Accurate characterization of the stellar and orbital parameters of the exoplanetary system WASP-33 b from orbital dynamics

TL;DR

This study precisely characterizes the orbital and stellar parameters of the WASP-33 b exoplanetary system using recent measurements and orbital dynamics modeling, revealing significant spin-orbit misalignment and star oblateness effects.

Contribution

It introduces a novel method combining Doppler tomography and orbital precession modeling to constrain the star's oblateness and spin orientation in the WASP-33 system.

Findings

Determined the star's spin axis inclination to the line of sight as 142+10-11 degrees.

Measured the star's J2 oblateness parameter as (2.1+0.8-0.5)×10^{-4}.

Found a large spin-orbit misalignment angle of about 100 degrees.

Abstract

By using the most recently published Doppler tomography measurements and accurate theoretical modeling of the oblateness-driven orbital precessions, we tightly constrain some of the physical and orbital parameters of the planetary system hosted by the fast rotating star WASP-33. In particular, the measurements of the orbital inclination $i_{\rm p}$ to the plane of the sky and of the sky-projected spin-orbit misalignment $\lambda$ at two epochs about six years apart allowed for the determination of the longitude of the ascending node $\Omega$ and of the orbital inclination $I$ to the apparent equatorial plane at the same epochs. As a consequence, average rates of change $\dot\Omega_{\rm exp},~\dot I_{\rm exp}$ of this two orbital elements, accurate to a $\approx 10^{-2}~{\rm deg}~{\rm yr}^{-1}$ level, were calculated as well. By comparing them to general theoretical expressions $\dot\Omega_{J_2},~\dot I_{J_2}$ for their precessions induced by an oblate star whose symmetry axis is arbitrarily oriented, we were able to determine the angle $i^{\star}$ between the line of sight the star's spin $S^{\star}$ and its first even zonal harmonic $J_2^{\star}$ obtaining $i^{\star} = 142^{+10}_{-11}~{\rm deg},~J_2^{\star} = (2.1^{+0.8}_{-0.5})\times 10^{-4}.$ As a by-product, the angle between $S^{\star}$ and the orbital angular momentum $L$ is as large as about $\psi \approx 100$ deg $(\psi^{2008} = 99^{+5}_{-4}~{\rm deg},~\psi^{2014} = 103^{+5}_{-4}~{\rm deg})$, and changes at a rate $\dot\psi = 0.7^{+1.5}_{-1.6}~{\rm deg}~{\rm yr}^{-1}$. The predicted general relativistic Lense-Thirring precessions, or the order of $\approx 10^{-3}~{\rm deg}~{\rm yr}^{-1}$, are, at present, about one order of magnitude below the measurability threshold.