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

Abstract

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 $\AA$ and 5895.92 $\AA$) 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 meter Magellan Telescopes. We exclude stellar flaring activity during the observations by analyzing the temporal variations of H$_{\alpha}$ and Ca II infra-red triplet (IRT) lines. Then we obtain the excess absorption light curves in wavelength bands of 0.75, 1, 1.5 and 3 $\AA$ 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 through 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 $\sigma_{\mathrm{Na}}$ = (0.128 $\pm$ 0.078) $\AA$ and A$_{\mathrm{Na}}$ = (1.7 $\pm$ 0.9)% in the excess light curve approach and $\sigma_{\mathrm{Na}}$ = (0.850 $\pm$ 0.034) $\AA$ and A$_{\mathrm{Na}}$ = (1.3 $\pm$ 0.6)% in the division approach. By comparing our measurements with a simple atmospheric model, we retrieve an atmospheric temperature of 1550 $^{+170} _{-200}$ K and radius (at 0.1 bar) of 1.81 $\pm$ 0.02 R$_{\rm Jup}$ for WASP-17b.