[PAE20-P05] Doppler tomographic measurement for the orbital obliquity of WASP-33b
Keywords:Exoplanet, planetary orbit
The previous study  found that the values of λ and b of the hot Jupiter WASP-33b, which has a 1.2-day period retrograde circular orbit around a rapidly rotating and pulsating A-type star, changed slightly from 2008 and 2014. They detected its orbital precession due to its slightly flattened central star. However, only two observational epochs, from 2008 and 2014, were used in the previous study . Thus, in this study, we aim to confirm and more precisely measure the precession using a dataset from 2011, which should show values of λ and b intermediate to those in the previous study.
In our research, we used observational data of WASP-33 which was obtained using the High Dispersion Spectrograph (HDS) on the 8.2m Subaru telescope on 19th October 2011 (UT). These data are composed of 35 spectral data taken from two hours before beginning of the planetary transit to one hour after the end of the transit. First we obtained the average line profile of each exposure from a large number of absorption lines by Least-Squares-Deconvolution (LSD). Then, considering the median of these line profiles as the stellar line profile, we calculated some necessary stellar parameters for calculation of λ and b of WASP-33b by χ2 fitting. Next, subtracting the average stellar line profile from the one from each exposure, we can get the planetary shadow showing a retrograde orbit and a component from stellar pulsations. In order to make the measurement of the planetary parameters more easily, we extracted only the planetary shadow by Fourier filtering used in the previous study . Finally, we calculated λ and b of WASP-33b by χ2 fitting of the planetary shadow.
As a result, λ and b in 2011 we measured are separated values from ones in 2008 and 2014 measured in the previous study . Thus, we also found the possibility of orbital precession of WASP-33b. Moreover, we verified that our values of λ and b are different from expected intermediate ones from the previous study , which shows the different orbital precession from the expected one of the previous study.
 Johnson et al. 2015