Pre-main-sequence variability across the radiative-convective gap

TL;DR

This study investigates variability in a 13-million-year-old star cluster, revealing that variable stars are mostly fully convective, with magnetic field topology changes linked to the transition to radiative cores, affecting observed variability.

Contribution

It provides observational evidence connecting magnetic field topology changes to the convective to radiative core transition in young stars, using variability as a diagnostic.

Findings

Variable stars are mainly on the convective side of the gap.

Magnetic field topology changes at the convective-radiative transition.

Variability surveys may only detect fully convective stars.

Abstract

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.