The atmosphere of WASP-17b: Optical high-resolution transmission spectroscopy

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    Sara Khalafinejad, Max Planck Inst Astron, Max Planck Society, Michael Salz, Hamburg Univ, University of Hamburg, Hamburg Observ, Patricio E. Cubillos, Austrian Acad Sci, Austrian Academy of Sciences, Space Res Inst, George Zhou, Harvard Smithsonian Ctr Astrophys, Harvard University, Smithsonian Astrophysical Observatory, Smithsonian Institution,
  • Carolina von Essen
  • Tim-Oliver Husser, Univ Gottingen, University of Gottingen, Inst Astrophys, Daniel D. R. Bayliss, Univ Warwick, University of Warwick, Dept Phys, Mercedes Lopez-Morales, Harvard Smithsonian Ctr Astrophys, Harvard University, Smithsonian Astrophysical Observatory, Smithsonian Institution, Stefan Dreizler, Univ Gottingen, University of Gottingen, Inst Astrophys, Juergen H. M. M. Schmitt, Hamburg Univ, University of Hamburg, Hamburg Observ, Theresa Lueftinger, Univ Wien, University of Vienna, Inst Astron

High-resolution transmission spectroscopy is a method for understanding the chemical and physical properties of upper exoplanetary atmospheres. Due to large absorption cross-sections, resonance lines of atomic sodium D-lines (at 5889.95 and 5895.92 angstrom) produce large transmission signals. Our aim is to unveil the physical properties of WASP-17b through an accurate measurement of the sodium absorption in the transmission spectrum. We analyze 37 high-resolution spectra observed during a single transit of WASP-17b with the MIKE instrument on the 6.5 m Magellan Telescopes. We exclude stellar flaring activity during the observations by analyzing the temporal variations of H-alpha and Ca II infrared triplet (IRT) lines. We then obtain the excess absorption light curves in wavelength bands of 0.75, 1, 1.5, and 3 angstrom around the center of each sodium line (i.e., the light curve approach). We model the effects of differential limb-darkening, and the changing planetary radial velocity on the light curves. We also analyze the sodium absorption directly in the transmission spectrum, which is obtained by dividing in-transit by out-of-transit spectra (i.e., the division approach). We then compare our measurements with a radiative transfer atmospheric model. Our analysis results in a tentative detection of exoplanetary sodium: we measure the width and amplitude of the exoplanetary sodium feature to be sigma(Na) = (0.128 +/- 0.078) angstrom and A(Na) = (1.7 +/- 0.9)% in the excess light curve approach and sigma(Na) = (0.850 +/- 0.034) angstrom and A(Na) = (1.3 +/- 0.6)% in the division approach. By comparing our measurements with a simple atmospheric model, we retrieve an atmospheric temperature of 1550-(200) K+170 and radius (at 0.1 bar) of 1.81 +/- 0.02 R-Jup for WASP-17b.

Original languageEnglish
Article number98
JournalAstronomy & Astrophysics
Volume618
Number of pages13
ISSN1432-0746
DOIs
Publication statusPublished - 17 Oct 2018

    Research areas

  • techniques: high angular resolution, planets and satellites: atmospheres, planets and satellites: composition, methods: observational, stars: activity, CAII INFRARED TRIPLET, HOT-JUPITER, EXOPLANET WASP-17B, EXTRASOLAR PLANET, RETROGRADE ORBIT, HD 189733B, MU-M, SODIUM, SPECTRA, STARS

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