DETECTION OF H2O AND EVIDENCE FOR TiO/VO IN AN ULTRA-HOT EXOPLANET ATMOSPHERE (2016, background): “Of these, TiO/VO absorption has been ruled out for WASP-12b (Teq ∼ 2600 K; Sing et al. 2013) and WASP-19b (Teq ∼ 2100 K; Huitson et al. 2013), while evidence for TiO emission has been reported for WASP-33b (Teq ∼ 2700 K; Haynes et al. 2015).”
• WATER VAPOR IN THE SPECTRUM OF THE EXTRASOLAR PLANET HD 189733b. I. THE TRANSIT (2014, background): “Similar WFC3 spectra have been reported for WASP-12b observed in staring mode underHST programs 12230 (P.I. Swain; Swain et al. 2013) and 12473 (P.I. Sing; Sing et al. 2013).”