TY - JOUR A1 - Alexoudi, Xanthippi A1 - Mallonn, Matthias A1 - Keles, Engin A1 - Poppenhäger, Katja A1 - von Essen, Carolina A1 - Strassmeier, Klaus T1 - Role of the impact parameter in exoplanet transmission spectroscopy JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - planets and satellites: atmospheres KW - planets and satellites: gaseous KW - planets Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-605378 SN - 0004-6361 SN - 1432-0746 VL - 640 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Keles, Engin A1 - Mallom, Matthias A1 - von Essen, Carolina A1 - Caroll, Thorsten A. A1 - Alexoudi, Xanthippi A1 - Pino, Lorenzo A1 - Ilyin, Ilya A1 - Poppenhäger, Katja A1 - Kitzmann, Daniel A1 - Nascimbeni, Valerino A1 - Turner, Jake D. A1 - Strassmeier, Klaus G. T1 - The potassium absorption on HD189733b and HD209458b JF - Monthly Notices of the Royal Astronomical Society: Letters N2 - In this work, we investigate the potassium excess absorption around 7699 Å of the exoplanets HD189733b and HD209458b. For this purpose, we used high-spectral resolution transit observations acquired with the 2 × 8.4 m Large Binocular Telescope (LBT) and the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI). For a bandwidth of 0.8 Å, we present a detection >7σ with an absorption level of 0.18 per cent for HD189733b. Applying the same analysis to HD209458b, we can set 3σ upper limit of 0.09 per cent, even though we do not detect a K-excess absorption. The investigation suggests that the K feature is less present in the atmosphere of HD209458b than in the one of HD189733b. This comparison confirms previous claims that the atmospheres of these two planets must have fundamentally different properties. Y1 - 2021 U6 - https://doi.org/10.1093/mnrasl/slz123 VL - 489 IS - 1 SP - L37 EP - L41 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Keles, Engin A1 - Kitzmann, Daniel A1 - Mallonn, Matthias A1 - Alexoudi, Xanthippi A1 - Fossati, Luca A1 - Pino, Lorenzo A1 - Seidel, Julia Victoria A1 - Caroll, Thorsten A. A1 - Steffen, M. A1 - Ilyin, Ilya A1 - Poppenhäger, Katja A1 - Strassmeier, Klaus G. A1 - von Essen, Carolina A1 - Nascimbeni, Valerio A1 - Turner, Jake D. T1 - Probing the atmosphere of HD189733b with the Na i and K i lines JF - Monthly Notices of the Royal Astronomical Society N2 - 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. Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa2435 VL - 498 IS - 1 SP - 1033 PB - Oxford Univ. Press CY - Oxford ER -