Measuring Rotation Periods with TESS in Crowded Star Clusters

TESS has observed a large fraction of the sky and produced full-frame images (FFIs) every 30 minutes (Cycles 1-2), 10 minutes (Cycles 3-4), and soon 200 seconds (Cycle 5+) for 27-day intervals. The light curves that can be extracted out of these FFIs (we use the causal pixel modeling procedure implemented in the Python unpopular package) can be used to measure stellar rotation periods. TESS light curves are perfect for studying young stars that are still rotating more rapidly than ~15 days. Since TESS has already surveyed most of the sky, dozens of young star clusters can be studied, and we can finally determine how stars converge onto slowly rotating sequence in the mass versus period diagram. However, the big challenge remaining is the large 21-arcsecond pixel size, which can cause multiple star cluster members to blend together in the crowded environments found in the richest, and often best studied, clusters. But how crowded is too crowded? How bright must a neighboring star be to interfere with period detection? To assess this, we gathered rotation period catalogs for star clusters with varying degrees of crowdedness, and periods measured from ground-based surveys with higher angular resolution than TESS, and then attempted period measurements from TESS light curves for the same stars. Based on these experiments, we will offer recommendations for avoiding source confusion, which balances maximizing the resulting sample size against spurious detections.