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 Å and 5895.92 Å ) 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 Å 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 ) Å and A _ { \mathrm { Na } } = ( 1.7 \pm 0.9 ) % in the excess light curve approach and \sigma _ { \mathrm { Na } } = ( 0.850 \pm 0.034 ) Å 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 _ { Jup } for WASP-17b .