Pre-main-sequence variability across the radiative-convective gap
  We use I band imaging to perform a variability survey of the 13 Myr-old
cluster h Per. We find a significant fraction of the cluster members to be
variable. Most importantly, we find that variable members lie almost entirely
on the convective side of the gap in the cluster sequence between fully
convective stars and those which have a radiative core. This result is
consistent with a scenario in which the magnetic field changes topology when
the star changes from being fully convective, to one containing a radiative
core. When the star is convective the magnetic field appears dominated by
large-scale structures, resulting in global-size spots that drive the observed
variability. For those stars with radiative cores we observe a marked absence
of variability due to spots, which suggests a switch to a magnetic field
dominated by smaller-scale structures, resulting in many smaller spots and thus
less apparent variability. This implies that wide field variability surveys may
only be sensitive to fully convective stars. On the one hand this reduces the
chances of picking out young groups (since the convective stars are the lower
mass and therefore fainter objects), but conversely the absolute magnitude of
the head of the convective sequence provides a straightforward measure of age
for those groups which are discovered.
