  Detailed observational characterization of transiting exoplanet systems has
revealed that the spin-axes of massive (M > ~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.
