Wednesday, September 9, 2015

Explaining Exoplanet Orbit and Host Star Rotation Misalignment

Magnetic Origins of the Stellar Mass-Obliquity Correlation in Planetary Systems


Spalding et al


Detailed observational characterization of transiting exoplanet systems has revealed that the spin-axes of massive (M greater than ~1.2 solar masses) stars often exhibit substantial misalignments with respect to the orbits of the planets they host. Conversely, lower-mass stars tend to only have limited obliquities. A similar trend has recently emerged within the observational dataset of young stars' magnetic field strengths: massive T-Tauri stars tend to have dipole fields that are ~10 times weaker than their less-massive counterparts. Here we show that the associated dependence of magnetic star-disk torques upon stellar mass naturally explains the observed spin-orbit misalignment trend, provided that misalignments are obtained within the disk-hosting phase. Magnetic torques act to realign the stellar spin-axes of lower-mass stars with the disk plane on a timescale significantly shorter than the typical disk lifetime, whereas the same effect operates on a much longer timescale for massive stars. Cumulatively, our results point to a primordial excitation of extrasolar spin-orbit misalignment, signalling consistency with disk-driven migration as the dominant transport mechanism for short-period planets. Furthermore, we predict that spin-orbit misalignments in systems where close-in planets show signatures of dynamical, post-nebular emplacement will not follow the observed correlation with stellar mass.

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