Pre-main-sequence variability across the radiative–convective gap |
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| Authors: | Eric S Saunders Tim Naylor Nathan Mayne S P Littlefair |
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| Institution: | Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Santa Barbara, CA 93117, USA;School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL;Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH |
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| 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. |
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| Keywords: | stars: fundamental parameters Hertzsprung–Russell (HR) diagram stars: magnetic fields stars: pre-main-sequence stars: spots stars: variables: other |
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