Spin–Orbit Alignment of Exoplanet Systems: Ensemble Analysis Using Asteroseismology

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The angle ψ between a planet’s orbital axis and the spin axis of its parent star is an important diagnostic of planet formation, migration, and tidal evolution. We seek empirical constraints on ψ by measuring the stellar inclination i s via asteroseismology for an ensemble of 25 solar-type hosts observed with NASA’s Kepler satellite. Our results for i s are consistent with alignment at the 2 σ level for all stars in the sample, meaning that the system surrounding the red-giant star Kepler-56 remains as the only unambiguous misaligned multiple-planet system detected to date. The availability of a measurement of the projected spin–orbit angle λ for two of the systems allows us to estimate ψ . We find that the orbit of the hot Jupiter HAT-P-7b is likely to be retrograde ( ##IMG## [http://ej.iop.org/images/0004-637X/819/1/85/apj522683ieqn1.gif] $psi =116rc. 4_-14.7^+30.2$ ), whereas that of Kepler-25c seems to be well aligned with the stellar spin axis ( ##IMG## [http://ej.iop.org/images/0004-637X/819/1/85/apj522683ieqn2.gif] $psi =12rc. 6_-11.0^+6.7$ ). While the latter result is in apparent contradiction with a statement made previously in the literature that the multi-transiting system Kepler-25 is misaligned, we show that the results are consistent, given the large associated uncertainties. Finally, we perform a hierarchical Bayesian analysis based on the asteroseismic sample in order to recover the underlying distribution of ψ . The ensemble analysis suggests that the directions of the stellar spin and planetary orbital axes are correlated, as conveyed by a tendency of the host stars to display large values of inclination.
TidsskriftThe Astrophysical Journal
Sider (fra-til)85
Antal sider1
StatusUdgivet - 2016

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