Institution(s): 1. LCOGT
For open cluster ages between about 100 Myr and 500 Myr, plots of rotational period vs. color (or equivalently, stellar mass) are almost bimodal, with distinct groups fast and slow rotators at all masses between roughly 0.5 and 1.3 M_sun. One cannot explain these diagrams without invoking some process with a lifetime of a few hundred Myr, that for some but not all stars isolates most of the stellar angular momentum from the torque caused by a magnetized stellar wind. The prevailing theory [e.g. Epstein & Pinsonneault 2014 (ApJ 780, 159) and references therein] locates this process at the base of the stellar convection zone, allowing the wind to spin down the convection zone without much affecting the core. In Brown 2014 (ApJ 789,101) I suggested rather that the break occurs above the stellar photosphere, with different spatial structures of the stellar dynamos accounting for drastically different degrees of magnetic coupling to the stellar wind. In this talk I will describe preliminary results from two observing programs that aim to test the latter hypothesis.
One program uses photometry from the LCOGT (ground-based, world-wide) telescope network to measure rotational periods of stars in fairly young open clusters, to improve comparisons between modeled and observed period-color diagrams by increasing sample sizes. The LCOGT network proves nearly ideal for this kind of work, having already provided good data sets for the clusters NGC 6281 and NGC 3532. These clusters are both about 300 Myr old, filling a gap in the current age distribution of observed clusters. The second program uses K2 photometry combined with multicolor photometry (from LCOGT) and spectroscopy (from the ARC 3.5m telescope) to search for rotation-dependent differences in possible proxies for the typical spatial scale of surface magnetic fields. These include the spot/photosphere temperature contrast, and short-timescale variations in various diagnostics of projected starspot area.