Combining low- to high-resolution transit spectroscopy of HD 189733b

Pino, Lorenzo; Ehrenreich, David; Wyttenbach, Aurélien; Bourrier, Vincent; Nascimbeni, Valerio; Heng, Kevin; Grimm, Simon; Lovis, Christophe; Malik, Matej; Pepe, Francesco; Piotto, Giampaolo (2018). Combining low- to high-resolution transit spectroscopy of HD 189733b. Astronomy and astrophysics, 612, A53. EDP Sciences 10.1051/0004-6361/201731244

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Space-borne low- to medium-resolution (ℛ ~ 10²–10³) and ground-based high-resolution spectrographs (ℛ ~ 10⁵) are commonly used to obtain optical and near infrared transmission spectra of exoplanetary atmospheres. In this wavelength range, space-borne observations detect the broadest spectral features (alkali doublets, molecular bands, scattering, etc.), while high-resolution, ground-based observations probe the sharpest features (cores of the alkali lines, molecular lines). The two techniques differ by several aspects. (1) The line spread function of ground-based observations is ~10³ times narrower than for space-borne observations; (2) Space-borne transmission spectra probe up to the base of thermosphere (P ≳ 10⁻⁶ bar), while ground-based observations can reach lower pressures (down to ~10⁻¹¹ bar) thanks to their high resolution; (3) Space-borne observations directly yield the transit depth of the planet, while ground-based observations can only measure differences in the apparent size of the planet at different wavelengths. These differences make it challenging to combine both techniques. Here, we develop a robust method to compare theoretical models with observations at different resolutions. We introduce πη, a line-by-line 1D radiative transfer code to compute theoretical transmission spectra over a broad wavelength range at very high resolution (ℛ ~ 10⁶, or Δλ ~ 0.01 Å). An hybrid forward modeling/retrieval optimization scheme is devised to deal with the large computational resources required by modeling a broad wavelength range ~0.3–2 μm at high resolution. We apply our technique to HD 189733b. In this planet, HST observations reveal a flattened spectrum due to scattering by aerosols, while high-resolution ground-based HARPS observations reveal sharp features corresponding to the cores of sodium lines. We reconcile these apparent contrasting results by building models that reproduce simultaneously both data sets, from the troposphere to the thermosphere. We confirm: (1) the presence of scattering by tropospheric aerosols; (2) that the sodium core feature is of thermospheric origin. When we take into account the presence of aerosols, the large contrast of the core of the sodium lines measured by HARPS indicates a temperature of up to ~10 000K in the thermosphere, higher than what reported in the literature. We also show that the precise value of the thermospheric temperature is degenerate with the relative optical depth of sodium, controlled by its abundance, and of the aerosol deck.

Item Type:

Journal Article (Original Article)


10 Strategic Research Centers > Center for Space and Habitability (CSH)
08 Faculty of Science > Physics Institute > Space Research and Planetary Sciences
08 Faculty of Science > Physics Institute
08 Faculty of Science > Physics Institute > NCCR PlanetS

UniBE Contributor:

Heng, Kevin and Grimm, Simon


500 Science > 520 Astronomy
500 Science > 530 Physics




EDP Sciences




Danielle Zemp

Date Deposited:

03 Jun 2019 12:55

Last Modified:

04 Nov 2019 18:21

Publisher DOI:





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