@article{KelesKitzmannMallonnetal.2020,
  author    = {Keles, Engin and Kitzmann, Daniel and Mallonn, Matthias and Alexoudi, Xanthippi and Fossati, Luca and Pino, Lorenzo and Seidel, Julia Victoria and Caroll, Thorsten A. and Steffen, M. and Ilyin, Ilya and Poppenh{\"a}ger, Katja and Strassmeier, Klaus G. and von Essen, Carolina and Nascimbeni, Valerio and Turner, Jake D.},
  title     = {Probing the atmosphere of HD189733b with the Na i and K i lines},
  series = {Monthly Notices of the Royal Astronomical Society},
  volume    = {498},
  journal   = {Monthly Notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  doi       = {10.1093/mnras/staa2435},
  pages     = {1033},
  year      = {2020},
  abstract  = {High spectral resolution transmission spectroscopy is a powerful tool to characterize exoplanet atmospheres. Especially for hot Jupiters, this technique is highly relevant, due to their high-altitude absorption, e.g. from resonant sodium (Na i) and potassium (K i) lines. We resolve the atmospheric K i absorption on HD189733b with the aim to compare the resolved K i line and previously obtained high-resolution Na i-D line observations with synthetic transmission spectra. The line profiles suggest atmospheric processes leading to a line broadening of the order of ∼10 km/s for the Na i-D lines and only a few km/s for the K i line. The investigation hints that either the atmosphere of HD189733b lacks a significant amount of K i or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences we see in the resolved absorption lines.},
  language  = {en}
}
@article{AlexoudiMallonnKelesetal.2020,
  author    = {Alexoudi, Xanthippi and Mallonn, Matthias and Keles, Engin and Poppenh{\"a}ger, Katja and von Essen, Carolina and Strassmeier, Klaus},
  title     = {Role of the impact parameter in exoplanet transmission spectroscopy},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {640},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202038080},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-605378},
  pages     = {9},
  year      = {2020},
  abstract  = {Context Transmission spectroscopy is a promising tool for the atmospheric characterization of transiting exoplanets. Because the planetary signal is faint, discrepancies have been reported regarding individual targets. Aims We investigate the dependence of the estimated transmission spectrum on deviations of the orbital parameters of the star-planet system that are due to the limb-darkening effects of the host star. We describe how the uncertainty on the orbital parameters translates into an uncertainty on the planetary spectral slope. Methods We created synthetic transit light curves in seven different wavelength bands, from the near-ultraviolet to the near-infrared, and fit them with transit models parameterized by fixed deviating values of the impact parameter b. First, we performed a qualitative study to illustrate the effect by presenting the changes in the transmission spectrum slope with different deviations of b. Then, we quantified these variations by creating an error envelope (for centrally transiting, off-center, and grazing systems) based on a derived typical uncertainty on b from the literature. Finally, we compared the variations in the transmission spectra for different spectral types of host stars. Results Our simulations show a wavelength-dependent offset that is more pronounced at the blue wavelengths where the limb-darkening effect is stronger. This offset introduces a slope in the planetary transmission spectrum that becomes steeper with increasing b values. Variations of b by positive or negative values within its uncertainty interval introduce positive or negative slopes, thus the formation of an error envelope. The amplitude from blue optical to near-infrared wavelength for a typical uncertainty on b corresponds to one atmospheric pressure scale height and more. This impact parameter degeneracy is confirmed for different host types; K stars present prominently steeper slopes, while M stars indicate features at the blue wavelengths. Conclusions We demonstrate that transmission spectra can be hard to interpret, basically because of the limitations in defining a precise impact parameter value for a transiting exoplanet. This consequently limits a characterization of its atmosphere.},
  language  = {en}
}