  We show that stars with transiting planets for which the stellar obliquity is
large are preferentially hot (T_eff > 6250 K). This could explain why small
obliquities were observed in the earliest measurements, which focused on
relatively cool stars drawn from Doppler surveys, as opposed to hotter stars
that emerged later from transit surveys. The observed trend could be due to
differences in planet formation and migration around stars of varying mass.
Alternatively, we speculate that hot-Jupiter systems begin with a wide range of
obliquities, but the photospheres of cool stars realign with the orbits due to
tidal dissipation in their convective zones, while hot stars cannot realign
because of their thinner convective zones. This in turn would suggest that hot
Jupiters originate from few-body gravitational dynamics, and that disk
migration plays at most a supporting role.
