Doppler tomographic measurement of the nodal precession of WASP-33b

Abstract

Abstract WASP-33b is a retrograde hot Jupiter with a period of 1.2 d orbiting a rapidly rotating and pulsating A-type star. A previous study found that the transit chord of WASP-33b had changed slightly from 2008 to 2014 based on Doppler tomographic measurements. They attributed the change to orbital precession caused by the non-zero oblateness of the host star and the misaligned orbit. We aim to confirm and more precisely model the precession behavior using additional Doppler tomographic data of WASP-33b obtained with the High Dispersion Spectrograph on the 8.2 m Subaru telescope in 2011, as well as the data sets used in the previous study. Using equations of long-term orbital precession, we constrain the stellar gravitational quadrupole moment J2 = (9.14 ± 0.51) × 10−5 and the angle between the stellar spin axis and the line of sight $i_{\star }=96^{+10}_{-14}$ deg. These updated values show that the host star is more spherical and viewed more equator than the previous study. We also estimate that the precession period is ∼840 yr. We also find that the precession amplitude of WASP-33b is ∼67° and WASP-33b transits in front of the host star for only ∼20% of the whole precession period.